On achievable performance of spatial diversity fading channels

Channel time-variation and frequency selectivity [causing intersymbol interference (SI)] are two major impairments in transmission for a wireless communication environment. Spatial diversity on the transmitter or the receiver side has been traditionally used to combat multipath fading, Recent results indicate significant gains in using multiple transmitter and receiver antenna diversity. By deriving the mutual information and cutoff rate we characterize the gains on these channels. We show that gains linear in the number of antennas can be achieved either when the signal-to-noise ratio (SNR) becomes very large or when the number of antennas becomes large. We show that some of these gains can be achieved by lower complexity linear receiver structures. By evaluating the cutoff rate for phase-shift keying (PSK) constellations we further quantify the gains of using spatial diversity at both the transmitter and the receiver. Next, we examine the expected mutual information for slowly fading ISI channels where the channel is assumed to be block time-invariant. We then examine the impact of fast channel time variation (time variation within a transmission block) on multicarrier transmission schemes. We derive the average mutual information for orthogonal frequency-division multiplexing (OFDM) in time-varying ISI environments. Using this we examine the impact of transmitter and receiver diversity on OFDM transmission over time-varying ISI channels. We also study the effect of time variation on OFDM packet-size design.