191 research outputs found
Achievability of Nonlinear Degrees of Freedom in Correlatively Changing Fading Channels
A new approach toward the noncoherent communications over the time varying
fading channels is presented. In this approach, the relationship between the
input signal space and the output signal space of a correlatively changing
fading channel is shown to be a nonlinear mapping between manifolds of
different dimensions. Studying this mapping, it is shown that using nonlinear
decoding algorithms for single input-multiple output (SIMO) and multiple input
multiple output (MIMO) systems, extra numbers of degrees of freedom (DOF) are
available. We call them the nonlinear degrees of freedom
A Coordinate System for Gaussian Networks
This paper studies network information theory problems where the external
noise is Gaussian distributed. In particular, the Gaussian broadcast channel
with coherent fading and the Gaussian interference channel are investigated. It
is shown that in these problems, non-Gaussian code ensembles can achieve higher
rates than the Gaussian ones. It is also shown that the strong Shamai-Laroia
conjecture on the Gaussian ISI channel does not hold. In order to analyze
non-Gaussian code ensembles over Gaussian networks, a geometrical tool using
the Hermite polynomials is proposed. This tool provides a coordinate system to
analyze a class of non-Gaussian input distributions that are invariant over
Gaussian networks
Fundamental Limits of Communication with Low Probability of Detection
This paper considers the problem of communication over a discrete memoryless
channel (DMC) or an additive white Gaussian noise (AWGN) channel subject to the
constraint that the probability that an adversary who observes the channel
outputs can detect the communication is low. Specifically, the relative entropy
between the output distributions when a codeword is transmitted and when no
input is provided to the channel must be sufficiently small. For a DMC whose
output distribution induced by the "off" input symbol is not a mixture of the
output distributions induced by other input symbols, it is shown that the
maximum amount of information that can be transmitted under this criterion
scales like the square root of the blocklength. The same is true for the AWGN
channel. Exact expressions for the scaling constant are also derived.Comment: Version to appear in IEEE Transactions on Information Theory; minor
typos in v2 corrected. Part of this work was presented at ISIT 2015 in Hong
Kon
On Non-coherent MIMO Channels in the Wideband Regime: Capacity and Reliability
We consider a multiple-input, multiple-output (MIMO) wideband Rayleigh block
fading channel where the channel state is unknown to both the transmitter and
the receiver and there is only an average power constraint on the input. We
compute the capacity and analyze its dependence on coherence length, number of
antennas and receive signal-to-noise ratio (SNR) per degree of freedom. We
establish conditions on the coherence length and number of antennas for the
non-coherent channel to have a "near coherent" performance in the wideband
regime. We also propose a signaling scheme that is near-capacity achieving in
this regime.
We compute the error probability for this wideband non-coherent MIMO channel
and study its dependence on SNR, number of transmit and receive antennas and
coherence length. We show that error probability decays inversely with
coherence length and exponentially with the product of the number of transmit
and receive antennas. Moreover, channel outage dominates error probability in
the wideband regime. We also show that the critical as well as cut-off rates
are much smaller than channel capacity in this regime
On capacity of optical communications over a lossy bosonic channel with a receiver employing the most general coherent electro-optic feedback control
We study the problem of designing optical receivers to discriminate between
multiple coherent states using coherent processing receivers---i.e., one that
uses arbitrary coherent feedback control and quantum-noise-limited direct
detection---which was shown by Dolinar to achieve the minimum error probability
in discriminating any two coherent states. We first derive and re-interpret
Dolinar's binary-hypothesis minimum-probability-of-error receiver as the one
that optimizes the information efficiency at each time instant, based on
recursive Bayesian updates within the receiver. Using this viewpoint, we
propose a natural generalization of Dolinar's receiver design to discriminate
coherent states each of which could now be a codeword, i.e., a sequence of
coherent states each drawn from a modulation alphabet. We analyze the
channel capacity of the pure-loss optical channel with a general
coherent-processing receiver in the low-photon number regime and compare it
with the capacity achievable with direct detection and the Holevo limit
(achieving the latter would require a quantum joint-detection receiver). We
show compelling evidence that despite the optimal performance of Dolinar's
receiver for the binary coherent-state hypothesis test (either in error
probability or mutual information), the asymptotic communication rate
achievable by such a coherent-processing receiver is only as good as direct
detection. This suggests that in the infinitely-long codeword limit, all
potential benefits of coherent processing at the receiver can be obtained by
designing a good code and direct detection, with no feedback within the
receiver.Comment: 17 pages, 5 figure
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