Publications
Chapters in Books
| 1 |
T. D. Abhayapala, "Broadband
Source Localization by Modal Space Processing", in the book titled Advances
in Direction-of-Arrival Estimation, edited
by S. Chandran, Artech House, 2006, ISBN 1-59693-004-7. (preprint) |
| 2 |
T. S.
Pollock, T.D. Abhayapala, and R. A. Kennedy, “Spatial
Characterization of Multiple Antenna Channels”, in the book titled Signal Processing for Telecommunications
and Multimedia, edited by
T.A. Wysocki, B. Honary and B.J. Wysocki, Springer,
2005, ISBN: 0387228470. preprint) |
PhD Thesis
| T.D. Abhayapala, ``Modal Analysis and Synthesis of Broadband Nearfield Beamforming Arrays",
PhD Thesis, The Australian National University, December 1999. (Download) Abstract: This thesis considers the design of a beamformer which can enhance desired signals in an environment consisting of broadband nearfield and/or farfield sources. The thesis contains: a formulation of a set of analysis tools which can provide insight into the intrinsic structure of array processing problems; a methodology for nearfield beamforming; theory and design of a general broadband beamformer; and a consideration of a coherent nearfield broadband adaptive beamforming problem. To a lesser extent, the source localization problem and background noise modeling are also treated. A set of analysis tools called modal analysis techniques which can be used to a solve wider class of array signal processing problems, is first formulated. The solution to the classical wave equation is studied in detail and exploited in order to develop these techniques. Three novel methods of designing a beamformer having a desired near eld broadband beampattern are presented. The first method uses the modal analysis techniques to transform the desired nearfield beampattern to an equivalent farfield beampattern. A farfield beamformer is then designed for a transformed farfield beampattern which, if achieved, gives the desired nearfield pattern exactly. The second method establishes an asymptotic equivalence, up to complex conjugation, of two problems: (i) determining the nearfield performance of a farfield beampattern specification, and (ii) determining the equivalent farfield beampattern corresponding to a nearfield beampattern specification. Using this reciprocity relationship a computationally simple nearfield beamforming procedure is developed. The third method uses the modal analysis techniques to find a linear transformation between the array weights required to have the desired beampattern for farfield and nearfield, respectively. An efficient parameterization for the general broadband beamforming problem is introduced with a single parameter to focus the beamformer to a desired operating radius and another set of parameters to control the actual broadband beampattern shape. This parameterization is derived using the modal analysis techniques and the concept of the theoretical continuous aperture. A design of an adaptive beamformer to operate in a signal environment consisting of broadband nearfield sources, where some of interfering signals may be correlated with desired signal is also considered. Application of modal analysis techniques to noise modeling and broadband coherent source localization conclude the thesis. (Bibtex Citation detail) |
<>©
199x (200x) IEEE. Personal use of
the following material is permitted. However, permission to
reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or
redistribution to servers or lists, or to reuse any copyrighted
component of this work in other works must be obtained from the
IEEE or other publishers of
the work concerned.
Peer Reviewed Papers
2011
| 168 |
P Leong, T.A. Lamahewa and T.D. Abhayapala, ``A Framework to Calculate Level Cross-
ing Rate and Average Fade Duration in 2-D and 3-D Scattering Environments",
in IET Proc. Communications. Accepted subject to Minor revision- Aug 2011), |
| 167 |
W. Zhang, M. Zhang, R.A. Kennedy and T.D. Abhayapala, ``On high resolution head-
related transfer function measurements: An efficient sampling scheme",
in IEEE Trans. Audio, Speech and Language Processing, Vol. 20, Issue 2, pp. 575 - 584, February 2012. Abstract: This paper deals with two important questions associated with HRTF measurement: (i) what is the required angular resolution, and (ii) what is the most suitable sampling scheme. The paper shows that a well-defined finite number of spherical harmonics can capture the HRTF spatial variations in sufficient detail, which is defined as the HRTF spatial dimensionality. For the 20 kHz audible frequency range, the value of the dimensionality means a high directional resolution HRTF measurement is required. Considering such a high resolution measurement, a number of sampling criteria have been identified from both mechanical setup and data processing aspects. Different sampling candidates are then compared to demonstrate that the best method which satisfies all requirements is the class termed as IGLOO. A fast spherical harmonic transform algorithm based on the IGLOO scheme is developed to accelerate the high resolution data analysis. The proposed method is validated through simulation and experimental data acquired from a KEMAR mannequin. Online) |
| 166 |
Y. J. Wu, and T.D. Abhayapala, ``Spatial Multizone Soundfield Reproduction: Theory and
Design",
in IEEE Trans. Audio, Speech and Language Processing, Vol. 19, Issue 6, pp. 1711
- 1720, August 2011. Abstract: Spatial multizone soundfield reproduction over an extended region of open space is a complex and challenging problem in acoustic signal processing. In this paper, we provide a framework to recreate 2-D spatial multizone soundfields using a single array of loudspeakers which encompasses all spatial regions of interest. The reproduction is based on the derivation of an equivalent global soundfield consisting of a number of individual multizone soundfields. This is achieved by using spatial harmonic coefficients translation between coordinate systems. A multizone soundfield reproduction problem is then reduced to the reproduction over the entire region. An important advantage of this approach is the full use of the available dimensionality of the soundfield. This paper provides quantitative performances of a 2-D multizone system and reveals some fundamental limits on 2-D multizone soundfield reproduction. The extensions of the multizone soundfield reproduction design in reverberant rooms are also included. (Online) / Download ) |
| 165 |
A. Gupta and T.D. Abhayapala, ``Three dimensional soundfield reproduction using multiple circular loudspeaker arrays", in IEEE Trans. Audio, Speech and Language Processing, Vol. 19, Issue 5, pp. 1149 - 1159, July 2011.
Abstract: Three dimensional (3D) spatial sound field reproduction enables enhanced immersive acoustic experience for a listener. Recreating an arbitrary 3D spatial sound field using a practically realizable array of loudspeakers is a challenging problem in acoustic signal processing. This paper exploits the underlying characteristics of wavefield propagation to devise a strategy for accurate 3D sound field reproduction inside a 3D region of interest with practical array geometries. Specifically we use the properties of the associated Legendre functions and the spherical Hankel functions, which are part of the solution to the wave equation in spherical coordinates, for loudspeaker placement on a set of multiple circular arrays and provide a technique for spherical harmonic mode-selection to control the repoduced sound field. We also analyze the artifacts of spatial aliasing due to the use of discrete loudspeaker arrays in the region of interest. As an illustration, we design a a third order reproduction system to operate at a frequency of 500 Hz with 18 loudspeakers arranged in a practically realizable configuration. (Online Download) < | 164 |
M. Poletti and T.D. Abhayapala, ``Interior and exterior sound field control using general two dimensional first-order sources",
journal of Acoustic Society of America (JASA), Vol. 129,
No. 1, pp. 234-44, Jan 2011. Abstract: Reproduction of a given sound field interior to a circular loudspeaker array without producing an undesirable exterior sound field is an unsolved problem over a broadband of frequencies. At low frequencies, by implementing the Kirchhoff-Helmholtz integral using a circular discrete array of line-source loudspeakers, a sound field can be recreated within the array and produce no exterior sound field, provided that the loudspeakers have azimuthal polar responses with variable first-order responses which are a combination of a two-dimensional (2D) monopole and a radially oriented 2D dipole. This paper examines the performance of circular discrete arrays of line-source loudspeakers which also include a tangential dipole, providing general variable-directivity responses in azimuth. It is shown that at low frequencies, the tangential dipoles are not required, but that near and above the Nyquist frequency, the tangential dipoles can both improve the interior accuracy and reduce the exterior sound field. The additional dipoles extend the useful range of the array by around an octave. (Online |
2010
| 163 |
S. Nordholm, T.D. Abhayapala, S. Doclo, S. Gannot, P. Naylor, and I. Tashev, ``Microphone
Array Speech Processing", Editorial
, , In EURASIP Journal on Advances in Signal Processing, Volume 2010 (2010), Article ID 694216, 3 pages doi:10.1155/2010/694216. |
| 162 |
D. Talagala, and T. D. Abhayapala, ``Novel Head Related Transfer Function Model for Sound Source Localization", in In Proc. IEEE 4th International Conference on
Signal Processing and Communication Systems , Gold Coast, Australia, Dec. 2010. |
| 161 |
K. Zhang, R.A. Kennedy, W. Zhang, and T. D. Abhayapala, ``Internal Structure Identification of Random Process by Using Principal Component Analysis", in In Proc. IEEE 4th International Conference on
Signal Processing and Communication Systems , Gold Coast, Australia, Dec. 2010. |
| 160 |
A. Salehin, and T. D. Abhayapala, ``Time-Frequency Domain Fundamental Frequency Estimation and Localization of Quasi periodic, Pulsatic Signal in a Correlated Mixture", in In Proc. IEEE 4th International Conference on Signal Processing and Communication Systems , Gold Coast, Australia, Dec. 2010. |
| 159 |
A. Salehin, and T. D. Abhayapala, ``Frequency Domain Method for Photoacoustic Tomography with an Arbitrary Detection Geometry", in In Proc. IEEE 4th International Conference on
Signal Processing and Communication Systems , Gold Coast, Australia, Dec. 2010. |
| 158 |
A. Salehin, and T. D. Abhayapala, ``Frequency Domain Photoacoustic Tomography with Sparse Frequency Samples", In Proc. 2010 IEEE Workshop on Signal Processing Systems, San Francisco Bay Area, California, USA, October 6-8, 2010. |
| 157 |
T.D. Abhayapala and A. Gupta, ``Spherical harmonic analysis of wavefields using multiple circular sensor arrays",
, , In IEEE Trans. Audio, Speech and Language Processing, Vol 18, Issue 6, pp. 1655-1666, Aug. 2010. Abstract: Spherical harmonic decomposition of wavefields is not only an active problem in acoustic signal processing but also a useful tool in a plethora of applications such as 3D beamforming, direction of arrival estimation, and spatial sound recording. This paper presents a novel array structure consisting of a set of parallel circular arrays of sensors to decompose a wavefield into spherical harmonic components. The new structure presented here provides an alternative design to the traditional spherical microphone arrays with increased flexibility on sensor locations. We use the underlying structure of the wave propagation together with the properties of the associated Legendre functions and the spherical Bessel functions to develop a systematic approach to place circular arrays and construct a hybrid array. As an illustration, we design a fifth order spherical harmonic decomposition array using 57 microphones to operate over a frequency band of an octave and compare it with a spherical array. We use computer simulations to show the performance of the array in a beamforming example.(Online Download) |
| 156 |
A. Salehin, and T. D. Abhayapala, ``Photoacoustic Image Reconstruction from a Frequency Invariant Source Localization Perspective", In Proc. 18th European Signal processing Conference (EUSIPCO), Aalborg, Denmark, Aug. 2010. |
| 155 |
Y.J. Wu and T. D. Abhayapala, ``Multizone Soundfield Reproduction using Multiple Loudspeaker Arrays", In Proc. The 20th International Congress on Acoustics, Sydney, August 2010. |
| 154 |
S. Kodituwakku, R.A. Kennedy, and T.D. Abhayapala, ``Radial Function based Kernel Design for Time-Frequency Distributions with Application", in
IEEE Transactions on Signal
Processing , Vol. 58, issue 6, pp. 3395-3400, June 2010. Abstract: A framework based on the n-dimensional Fourier transform of a radially symmetric function is introduced to design kernels for Cohen time-frequency distributions. Under this framework, we derive a kernel formula which generalizes and unifies Margenau-Hill, Born-Jordan, and Bessel distributions, using a realization based on a n-dimensional radial delta function. The higher order radial kernels suppress more cross-term energy compared with existing lower order kernels, which is illustrated by the time-frequency analysis of atrial fibrillation from surface electro cardiogram data. (Online) |
| 153 |
T.A. Lamahewa, T.D. Abhayapala, R. Iqbal, and C. Athawudage, ``Framework to Calculate Space-Frequency Correlation in Multi-carrier Systems", in IEEE Transactions in Wireless Communications
, Vol. 9, Issue 6, pp. 1825-1831, June 2010. Abstract: This paper proposes a general framework for the calculation of space-frequency correlation at the receiver in time-invariant frequency selective channel environments. The proposed framework incorporates antenna locations to capture the spatial information of the channel and a general joint angle-delay power distribution to capture the frequency selectivity of the multipath channel. To demonstrate the strength of the proposed framework, we apply it on an orthogonal frequency division multiplexing (OFDM) system to obtain the space-frequency correlation between two antenna elements for two analytically derived joint angle-delay power distributions. (Online) |
| 152 |
T.D. Abhayapala, and A. Gupta, ``Higher order differential-integral microphone arrays", in
Journal of Acoustic Society of America (JASA) , Vol. 127, No. 5, pp. EL227–EL233, May 2010 Abstract: This paper develops theory to design higher order directional microphone arrays. The proposed higher order designs have similar inter sensor spacings as traditional first and second order differential arrays. The Jacobi–Anger expansion is used to exploit the underlying structure of microphone signals from pairs of closely spaced sensors. Specifically, the difference and sum of these microphone signals are processed to design the novel directional array. (Online ) |
| 151 |
Y.J. Wu and T. D. Abhayapala, ``Simultaneous Soundfield Reproduction at Multiple Spatial Regions", in In Proc. AES 128th Convention, London, May 2010. |
| 150 |
W. Zhang, T.D. Abhayapala, R.A. Kennedy, and R. Duraiswamy, ``Insights into Head Related Transfer Function: Spatial Dimensionality and Continuous Representation", in
Journal of Acoustic Society of America (JASA) , 127(4), pp. 2347-57, Apr. 2010 Abstract: This paper studies HRTF sampling and synthesis in a three-dimensional auditory scene based on a general modal decomposition of the HRTF in all frequency-range-angle domains. The main finding is the HRTF decomposition with the derived spatial basis function modes can be well approximated by a finite number, which is defined as the spatial dimensionality of the HRTF. The dimensionality determines the minimum number of parameters to represent the HRTF corresponding to all directions and also the required spatial resolution in HRTF measurement. The general model is further developed to a continuous HRTF representation, in which, the normalized spatial modes can achieve HRTF nearfield and farfield representations in one formulation. The remaining HRTF spectral components are compactly represented using Fourier Spherical Bessel (FSB) series, where the aim is to generate the HRTF with much higher spectral resolution in fewer parameters from typical measurements which usually have limited spectral resolution constrained by sampling conditions. A low-computation algorithm is developed to obtain the model coefficients from the existing measurements. The HRTF synthesis using the proposed model is validated by three sets of data, (i) synthetic HRTFs from the spherical head model, (ii) MIT KEMAR data, and (iii) 45-subject CIPIC measurements. (Online /Preprint) | 149 |
A. Gupta and T.D. Abhayapala, ``Double sided cone array for spherical harmonic analysis of wavefields", in In Proc. IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP), Dallas, TX, USA, pp. 77-80, Mar. 2010 |
| 148 |
S.M. A. Salehin and T.D. Abhayapala , ``Localizing Lung Sounds: Eigen Basis Decomposition for Localizing Sources within a Circular Array of Sensors", in
Journal of Signal Processing , (To Appear) DOI 10.1007/s11265-009-0435-3. Abstract: Lung disorders or injury can result in changes in the production of lung sounds both spectrally and regionally. Localizing these lung sounds can provide information to the extent and location of the disorder. Difference in arrival times at a set of sensors and triangulation were previously proposed for acoustic imaging of the chest. We propose two algorithms for acoustic imaging using a set of eigen basis functions of the Helmholtz wave equation. These algorithms remove the sensor location contribution from the multi sensor recordings using either an orthogonality property or a least squares based estimation after which a spatial minimum variance (MV) spectrum is applied to estimate the source locations. The use of these eigen basis functions allows possible extension to a lung sound model consisting of layered cylindrical media. Theoretical analysis of the relationship of resolution to frequency and noise power was derived and simulations verified the results obtained. Further, a Nyquist’s criteria for localizing sources within a circular array shows that the radius of region where sources can be localized is inversely proportional to the frequency of sound.The resolution analysis and modified Nyquist criteria can be used for determining the number of sensors required at a given noise level, for a required resolution, frequency range, and radius of region for which sources need to be localized. (Online /Download ) |
| 147 |
T.D. Abhayapala and D.B.Ward, ``Wideband Beamspace Processing Using Orthogonal Modal Beamformers", in Journal of Signal Processing Systems , DOI 10.1007/s11265-009-0421-9,
(Accepted Oct. 2009) To appear in 2010. Abstract: We introduce a novel beamspace processing structure that can be used for narrowband or wideband sources located either in nearfield or farfield of a sensor array. Main features of the new structure are: (i) a single parameter is used to steer the processor to any desired radial distance; (ii) a set of fixed frequency invariant orthogonal beamformers are used to transform array data into beamspace; and (iii) consequently, only a single set of beamspace weights are needed to process wideband beamspace data. The utility of the novel structure is illustrated by applications in interference cancellation and direction/range estimation. (Online / preprint) |
| 146 |
P.T. Samarasinghe, T.A. Lamahewa, T.D. Abhayapala and R.A. Kennedy, ``3D Mobile-to-Mobile Wireless Channel Model," in Proc. 2010 Australian Communications Theory Workshop (AusCTW), pp. 30-34, Canberra, Australia, Feb. 2010 |
2009
| 145 |
R.Iqbal, T.D. Abhayapala, J. Ahmed, and T.A. Lamahewa, ``Wigner-Ville Distribution of a Type of Non-Stationary Mobile Rayleigh Fading Channels'', Proc. 13th IEEE International Multitopic Conference 2009 (INMIC-2009) , Dec., 2009 (accepted). Abstract:The classical Clarke model of mobile radio reception assumes a constant mobile velocity. We, in this paper, relax the assumption of constant mobile velocity to allow the mobile to have constant acceleration and derive expression for the non-stationary autocorrelation function of the channel process in general 2-dimensional (2D) scattering environments. Under suitable assumptions, an expression for Wigner-Ville spectrum is obtained in isotropic scattering environment which suggests that the Wigner-Ville spectrum is a natural generalization of the Clarke’s model to constant mobile acceleration scenario. (Download) |
