Digital Communications Over Fading Radio Channels

A major contribution to system outage in a terrestrial digital radio channel is deep fading of the frequency transfer characteristic, which in addition to causing a precipitous drop in received signal-to-noise ratio (s/n) also causes signal dispersion that can result in severe intersymbol interference. Because the temporal variation of the channel is slow compared to the signaling rate, the information theoretic channel capacity and the “Efficiency Index” in bits/cycle — a figure-of-merit we use for the communication techniques considered — can be viewed as random processes. Starting from an established mathematical model characterizing fading channels (derived from extensive measurements), we estimate the probability distribution of channel capacity and the distributions of efficiency indices for different communications techniques. The repertoire of communication methods considered involves quadrature amplitude modulation with adaptive linear and decision feedback equalization, and maximum likelihood sequence estimation. For specific outage objectives the maximum number of bits per cycle achievable by each technique is estimated. The sensitivity of the distributions to bit-error-rate objective and unfaded s/n is assessed. For certain desired operating points the efficacy of adaptive equalization is demonstrated. There are some operating points where adaptive equalization alone is not adequate and therefore space diversity should be considered. An estimate of the effect of frequency diversity is also included.