The Noncoherent Rician Fading Channel -- Part II : Spectral Efficiency in the Low-Power Regime

Transmission of information over a discrete-time memoryless Rician fading channel is considered where neither the receiver nor the transmitter knows the fading coefficients. The spectral-efficiency/bit-energy tradeoff in the low-power regime is examined when the input has limited peakedness. It is shown that if a fourth moment input constraint is imposed or the input peak-to-average power ratio is limited, then in contrast to the behavior observed in average power limited channels, the minimum bit energy is not always achieved at zero spectral efficiency. The low-power performance is also characterized when there is a fixed peak limit that does not vary with the average power. A new signaling scheme that overlays phase-shift keying on on-off keying is proposed and shown to be optimally efficient in the low-power regime.Comment: To appear in the IEEE Transactions on Wireless Communication

Covert Wireless Communication with a Poisson Field of Interferers

In this paper, we study covert communication in wireless networks consisting of a transmitter, Alice, an intended receiver, Bob, a warden, Willie, and a Poisson field of interferers. Bob and Willie are subject to uncertain shot noise due to the ambient signals from interferers in the network. With the aid of stochastic geometry, we analyze the throughput of the covert communication between Alice and Bob subject to given requirements on the covertness against Willie and the reliability of decoding at Bob. We consider non-fading and fading channels. We analytically obtain interesting findings on the impacts of the density and the transmit power of the concurrent interferers on the covert throughput. That is, the density and the transmit power of the interferers have no impact on the covert throughput as long as the network stays in the interference-limited regime, for both the non-fading and the fading cases. When the interference is sufficiently small and comparable with the receiver noise, the covert throughput increases as the density or the transmit power of the concurrent interferers increases

On the Diversity-Multiplexing Tradeoff of Unconstrained Multiple-Access Channels

In this work the optimal diversity-multiplexing tradeoff (DMT) is investigated for the multiple-input multiple-output fading multiple-access channels with no power constraints (infinite constellations). For K users (K>1), M transmit antennas for each user, and N receive antennas, infinite constellations in general and lattices in particular are shown to attain the optimal DMT of finite constellations for the case N equals or greater than (K+1)M-1, i.e., user limited regime. On the other hand for N<(K+1)M-1 it is shown that infinite constellations can not attain the optimal DMT. This is in contrast to the point-to-point case in which infinite constellations are DMT optimal for any M and N. In general, this work shows that when the network is heavily loaded, i.e. K>max(1,(N-M+1)/M), taking into account the shaping region in the decoding process plays a crucial role in pursuing the optimal DMT. By investigating the cases where infinite constellations are optimal and suboptimal, this work also gives a geometrical interpretation to the DMT of infinite constellations in multiple-access channels

Communication for wideband fading channels : on theory and practice

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 163-167).This dissertation investigates some information theoretic aspects of communication over wideband fading channels and their applicability to design of signaling schemes approaching the wideband capacity limit. This work thus leads to enhanced understanding of wideband fading channel communication, and to the proposal of novel efficient signaling schemes, which perform very close to the optimal limit. The potential and limitations of such signaling schemes are studied. First, the structure of the optimal input signals is investigated for two commonly used channel models: the discrete-time memoryless Rician fading channel and the Rayleigh block fading channel. When the input is subject to an average power constraint. it is shown that the capacity-achieving input amplitude distribution for a Rician channel is discrete with a finite number of mass points in the low SNR regime. A similar discrete structure for the optimal amplitude is proven to hold over the entire SNR range for the average power limited Rayleigh block fading channel. Channels with a peak power constraint are also analyzed. When the input is constrained to have limited peak power, we show that if its Kuhn-Tucker condition satisfies a sufficient condition, the optimal input amplitude is discrete with a finite number of values.(cont.) In the low SNR regime, the discrete structure becomes binary. Next, we consider signaling over general fading models. Multi-tone FSK, a signaling scheme which uses low duty cycle frequency-shift keying signals (essentially orthogonal binary signals, is proposed and shown to be capacity achieving in the widceband limit. Transmission of information over wideband fading channels using Multi-tonc FSK is considered by using both theoretic analysis and numerical simulation. With a finite bandwidth and noncoherent detection, the achievable data rate of the Multi-tone FSK scheme is close to the wideband capacity limit. furthermore, a feedback scheme is proposed for Multi-tone FSK to improve the codeword error performance. It is shown that if the receiver can feedback received signal quality to the transimitter. a significant improvement in codeword error probability can be achieved. Experimental results are also obtained to dlenlonstrate features and practicality of Multi-tone FSK.by Cheng Luo.Ph.D

The Noncoherent Rician Fading Channel -- Part I : Structure of the Capacity-Achieving Input

Transmission of information over a discrete-time memoryless Rician fading channel is considered where neither the receiver nor the transmitter knows the fading coefficients. First the structure of the capacity-achieving input signals is investigated when the input is constrained to have limited peakedness by imposing either a fourth moment or a peak constraint. When the input is subject to second and fourth moment limitations, it is shown that the capacity-achieving input amplitude distribution is discrete with a finite number of mass points in the low-power regime. A similar discrete structure for the optimal amplitude is proven over the entire SNR range when there is only a peak power constraint. The Rician fading with phase-noise channel model, where there is phase uncertainty in the specular component, is analyzed. For this model it is shown that, with only an average power constraint, the capacity-achieving input amplitude is discrete with a finite number of levels. For the classical average power limited Rician fading channel, it is proven that the optimal input amplitude distribution has bounded support.Comment: To appear in the IEEE Transactions on Wireless Communication