| 144 |
P.T. Samarasinghe, R.A. Kennedy, T.D. Abhayapala and T.A. Lamahewa, ``Space-Time Cross Correlation between Signals at Moving Receiver'', Proc. Seventh International Conference
on Information, Communications and Signal Processing (ICICS 2009) , Dec, 2009. |
| 143 |
M. Zhang, W. Zhang, R. A. Kennedy, and T. D. Abhayapala, ``HRTF Measurement on KEMAR Manikin'', Proc. ACOUSTICS 2009 (Australian Acoustical Society), Nov, 2009. |
| 142 |
Y.J. Wu and T. D. Abhayapala, ``Multizone 2D soundfield reproduction via spatial band stop filters'', Proc. 2009 IEEE Workshop on Applications of Signal Processing to Audio and
Acoustics (WASPAA 2009), Oct, 2009. Abstract: Any attempt to create multiple independent soundfields in separate zones over an extended region of open space results in unintended interference in a given zones from other zones. In this paper, we design spatial band stop filters to suppress interzone interference in the regions of interests and pass the desired soundfields with no distortion. This is achieved by using the higher order spatial harmonics of one zone to cancel the undesirable effects of the lower order harmonics of the same zone on the other zones. We illustrate the work by designing and simulating a 2D two-zone soundfield. (Download) |
| 141 |
T. D. Abhayapala and Y.J. Wu, ``Spatial soundfield reproduction with zones of quiet", in
Proc. AES 127th Convention , New York, Oct., 2009. Abstract: Reproduction of a spatial soundfield in an extended region of open space with a designated quiet zone is a challenging problem in audio signal processing. We show how to reproduce a given spatial soundfield without altering a nearby quiet zone. In this paper, we design a spatial band stop filter over the zone of quiet to suppress the interference from the nearby desired soundfield. This is achieved by using higher order spatial harmonics to cancel the undesirable effects of the lower order harmonics of the desired soundfield on the zone of quiet. We illustrate the theory and design by simulating a 2D spatial soundfield.(Download) |
| 140 |
M. Zhang, T. D. Abhayapala, D. Jayalath, D. Smith, C. Athaudage, ``Space-Frequency Block Code with Matched Rotation for MIMO-OFDM System with Limited Feedback'', EURASIP Journal on Advances in Signal Processing , Volume 2009 (2009), Article ID 231587, 15 pages
doi:10.1155/2009/231587 Abstract: This paper presents a novel matched rotation precoding (MRP) scheme to design a rate one space-frequency block code (SFBC) and a multirate SFBC for MIMO-OFDM systems with limited feedback. The proposed rate one MRP and multirate MRP can always achieve full transmit diversity and optimal system performance for arbitrary number of antennas, subcarrier intervals, and subcarrier groupings, with limited channel knowledge required by the transmit antennas. The optimization process of the rate one MRP is simple and easily visualized so that the optimal rotation angle can be derived explicitly, or even intuitively for some cases. The multirate MRP has a complex optimization process, but it has a better spectral efficiency and provides a relatively smooth balance between system performance and transmission rate. Simulations show that the proposed SFBC with MRP can overcome the diversity loss for specific propagation scenarios, always improve the system performance, and demonstrate flexible performance with large performance gain. Therefore the proposed SFBCs with MRP demonstrate flexibility and feasibility so that it is more suitable for a practical MIMO-OFDM system with dynamic parameters. (Download) |
| 139 |
S. Kodituwakku, T. D. Abhayapala, R.A. Kennedy, ``Orthogonal Basis Expansion based Atrial Activity Reconstruction for Atrial Fibrillation Electrocardiogram Analysis'', In Proc. 31st International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC'09)., Minneapolis, Minesota, USA, Sep. 2009. Abstract: We propose a novel algorithm for extracting atrial activity from single lead electrocardiogram (ECG) signal sustained with atrial fibrillation (AF), based on a short-time expansion of an orthogonal basis function set. The method preserves the time variation of spectral content of the underlying AF signal, thus time-frequency analysis of the AF signal can be successfully performed. The new method is compared to the standard average beat subtraction (ABS) method using synthetic AF sustained ECG data. The orthogonal basis expansion method has a higher correlation with the original AF signal compared to the ABS method for a range of signal to noise ratio (SNR) levels, and correlation is improved by 16% at an SNR of 0dB. Time-frequency analysis of the reconstructed AF signal based on Bessel distribution also shows the superiority of the orthogonal basis expansion method over ABS. (Download) |
| 138 |
A. Gupta and T. D. Abhayapala, ``3D soundfield reproduction using non-spherical loudspeaker arrays'', In Proc. 17th European Signal processing Conference ., Glasgow, Scotland, Aug. 2009. Abstract: Three dimensional (3D) spatial soundfield reconstruction techniques typically need multiple loudspeakers placed on the surface of a sphere covering a desired region of reconstruction. Such array geometries have certain limitations that make them impractical in most of the intended applications. In this paper, we show how to reconstruct 3D soundfields using a set of circular arrays. The proposed method uses circular harmonics alongwith the underlying theory of wavefield propagation. Specifically, we use the properties of the associated Legendre functions and the spherical Hankel functions for loudspeaker placement. As an illustration, we design a third order spherical harmonic reconstruction system using 16 loudspeakers. (Download) |
| 137 |
M. Zhang, T. D. Abhayapala, D. Jayalath, D. Smith, C. Athaudage, ``Matched Rotation
Precoding: A new Paradigm in Space-Frequency Coding'', In Proc. IEEE International
Conference on Communications (ICC), Dresden, Germany, June 2009. Abstract: This paper proposes an efficient rate one space frequency block code (SFBC) for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. The proposed SFBC incorporates concept of matched rotation precoding (MRP) to achieve full transmit diversity and optimal system performance for arbitrary number of transmit antennas, subcarrier interval and subcarrier grouping. The MRP exploits the inherent rotation property of SFBC and has relaxed restrictions on subcarrier interval and subcarrier grouping, making it ideal for adaptive/time varying systems. The lower bound of the coding gain for MRP is derived and shown that it is useful when designing a SFBC for practical scenarios, e.g. when transmitters have only partial knowledge of power delay profile or when the power delay profile has only a few dominant delayed paths. Simulation results show that the MRP can achieve a similar or better performance than existing SFBCs. (download) |
| 136 |
R. Iqbal, T. Abhayapala, P. Sadeghi, `` A technique to improve MMSE performance of PACE in time-selective Rayleigh fading channels'', In Proc. 11th Canadian Workshop on Information Theory (CWIT) , pp. 175-160, Ottawa, Canada, May 2009. Abstract: The so-called peaky signaling has been shown to improve the performance of a communication system over noncoherent fading channels in the low signal-to-noise ratio (SNR) regime. We consider a signaling scheme which is equivalent to peaky signaling over a time-selective Rayleigh fading channel and make use of the unused degrees of freedom to improve the minimum mean square error (MMSE) performance of pilot-aided channel estimation (PACE) by a processing technique incorporating low pass filtering and downsampling. We assume infinite pilot symbols and optimal Wiener smoother at the receiver and give some analytical as well numerical results suggesting significant gains in MMSE performance of PACE. IEEEXplore) |
| 135 |
W. Zhang, T. D. Abhayapala, R. A. Kennedy, R. Duraiswami, ``Modal Expansion of HRTFs:
Continuous Representation in Frequency-Range-Angle'', In Proc. IEEE International Conference on Acoustics, Speech, and Signal Pro-
cessing (ICASSP),pp. 285-288, Taipei, Taiwan, April, 2009. Abstract: This paper proposes a continuous HRTF representation in both 3D spatial and frequency domains. The method is based on the acoustic reciprocity principle and a modal expansion of the wave equation solution to represent the HRTF variations with different variables in separate basis functions. The derived spatial basis modes can achieve HRTF near-field and far-field representation in one formulation. The HRTF frequency components are expanded using Fourier Spherical Bessel series for compact representation. The proposed model can be used to reconstruct HRTFs at any arbitrary position in space and at any frequency point from a finite number of measurements. Analytical simulated and measured HRTFs from a KEMAR are used to validate the model. IEEEXplore) |
| 134 |
Y.J.Wu and T. D. Abhayapala, ``Spatial Multizone Sound eld Reproduction'', In Proc. IEEE International Conference on Acoustics, Speech, and Signal Pro-
cessing (ICASSP),Taipei, Taiwan, pp. 93-96, April, 2009. Abstract: Spatial multizone soundfield reproduction is a difficult problem, which has many potential applications. This paper provides a framework to recreate 2D spatial multizone soundfields using an array of loudspeakers. We derive the desired global soundfield by translating individual desired soundfields to a single global co-ordinate system and applying appropriate angular window functions. We reveal some of the fundamental limits of 2D multizone soundfield reproduction. We show that the ability of multizone reproduction is dependent on (i) maximum radius of multizones, (ii) window length (size, and nature), and (iii) radial distance to the furthermost zone. We illustrate the framework by designing and simulating a two dimensional two zone soundfield. IEEEXplore) |
| 133 |
T. D. Abhayapala and A. Gupta, ``Alternatives to spherical microphone arrays: hybrid
geometries'', In Proc. IEEE International Conference on Acoustics, Speech, and Signal Pro-
cessing (ICASSP),Taipei, Taiwan, pp. 81-84, April, 2009. Abstract: We present a novel theory and design for constructing microphone arrays to extract spherical harmonic components from soundfields. The proposed non-spherical array structure provides a flexible and alternative design to the traditional spherical microphone arrays with lesser restriction on sensor locations. We use the properties of the associated Legendre functions and the spherical Bessel functions to develop a systematic approach to place circular microphone arrays in three dimensions for hybrid array geometries. As an illustration, we design and simulate a fifth order spherical harmonic decomposition array using 70 microphones to operate over a frequency band of an octave. IEEEXplore) |
| 132 |
R. Iqbal, P. Sadeghi, and T. D. Abhayapala, ``Constant Power Signaling in Rayleigh Fading
Channels: Joint Output Probability Distribution and Information Rate Bounds'', In Proc.
2009 World Congress on Computer Science and Information Engineering (CSIE 2009) ,Los
Angeles/Anaheim, USA, March 31 to April 2, 2009. |
| 131 |
W. Zhang, R.A. Kennedy, and T.D. Abhayapala, ``Efficient continuous HRTF model using data independent basis functions: Experimentally guided approach'', IEEE Trans. Audio, Speech and Language Processing, Vol. 17, No. 4, pp.819--829, May, 2009. Abstract: This paper introduces a continuous functional model for head-related transfer functions (HRTFs) in the horizontal auditory scene. The approach uses a separable representation consisting of a Fourier–Bessel series expansion for the spectral components and a conventional Fourier series expansion for the spatial components. Being independent of the data, these two sets of basis functions remain unchanged for all subjects and measurement setups. Hence, the model can transform an individualized HRTF to a subject specific set of coefficients. A continuous functional model is also developed in the time domain. We show the efficient model performance in approximating experimental measurements by using the HRTF measurements from a KEMAR manikin and the synthetic data from the spherical head model. The statistical results are determined from a 50-subject HRTF data set. We also corroborate the predictive capability of the proposed model. The model has near optimal performance, which can be ascertained by comparison with the standard principle component analysis (PCA) and discrete Karhunen–Loeve expansion (KLE) methods at the measurement points and for a given number of parameters. (IEEEXplore) |
| 130 |
J. Zhang, R.A. Kennedy, and T.D. Abhayapala, ``Reduced Rank Shift Invariant Technique and its Application for Synchronization and Channel Identification in UWB Systems'', EURASIP Journal on Wireless Communications and Networking, Volume 2008, Article ID 892193, 13 pages
doi:10.1155/2008/892193 Abstract: We investigate reduced-rank shift-invariant technique and its application for synchronization and channel identification in UWB systems. Shift-invariant techniques, such as ESPRIT and the matrix pencil method, have high resolution ability, but the associated high complexity makes them less attractive in real-time implementations. Aiming at reducing the complexity, we developed novel reduced-rank identification of principal components (RIPC) algorithms. These RIPC algorithms can automatically track the principal components and reduce the computational complexity significantly by transforming the generalized eigen-problem in an original high-dimensional space to a lower-dimensional space depending on the number of desired principal signals. We then investigate the application of the proposed RIPC algorithms for joint synchronization and channel estimation in UWB systems, where general correlator-based algorithms confront many limitations. Technical details, including sampling and the capture of synchronization delay, are provided. Experimental results show that the performance of the RIPC algorithms is only slightly inferior to the general full-rank algorithms. (Online) |
| 129 |
R. Iqbal, T.D. Abhayapala, and T.A. Lamahewa, `` Generalised clarke model for mobile radio reception'', IET Communications., pp. 644-654, Apr. 2009. Abstract: Clarke’s classical model of mobile radio reception assumes isotropic rich scattering around the mobile receiver antenna. The assumption of isotropic scattering is valid only in limited circumstances. In this contribution we develop a generalized Clarke model, which is applicable to mobile radio reception in general scattering environments. We give expressions for the autocorrelation and power spectral density (PSD) of the channel fading process and demonstrate the generality of the model by applying it to different non-isotropic scattering scenarios. Using the generalized model, we analyze the effect of mobile direction of travel and the non-isotropicity on the statistics of the channel fading process. We also show that if the mobile direction of travel is equiprobable in all directions, a non-isotropic scattering environment on average is as good as an isotropic scattering environment. (preprint)/ Online |
| 128 |
S. Kodituwakku, R.A. Kennedy and T.D. Abhayapala, ``Kaiser window based kernel for time-frequency distributions'', Electronics Letters, Vol. 45, No.4, Feb. 2009. Abstract: A novel sinhc kernel, which generates a Kaiser window based time-frequency distribution for non-stationary signal analysis is introduced. The Kaiser distribution belongs to the Cohen class of time-frequency distributions, and satisfies desirable distribution properties. By controlling the shape of the Kaiser window, auto-term resolutions and crossterm magnitudes can be successfully traded. (IEEEXplore) |
| 127 |
Y. J. Wu and T.D. Abhayapala,``Theory and Design of Soundfield Reproduction using Continuous Loudspeaker Concept'', IEEE Trans. Audio, Speech and Language Processing, Vol. 17, No. 1. pp. 107-116, Jan. 2009 , Abstract: Reproduction of a soundfield is a fundamental problem in acoustic signal processing. A common approach is to use an array of loudspeakers to reproduce the desired field where the least squares method is used to calculate the loudspeaker weights. However, the least squares method involves matrix inversion which may lead to errors if the matrix is poorly conditioned. In this paper, we use the concept of theoretical continuous loudspeaker on a circle to derive the discrete loudspeaker aperture functions by avoiding matrix inversion. In addition, the aperture function obtained through continuous loudspeaker method reveals the underlying structure of the solution as a function of the desired soundfield, the loudspeaker positions, and the frequency. This concept can also be applied for the 3D soundfield reproduction using spherical harmonics analysis with a spherical array. Results are verified through computer simulations. (IEEEXplore / preprint) |
2008
2005
2004
2003
2002
2001
2000
1999
1998
1996-1997
| 126 |
A. Salehin and T.D. Abhayapala``Lung sound localization using array of acoustic sensors'', The 2nd International Conference on Signal Processing and
Communication Systems ICSPCS'2008 , Gold Coast, Australia, 5 - 17 December 2008, Abstract: This paper presents a localization algorithm to detect lung sounds using an circular array of microphones. We use the natural basis functions of propagation waves in height invariant wavefields to form a spatial minimum variance (MV) problem in eigen space. We also derive a Nyquist criteria for localizing sources within a circular. Further, the Nyquist criteria shows that the radius of the region where sources can be localized is inversely proportional to the frequency of sound. The modified Nyquist criteria can be used for determining the number of sensors required for a required resolution, frequency range, and radius of the region for which sources need to be localized. The results are corroborated by computer simulations. (preprint) |
| 125 |
R.A. Kennedy, W. Zhang, and T.D. Abhayapala, ``Spherical Harmonic Analysis and Model-Limited Extrapolation on the Sphere: Integral Equation Formulation'', The 2nd International Conference on Signal Processing and
Communication Systems ICSPCS'2008 , Gold Coast, Australia, 5 - 17 December 2008. Abstract: The classical problem of extrapolation of a bandlimited signal from limited time domain data is revisited for signals defined on the sphere. That is, given limited or incomplete measurements of an isotropic low pass signal on the unit sphere, S2, find the unique extrapolation to the complete unit sphere. Signals defined on the unit sphere arise in a number of applications, such as beampatterns in azimuth and elevation and head related transfer functions. Our investigations explore the role of integral equation operators in characterizing the extrapolation problem which leads to an iterative algorithm analogous to that obtained in the time-frequency case. (preprint) |
| 124 |
T.D. Abhayapala and A. Gupta, ``Non-spherical microphone array structures for 3D beamforming and spherical harmonic analysis'', The 11th International Workshop on Acoustic Echo and Noise Control , Seattle, Washington, USA, Sep. 2008 Abstract: Decomposition of a soundfield into spherical harmonics is a fundamental problem in acoustic signal processing. This paper shows how to design non-spherical microphone array structures in 3D space to decompose a soundfield into spherical harmonic components. We use the mode limiting property of the Bessel functions and zeros of the associated Legendre functions together with orthogonality of the exponential functions over circles to construct an array of parallel circles of microphones. The result provides flexible design guidelines to construct 3D arrays than traditional spherical arrays. A simple beamforming example is given to verify the result. (preprint) / Online) |
| 123 |
S. Kodituwakku, T.D. Abhayapala, and R.A. Kennedy,``Atrial Fibrillation Analysis using Bessel Kernel based Time Frequency Distribution Technique'', in In Proc. 35th Computers in Cardiology, pp.
