145 research outputs found
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
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
Band Limited Signals Observed Over Finite Spatial and Temporal Windows: An Upper Bound to Signal Degrees of Freedom
The study of degrees of freedom of signals observed within spatially diverse
broadband multipath fields is an area of ongoing investigation and has a wide
range of applications, including characterising broadband MIMO and cooperative
networks. However, a fundamental question arises: given a size limitation on
the observation region, what is the upper bound on the degrees of freedom of
signals observed within a broadband multipath field over a finite time window?
In order to address this question, we characterize the multipath field as a sum
of a finite number of orthogonal waveforms or spatial modes. We show that (i)
the "effective observation time" is independent of spatial modes and different
from actual observation time, (ii) in wideband transmission regimes, the
"effective bandwidth" is spatial mode dependent and varies from the given
frequency bandwidth. These findings clearly indicate the strong coupling
between space and time as well as space and frequency in spatially diverse
wideband multipath fields. As a result, signal degrees of freedom does not
agree with the well-established degrees of freedom result as a product of
spatial degrees of freedom and time-frequency degrees of freedom. Instead,
analogous to Shannon's communication model where signals are encoded in only
one spatial mode, the available signal degrees of freedom in spatially diverse
wideband multipath fields is the time-bandwidth product result extended from
one spatial mode to finite modes. We also show that the degrees of freedom is
affected by the acceptable signal to noise ratio (SNR) in each spatial mode.Comment: Submitted to IEEE Transactions on Signal Processin
MIMO Channel Correlation in General Scattering Environments
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.Comment: Australian Communication Theory Workshop Proceedings 2006, Perth
Western Australia. (accepted
Real-time separation of non-stationary sound fields on spheres
The sound field separation methods can separate the target field from the
interfering noises, facilitating the study of the acoustic characteristics of
the target source, which is placed in a noisy environment. However, most of the
existing sound field separation methods are derived in the frequency-domain,
thus are best suited for separating stationary sound fields. In this paper, a
time-domain sound field separation method is developed that can separate the
non-stationary sound field generated by the target source over a sphere in
real-time. A spherical array sets up a boundary between the target source and
the interfering sources, such that the outgoing field on the array is only
generated by the target source. The proposed method decomposes the pressure and
the radial particle velocity measured by the array into spherical harmonics
coefficients, and recoveries the target outgoing field based on the time-domain
relationship between the decomposition coefficients and the theoretically
derived spatial filter responses. Simulations show the proposed method can
separate non-stationary sound fields both in free field and room environments,
and over a longer duration with small errors. The proposed method could serve
as a foundation for developing future time-domain spatial sound field
manipulation algorithms.Comment: 34 pages, 15 figure
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