Communication for wideband fading channels: on theory and practice

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. 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 wideband limit. Transmission of information over wideband fading channels using Multi-tone 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 transmitter. a significant improvement in codeword error probability can be achieved. Experimental results are also obtained to demonstrate features and practicality of Multi-tone FSK. 3 Thesis Supervisor: Muriel Medard Title: Harold E. and Esther Edgerton Associate Professor Thesis Supervisor: Lizhong Zheng Title: Assistant Professor