, September 2008, Bologna, Italy. Abstract: We propose a Bessel kernel based time frequency distribution technique for identification and tracking of spectrum of Atrial Fibrillation (AF) in ECG. The algorithm shows a good frequency resolution and a low RMS error even when the noise dominates the signal which is critical for detecting the low amplitude AF activity within the ECG. In comparison with other time frequency distributions, the Bessel kernel reduces cross terms between frequencies in the multi-component ECG signal. Superiority of the Bessel kernel method over the short time Fourier transform (STFT) is demonstrated using a frequency modulated sinusoidal model and using real AF data. At low signal to noise levels the Bessel distribution outperforms the STFT and at an SNR of -5dB the RMS error is reduced from 1.8Hz to 0.8Hz. Also it achieves a frequency resolution of 0.5Hz at an SNR of 0dB which is four times better than that of the STFT. (preprint) / Online) |
| 122 |
Min Zhang, Thushara D.
Abhayapala, Dhammika Jayalath, David Smith, and Chandra Athaudage ``Multirate Space-Time-Frequency Linear Block Coding'', in In Proc. IEEE International Conference on
Communications (ICC), pp. 4084-4089, May 2008,
Beijing, China. Abstract: This paper presents a multirate space-timefrequency linear block coding scheme (STFBC) with full transmit diversity for a variety of transmission rates. The proposed multirate STFBC can achieve relatively smooth balance between the performance and the transmission rate for a given constellation size. Design of a space-time linear block coding (STBC) scheme is presented as a special case of the proposed multirate STFBC, which perform better than some of the existing STBCs. Moreover, optimized multirate STFBCs have also been compared with some of the existing STFBCs. Simulations results show that the design parameter has sufficient flexibility to achieve improved performance with reduced computational complexity. (preprint) |
| 121 |
Wee Gin Lim, Rodney A. Kennedy, and Thushara D. Abhayapala, ``Adaptive Decision Feedback Equalization Under Parallel Adaptation'', in In Proc. IEEE International Conference on
Communications (ICC), pp. 495--499, May 2008,
Beijing, China. Abstract: In this paper, we first propose an alternative decision feedback equalizer (DFE) scheme to the DFE scheme of Labat, Macchi and Laot. Both schemes are broken into two distinct operation modes which feature a linear equalizer during acquisition and a DFE after the initial convergence has been achieved. While avoiding some potential problems with the latter DFE, our scheme achieves similar overall performance in terms of convergence speed and steady state error. A second contribution is the development of a strategy that simultaneously adapts the linear equalizer and the DFE in a parallel fashion such that the switch-over between modes is no longer necessary. Consequently, smoother and usually faster convergence is achieved as switching disruptions are reduced. This scheme is especially advantageous under noisy and severe conditions as supported by our simulations. (IEEE Xplore) |
| 120 |
Wen Zhang, Rodney A. Kennedy, and Thushara D. Abhayapala, ``Iterative Extrapolation Algorithm for Data Reconstruction over Sphere'', in In Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), pp. 3733-3736
, April 2008,
Las vegas, USA. Abstract: Given limited or incomplete measurement data on a sphere, a new iterative algorithm is proposed on how to extrapolate signal over the whole sphere. The algorithm is based on a priori assumption that the Fourier decomposition of the signal on the sphere has finite degree of spherical harmonic coefficients, that is, the signal is modelimited. The algorithm is a simple iteration involving only the spherical harmonic decomposition. It is proven that the algorithm converges to the original signal over observation region and the convergence rate is lower bounded by the largest eigenvalue of an associated Fredholm integral equation. (download) |
| 119 |
Yan Jennifer WU and Thushara D. Abhayapala, ``Soundfield reproduction using theoretical continuous loudspeaker'', in In Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), pp. 377--380, April 2008,
Las vegas, USA. Abstract: Reproduction of a soundfield is a fundamental problem in acoustic signal processing. A common approach is to use an array of loudspeakers to reproduce the desired field where the least square method is used to calculate the loudspeaker weights. However, the least square method involves matrix inversion which may lead to errors if the matrix is poorly conditioned. In this paper, we derive a new theoretical continuous loudspeaker method to obtain the loudspeaker aperture function in order to avoid matrix inversion. In addition, the aperture function obtained through continuous loudspeaker method reveals the underlying structure of the solution as a function of the desired soundfield, the loudspeaker positions and the frequency. Results are verified through simulations. (download) |
| 118 |
Thushara D. Abhayapala, ``Generalized
framework for spherical microphone arrays: Spatial and Frequency Decomposition'', in In Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), (accepted) Vol. X, pp.
, April 2008,
Las vegas, USA. Abstract: This paper provides a generalized framework to decompose a sourcefield into its spatial and frequency components using a spherical microphone array. Spherical arrays are used to decompose a soundfield into spherical harmonics which are the natural basis functions for space over directions. This paper extends this theory by including basis functions in frequency to decompose a soundfield into spatial-frequency components. Specifically, we use spherical Bessel functions as a set of basis in frequency. The paper also shows how to avoid numerical ill-conditioning inherited in open spherical arrays at frequencies corresponding to Bessel zeros. (download) |
| 117 |
Tharaka Lamahewa, Thushara
D. Abhayapala, and Tony Pollock “Capacity Optimizing Power Loading Scheme for Spatially Constrained Antenna Arrays: Channels with Narrow Angular Spread”, In In Proc. IEEE 9th
Australian Communication Theory Workshop, AusCTW07, Christchurch, New Zealand, Feb. 2008. Abstract: This paper proposes a capacity optimizing power loading scheme for spatially constrained antenna arrays. The proposed power loading scheme is designed mainly for MIMO systems that are operating over channels with narrow angular spread values. The design problem is approached from a physical wave field perspective, in particular using a modal expansion for free space wave propagation. When a signal is transmitted from a spatially constrained antenna array, there will be an infinite number of modes excited at the transmit antenna region and only a finite number of those excited modes will have sufficient power to carry information to the other end of the channel. We show that in a non-isotropic scattering environment, correlation between adjacent modes is significantly higher than the correlation between well separated mode orders. Based on this finding, the power loading scheme is designed to eliminate the correlation between adjacent modes by allocating zero power to every second effective mode at the transmitter region. The capacity performance of the proposed scheme is evaluated for 1-D and 2-D antenna array geometries and results are compared with the traditional equal power loading scheme. We show that the proposed scheme gives more scope for capacity improvements for 1-D arrays at low angular spread values than for 2-D arrays. (download) |
| 116 |
Wen Zhang, Rodney Kennedy, and Thushara
D. Abhayapala, “Signal Estimation from Incomplete Data on the Sphere”, In In Proc. IEEE 9th
Australian Communication Theory Workshop, AusCTW07, Christchurch, New Zealand, Feb. 2008. Abstract: An iterative algorithm is proposed to estimate a signal on the sphere from limited or incomplete measurements. The algorithm is based on a priori assumption that the Fourier decomposition of the signal on the sphere has finite degree of spherical harmonic coefficients, that is, the signal is mode-limited or low-pass in character. The iteration algorithm is to reduce the mean square error between the spherical harmonic coefficients of the estimated and that of the original signal. Convergence and its numerical properties are determined using spherical harmonic analysis. A practical example of antenna radiation pattern reconstruction is given with detailed analysis. (download) |
| 115 |
Chandra Athaudage, Min Zhang, Dhammika Jayalath and Thushara
D. Abhayapala, “Classifier Based Low-Complexity MIMO Detection for Spatial Multiplexing Systems”, In In Proc. IEEE 9th
Australian Communication Theory Workshop, AusCTW07, Christchurch, New Zealand, Feb. 2008. Abstract: In this paper, we propose a low complexity detection scheme for MIMO systems incorporating spatial multiplexing. Optimal detection schemes such as maximum-likelihood (ML) detection of MIMO signals demands computational resources that are beyond the capabilities of most practical systems. Alternative reduced complexity MIMO detection techniques have been proposed, but the complexity of algorithmic schemes are in general much higher than that of the equalizer-based techniques, e.g. zero-forcing (ZF) or MMSE. On the other hand, equalizer based techniques perform relatively poor in terms of error rate. In this paper, we propose a hybrid of an equalizer-based technique and an algorithmic search stage. Based on an error matric and its probability density functions for different classes of error, a particular search region is selected for the algorithmic stage. As the probability of occurrence of error classes with larger search regions is small, overall complexity of the proposed technique remains low while providing a significant improvement in the error performance. (download) |
| 114 |
Aastha Gupta, and Thushara
D. Abhayapala, “Body Area Networks: Radio Channel Modelling and Propagation Characteristics”, In In Proc. IEEE 9th
Australian Communication Theory Workshop, AusCTW07, Christchurch, New Zealand, Feb. 2008. Abstract: Many current and future wireless devices are wearable and use the human body as a carrier. This has made the body an important part of the transmission channel of these wireless devices. Inclusion of the body as a transmission channel will see future wireless networks rely heavily on Body Area Networks (BAN). BAN will be used not only in medical applications but also in personal area network applications. In order to build BAN devices, it is imperative to model the channel accurately. Channel measurements are important, however, a closer look on the body channel can only be attained through Electromagnetic (EM) propagation modelling. This paper presents an analytical EM channel model for BAN. Specifically, the dyadic Green’s function for a cylindrical human body model is used to propose a simple channel model. Four possible cases are considered, where the transmitter and receiver are either inside or outside the body. An exact analytical expression is derived for the case where both the transmitter and receiver are outside the body. This case is then used to show how the received signal power varies around the body, with the receiver at a constant radial distance from the cylindrical axis of the body. (download) |
2007
2006
| 113 |
Thushara
D. Abhayapala, Mérouane Debbah, and Rodney A. Kennedy, “Space-Time Channel Modeling for Wireless Communications (Editorial) ”, In In EURASIP Journal on Wireless Communications and Networking, Vol. 2007, Article ID 24595, doi:10.1155/2007/24595
, 2007 |
| 112 |
Aastha Gupta, Thushara
D. Abhayapala, and Tony S. Pollock, “On wireless channel modelling: Extraction of channel from channel measurements”, In In Proc. IEEE 7th International Symposium on Communications and Information Technologies
, pp. 819-823, Oct. 2007 Abstract: Next generation systems are using directional antennas for indoor high data rate devices. This arises a need for measurement and modelling of directional channels. Indoor environments also introduce extreme multipath behaviour into the directional channel. Directional measurements raise the problem of complicated data sets with 2-D spatio-temporal information. In order to analyse these complex data sets, it is first imperative to make sure the data set actually represents the propagation channel and is not influenced by themeasuring apparatus. In order to construct the propagation channel, we discuss a novel technique of eliminating such effects from the measured data set. The method involves solving sets of simultaneous equations in a three stage process. Firstly, the data is cleaned in the spatial domain, secondly in the in the time domain and finally the channel parameters are extracted. The technique is verified on real and simulation data sets.(download) |
| 111 |
Rauf Iqbal and Thushara
D. Abhayapala, “On statistics of the mobile Rayleigh fading channel in non-isotropic scattering environments”, In In Proc. IEEE 7th International Symposium on Communications and Information Technologies
, pp. 814-818, Oct. 2007 Abstract: Scattering encountered in many wireless communications scenarios is non-isotropic. Assumption of uniform distribution of Power Azimuth Spectrum (PAS) in a non-isotropic scattering environment introduces significant errors on the second order channel statistics which are the basis for the estimation of some important receiver parameters. Moreover, there are certain applications in communications that rely solely on the statistics of the channel. In this contribution, we use the wellknown Jacobi-Anger expansion of the plane wave to develop a discrete-time generalized Rayleigh fading channel model that models the statistics of the channel in closed form in general, nonisotropic and isotropic, scattering environments. We compare the statistics of the channel for different commonly used non-isotropic scattering distributions, first, on the basis of autocorrelation, and, then, using a function (mutual information) of the correlational properties of the channel. In the latter case, through simulations, we observe the effect of varying different parameters like the angular spread, the block length of transmission and the mobile velocity which gives some interesting insights. (download) |
| 110 |
Min Zhang, A.D.S. Jayalath,
Thushara D. Abhayapala, David Smith and Chandra Athaudage,
``Compensation Decoding of Space Time Frequency Block Codes'', In IEEE Communication Letters, Vol. 11, Issue 7, pp. 610-612, July 2007 Abstract: A novel compensation decoding scheme for a given space time frequency linear block code is presented, exploiting the simplicity of zero forcing equalization, and special characteristics of the precoding matrix. The proposed decoding procedure is relatively simple and straightforward in comparison to maximum likelihood decoding (MLD) and sphere decoding (SD). The biterror-rate performance of the proposed scheme is better than zero forcing decoding and close to MLD and SD for low to medium signal-to-noise ratio range. ( Online download) |
| 109 |
Thushara D. Abhayapala, Michael
C.T. Chan, ``Limitation and Errior Analysis of Spherical Microphone
Arrays'', In Proc. 14th
International Congress on Sound and Vibration (ICSV14), pp.
Cairns, Australia, July 2007 Abstract: There has been a strong interest in theory and design of spherical microphone arrays in recent years due to their possible use in recording directional sounds for surround sound creation. The theory of such arrays is based on decomposition of sound fields to spherical harmonics which are the natural basis functions for valid sound fields over three dimensional space. However, due to physical size of practical spherical arrays, number of extractable spherical harmonic components are limited to the third order. This paper investigates the various design issues including the inherent limitations of the spherical microphone, discretization of the theoretical continuous spherical microphone into a microphone array and associated spatial aliasing problems, calibration errors of these microphones and signal processing issues. A fourth order microphone design was presented and analysed, which allowed the verification, integration and evaluation of the design issues mentioned earlier, in the context of this design. Overall, the design was capable of recording a frequency range of [340; 3400] Hz. The work presented in this paper has made the following main contributions to sound field recording: (i) Analysis of the role of rigid spheres in improving microphone design; (ii) Analysis of various microphone arrangements, which are applicable to microphone array design; (ii) A model for analysing the error due to inexact positioning of microphones in a spherical array; (iii) A set of error measures for error analysis of spherical microphones; and (iv) Design and analysis of a fourth order spherical microphone. (download) |
| 108 |
T. Lamahewa, R.A. Kennedy, T.D.
Abhayapala, V.K. Nguyen , " Spatial precoder Design for
Space-Time
Coded MIMO Systems: Based on Fixed Parameters of MIMO
Channels", In Wireless
Personal Communications, 10.1007/s11277-007-9281-4, (online). Abstract: In realistic channel environments the performance of space–time coded multiple-input multiple output (MIMO) systems is significantly reduced due to non-ideal antenna placement and non-isotropic scattering. In this paper, by exploiting the spatial dimension of a MIMO channel we introduce the novel idea of linear spatial precoding (or power-loading) based on fixed and known parameters of MIMO channels to ameliorate the effects of non-ideal antenna placement on the performance of coherent (channel is known at the receiver) and non-coherent (channel is un-known at the receiver) space–time codes. Antenna spacing and antenna placement (geometry) are considered as fixed parameters of MIMO channels, which are readily known at the transmitter. With this design, the precoder is fixed for fixed antenna placement and the transmitter does not require any feedback of channel state information (partial or full) from the receiver. We also derive precoding schemes to exploit non-isotropic scattering distribution parameters of the scattering channel to improve the performance of space–time codes applied on MIMO systems. However, these schemes require the receiver to estimate the non-isotropic parameters and feed them back to the transmitter. Closed form solutions for precoding schemes are presented for systems with up to three receive antennas. A generalized method is proposed for more than three receive antennas. (download) |
| 107 |
W. Zhang, T.D. Abhayapala, R.A. Kennedy,
``Horizontal Plane HRTF Reproduction Using Continuous Fourier-Bessel
Functions'', in Proc. 31st AES International Conference,
London, Uk, June 2007. Abstract: This paper proposes a method to reproduce the Head-Related-Transfer-Function (HRTF) in the horizontal plane. The method is based on a functional representation for HRTFs being a conventional Fourier series expansion for spatial dependence and a Fourier Bessel series expansion for the frequency components. The proposed representation can be used to predict HRTFs at any azimuth source position and at any frequency point from a finite number of parameters. Measured HRTFs from a KEMAR are used to validate the fidelity and predictive capabilities of the method. Errors between measured and modeled HRTFs are generally less than 2 percent. (download) |
| 106 |
L.
Hanlen, and T. D. Abhayapala, ` ``Space-Time-Frequency
Degrees of Freedom: Fundamental Limits for Spatial Information'', in
Proc. IEEE International
Symposium on Information Theory (ISIT2007),
pp-701-705, June 2007. Abstract: We bound the number of electromagnetic signals which may be observed over a frequency range [F −W, F +W] a time interval [0, T] within a sphere of radius R. We show that the such constrained signals may be represented by a series expansion whose terms are bounded exponentially to zero beyond a threshold. Our result implies there is a finite amount of information which may be extracted from a region of space via electromagnetic radiation. (download) |
| 105 |
R.A. Kennedy, P. Sadeghi, T.D.
Abhayapala, and H. Jones, ``Intrinsic Limits of Dimensionality and
Richness in Random Multipath Fields'', In IEEE Trans. Signal Processing, Vol.
55, Issue 6, Part 1, pp. 2542-2556, June 2007 ( Online Journal ) Abstract: We study the dimensions or degrees of freedom of farfield multipath that is observed in a limited, source-free region of space. The multipath fields are studied as solutions to the wave equation in an infinite-dimensional vector space. We prove two universal upper bounds on the truncation error of fixed and random multipath fields. A direct consequence of the derived bounds is that both fixed and random multipath fields have an effective finite dimension. For circular and spherical spatial regions, we show that this finite dimension is proportional to the radius and area of the region, respectively. We use the Karhunen-Loeve (KL) expansion of random multipath fields to quantify the notion of multipath richness. The multipath richness is defined as the number of significant eigenvalues in the KL expansion that achieves 99% of the total multipath energy. We prove a lower bound on the largest eigenvalue. This lower bound quantifies, to some extent, the well-known reduction of multipath richness with reducing the angular spread of multipath angular power spectrum. We also provide a numerical algorithm to find multipath eigenvalues, which unlike the Fredholm equation method, does not require selecting quadrature points. (preprint) |
| 104 |
M.I.Y. Williams, T.D.
