Modal Analysis and Synthesis of Broadband Nearfield Beamforming Arrays

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 nearfield broadband beampattern are presented. ..

Achieving secrecy without knowing the number of eavesdropper antennas

The existing research on physical layer security commonly assumes the number of eavesdropper antennas to be known. Although this assumption allows one to easily compute the achievable secrecy rate, it can hardly be realized in practice. In this paper, we provide an innovative approach to study secure communication systems without knowing the number of eavesdropper antennas by introducing the concept of spatial constraint into physical layer security. Specifically, the eavesdropper is assumed to have a limited spatial region to place (possibly an infinite number of) antennas. From a practical point of view, knowing the spatial constraint of the eavesdropper is much easier than knowing the number of eavesdropper antennas. We derive the achievable secrecy rates of the spatially-constrained system with and without friendly jamming. We show that a non-zero secrecy rate is achievable with the help of a friendly jammer, even if the eavesdropper places an infinite number of antennas in its spatial region. Furthermore, we find that the achievable secrecy rate does not monotonically increase with the jamming power, and hence, we obtain the closed-form solution of the optimal jamming power that maximizes the secrecy rate.Comment: IEEE transactions on wireless communications, accepted to appea

On statistics of the mobile Rayleigh fading channel in non-isotropic scattering environments

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

Performance analysis of spatially distributed MIMO systems

With the growing popularity of ad-hoc sensor networks, spatially distributed multiple-input multiple-output (MIMO) systems have drawn a lot of attention. This work considers a spatially distributed MIMO system with randomly distributed transmit and receive antennas over spatial regions. The authors use the modal decomposition of wave propagation to analyse the performance limits of such system, since the sampling of the spatial regions populated with antennas is a form of mode excitation. Specifically, they decompose signals into orthogonal spatial modes and apply concepts of MIMO communications to quantify the instantaneous capacity and the outage probability. The authors’ analysis shows that analogous to conventional point-to-point MIMO system, the instantaneous capacity of spatially distributed MIMO system over Rayleigh fading channel is equivalent to a Gaussian random variable. Afterwards, they derive an accurate closed-form expression for the outage probability of proposed system utilising the definition of instantaneous capacity. Besides, in rich scattering environment, the spatially distributed MIMO system provides best performance when the spatial regions are of same size, and each region is equipped with equal number of antennas. Furthermore, to facilitate the total transmit power allocation among the channels, they propose an algorithm which indicates a significant performance improvement over conventional equal transmit power allocation scheme, even at low signal-to-noise ratio

Room transfer function measurement from directional loudspeaker

Room transfer function (RTF) is the room response observed at a particular listening point due to an impulse generated from an omnidirectional point source. Typically, measured RTFs in practice are often erroneous due to the directivity of the measurement loudspeaker. This paper formulates a spherical harmonic based parameterization of the room response for a directional loudspeaker, and provides a direct approach to derive the point to point RTF using measurements from a directional loudspeaker. Simulation results are presented for 2 directional loudspeakers with an active frequency bandwidth of 200 - 4000 Hz.This work is supported by Australian Research Council (ARC) Discovery Projects funding scheme (project no. DP140103412)

PSD Estimation of Multiple Sound Sources in a Reverberant Room Using a Spherical Microphone Array

We propose an efficient method to estimate source power spectral densities (PSDs) in a multi-source reverberant environment using a spherical microphone array. The proposed method utilizes the spatial correlation between the spherical harmonics (SH) coefficients of a sound field to estimate source PSDs. The use of the spatial cross-correlation of the SH coefficients allows us to employ the method in an environment with a higher number of sources compared to conventional methods. Furthermore, the orthogonality property of the SH basis functions saves the effort of designing specific beampatterns of a conventional beamformer-based method. We evaluate the performance of the algorithm with different number of sources in practical reverberant and non-reverberant rooms. We also demonstrate an application of the method by separating source signals using a conventional beamformer and a Wiener post-filter designed from the estimated PSDs.Comment: Accepted for WASPAA 201

Analysis of Degrees of Freedom of Wideband Random Multipath Fields Observed Over Time and Space Windows

In multipath systems, available degrees of freedom can be considered as a key performance indicator, since the channel capacity grows linearly with the available degrees of freedom. However, a fundamental question arises: given a size limitation on the observable region, what is the intrinsic number of degrees of freedom available in a wideband random multipath wavefield observed over a finite time interval? In this paper, we focus on answering this question by modelling the wavefield as a sum of orthogonal waveforms or spatial orders. We show that for each spatial order, (i) the observable wavefield is band limited within an effective bandwidth rather than the given bandwidth and (ii) the observation time varies from the given observation time. These findings show the strong coupling between space and time as well as space and bandwidth. In effect, for spatially diverse multipath wavefields, the classical degrees of freedom result of "time-bandwidth" product does not directly extend to "time-space-bandwidth" product.Comment: 9 pages, 2 figures, Accepted in 2014 IEEE Workshop on Statistical Signal Processin