Coherence diversity: A novel source of gain in wireless multiuser networks
Shady, Mohamed Fadel
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This dissertation investigates multiuser networks where the fading links experience unequal coherence conditions as well as dissimilar link CSI availability. It is shown that the disparity in coherence conditions for multiple users leads to a novel gain in the transmission rates compared with techniques that do not explicitly take advantage of this disparity. This gain is denoted coherence diversity and is demonstrated by product superposition transmission. First, a frequency-selective broadcast channel is considered, where two users have a disparity in coherence time and coherence bandwidth. This channel is analyzed under three broad scenarios of the disparity between the link qualities: when the disparity is in coherence time, in coherence bandwidth, and in both coherence time and coherence bandwidth. For each scenario, an analysis is provided and coherence diversity gain is demonstrated. The results are obtained in the framework of OFDM transmission covering a variety of pilot transmission schemes and different channel estimation techniques. Numerical simulations are presented to show coherence diversity gains. Second, coherence diversity is investigated in broadcast and multiple access channels with an arbitrary number of users. The users experience unequal fading block lengths, and CSI is not available. In the broadcast channel, product superposition is employed to find the achievable degrees of freedom. The case of multiple users experiencing fading block lengths of arbitrary ratio or alignment is studied. Also, in the multiple-access channel with unequal coherence times, achievable and outer bounds on the degrees of freedom are obtained. Third, a MISO broadcast channel is considered where some receivers experience longer coherence intervals and have CSIR, while some other receivers experience shorter coherence intervals and do not enjoy free CSIR. A variety of CSIT availability models is considered, including no CSIT, delayed CSIT, or hybrid CSIT. For each model, coherence diversity gains are merged with interference alignment and beamforming to achieve degrees of freedom. For several cases, inner and outer bounds are established that either partially meet, or the gap diminishes with increasing coherence times.