Abhayapala, and R.A. Kennedy, “Generalized Broadband Beamforming using
a Modal Subspace Decomposition,” in
EURASIP Journal on Advances in Signal Processing, Vol. 2007,
Article ID 68291, 9 pages, doi:10.1155/2007/68291 Abstract: We propose a new broadband beamformer design technique which produces an optimal receiver beampattern for any set of field measurements in space an, d time. The modal subspace decomposition (MSD) technique is based on projecting a desired pattern into the subspace of patterns achievable by a particular set of space-time sampling positions. This projection is the optimal achievable pattern, in the sense that it minimizes the mean-squared error (MSE) between the desired and actual patterns. The main advantage of the technique is versatility as it can be applied to both sparse and dense arrays, non-uniform and asynchronous time sampling, and dynamic arrays where sensors can move throughout space. It can also be applied to any beampattern type, including frequencyinvariant and spot pattern design. A simple extension to the technique is presented for oversampled arrays which allows highresolution beamforming whilst carefully controlling input energy and error sensitivity. (download) ( Online Journal ) |
| 103 |
L. Hanlen, and T. D.
Abhayapala, ``Bounds on Space-Time-Frequency Dimensionality”, in Proc. IEEE 8th Australian Communication
Theory Workshop, AusCTW07, Adelaide, Australia, pp. 144-149,
Feb. 2007. Abstract: We bound the number of electromagnetic signals which may be observed over a frequency range 2W for a time T within a region of space enclosed by a radius R. Our result implies that broadband fields in space cannot be arbitrarily complex: there is a finite amount of information which may be extracted from a region of space via electromagnetic radiation. Three-dimensional space allows a trade-off between large carrier frequency and bandwidth. We demonstrate applications in super-resolution and broadband communication. (download) |
| 102 |
Yan Jennifer Wu, and T. D.
Abhayapala, “Spatial Decorrelation Receiving technique for Frequency
Selective SIMO Channels”, in Proc.
IEEE 8th Australian Communication Theory Workshop, AusCTW07,
Adelaide, Australia, pp. 122-128, Feb. 2007. Abstract: In multi-antenna system, the correlations between the receiver antennas degrades the systems performance. In this paper, we propose a spatial decorrelation receiving technique in frequency-selective SIMO channels. A spatial-to-modal matrix is placed after the SIMO channel output to remove the correlations between the receiver antennas caused by the limited antenna separations. Simulation results of this newly proposed spatial decorrelation design demonstrate favourable BER improvement and better system performance. (download) |
| 101 |
R. Iqbal, and T. D. Abhayapala,
``Impact of Mobile Acceleration on the Statistics of Rayleigh Fading
Channel'', in Proc. IEEE 8th
Australian Communication Theory Workshop, AusCTW07, Adelaide,
Australia, pp. 129-133, Feb. 2007. Abstract: Clarke's model of the received signal statistics in a mobile isotropic scattering environment assumes a constant mobile velocity, a consequence of which is that the autocorrelation function of the received signal and the Power Spectral Density (PSD) are independent of the absolute time. In this contribution we relax the assumption of constant mobile velocity and analyze the statistics of the channel when the mobile receiver has a constant acceleration. First, we derive expressions for a general scattering environment and, then, specialize them to the case of isotropic scattering environment. The autocorrelation and PSD of the channel are not only a function of the lag, τ, but the absolute time index, n as well. There are now two kinds of PSDs: The conventional PSD , based on the well-known Wiener-Khintchine theorem, gives the spectrum in the τ domain. The second PSD is concerned with the variation of the channel with time and gives the spectrum in the n domain. The simulation results suggest that the two PSDs show a pattern of periodicity which can be explained by considering asymptotic approximation of the Bessel function. Moreover, the magnitudes of the PSDs diminish with increasing τ or n such that the conventional PSD approaches uniform distribution over 0 to 2π when time n is large whereas the PSD of channel variation with time approaches zero for large τ. We give results on the basis of simulations and justify analytically, or heuristically. We also discuss different implications of these results. (download) |
| 100 |
Rasika
R. Perera, Tony S. Pollock, and Thushara
D. Abhayapala, ``Gaussian inputs: Performance limits over
non-coherent SISO and MIMO channels'', in European
Transactions on Telecommunications,
Vol 18, pp 235–244, 2007 (download) Abstract: Performance limits of information transfer over a discrete time memoryless Rayleigh fading channel with neither the receiver nor the transmitter knowing the fading coefcients except its statistics is an important problem in information theory. We derive closed form expressions for the mutual information of single input single output (SISO) and multiple input multiple output (MIMO) Rayleigh fading channels for any antenna number at any signal to noise ratio (SNR). Using these expressions, we show that the maximum mutual information of non-coherent Rayleigh fading MIMO channels is achieved with a single transmitter and multiple receivers when the input distribution is Gaussian. We show that the addition of transmit antennas for a xed number of receivers result in a reduction of mutual information. Furthermore, we argue that the mutual information is bounded by the SNR in both SISO and MIMO systems showing the sub optimality of Gaussian signalling in non-coherent Rayleigh fading channels. ( online ) |
2006
| 99 |
R.R. Perera, K. Nguyen, T.P.
Pollock, T.D.
Abhayapala, ``Capacity of non-coherent Rayleigh fading MIMO
channels'', in IEE Proc.
Communications, vol. 6, pp. 976-983,
Dec. 2006. Abstract: This paper investigates the capacity of discrete time uncorrelated Rayleigh fading multiple input multiple output (MIMO) channels without channel state information (CSI) at neither the transmitter or the receiver. We prove that to achieve the capacity, the amplitude of the multiple input needs to have a discrete distribution with a finite number of mass points with one of them located at the origin. We show how to compute the capacity numerically in multi antenna configuration at any signal to noise ratio (SNR) with the discrete input using the Kuhn-Tucker condition for optimality. Furthermore, we show that at low SNR, the capacity with two mass points is optimal. As the number of receiver antennas increases, the maximum SNR at which two mass points are optimal decreases. Using this result we argue that on-off keying is optimal in non-coherent Rayleigh fading MIMO channels at low SNR. (download) ( Online ) |
| 98 |
Rauf Iqbal, Thushara D.
Abhayapala, and Tharaka A. Lamaheva, ``Information Rates of
Time-Varying Rayleigh Fading Channels in Non-Isotropic Scattering
Environments'', in Proc. 5th
Workshop of the Internet Telecommunications and Signal Processing,
WITSP'2006, Hobart, Australia Dec. 2006
(ISBN: 0 9756934 2 5) Abstract: In this contribution a temporally correlated rayleigh fading channel model applicable in general scattering environments, with Clarke's isotropic scattering environment as a special case, is considered. For a fixed direction of mobile travel, the results show that the information rate penalty incurred for not knowing the channel state information (CSI) in a non-isotropic scattering environment can be significantly less than that for the isotropic scattering environment for the same average received signal-to-noise power ratio (SNR). The results show that for a fixed mean angle of arrival, fading rate and direction of mobile travel, higher information rates are achievable in case of more non-isotropic scattering environment. The results are presented for di®erent non-isotropic scattering environments in terms of the normalized fading rate and SNR, and are compared with those for isotropic scattering environments. (paper) |
| 97 |
D. Smith and T.D.
Abhayapala, ``Maximal Ratio Combining Performance Analysis in
Rayleigh Fading Channels'', in IEE
Proc. Communications, Vol. 153, Issue 5, pp. 755-761, Oct 2006 Abstract: This paper presents a novel theoretical method to analyse the maximal ratio combining (MRC) receivers using BPSK and M-PSK modulation in spatially correlated Rayleigh fading channels, with perfect and imperfect channel knowledge, in terms of antenna array configuration and parameters of scatterer distributions. In this analysis we derive closed form expressions for error probabilities of these modulations with MRC, allowing for non-distinct eigenvalues from the channel gain correlation matrix. The results of performance analysis assuming different receiver configurations and scattering scenarios presented give valuable insight into the performance of MRC in practical Rayleigh fading scenarios for isotropic and non-isotropic scatterer distributions. (online/ preprint) |
| 96 |
T. Lamahewa, T.D. Abhayapala,
R.A. Kennedy, V. K. Nguyen ''Spatial Precoder Design for
Differential Space-Time Coded Systems: Based on fixed parameters of
MIMO channels'', in Proc. The
Seventh IEEE International
Workshop
on Signal Processing Advances in Wireless Communications, SPWAC
2006, July, 2006. (Online)/ download) Abstract: In this paper, we introduce the novel idea of linear spatial precoding based on fixed and known parameters of MIMO channels to improve the performance of differential space-time coding schemes. Antenna spacing and antenna placements (geometry) are considered as fixed parameters of MIMO channels, which are known at the transmitter. This new linear spatial precoding scheme exploits the antenna placement information at both ends of the MIMO channel to ameliorate the effect of non-ideal antenna placement on the performance of differential space-time coded systems. With this design, the precoder is fixed for fixed antenna placement and the transmitter does not require any feedback of channel state information (partial or full) from the receiver. A second precoding scheme is proposed by exploiting the non-isotropic scattering distribution parameters of the scattering channel. Unlike in the first scheme, the second scheme requires the receiver to estimate the non-isotropic distribution parameters and feed them back to the transmitter. Simulation results show that first scheme provides significant performance improvements at low SNRs and the second scheme provides significant performance improvements both at low and high SNRs. |
| 95 |
Terence Betlehem, Thushara D.
Abhayapala, ``Dependence of MIMO System Performance on the Joint
Properties of Angular Power'', In Proc. 2006 IEEE International
Symposium on Information Theory (ISIT), pp. 2849-2853, Seattle,
Washington, July, 2006. (online) Abstract: In this paper, we use a novel MIMO channel model to characterize the dependence of ergodic capacity and diversity order on the joint statistics of the angular power density. The scattering environment of a MIMO channel is characterized by a double directional angular power distribution, describing the power transferred in each direction from transmitter aperture to receiver aperture. Angular power, which is typically separable Kronecker-modelled, is here generalized to include joint distribution properties using well-known bivariate probability density functions. We show that the joint properties of the power density, namely the shape and the orientation of power distribution contours, have significant impact on capacity and diversity of non-line-of-sight (NLOS) channels (download) |
| 94 |
Parastoo Sadeghi, Predrag
Rapajic, Rodney Kennedy, Thushara Abhayapala ``Autoregressive
Time-Varying Flat-Fading Channels: Model Order and Information Rate
Bounds'', In Proc. 2006 IEEE
International Symposium on Information
Theory, pp. 1061–1065, Seattle,
Washington, July, 2006. (online) Abstract: In this paper, we study the effect of channel memory order on the information rate bounds in time-varying flat-fading (FF) channels. We model the time variations of FF channel with autoregressive (AR) processes with varying degrees of model order. We observe that in high SNR conditions (SNR & 20 dB), the information rate penalty of not knowing the AR channel monotonically decreases with increasing the AR model order. This is expected, since AR channel predictability increases with its order. However, in low SNR conditions, the information rate penalty is not monotonically decreasing with increasing the model order. Likewise, the intuitive, universal, monotonic increase of the information rate bounds with AR model order is only observed in almost noiseless conditions. In the low SNR regime, however, the achievable information rate bounds in low-order AR channels can be higher than those in high-order AR channels. /download) |
| 93 |
Parastoo
Sadeghi, Thushara D. Abhayapala, Rodney Kennedy, ``Intrinsic Finite
Dimensionality of Random Multipath Fields'', in Proc. IEEE
International Conference on Acoustics, Speech, and Signal Processing
(ICASSP), Vol. 4, pp 17-20, May 2006, Toulouse, France. (online) Abstract: We study the dimensions or degrees of freedom of random multipath fields in wireless communications. Random multipath fields are presented as solutions to the wave equation in an infinite-dimensional vector space. We prove a universal bound for the dimension of random multipath field in the mean square error sense. The derived maximum dimension is directly proportional to the radius of the two-dimensional spatial region where the field is coupled to. Using the Karhunen-Loeve expansion of multipath fields, we prove that, among all random multipath fields, isotropic random multipath achieves the maximum dimension bound. These results mathematically quantify the imprecise notion of rich scattering that is often used in multiple-antenna communication theory and show that even the richest scatterer (isotropic) has a finite intrinsic dimension. ( download) |
| 92 |
Terence Betlehem, Thushara D.
Abhayapala, ''Spatial Correlation for Correlated Scatterers'', in Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), May 2006,
Vol 4, pp. 5-8, Toulouse, France. (online) Abstract: This paper investigates the correlations between sensor signals in multipath environments created by correlated scatterers. We derive a closed form expression for the correlation in fields created by arbitrary scatterer correlations and scatterer powers, using Fourier techniques, and propose reasonable function forms for scatterer correlation. Simulations show notable differences from the uncorrelated scatterer case. (download) |
| 91 |
Michael Williams, Thushara D.
Abhayapala, Rodney Kennedy, ``Generalized
Broadband Beamforming Using a Modal Decomposition'', in Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), Vol. 4, pp.
1021-1024, May 2006,
Toulouse, France. (online) Abstract: We propose a new broadband beamformer design technique which produces an optimal beampattern for any set of samples in space and time. The modal subspace decomposition (MSD) technique is based on projecting a desired pattern into the subspace of patterns achievable by a particular set of space-time sampling positions. This projection is the optimal achievable pattern, in the sense that it minimizes the mean-squared error (MSE) between the desired and actual patterns. The main advantage of the technique is versatility as it can produce optimal beamformers for both sparse and dense arrays, non-uniform and asynchronous time sampling, and dynamic arrays where sensors can move throughout space. It can also be applied to any beampattern type, including frequency-invariant and spot pattern design. (download) |
| 90 |
Tharaka Lamahewa, Thushara D.
Abhayapala, Rodney A. Kennedy, Jaunty Ho,
``Space-Time Cross Correlation and Space-Frequency Cross Spectrum in
Non-Isotropic Scattering Environments'', in Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing (ICASSP), May 2006,
Vol. 4, pp. 609-612, Toulouse, France. Online) Abstract: This paper presents a method which allows to obtain expressions for the space-time cross correlation (STCC) and the space-frequency cross spectrum (SFCS) between signals at two receiver antennas on a mobile unit (MU) for an arbitrary scattering distribution surrounding the MU. (download) |
| 89 |
Parastoo Sadegi, Thushara D.
Abhayapala, and Rodney A. Kennedy,
``Directional Random Scattering MIMO Channels: Entropy Analysis and
Capacity Optimization'', in Proc.
2006 IEEE International Conference on Communications (ICC 2006),
vol. 3, pp. 1331-1336, Jun, 2006. Abstract: In this paper, we study the effect of directional random scattering on the capacity of multiple-input multiple-output (MIMO) systems. First, we use the spatial decomposition of the MIMO channel matrix to analyze the randomness (entropy) of directional scattering. The analysis shows that directional scatterers (with at least a null in the angular power spectrum) will no longer be random when the receiver observation radius is sufficiently large. Therefore, directional scattering limits the expected linear increase of MIMO capacity with increasing the number of antennas. Second, we consider the effect of receiver antenna arrangement (positions) on the capacity of MIMO systems. For any random scatterer with a given angular power spectrum, we show that it is possible to choose the receiver antenna arrangement with the optimum whitening of the MIMO channel matrix that, in turn, maximizes MIMO channel capacity. (Online) (download) |
| 88 |
Tharaka A.
Lamahewa, Van Khanh Nguyen, and Thushara D. Abhayapala, ``Exact
Pairwise Error Probability of Differential Space-Time Codes in
Spatially Correlated Channels,'' in Proc.
2006 IEEE International Conference on Communications (ICC 2006),
vol. 10, pp. 4853-4858, Jun, 2006. Abstract: In this paper, we derive an analytical expression for the exact pairwise error probability (PEP) of a differential space-time coded system operating over a spatially correlated slow fading channel. An analytic model for spatial correlation is used which fully accounts for antenna spacing, antenna geometry and non-isotropic scattering distributions. Inclusion of spatial information in error performance analysis provides valuable insights into the physical factors determining the performance of a differential space-time code (DSTC). Using this new PEP expression, we investigate the effects of antenna spacing, antenna geometries and azimuth power distribution parameters (angle of arrival/departure and angular spread) on the performance of a differential space-time block code (DSTBC) proposed in the literature for two transmit antennas. (Online) (download) |
| 87 |
Rasika
R. Perera, Tony S. Pollock, and Thushara D. Abhayapala, ``Non-coherent
Rayleigh fading MIMO channels: Capacity and optimal input'', in Proc. IEEE International Conference
on Communications (ICC 2006), vol. 9, pp. 4180-4185, Jun, 2006. Abstract: Information transfer over a discrete time uncorrelated Rayleigh fading multiple input multiple output (MIMO) channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information (CSI) except the fading statistics. We derive a capacity supremum with the receive antenna number at any SNR using Lagrange optimisation. We show that the asymptotic capacity is double logarithmic when the input power is large. We prove that to achieve the capacity, the amplitude of the multiple input needs to have a discrete distribution with a finite number of mass points, one of them necessarily located at the origin. We show how to compute the capacity numerically in multi-antenna configuration at any signal to noise ratio (SNR) with the discrete input using the Kuhn-Tucker condition for optimality. Furthermore, we show that the capacity with two mass points is optimal at low SNR signifying on-off keying. As the number of receive antennas increases, the maximum SNR at which two mass points are optimal decreases. (download) (Online) |
| 86 |
Terence Betlehem, Thushara D.
Abhayapala, ''A Space-time Channel Simulator using Angular Power
Distributions'' in Proc. 63rd IEEE
Vehicular Technology Conference (VTC), vol. 6, pp.
2891-2895, May 2005, Melbourne,
Australia. Abstract: In this paper, we develop a channel simulator to generate the channel gains to an arbitrary array of receiver antennas, for a general class of non-line-of-sight channels. The channel scattering environment is defined by the angular power distribution as seen by the receiver. We derive the second order statistics of the channel gains in terms of the parameters of the angular power distribution. As an illustration of the channel simulator, we compare the performance of different direction-of-arrival techniques. (Online)( download) |
| 85 |
Wen Zhang,
Thushara D. Abhayapala, and Jian Zhang, ''UWB Spatial-Frequency
Channel Characterization'', in Proc.
63rd IEEE Vehicular Technology Conference (VTC), vol.6, pp.
2731-2736, May 2005,
Melbourne, Australia. Abstract: This paper investigates the spatial-frequency channel characterization of Ultra-wideband (UWB) wireless communication systems. Firstly, a novel frequency dependent UWB channel model is constructed based on the theory of electromagnetic diffraction mechanism, which causes the field strength to vary with the frequency in each multipath. Secondly, we build a space-frequency model, which includes spatial characteristics such as angular power spectrum, and physical sampling points in space. The space-frequency model has two special cases (i) discrete multipath model, and (ii) cluster model, which can be readily used to generate channel data for any arbitrary set of sensor locations. The reconstruction results from channel measurements show the accurateness of the novel frequency dependent model, with reconstruction error decreasing by 40%, compared to the traditional Turin model. (download) |
| 84 |
X. Yuan, J. Zhang, D. Jayalath
and T. D. Abhayapala, ``Influence of Timing Offset in Multiband
OFDM Systems'',
First International Conference on Wireless Broadband and Ultra
Wideband Communications (AuS Wireless 2006), Feb.
2006, http://epress.lib.uts.edu.au/ocs/viewrecord.php?id=73 Abstract: In Cyclix prefixed OFDM (CP) systems, synchronization to the maximal multipath signal generally causes no problem, however, interchip interference (ICI) will be introduced in Zero-padded OFDM (ZP-OFDM) systems, e.g, multiband OFDM UWB (MB-OFDM) systems. In this paper, based on the overlap-and-add (OLA) method typically applied in MB-OFDM, we analyze the cause of the ICI and the performance degradation due to the ICI. The expressions of the channel estimation error, signal-to-noise ratio and ICI are derived. Simulation results are also provided, which show good match with the theoretical results. (download) |
| 83 |
T.A.
Lamahewa, T.D. Abhayapala, R.A. Kennedy, and Marvin Simon, ``Exact
Pairwise Error Probability Analysis of Space-Time
Codes in Realistic Propagation Environments'', in Journal of Telecommunications and
Information Technology, pp. 60-68, Q1, 2006. Abstract: In this paper, we derive an analytical expression for the exact pairwise error probability (PEP) of a space-time coded system operating over a spatially correlated slow fading channel using a moment-generating function-based approach. This analytical PEP expression is more realistic than previously published exact-PEP expressions as it fully accounts for antenna spacing, antenna geometries (Uniform Linear Array, Uniform Grid Array, Uniform Circular Array, etc.) and scattering models (Uniform, Gaussian, Laplacian, Von-mises, etc). Inclusion of spatial information provides valuable insights into the physical factors determining the performance of a spacetime code. We demonstrate the strength of our new analytical PEP expression by evaluating the performance of two spacetime trellis codes proposed in the literature for different spatial scenarios. (download)/ Link to Journal |
| 82 |
Rasika
R. Perera, Tony S. Pollock, and Thushara D. Abhayapala, ''On
Capacity of Non-Coherent Rayleigh Fading MIMO Channels'' in Proc. 7th
Australian Communication Theory Workshop AusCTW05,
Perth, Australia, pp. 29-35, Feb 2006. Abstract: This paper investigates the capacity of discrete time uncorrelated Rayleigh fading multiple input multiple output (MIMO) channels with no channel state information (CSI) at both the transmitter and the receiver. We prove that to achieve the capacity, the amplitude of the multiple input needs to have a discrete distribution with a finite number of mass points with one of them located at the origin. We show how to compute the capacity numerically in multi antenna configuration at any signal to noise ratio (SNR) with the discrete input using the Kuhn-Tucker condition for optimality. Furthermore, we show that at low SNR, the capacity with two mass points is optimal. Since the first mass point is necessarily located at the origin, we argue that at low SNR, on-off keying is optimal for any antenna number. As the number of receiver antennas increases, the maximum SNR at which two mass points are optimal decreases. (Online (download) |
| 81 |
Thraka Lamahewa, Thushara D.
Abhayapala, Rodney A. Kennedy, and Terence
Betlehem, ''MIMO Channel Correlation in General Scattering
Environments,'' in Proc. 7th Australian Communication Theory Workshop
AusCTW05, Perth, Australia, pp. 91-96, Feb 2006. Abstract: This paper presents an analytical model for the fading channel correlation in general scattering environments. In contrast to the existing correlation models, our new approach treats the scattering environment as non-separable and it is modeled using a bi-angular power distribution. The bi-angular power distribution is parameterized by the mean departure and arrival angles, angular spreads of the univariate angular power distributions at the transmitter and receiver apertures, and a third parameter, the covariance between transmit and receive angles which captures the statistical interdependency between angular power distributions at the transmitter and receiver apertures. When this third parameter is zero, this new model reduces to the well known ``Kronecker" model. Using the proposed model, we show that Kronecker model is a good approximation to the actual channel when the scattering channel consists of a single scattering cluster. In the presence of multiple remote scattering clusters we show that Kronecker model over estimates the performance by artificially increasing the number of multipaths in the channel. (download)( online) |
| 80 |
Terence Betlehem, Thushara D.
Abhayapala, and Tharaka Lamahewa,
"Space-Time MIMO Channel Modelling using Angular Power Distributions,"
in Proc. 7th Australian Communication Theory Workshop AusCTW05, Perth,
Australia, pp. 163-168, Feb 2006. Abstract: In this paper, we develop a MIMO channel model for generating the channel gains between arbitrary arrays of transmitter and receiver antennas, for a general class of non-line-of-sight (NLOS) channels. We derive the second order statistics of the channel gains in terms of the angular power distribution parameters in a flat-fading channel. The channel scattering environment is defined by a double directional angular distribution describing the power transferred from transmitter aperture to receiver aperture in each direction. We propose several parametrized bivariate distributions that are consistent with univariate scatterer distributions separately observed at the transmitter and receiver. We characterize MIMO system performance as a function of the distribution parameters. ( Online) (download) |
| 79 |
Zhiquo Ding, Thushara D.
Abhayapala, and Darren Ward, "Semi-blind
Channel estimation for precoded STBC systems over correlated MIMO
channels", in Proc. 7th Australian Communication Theory
Workshop AusCTW05, Perth,
Australia, pp. 169-172 Feb 2006. Abstract: Most existing techniques for space time block coding (STBC) systems have assumed full decorrelated radio channels which may not be the case in practice. In this paper a subspace based approach of semi-blind channel estimation is proposed for precoded STBC systems, in which multiple-input multiple-output (MIMO) channels are partially correlated. By carefully designing the transceiver of the precoded STBC systems, correlated MIMO channels (where correlation is due to antenna geometry ) can be converted to full independent MIMO channels. Then a conventional subspace based channel estimation method is applied to such precoded STBC systems. Simulation results demonstrate the performance of the proposed algorithm. (download) |
| 78 |
Rasika
R. Perera, Tony S. Pollock, and Thushara
Abhayapala, '' On non-coherent Rician fading channels with average
and peak power limited input'', in Proc. 8th
International
Symposium on DSP and Communication Systems, DSPCS'2005 & 4th
Workshop on the
Internet, Telecommunications and Signal Processing,
WITSP'2005, pp. 172-177, Dec,
2005. Abstract: Communication over a discrete time-varying Rician fading channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information (CSI) except the fading statistics. We provide closed form expressions to the mutual information under both average and peak input power constraint when its distribution is complex Gaussian. The results are compared with the existing capacity results showing the importance of the perfect CSI at the receiver at moderate and high signal to noise ratio (SNR). Furthermore, we show that the peak input power constraint gives better mutual information compared to the average power constraint input in Rician fading. Also, we show the information loss with a Gaussian distributed input compared to channel capacity is negligible at low SNR. Hence, there is no penalty for not knowing the channel perfectly at the receiver in the low SNR regime. (download) |
| 77 |
Wen Zhang,
Thushara D. Abhayapala, and Jian Zhang, ``Frequency Dependent UWB
Channel Modelling'', in Proc. 8th
International
Symposium on DSP and Communication Systems, DSPCS'2005 & 4th
Workshop on the
Internet, Telecommunications and Signal Processing,
WITSP'2005, pp. 248-252, Dec. 2005. Abstract: This paper presents a novel parametric Ultrawideband (UWB) channel model to characterize frequency dependency of small scale fading. The model is based on electromagnetic diffraction mechanism, which is a major factor of frequency dependent fading in UWB systems, geometric theory of diffraction and scattering center analysis. The statistical distribution of model parameters can be easily estimated for any site specific measurements. The Intel channel measurements [14] are used to demonstrate the effectiveness and robustness of the novel model. (download) |
| 76 |
Tharaka A.
Lamahewa, Tony S. Pollock, and Thushara D. Abhayapala, ``Achieving
Maximum Capacity from a Fixed Region of
Space'', in Proc. 8th
International
Symposium on DSP and Communication Systems, DSPCS'2005 & 4th
Workshop on the
Internet, Telecommunications and Signal Processing,
WITSP'2005, pp 38-43, Dec. 2005. Abstract: Previous results have shown channel capacity of multiple-antenna array communication systems linearly scales with the number of antennas. In reality, by increasing the number of antennas within a fixed region of space the antenna array become dense and spatial correlation (non-ideal antenna placement) significantly limits the capacity. In this paper, we derive a spatial precoder which eliminates the effects of nonideal antenna placement on the capacity performance of spatially constrained dense MIMO systems. The precoder is derived based on fixed and known parameters of MIMO channels, namely the antenna spacing and antenna placement which are known at the transmitter. Therefore, with this design, the precoder is fixed for fixed antenna placement and the transmitter does not require any feedback of channel state information (partial or full) from the receiver. Closed form solutions for the spatial precoder is derived and numerical results are presented to show the capacity improvements obtained for two types of spatially constrained antenna arrays. (download) |
| 74 |
Rasika R. Perera, Tony S.
Pollock and Thushara D. Abhayapala, ``Performance of Gaussian
Distributed Input in Non Coherent Rayleigh Fading MIMO Channels'', in
Proc. IEEE Fifth International
Conference on Information, Communications and Signal Processing (ICICS
2005), pp. 791-795, Dec.
2005. Abstract: The mutual information of a discrete time Rayleigh fading channel is considered, where neither the transmitter nor the receiver has knowledge of the channel state information. We show the mutual information of uncorrelated multiple input multiple output Rayleigh fading channel in closed form when the input distribution is complex Gaussian for any transmit and receive antenna configuration at any signal to noise ratio. In addition, we show the maximum mutual information is achieved with a single transmitter and multiple receivers with a reduction in mutual information with additional transmitters. Also, we argue that the mutual information is bounded on the signal to noise ratio similar to non-coherent SISO systems. (download/ Online) |
| 75 |
Ying Chen, Dhammika Jayalath
and Thushara Abhayapala,''Low Complexity Decision Directed Channel
Tracking for MIMO WLAN System'', in IEEE
2005 International Symposium on Intelligent Signal Processing and
Communications Systems (ISPACS 2005), Dec. 2005.
Abstract: It is expected that the next generation wireless local area networks will be predominantly based on MIMO-OFDM physical layer. The performance of coherent MIMO-OFDM based systems critically depends on the availability of accurate channel estimations. Preambles, at the beginning of each packet, are used to estimate the channels in IEEE 802.11n Systems. However, in mobile communication, when the channel is time varying, the channel variations within a given packet increases the packet error rate (PER) in the system. So the channel variations have to be tracked by the receiver. This paper proposes a low complexity channel tracking scheme for 802.11n systems to improve the PER. The delay introduced by the channel tracking scheme is within the acceptable limits. (download) |
| 73 |
T. A.
Lamahewa, Marvin K. Simon, Rodney
A. Kennedy and Thushara D.
Abhayapala, “Performance Analysis of Space time Codes in Realistic
Propagation Environments: A Moment Generating Function Based Approach”,
in International
Journal on
Communications and Networks, vol. 7, pp. 438-449 ,
December, 2005. Abstract: In this paper, we derive analytical expressions for the exact pairwise error probability (PEP) of a space-time coded system operating over spatially correlated fast (constant over the duration of a symbol) and slow (constant over the length of a code word) fading channels using a momentgenerating function-based approach. We discuss two analytical techniques that can be used to evaluate the exact-PEPs (and therefore approximate the average bit error probability (BEP)) in closed form. These analytical expressions are more realistic than previously published PEP expressions as they fully account for antenna spacing, antenna geometries (Uniform Linear Array, Uniform Grid Array, Uniform Circular Array, etc.) and scattering models (Uniform, Gaussian, Laplacian, Von-mises, etc). Inclusion of spatial information in these expressions provides valuable insights into the physical factors determining the performance of a space-time code. Using these new PEP expressions, we investigate the effect of antenna spacing, antenna geometries and azimuth power. (preprint/ link to Journal) |
| 72 |
D. Smith and T.D. Abhayapala,
“Maximal Ratio Combining Performance Analysis in Spatially Correlated
Rayleigh Fading Channels with Imperfect Channel Knowledge”, in
Proc. 11th Asia-Pacific
Conference on Communications APCC2005, pp. 549-553,
Oct, 2005, Perth,
Australia. Abstract: New Results for the performance analysis of maximal ratio combining (MRC) using BPSK modulation in spatially correlated Rayleigh fading channels with imperfect channel knowledge are presented in terms of antenna array configuration and parameters of scatterer distributions. The utility of recently developed expressions for bit error probability is enhanced by application of a spatial correlation formulation for arbitrarily spaced antennas and general scatterer distributions. The results of performance analysis give valuable insight into the performance of MRC in realistic Rayleigh fading scenarios with imperfect channel state information for both isotropic and non-isotropic fading scenarios. (download) |
| 71 |
Michael I. Y. Williams, Rodney
A. Kennedy, Tony S. Pollock and Thushara D. Abhayapala, “A Novel Scheme
for Spatial Extrapolation of Multipath”, in Proc. 11th Asia-Pacific Conference on
Communications APCC2005, Perth, Australia, pp. 784-787, Oct 3-5,
2005. Abstract: It has been shown that the effects of multipath propagation in a mobile wireless communications system can be mitigated if the receiver can make predictions about the multipath fading. In this paper, we introduce a novel scheme for extrapolating multipath fields outwards in space, given field observations within a limited region. Whereas previous work has concentrated on simple multipath propagation with a finite number of plane wave scatterers, we use a less restrictive continuous model of scattering. The extrapolation scheme is based on a information-weighted modal expansion of the field, where modes containing too little information are penalized to minimize the extrapolation error. The performance of this scheme is shown to be far better than pessimistic error bounds derived in previous work. (download) |
| 70 |
T.A. Lamahewa, R.A. Kennedy and
T.D. Abhayapala, “Spatial Precoder Design Using Fixed Parameters of
MIMO Channels”, in Proc. 11th
Asia-Pacific Conference on Communications APCC2005, Perth,
Australia, pp. 82-86, Oct 3-5, 2005. Abstract: In this paper, we introduced the novel idea of linear spatial precoding based on fixed and known parameters of a MIMO channel where antenna spacing and antenna placement at the transmitter and receiver arrays are considered as fixed parameters. This precoder reduces the effects of antenna spacing and antenna placement and improves the performance of spacetime coded MIMO systems. Unlike the previous precoder designs found in the literature, this precoder does not require frequent feedback of channel state information (partial or full) from the receiver. Closed form solutions for the precoder is presented for systems with up to three receiver antennas. A generalized method is proposed for more than three receiver antennas. Simulation results show that at low SNRs, this precoder provides significant performance improvement over a non-precoded system for small antenna aperture sizes. (download) |
| 69 |
Rasika R. Perera, Tony S.
Pollock and Thushara D. Abhayapala, “ Upper bound on non coherent MIMO
channel capacity in Rayleigh fading” in Proc. 11th Asia-Pacific Conference on
Communications APCC 2005, Perth Australia, pp. 72-76, Oct 3-5,
2005. Abstract: Limits of information transfer over a discrete time uncorrelated Rayleigh fading MIMO channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information (CSI) except the fading statistics. We show the capacity supremum with the receive antenna number at any SNR using Lagrange optimisation. Furthermore, we show the asymptotic capacity when the input power is large, and compare with the existing capacity results when the receiver is equipped with large number of antennas. (download) |
| 68 |
J. Zhang, and T.D. Abhayapala, and
R.A. Kennedy.,
``Role of Pulses in Ultra
Wideband Systems'', in Proc 2005
IEEE International Conference on Ultra-Wideband
(ICU2005), sep 5-8 2005. Abstract: UWB pulses are the unique labels of UWB systems. This paper investigates the role of pulse systematically and highlights the central position of the pulse in UWB systems. Four system properties related closely to the pulse are discussed: propagation properties, capacity, interference to existing systems and performance of correlation receivers. The properties of pulses which function directly on every aspect are highlighted. Novel viewpoint is provided for the evaluation of capacity and interference. Suggestions are given on the pulse design, with emphasis on the whole system performance. (download) |
| 67 |
R. Perera,
T.S. Pollock, and T.D. Abhayapala, ``Mutual Information of Non-coherent
Fading Channels with Gaussian Input'', in Proc. IEEE 2005 Information Theory
Workshop (ITW2005), pp. 162-165, Aug, 2005. Abstract: In this paper, the mutual information of a discrete time Rayleigh fading channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information. We derive a closed form expression for the mutual information of single input single output Rayleigh fading channel when the input distribution is complex Gaussian. We compare the channel capacity and the mutual information attained with Gaussian input using the derived closed form expressions. Furthermore, we argue that the mutual information is bounded by the signal to noise ratio with the simulation results showing the sub optimality of Gaussian signalling in non-coherent Rayleigh fading channels. (download) |
| 66 |
Zhang, J.,
Kennedy, R. A., and Abhayapala, T. D., “Performance and
Parameter Optimization of UWB RAKE Reception with Interchip
Interference”, in Proc. IEEE
International Conference on Communications,
ICC’2005, pp 2830-2834, May 2005. (accepted) Abstract: In this paper, performance and parameter optimization of RAKE reception for time-hopping Ultra Wideband (THUWB) systems is investigated when interchip interference (ICI) is taken into consideration. For a TH-UWB system, ICI is closely related to the relationship between the number of chips in a frame (N_c), and the period of the TH code (N_f ). In a fixed datarate case, larger Nf implies higher transmitted symbol signal-tonoise ratio (SNR) and larger ICI in RAKE fingers. So there is a tradeoff between N_c and N_f to optimize the RAKE performance. In this paper, two models are suggested to describe the ICI, and this tradeoff is investigated in the single user case based on the traditional Gaussian approximation method and other flexible methods. Given the lack of explicit knowledge of the relationship among N_f , N_c and the interference, a rule of thumb is proposed to configure N_f and N_c. (download) |
| 65 |
T. Betlehem, and T.D.
Abhayapala, “Theory
and design of soundfield
reproduction in reverberant rooms,” Journal of the Acoustic Society
of America, Vol. 117, Issue 4, Apr. 2005. Abstract: With the recent emergence of surround sound technology, renewed interest has been shown in the problem of sound field reproduction. However, in practical acoustical environments, the performance of sound reproduction techniques are significantly degraded by reverberation. In this paper, we develop a method of sound field reproduction for reverberant environments. The key to this method is an efficient parametrization of the acoustic transfer function over a region of space. Using this parametrization, a practical method has been provided for determining the transfer function between each loudspeaker and every point in the reproduction region. Through several simulation examples, the reverberant field designs have been shown to yield a reproduction accuracy as good as conventional free-field designs, and better than multipoint least squares designs when loudspeaker numbers are limited. The successful reproduction of sound over a wide frequency range has also been demonstrated. This approach reveals the appropriate choices for fundamental design parameters. (Link to Journal/ download) |
| 64 |
J.
Zhang, R.A. Kennedy, T.D. Abhayapala, “CramerRao lower bounds for the
synchronization of UWB signals,”
EURASIP
Journal on Applied Signal Processing, 2005:3 (2005)
pp. 426-438. Abstract: We present Cramér-Rao lower bounds (CRLBs) for the synchronization of UWB signals which should be tight lower bounds for the theoretical performance limits of UWB synchronizers. The CRLBs are investigated for both single-pulse systems and time-hopping systems in AWGN and multipath channels. Insights are given into the relationship between CRLBs for different Gaussian monocycles. An approximation method of the CRLBs is discussed when nuisance parameters exist. CRLBs in multipath channels are studied and formulated for three scenarios depending on the way multipath interference is treated. We find that a larger number of multipaths implies higher CRLBs and inferior performance of the synchronizers, and multipath interference on CRLBs cannot be eliminated completely except in very special cases. As every estimate of time delay could not be perfect, the least influence of the synchronization error on the performance of receivers is quantified. (Link to Journal/ download paper) |
| 63 |
T.S. Pollock, M.I.Y. Williams, T.D. Abhayapala
“Spatial Limits to
Mutual Information Scaling in MultiAntenna Systems”, in Proc. IEEE
International Conference on Acoustics, Speech, and Signal Processing,
ICASSP’2005, vol. III , pp. 389-392, March 2005. Abstract: Previous results have shown significant capacity gains by employing multiple antennas at both transmitter and receiver, however, due to physical size restraints (particularly at the receiver) these may not be obtained. In this paper we consider the capacity behaviour of multi-antenna systems when the receiver sampling is constrained to a finite region of space. By characterizing the wavefield generated at the receiver due to transmitted signals and the scattering environment, a theoretically derived sampling threshold is shown to exist, at which the capacity growth is reduced from linear to logarithmic with increasing number of sampling outputs. Furthermore, this threshold is shown to be linearly dependent on the receiver region radius within which the sampling is constrained, and is independent of the sampling characteristic ssuch as the antenna properties, array geometry, and/or array signal processing. (download) |
| 62 |
T. Betlehem,
and T.D. Abhayapala, “A Modal Space Approach to Soundfield
Reproduction in Reverberant Rooms”, in Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing, ICASSP’2005, vol. III
, pp. 289-292,
March 2005. Abstract: In this paper, we present a novel method of soundfield reproduction (SFR) for reverberant acoustic environments. Using an efficient parametrization of the acoustic transfer function (ATF) over a region of space, we devise a method for accurate SFR over the whole of the reproduction region. This method is based on a practical method of determining the ATF between each loudspeaker and the reproduction region. (download) |
| 61 |
M.I.Y. Williams, Glenn Dickins,
Rodney A. Kennedy and Thushara
D. Abhayapala, “Spatial Limits on the Performance of Direction of
Arrival Estimation” in Proc. 6th
Australian Communication Theory
Workshop AusCTW05, pp. 175–179, Feb. 2005. Abstract: In this paper, we demonstrate that the performance of a direction of arrival (DOA) estimator is fundamentally limited by the size of the region over which we measure a wavefield. That is, even assuming continuous field measurements across the region, we still cannot achieve perfect performance. We use an approach based on modal decomposition of a spatially truncated field, and completely independent of sensor geometry, to derive the Cram´er-Rao Bound (CRB) for spatially-limited DOA estimators. The model is validated by comparison with results from a uniform circular array (UCA) as the number of sensors goes to infinity. Simulations of the spatial CRB show how DOA performance improves as the measurement region expands. Simulations of the bound also indicate that P sources can only be effectively resolved once a certain threshold region size is reached. (download) |
| 60 |
D.B. Smith and T.D. Abhayapala,
“Generalised SpaceTime
Modelling of Rayleigh MIMO channels”, in Proc. 6th Australian
Communication Theory Workshop AusCTW05,
pp.
135–140, Feb. 2005. Abstract: A recently proposed space-time model for Rayleigh fading for a multiple-input multiple-output (MIMO) channel with arbitrary spatial separation at the transmitter and the receiver is extended. A space-frequency cross spectrum is derived from a space-time cross correlation for a non-isotropic scatterer distribution around the receiver. A simulation study is performed into the space-time correlation formulation to gain understanding of the changing of some relevant parameters and applying different non-isotropic distributions. The formulations presented can be applied to analyse any numbers of transmit and receive antennas operating in typical time-selective MIMO fading channels, and hence can be used in the design of space-time modems. (download) |
| 59 |
D.B. Smith and T.D. Abhayapala ,
“Maximal Ratio Combining
Performance Analysis in Spatially Correlated Rayleigh Fading Channels”,
in Proc. 6th Australian
Communication Theory Workshop AusCTW05, pp. 130–134,
Feb. 2005. Abstract: New Results for the performance analysis of maximal ratio combining (MRC) using BPSK and M-PSK modulation in spatially correlated Rayleigh fading channels are presented in terms of antenna array configuration and parameters of scatterer distributions. Revised closed-form expressions for probabilities of bit error and symbol error of BPSK and M-PSK modulations respectively are given with MRC which allow for non-distinct eigenvalues (or closely spaced eigenvalues) from the correlation matrix at the receiver. The results of performance analysis assuming different receiver configurations and scattering scenarios give valuable insights into the performance of MRC in realistic Rayleigh fading scenarios for isotropic and non-isotropic scatterer distributions. (download) |
| 58 |
R.R. Perera, T.S. Pollock and
Thushara D. Abhayapala, “Bounds
on Mutual Information of Rayleigh Fading Channels with Gaussian Input”,
in Proc. 6th Australian
Communication Theory Workshop AusCTW05, pp.57–62, Feb.
2005. Abstract: The mutual information of a discrete time Rayleigh fading channel is considered, where neither the transmitter nor the receiver has the knowledge of the channel state information. We specically derive a lower bound for the mutual information of this channel when the input distribution is Gaussian. The bound is expressed in terms of the capacity of the corresponding non fading channel and the capacity when the perfect channel state information is known at the receiver. http://ieeexplore.ieee.org/xpls/abs_all.jsp?isnumber=34094&arnumber=1624222&count=54&index=9 (download) |
| 57 |
T.A. Lamahewa, R.A. Kennedy and T.D. Abhayapala,
“Upperbound
for the Pairwise Error Probability of Spacetime Codes in Physical
Channel Scenarios”, in Proc. 6th
Australian Communication Theory
Workshop AusCTW05,
pp. 2632, Feb. 2005.
Abstract: In
this paper,
we derive an upper-bound for the pair-wise error probability of
space-time codes which captures the effects of the transmitter and the
receiver antenna configurations (antenna separation and antenna
geometry) and the surrounding scattering distributions at the
transmitter and the receiver antenna arrays. This new upper-bound
allows investigation of the individual effects of antenna configuration
and scattering environment parameters on the performance of spacetime
codes. Using this upper-bound, we quantify the degree of the effect of
antenna configuration on the diversity advantage given by a space-time
code. Simulation results show that as the number of antennas increase
within a fixed aperture, the diversity advantage of a space-time code
is upper-limited by the size of the antenna aperture. (
(IEEE Explorer)
download) |
2004
| 56 |
T.A.
Lamahewa, T.D. Abhayapala, R.A. Kennedy, and Marvin Simon, “Exact
Pairwise Error Probability Analysis of Space-Time Codes in Realistic
Propagation Environments ”, in
Proc. 3rd Workshop on the Internet,
Telecommunications and Signal Processing.
WITSP-2004, pp. 170-175, Dec. 2004.
Abstract: In this paper, we derive an analytical expression for the exact pairwise error probability (PEP) of a space-time coded system operating over a spatially correlated slow fading channel using a moment-generating function-based approach. This analytical PEP expression is more realistic than previously published exact-PEP expressions as it fully accounts for antenna spacing, antenna geometries (Uniform Linear Array, Uniform Grid Array, Uniform Circular Array, etc.) and scattering models (Uniform, Gaussian, Laplacian, Von-mises, etc). Inclusion of spatial information provides valuable insights into the physical factors determining the performance of a space-time code. We demonstrate the strength of our new analytical PEP expression by evaluating the performance of two space-time trellis codes proposed in the literature for different spatial scenarios. (download paper) |
| 55 |
T.A.
Lamahewa, T.D. Abhayapala, and R.A. Kennedy, “Effect of Transmit
Antenna Configuration on Rank-Determinant Criteria of Space-time
Trellis Codes”, in Proc. IEEE 8th
International Symposium on Spread Spectrum Systems and Applications
(ISSSTA), Sydney, Australia, Aug. 2004, pp. 750-754. Abstract: We derive a new upper bound for the pairwise error probability of space-time codes in a quasi-static Rayleigh fading channel, considering antenna configuration at the transmitter side. The design criterion for existing space-time trellis codes is based on the rank and the determinant of the distance matrix between two code words. In particular, the diversity advantage of the space-time code is associated to the rank of the distance matrix. We show that when the transmit antenna region is small, the diversity advantage given by the space-time code is reduced by the transmit antenna configuration and the diversity advantage of the code depends on the rank of the antenna configuration matrix. We also show that the uniform linear array antenna configuration diminishes the diversity advantage provided by the space-time code while the uniform circular array antenna configuration does not affect the diversity advantage of the space-time code. (Link to IEEE Xplore/ download paper) |
| 54 |
J.
Zhang, T. D. Abhayapala and R. A. Kennedy, “Principal Components
Tracking Algorithms for Synchronization and Channel Identification in
UWB systems,” in Proc. IEEE 8th
International Symposium on
Spread Spectrum Systems and Applications (ISSSTA), Sydney,
Australia, Aug. 2004, pp. 369-373. Abstract: We investigate the reduced rank shift invariant techniques in the application of synchronization and channel identification of UWB signals. The proposed reduced rank techniques can track the principal components automatically and reduce the computational complexity significantly by transforming the generalized eigen-problem in an original high dimensional space to a lower dimensional space depending on the number of desired principal signals. Technical details in the application, including the operations of sampling, fast Fourier transform (FFT) and the capture of synchronization delay, are given. Experiments show the performance is only slightly inferior to the general full rank algorithms. (IEEE Explorer) (download paper) |
| 53 |
T.
S. Lamahewa, T.D. Abhayapala, and R.A. Kennedy, “Fading
Resistance of Orthogonal Space-Time Block Codes Under Spatial
Correlation”, in Proc. IEEE 5th
Workshop on Signal Processing
Advances in Wireless Communications, SPAWC'05, July
2004, pp. xx-xx. Abstract: In this paper,
impact on the bit-error rate performance of orthogonal space-time block
codes (O-STBC) due to spatial correlation is investigated. An analytic
model for spatial correlation is used which fully accounts for: i)
antenna placement and separation, and ii) scattering distributions
(Isotropic, Uniform-limited, etc.). We show that the impact of the
space is limited on the bit-error rate performance of O-STBC, that is,
most of the error rate improvement is due to the 'time-coding' than to
'space-coding'. Further, we investigate how the non-isotropic
parameters of an azimuth power distribution, including the angular
spread and the mean angle of arrival (AOA) of an impinging signal
e®ect the bit-error rate performance of O-STBC. A rule of thumb for
the antenna separation is proposed, where the performance of the O-STBC
is optimal under a given scattering environment.
(IEEE Explorer)
(download paper)
|
| 52 |
R.A.
Kennedy, R.A., T.D. Abhayapala, “Source-fieldWave-field
Concentration and Dimension: Towards Spatial Information Content” , in Proc. IEEE International Symposium on
Information Theory, ISIT 2004,
pp
243, June 2004. Abstract: Using a spatial region as an information bearing resource is considered and an analogy drawn with the work on essentially time- and band-limited signals by Slepian, Landau and Pollack. This paper considers the fundamental limits to the use of free-space as an information bearing resource, a concept for wireless communication which exploits space to achieve information transfer. (Link to IEEE Xplore/ download paper) |
| 51 |
T.S.
Pollock, T.D. Abhayapala, R.A Kennedy, “Limits to Multi-Antenna
Capacity of Spatially Selective Channels”, in Proc. IEEE International Symposium on
Information Theory, ISIT 2004, pp. 244, June 2004. Abstract: In this paper we present a new upper bound on the mutual information of MIMO systems. By characterizing the fundamental communication modes between two physical regions, we develop an intrinsic capacity which is independent of antenna array geometries and array signal processing, and depends only on the size of the regions and the statistics of the scattering environment. (Link to IEEE Xplore/ download paper) |
| 50 |
D.B.
Smith, and T.D. Abhayapala, “On space time cross correlation of a
(2, 2) MIMO Rayleigh channel model,” URSI-F
2004 conference on Radio Wave Propagation and Remote Sensing,
June 2004. (http://www.ursi-f2004.com/) Abstract: This paper extends a recently proposed space-time model for Rayleigh fading to a 2 transmit, 2 receive antenna (2, 2) multiple-input multiple-output (MIMO) channel with arbitrary spatial separation at the transmitter and at the receiver. The space-time correlation function for a non-isotropic scatterer distribution around the receiver is derived for the arbitrary configuration as a further extension of a result for a (2, 2) MIMO radio channel with a non-isotropic scatterer but co-located receive antennas. (Link to proceedings/ download paper) |
| 49 |
L.
Wee Gin, R.A. Kennedy, and T.D.
Abhayapala, “Reliability based soft transition
technique for dual-mode blind equalizers”, in Proc. International Conference on
Communications, ICC’2004, vol.
5, pp.
2631–2635, June 2004. Abstract: We propose a new technique that facilitates the soft transition between a startup algorithm and a decision directed (DD) algorithm in blind adaptive equalizers. The algorithm-pair is combined using a reliability measure that is proportional to an estimate of the probability of the equalizer detecting a correct symbol. This measure takes into account both the equalized signal and its statistical distribution. The main feature of the technique is in the smooth and automatic switching from the startup algorithm to the DD algorithm and vice versa depending on the value of the reliability measure. This technique has been compared with the popular Benveniste-Goursat and stop-and-go algorithms and is shown to exhibit a faster rate of convergence and lower steady state error. (Link to IEEE Xplore/ download paper) |
| 48 |
J.
Zhang, R.A. Kennedy, and T.D. Abhayapala, “Cram´er-Rao Lower
Bounds for the Time Delay
Estimation of UWB Signal”, Proc.
IEEE International Conference on Communications, ICC’2004, vol. 6, pp. 3424–3428, June 2004. Abstract: In this paper, we present the Cramer-Rao lower bounds (CKLBs) for the time delay estimation of UWB signals which could be tight lower bounds for the theoretical performance limits of UWB synchronizers. The CRLBs are investigated for both single pulse systems and time hopping systems in AWGN and multipath channels. Insights are given into the relationship between CRLBs for different Gaussian monocycles. It is found that larger number of multipath signals implies higher CRLBs and inferior performance of synchronizers, and multipath interference on CRLBs can not be eliminated completely except in very special cases. As every estimate of time delay could not be perfect, the least influence of the synchronization error on the performance of receivers is quantified. (Link to IEEE Xplore/ download paper) |
| 47 |
D.
B. Ward and T.D. Abhayapala, “Range and bearing estimation for wideband
sources using an orthogonal
beamspace processing structure,” in Proc. IEEE International Conference on Acoustics,
Speech, and Signal Processing, ICASSP’2004, vol. 2 , pp. 109–12, May 2004. Abstract: We propose a new method for range and bearing estimation of wideband sources based on a novel beamspace structure. The proposed structure consists of an orthogonal set of beamspace processors that have a frequency-invariant beampattern property, and can be steered easily to any nearfield range while maintaining the frequency-invariance of the spatial response. These properties mean that the proposed structure can perform coherent signal subspace processing for nearfield wideband sources, making it ideal for wideband range and bearing estimation. Performance of the estimator is demonstrated through Monte Carlo simulations. (Link to IEEE Xplore/ download paper) |
| 46 |
T.
Betlehem, and T.D. Abhayapala, “Spherical Harmonic Analysis of
Equalization in a Reverberant
Room,” in Proc.IEEE International
Conference on Acoustics, Speech, and Signal Processing, ICASSP’2004, vol. 689–92, May 2004. Abstract: We investigate the performance of acoustic equalization in reverberant environments. We first highlight an efficient general representation of a sound field using spherical harmonics. We then use this representation to develop a concise closed-form expression for robustness of equalization to sensor movement. This expression is used (i) to characterize equalization performance for a general class of non-isotropic sound fields and (ii) to quantify the improvements to equalizer robustness that can be obtained by using a directional microphone. This approach does not use any of the assumptions of statistical acoustics, but instead exploits the inherent properties of a sound field as described by the wave equation. (Link to IEEE Xplore/ download) |
| 45 |
D.B.
Smith, and T.D. Abhayapala, T. Aubrey, “A generalized (M, NR) MIMO
Rayleigh channel model for
nonisotropic scatterer distributions,” in Proc. Workshop on Applications of Radio Science
(WARS’04), Feb., 2004.
(http://www.ips.gov.au/IPSHosted/NCRS/wars/wars2004/proceedings/final/c/smith
f.pdf). Abstract: This paper extends a recently proposed space-time model for Rayleigh fading to include an arbitrary transmit antenna configuration of any shape and size transmitting simultaneously in a multiple-input multiple-output (MIMO) channel. The space-time correlation function and spacefrequency cross spectrum function for a non-isotropic scatterer distribution around the receiver is derived for the arbitrary configuration as a further extension of a previous result of a multipleinput single-output Rayleigh wireless channel which used a ring of uniformly distributed scatterers model. Analysis based on achievable spectral efficiency for typical arbitrary transmit antenna configurations is given. The analysis demonstrates the utility of the correlation function. (download paper) |
| 44 |
J.
Zhang, R.A. Kennedy, and T.D. Abhayapala, “Reduced-Rank ESPRIT
Techniques in the Estimation
of Principle Signal Components”, in Proc.
Fifth Australian Communications Theory Workshop,
AusCTW’2004, pp. 13–17,
Newcastle, Australia. Abstract: In this paper, we present novel rank reduced ESPRIT algorithms to estimate principle signal components with low computational complexity, which can typically be applied in the high resolution identification of closely spaced wireless multipath channels. These algorithms transform the generalized eigen-problem from an original high dimensional space to a lower dimensional space depending on the number of desired principle signals. As only principle singular values and vectors are required, fast algorithms such as the power method can be applied to greatly simplify the proposed algorithm and make it implementable in real time. (download) |
2003
| 43 |
T.S. Pollock, T.D.
Abhayapala,. and R.A. Kennedy, “Spatial Limits to MIMO Capacity
in General Scattering Environments”, in
Proc. 7th International Symposium on DSP for Communication Systems (DSPCS’03), pp
49-54, December 2003 Abstract: In this paper we present a new upper bound on the capacity of MIMO systems. By characterizing the fundamental communication modes of a physical aperture, we develop an intrinsic capacity which is independent of antenna array geometries and array signal processing. Using a modal expansion for free-space wave propagation we show that there exists a maximum achievable capacity for communication between spatial regions of space, which depends on the size of the regions and the statistics of the scattering environment. (download) |
| 42 |
J. Zhang, R.A. Kennedy, and T.D.
Abhayapala, “Conditions and performance of ideal RAKE reception for UWB
signals in Lognormal-fading channels”, International Journal of Wireless Information Network (Kluwer
Academic Publishers), pp. 193-200, vol. 10(4), Oct. 2003. Abstract: In this paper, the conditions and performance of ideal RAKE reception for time hopping Ultra-Wideband (UWB) investigated. Owing to the complex propagation phenomena and specific structure of UWB signals, new problems relevant to the operation of RAKE receivers arise. This motivates us to reconsider the conditions under which a RAKE receiver can work effectively with negligible interference between fingers. Key findings are that the conditions not only relate to the property of time hopping codes, but also modulation methods. An analytical technique is introduced to derive explicit expressions of RAKE performance for various combining methods for a lognormal fading channel. Numerical results show that RAKE reception can largely improve the performance, and equal gain combining has comparable performance to maximum ratio combining. (Link to online journal)/ download |
| 41 |
D.B. Smith, and T.D. Abhayapala,
“A Generalised Space-Time Model for Rayleigh fading channels with a
non-isotropic scatterer distribution,” IEE Electronics Letters, vol. 39,
no. 21, pp 1541–1543, Oct. 2003. Abstract: A generalised space-time model based on Clarkes/Jakes model is extended to the case of a non-isotropic distribution of scatterers. The respective space-time correlation function and space-frequency cross spectrum around the receiver are derived. A 3-D space-time plot of the correlation function is shown for a specific scatterer distribution based on a typical mobile radio scenario. (IEEE Xplore/ download) |
| 40 |
T.S. Pollock, T.D.
Abhayapala,. and R.A. Kennedy, “Introducing Space into MIMO Capacity
Calculations,” Journal on
Telecommunications Systems (Kluwer Academic Publishers), 24:2,
pp. 415436, 2003. Abstract : The large
spectral efficiencies promised for multiple-input multiple-output
(MIMO) wireless fading channels are derived under certain conditions
which do not fully take into account the spatial aspects of the
channel. Spatial correlation, due to limited angular spread or
insufficient antenna spacing, significantly reduces the performance of
MIMO systems. In this paper we explore the effects of spatially
selective channels on the capacity of MIMO systems via a new capacity
expression which is more general and realistic than previous
expressions. By including spatial information we derive a closed-form
expression for ergodic capacity which uses the physics of signal
propagation combined with the statistics of the scattering environment.
This expression gives the capacity of a MIMO system in terms of antenna
placement and scattering environment and leads to valuable insights
into the factors determining capacity for a wide range of scattering
models.
(Link
to online journal/ download) |
| 39 |
T.D Abhayapala, T.S. Pollock,
and R.A. Kennedy, “Characterization of 3D Spatial Wireless
Channels ”, in
Proc. IEEE Vehicular Technology
Conference (Fall), VTC’2003, Vol 1, pp. 123-127, Orlando,
Florida, USA, Oct. 4-9,
2003. Abstract: In this paper a novel three dimensional spatial channels model is developed to provide insight into spatial aspects of multiple antenna communication systems. The spherical harmonic representation of wavefields is used to decompose the spatial channel matrix into a product of known and random matrices where the known portion shows the effects of the physical configuration of antenna elements. The model supports any arbitrary antenna array configurations as well as any distribution of scatterers. Possible applications of the model and its usefulness are outlined. (IEEE Xplore/ download) |
| 38 |
H.M. Jones, A. Saha, and T.D.
Abhayapala, “The effect of finite antenna separation on the performance
of spatial diversity receivers” , in Proc.
Seventh International Symposium on Signal Processing and its
Applications, ISSPA 2003, Paris, FRANCE, pp 515 -518, Vol. 2,
July 2003. Abstract: The wireless communications channel is commonly modelled as imposing a Rayleigh distributed envelope onto any incumbent signals. Antenna arrays have long been recognised as enabling exploitation of signal spatial diversity in such an environment based upon the assumption of uncorrelated signals at the different antennas. However, it is precisely the relative positions of the antennas which determine the extent of the correlation. We consider the effects on receiver output SNR performance, of the maximum ratio combining and equal gain combining diversity schemes, of the relative positions of antennas in a uniform linear array compared with the standard Rayleigh fading model. Such a comparison is possible via a spatially based SIMO model. The insights gained allow the possibility of determining optimal antenna configurations for given output performance measures. (IEEE Xplore/ download) |
| 37 |
T.D Abhayapala, T.S. Pollock,
and R.A. Kennedy, “Spatial Decomposition of MIMO wireless Channels”, in
Proc. Seventh International
Symposium on Signal Processing and its Applications, ISSPA 2003, Paris, FRANCE, pp 309
-312, Vol. 1, July 2003. Abstract: In this paper a novel decomposition of spatial channels is developed to provide insight into spatial aspects of multiple antenna communication systems. The underlying physics of the free space propagation is used to model the channel in scatterer free regions around the transmitter and the receiver, and the rest of the complex scattering media is represented by a parametric model. The channel matrix is separated into a product of known and random matrices where the known portion shows the effects of the physical configuration of antenna elements. We use the model to show the intrinsic degrees of freedom in a multiantenna system. Potential applications of the model are briefly discussed. (IEEE Xplore/ download) |
| 36 |
J. Zhang, R.A. Kennedy, and T.D.
Abhayapala, “Performance of RAKE Reception for Ultra Wideband Signals
in a Lognormal-fading Channel”, in Proc. 2003 International Workshop on Ultra
Wideband (IWUWBS), Finland, 2-5 June, 2003. Abstract: RAKE reception for Ultra Wideband is crucial as the radiated power is restricted to be very low and there exist many resolvable multipath components. In this paper, the effectiveness of RAKE reception is investigated analytically. First, the conditions under which the interference in the RAKE fingers can be ignored are discussed. Then a method is introduced to derive the explicit expression of RAKE performance for the various combining methods for a lognormal fading channel. Numerical results show that RAKE reception can greatly improve the performance, and equal gain combining has comparable performance with maximal ratio combining. (download) |
| 35 |
J. Zhang, R.A. Kennedy, and T.D.
Abhayapala, “New Results on the Capacity of Mary PPM Ultra- WideBand
Systems”, in Proc.IEEE International
Conference on Communications, ICC’2003, pp. 2867–2871,
Anchorage, Alaska, May 11-15, 2003. Abstract: In this paper, some new results on the capacity of a typical M-ary pulse position modulation (M-PPM) time hopping (TH) ultra-wideband (UWB) systems are presented. Previous results using a "pure PPM model" are proven to exaggerate the real capacity of a UWB system. Based on an extended model containing correlator and soft decision decoding, the capacity is evaluated in the single-user case and in the case of a system with asynchronous multiple user interference (MUI) when the inputs are equiprobable. It is found that only when bit- signal-to-noise ratio (bit-SNR) is high enough, larger M leads to higher capacity; and for a specific M, the optimal values of PPM time offset parameter T/sub d/, which maximizes the capacity, are independent of bit-SNR. The influence of MUI on capacity is detrimental, especially in the case of high bit-SNR. (IEEE Xplore/ download) |
| 34 |
J. Zhang, T.D Abhayapala, and
R.A. Kennedy, “Performance of Ultra- Wideband Correlator Receiver Using
Gaussian Monocycles”, in Proc.IEEE
International Conference on Communications, ICC’2003, pp. 2192–2196,
Anchorage, Alaska, May 11-15, 2003. Abstract: This paper investigates the performance of ultra-wideband (UWB) correlator receivers for Gaussian monocycles under the condition of equal mean power and provides constructive reference to the selection of pulses. Several channel situations are examined including ideal single user AWGN channel, non-ideal synchronous, multipath fading and multiple access interference. Both numerical and analytical techniques show that the shapes of pulses have notable impact on the performance of correlator receivers, especially on the interference resistance ability and signal-to-noise ratio (SNR) of the output. he results are also extended to the field of fractional bandwidth to better understand the possible relationship between fractional bandwidth and correlator receivers. (IEEE Xplore/ download) |
| 33 |
T.S. Pollock, T.D.
Abhayapala,. and R.A. Kennedy, “Antenna Saturation Effects
on MIMO Capacity ”, in Proc. IEEE
International Conference on Communications, ICC’2003, Anchorage,
Alaska, May 11-15, 2003. Abstract: A theoretically derived antenna saturation point is shown to exist for MIMO systems, at which the system suffers a capacity growth decrease from linear to logarithmic with increasing antenna numbers. We show this saturation point increases linearly with the radius of the region containing the receiver antennas and is independent of the number of antennas. Using an alternative formulation of capacity for MIMO systems we derive a closed form capacity expression, which uses the physics of signal propagation combined with statistics of the scattering environment. This expression gives the capacity of a MIMO system in terms of antenna placement and scattering environment and show that the saturation effect is due to spatial correlation between receiver antennas. IEEE Xplore/ download) |
| 32 |
T.S. Pollock, T.D.
Abhayapala,. and R.A. Kennedy., “Antenna Saturation Effects on
Dense Array MIMO Capacity ”, in Proc.
IEEE International Conference on Acoustics, Speech, and Signal
Processing, ICASSP’2003, vol. 4, pp. 361-364, Hong Kong, April
6-10, 2003. Abstract: We investigate the behaviour of MIMO capacity when the size of the antenna array is constrained. By increasing the number of antennas within a small region in space the antenna array becomes dense and spatial correlation inhibits capacity growth. A theoretically derived antenna saturation point is shown to exist for dense array MIMO systems, at which there is no capacity growth with increasing antenna numbers. We show this saturation point increases linearly with the radius of the region containing the antenna array and is independent of the number of antennas. (IEEE Xplorer/ download) |
| 31 |
R.A. Kennedy, T.D Abhayapala,.
and T.S. Pollock, “Generalized Herglotz Wave Functions for Modelling
Wireless Nearfield Multipath Scattering Environments”, in Proc. IEEE International Conference on
Acoustics, Speech, and Signal Processing, ICASSP’2003, vol. 4,
pp.660–663, Hong Kong, April 6-10, 2003. Abstract: We develop a general mathematical model for nearfield multipath scattering as a basis for studying the spatial limits imposed on multi-antenna wireless communication systems. This model generalizes the Herglotz wave function, which is an important tool in the study of inverse scattering problems, to a form where the scatterers can be nearfield. This permits the development of the most general form of spatial correlation which is known to constrain the capacity of wireless systems. (IEEE Xplorer/ download) |
| 30 |
T.S. Pollock, T.D. Abhayapala,
and R.A. Kennedy, “On Introducing Space into Space-Time MIMO Capacity
Calculations: A New Closed Form Upper Bound”, in Proc. IEEE 10th International Conference
on Telecommunications(ICT’2003), vol. 2, pp 1536–1541, February
23 - March 1, 2003, Tahiti. Abstract: We present a new upper bound on capacity for multiple-input multiple-output (MIMO) wireless fading channels, which is more general and realistic than previous capacity expressions. By including spatial information at the antenna arrays, a closed form upper bound on capacity, which uses the physics of signal propagation combined with statistics of the scattering environment, is derived. This expression gave the capacity of MIMO system in terms of antenna placement and scattering environment and lead to valuable insights into the factors determining capacity for a wide range of scattering models. (IEEE Xplorer/ download) |
| 29 |
T.D. Abhayapala, and H. Bhatta
“On Coherent Broadband Source Localization by Modal Space Processing”, Proc. IEEE 10th International Conference
on Telecommunications (ICT’2003), vol. 2, pp. 1617–1623,
February 23 - March 1, 2003, Tahiti. Abstract: A novel method for coherent broadband direction of arrival (DOA) estimation is introduced based on physics of signal propagation. This technique does not require any preliminary knowledge of DOA angles nor the number of sources to be estimated. As an illustration, two simulation examples covering single and multi-group scenarios are presented. (IEEE Xplorer/ download) |
| 28 |
Kennedy, R.A., and Abhayapala,
T.D. “ Spatial Concentration of Wave-Fields: Towards Spatial
Information Content in Arbitrary Multipath Scattering ” , in Proc. Fourth Australian Communications Theory Workshop,
AusCTW’2003, pp 38–45, Melbourne, Australia, Feb 5-7, 2003. Abstract: The fundamental limitation of using a spatial channel as an information bearing resource is considered. Such theoretical foundations are largely absent in the development of wireless communication systems which unduly focus on implementation and as a consequence there are a number of misconceptions and mal-implementations of communication systems which employ the MIMO principle. The notion of essential dimensionality of spatial systems is examined in detail for the case of a spherical region in three dimensional space in a nearfield scattering environment as a pointer to a more general theory nearing completion. A non-trivial but tight analogy is drawn with the classical work on time-frequency concentration and the dimensionality of essentially time- and band-limited signals by Slepian, Landau and Pollack. (download) |
| 27 |
Kennedy, R.A., Abhayapala, T.D.
and Pollock, T.S., “ Modelling Multipath Scattering Environments Using
Generalized Herglotz Wave Functions ” , in Proc. Fourth Australian Communications Theory Workshop, AusCTW’2003,
pp 87-92, Melbourne,
Australia, Feb 5-7, 2003. Abstract: We develop a general mathematical model for nearfield multipath scattering as a basis for studying the spatial limits imposed on multi-antenna wireless communication systems. This model generalizes the Herglotz Wave Function, which is an important tool in the study of inverse scattering problems, to a form where the scatterers can be nearfield. This permits the development of the most general form of spatial correlation which is known to be the principle factor in the determination of the capacity of wireless systems. (download) |
| 26 |
T.S. Pollock, T.D. Abhayapala,
and R.A. Kennedy, “ Fundamental Limits of Constrained Array Capacity, ”
in Proc. Fourth Australian
Communications Theory Workshop, AusCTW’2003, pp. 7–12,
Melbourne, Australia, Feb 5-7, 2003. Abhstract: In this paper we investigate the capacity behavior of spatial constrained multiple-antenna array communications. By increasing the number of antennas within a fixed region of space the antenna array becomes dense and spatial correlation inhibits capacity growth. Using a novel spatial channel model we show that the underlying physics of wave propagation limits the capacity of constrained arrays. A theoretically derived antenna saturation point is shown to exist for dense array MIMO systems, at which there is no capacity growth with increasing antenna numbers. We show this saturation point increases linearly with the radius of the region, and that it naturally lends itself to a definition for the theoretical maximum capacity for a fixed region of space. (download) |
2002
| 25 |
Jaunty T.Y. Ho, R.A. Kennedy,
and T.D. Abhayapala, “Selection Diversity in General Scattering
Environments,” in Proc. IEEE Global
Telecommunications Conference, GLOBECOM 2002, Vol.
1, pp. 370-373, Taipei, Taiwan, R.O.C., November 17-21, 2002. Abstract: We derive an infinite series expression of the average signal-to-noise ratio (SNR) gain for a dual selection combining system under Nakagami-m fading using a series representation of the bivariate cumulative probability distribution function. The average SNR gain expression is shown to relate to the Nakagami shape factor m, the power correlation and the power allocation ratio. (Link to IEEE Xplore/ download) |
| 24 |
P.Teal, T.D. Abhayapala, and
R.A. Kennedy, “Spatial Correlation for General Distributions of
Scatterers,” IEEE Sig. Proc. Let.,
vol. 9, issue 10, pp 305-308 Oct. 2002. Abstract: The well-known results of the spatial correlation function for two-dimensional and three-dimensional diffuse fields of narrowband signals are generalized to the case of general distributions of scatterers. A method is presented that allows closed-form expressions for the correlation function to be obtained for arbitrary scattering distribution functions. These closed-form expressions are derived for a variety of commonly used scattering distribution functions. (Link to IEEE Xplore/ download)  |
| 23 |
Jaunty T.Y. Ho, R.A. Kennedy,
and T.D. Abhayapala, “Dual Selection Diversity SNR Performance in
Spatially Correlated Scattering Environments,” in IEEE International Conference of
Telecommunications 2002, Bejing, China, 23-26 June, 2002. Abstract: A new expression is given for the average signal-to-noise ratio (SNR) of a dual selection-combining receiver where the antenna signals are correlated in a Rayleigh fading multipath environment. This formula is based on a recently developed expression for the bivariate Rayleigh cumulative distribution function by associating the correlation parameter of the distribution with the spatial correlation of the multipath field. By examining various multipath and receiver antenna geometries we can determine the critical parameters which affect the average SNR performance. Our results are shown to generalize and subsume well-known expressions based on an independence assumption of the signals at the antennas. We relate the average SNR diversity gain to critical parameters such as the spatial correlation, beamwidth, angle of arrival and antenna separation. These results show the subtle interplay between these parameters and reveal there is a reduction in performance relative to unrealistic models previously studied. (download) |
| 22 |
T.D. Abhayapala and D. B. Ward,
“Theory and Design of High Order Sound Field Microphones using
Spherical Microphone Array,” in Proc.
IEEE Int. Conf. Acoust. Speech Sig. Process. (ICASSP’2002), Vol.
2, 1949-1952,
May 2002. Abstract: A major problem in sound field reconstruction systems is how to record the higher order (> 1) harmonic components of a given sound field. Spherical harmonics analysis is used to establish the theory and design of a higher order recording system, which comprises an array of small microphones arranged in a spherical configuration and associated signal processing. This result has implications to the advancement of future sound field reconstruction systems. An example of a third order system for operation over a 10:1 frequency range of 340 Hz to 3.4 kHz is given. (Link to IEEE Xplore/ download) |
| 21 |
P.Teal T.D. Abhayapala and R.A.
Kennedy, “Spatial Correlation in Non-Isotropic Scattering,” in Proc. IEEE Int. Conf. Acoust. Speech
Sig. Process. (ICASSP’2002), vol. 3, pp. 2833–2836, May 2002. Abstract: The well known results of the spatial correlation function for 2-dimensional and 3-dimensional diffuse fields of narrowband signals are generalised to the case of general distributions of scatterers. A method is presented which allows closed form expressions for the correlation function to be obtained for arbitrary scattering distribution functions. These closed form expressions are derived for a variety of commonly used scattering distribution functions. (Link to IEEE Xplore/ download) |
| 20 |
H. Jones, R.A. Kennedy, and T.D.
Abhayapala, “On Dimensionality of Multipath Fields: Spatial Extent and
Richness,” in Proc. IEEE Int. Conf.
Acoust. Speech Sig. Process.
(ICASSP’2002), vol. 3, pp. 2837–2840, May
2002. Abstract: We establish that an arbitrary narrowband multipath field in any circular region in two dimensional space has an intrinsic functional dimensionality of pi.e.R/lambda = 8.54R/lambda that scales only linearly with radius R/lambda in wavelengths. This result implies there is no such thing as an arbitrarily complicated multipath field. That is, a field generated by any number of nearfield and farfield, specular and diffuse multipath reflections is no more complicated than a field generated by a limited number plane waves. As such, there are limits on how \emph{rich} multipath can be. This result has significant implications including means: i) to determine a parsimonious parameterization for arbitrary multipath fields, ii) of synthesizing arbitrary multipath fields with arbitrarily located nearfield or farfield, spatially discrete or continuous sources. We give examples of multipath field analysis and synthesis. (Link to IEEE Xplore/ download) |
| 19 |
J. Zhang, R.A. Kennedy, and
T.D. Abhayapala, “Analytical Description of Signal Characteristics and
Interference for time Hopped UWB System,” in Proc. 3rd Australian Communications Theory Workshop, AusCTW 2002, February
2002, pp. 114-118. Abstract: In this paper, signal and interference characterisation of Time Hopped Ultra Wideband (TH-UWB) systems is considered. Some special problems often neglected in theoretical studies are discussed, such as the bound between nearfield and farfield and the distortions of wave shape. The transmitted TH-UWB signal is shown to be a cyclostationary process, as is the interference caused by TH-UWB signal on another (victim’s) receiver. Further, the statistical properties of the system output are investigated when the input is cyclostationary process and when the system is either linear timeinvariant or a stationary process independent of the input. To evaluate the influence of interference, the standard method of Power Spectral Density (PSD) is used. The result shows that the PSD of UWB interference signal is made up of many discrete spectral lines. Since the PSD measure, originating in harmonic analysis, has been argued to be an inappropriate tool to analyze transient signals like UWB, a new method, focusing on the aggregate of time jitter, is developed and tested. (download) |
| 18 |
R.A. Kennedy, T.D. Abhayapala
and H. Jones, “Bounds on the Spatial Richness of Multipath,” in Proc. 3rd Australian Communications Theory
Workshop, AusCTW 2002, February 2002, pp. 76-80. Abstract: In this paper wireless multipath fields are modelled using classes of orthogonal functional expansions based on the solutions to the Helmholtz (wave) equation. These expansions permit a multipath field-generated by any number of nearfield, farfield, specular and diffuse multipath reflections-to be modelled to any precision in a region of interest. Two expansions are provided, one suitable for multipath fields which show no variation with height, and the other suitable for general 3D fields. We establish that the dimensionality of the functional expansion, and thereby the multipath field, scales with the size of the boundary of the region in space. When the region of interest is small in wavelengths, typical of antenna arrays apertures in practice, then the multipath field can be modelled by only a small number, given by the dimensionality, of arbitrarily located sources. Multipath field synthesis can be done using combinations of arbitrarily located distinct farfield, nearfield, point and spatially distributed sources. These results establish rigorous bounds on the spatial richness of multipath. (download) |
| 17 |
Jaunty T.Y. Ho, R.A. Kennedy,
and T.D. Abhayapala, “Analytical Expression for Average SNR of
Correlated Dual Selection Diversity System,” in Proc. 3rd Australian Communications Theory Workshop, AusCTW 2002, February
2002, pp. 90-94. Abstract: We use a recently developed expression for the bivariate Rayleigh cumulative distribution function (CDF) to derive a novel analytical formula for the average signal-to-noise ratio (SNR) of the correlated dual selection combining (SC) system under slow Rayleigh fading. We relate the average SNR diversity gain to spatial correlation as well as to antenna separation. Furthermore, we also develop a more practical channel model in which multipath scatters are from more than one incident angle, each with a certain beamwidth. For any scattering field from our model, the average SNR diversity gain can be easily related to the antenna separation. Finally, we corroborate our theory with a simulation study. (download) |
| 16 |
T.D. Abhayapala, R.A. Kennedy,
and Jaunty T.Y. Ho, “On capacity of multi-antenna wireless channels:
Effects of antenna separation and spatial correlation,” in Proc. 3rd Australian Communications Theory Workshop,
AusCTW 2002 , February 2002, pp. 100-104. Abstract: The channel capacity of multi-element communication systems in independent Rayleigh channels has been shown to scale linearly with the number of antennas. In reality, the signals received by different receiver antennas can be correlated with each other due to the non-uniform scattering environment and limited aperture of the antenna system. In this paper, the effect of spatial correlation between receiver antennas on capacity is investigated for various scattering environments. The physics of signal propagation is combined with statistics of the scattering environment to derive a capacity expression in terms of spatial correlation, antenna spacings/ placement, aperture size, and power distribution of scatters. This result is used to show that for a given aperture size, one can increase the capacity approximately linearly up to a certain value by increasing the number of antennas but further increase will not give any significant capacity gain. (download) |
2001
| 15 |
. D.B. Ward and T.D. Abhayapala,
“Reproduction of a plane-wave sound field using an array of
loudspeakers,” IEEE Trans. Speech
and Audio Proc., vol. 9, no. 6, pp 697-707, Sep. 2001. Abstract: Reproduction of a sound field is a fundamental problem in acoustic signal processing. In this paper, we use a spherical harmonics analysis to derive performance bounds on how well an array of loudspeakers can recreate a three-dimensional (3-D) plane-wave sound field within a spherical region of space. Specifically, we develop a relationship between the number of loudspeakers, the size of the reproduction sphere, the frequency range, and the desired accuracy. We also provide analogous results for the special case of reproduction of a two-dimensional (2-D) sound field. Results are verified through computer simulations (IEEE Xplore / download) |
| 14 |
D. B. Ward and T.D. Abhayapala,
“Performance bounds on sound field reporduction using a loudspeaker
array,” in Proc. 2001 IEEE Workshop
on Applications of Signal Processing to Audio and Acoustics, pp.
187--190, Oct.
2001. Abstract: A fundamental problem in acoustic signal processing is how best to use an array of loudspeakers to reproduce a sound field. We derive performance bounds on how well a given loudspeaker array can reproduce a plane-wave sound field within a spherical region of space. The development is based on spherical harmonics analysis (download) |
2000
| 13 |
T.D. Abhayapala, R.A. Kennedy,
and R.C. Williamson, “Nearfield broadband array design using a radially
invariant modal expansion,” J. Acoust. Soc. Amer., vol. 107, pp
392–403, Jan. 2000. Abstract: This paper introduces an efficient parameterization for the nearfield broadband beamforming problem with a single parameter to focus the beamformer to a desired operating radius and another set of parameters to control the actual broadband beampattern shape. The parameterization is based on an orthogonal basis set of elementary beampatterns by which an arbitrary beampattern can be constructed. A set of elementary beamformers are then designed for each elementary beampattern and the desired beamformer is constructed by summing the elementary beamformers with frequency and source-array distance dependent weights. An important consequence of our result is that the beamformer can be factored into three levels of filtering: (i) beampattern independent elementary beamformers; (ii) beampattern shape dependent filters; and (iii) radial focusing filters where a single parameter can be adjusted to focus the array to a desired radial distance from the array origin. As an illustration the method is applied to the problem of producing a practical array design that achieves a frequency invariant beampattern over the frequency range of 1:10 ~which is suitable for speech acquisition using a microphone array, and with the array focused either to farfield or nearfield where at the lowest frequency the radial distance to the source is only three wavelengths. (Link to Journal/download) |
1999
| 12 |
R.A. Kennedy, D.B. Ward, and
T.D. Abhayapala, “Nearfield beamforming using radial reciprocity,” IEEE Trans. Sig. Proc., vol. 47,
pp. 33–40, Jan. 1999. Abstract: We establish the asymptotic equivalence, up to complex conjugation, of two problems: 1) determining the near-field performance of a far-field beam pattern specification and 2) determining the equivalent far-field beam pattern corresponding to a near-field beam pattern specification. Using this reciprocity relationship, we develop a computationally simple procedure to design a beamforming array to achieve a desired near-field beam pattern response. The superiority of this approach to existing methods, both in ease of design implementation and performance obtained, is analyzed and then illustrated by a design example. (Link to Journal/ download) |
| 11 |
T.D. Abhayapala, R.A. Kennedy,
and R.C. Williamson, “Spatial aliasing for nearfield sensor arrays,” IEE Electronics Letters, vol. 35,
no. 10, pp 764–765, May 1999. Abstract: An investigation is presented into the presence of spatial aliasing due to the operation of a linear array in the near-field. It shows that the standard half wavelength sensor spacings rule, which guarantees that no aliasing will occur in the operation of far-field arrays, is not sufficient to prevent aliasing in the near-field. This claim is justified by theoretical considerations and corroborated by simulation results. (Link to Journal/ download) |
| 10 |
T.D. Abhayapala, R.A.
Kennedy, and R.C. Williamson, “Noise modelling for nearfield
array optimisation,” IEEE Sig.
Proc. Let., vol. 6. no. 8, Aug. 1999. Abstract: An exact series representation for a nearfield spherically isotropic noise model is introduced. The proposed noise model can be utilized effectively to apply well-established farfield array processing algorithms for nearfield applications of sensor arrays. A simple array gain optimization is used to demonstrate the use of the new noise model. (Link to Journal/ download) |
| 9 |
T.D. Abhayapala, R.A. Kennedy,
D.B. Ward and R.C. Williamson, “Nearfield Broadband Adaptive
beamforming,” in Proc. IEEE Fifth
International Symposium on Signal Processing and its Applications, Vol.
2, pp. 839-842, Aug
1999. Abstract: A nearfield broadband adaptive beamforming approach based on the modal expansion of the solution to the spherical wave equation is formulated. It provides an efficient parameterization for the nearfield beamforming problem with a single parameter to focus the beamformer to a desired operating radius and a set of modal coefficients to control the angular response. As a consequence, the adaptive beamformer has a frequency invariant response for nearfield sources, decorrelates coherent interferers (thus combating signal cancellation) and has a smaller number of parameters to be adapted compared to a conventional beamformer. A simulation example is presented to demonstrate the use of the proposed method in microphone array applications in speech acquisition systems (Link to Journal/ download) |
| 8 |
T.D. Abhayapala, R.A. Kennedy,
and R.C. Williamson, “Isotropic noise modelling for nearfield array
processing,” in Proc. 1999 IEEE
Workshop on Applications of Signal Processing to Audio and Acoustics, pp.
11–14, Oct. 1999. Abstract: An exact series representation for a nearfield spherically isotropic noise model is introduced. The methodology uses the spherical harmonics expansion of the wavefield at a sensor to obtain the correlation between two sensors due to the nearfield isotropic noise field. The result is useful in nearfield application of sensor arrays. The proposed noise model can be utilized effectively to apply well established farfield array processing algorithms for nearfield applications. Specifically, any signal processing criterion based on farfield isotropic noise correlation can be reformulated with nearfield noise with this representation. A simple array gain optimization is used to demonstrate the new noise model . (Link to Journal/ download) |
1998
| 7 |
T.D. Abhayapala, R.A. Kennedy,
and R.C. Williamson, “Farfield array weight redesign for nearfield
beamforming,” in Proc. 6th IEEE Int.
Workshop on Intelligent Signal Processing and Communication Systems, pp. 537–540,
1998. Abstract: This paper presents a new method for nearfield linear array beamforming that achieves a desired beampattern (as a function of direction) at any nominal finite distance from the array origin. Given a set of array weights which achieves the desired beampattern for farfield sources, we device a linear transformation to obtain another set of array weights which achieves the same beampattern for sources in the nearfield. A simulation example is presented to demonstrate the effectiveness of this method in producing a nearfield beampattern using a standard farfield design technique and the proposed transformation. (download) |
| 6 |
R.A. Kennedy, T.D. Abhayapala,
and D.B. Ward, “Broadband nearfield beamforming using a radial
beampattern transformation,” IEEE
Trans. Sig. Proc., vol. 46, pp. 2147–2156, Aug. 1998. (download) |
| 5 |
T.D. Abhayapala, R.A. Kennedy,
and R.C. Williamson, “Broadband beamforming using elementary shape
invariant beampatterns,” in Proc.
IEEE Int. Conf. Acoust. Speech Sig. Process. ICASSP98, vol. 5,
pp. 2041–2044, 1998. |
1996-1997
| 4 |
D. B.
Ward, R. A. Kennedy, and T.D. Abhayapala, “A beamforming paradox: Using
far-field techniques to design a near-field microphone array,” J. Acoust. Soc. Amer., vol. 102, pp. 3208, 1997. |
| 3 |
R.A.
Kennedy, D.B. Ward, and T.D. Abhayapala, “Nearfield beamforming using
nearfield farfield reciprocity,” in Proc.
IEEE Int. Conf. Acoust. Speech Sig. Process. ICASSP97, vol. 5,
pp. 3741–3744, 1997. |
| 2 |
R.
A. Kennedy, T.D. Abhayapala, and D. B. Ward, “Nearfield broadband array
beamforming using wave equation transformation,” J. Acoust. Soc. Amer., vol. 100,
pp. 2696, 1996. |
| 1 |
R.A.
Kennedy, T.D. Abhayapala, D.B. Ward, and R.C. Williamson, “Nearfield
broadband frequency invariant beamforming,” in Proc. IEEE Int. Conf. Acoust. Speech Sig.
Process. ICASSP96, vol. 2, pp. 905–908, 1996 |
© 2007 Thushara Abhayapala. Web page template designed by Andreas Viklund