| This dissertation has two parts: the first part studies multi-antenna broadcast channels with nodes of varying mobility, and the second part studies capacity limits of spectrum-sharing networks.;In the multi-antenna broadcast channel without transmit-side channel state information (CSIT), it has been known that when all receivers have channel state information (CSIR), the degrees of freedom (DoF) cannot be improved beyond what is available via TDMA. The same is true if none of the receivers possess CSIR. This dissertation shows that an entirely new scenario emerges when receivers have unequal CSIR: orthogonal transmission is no longer DoF-optimal. In particular, when one receiver has CSIR and the other does not, two product superposition methods based on Grassmannian signaling are proposed and analyzed, and are shown to attain the optimal degrees of freedom for a wide set of antenna configurations and channel coherence times. Furthermore, the product superposition is extended to the domain of coherent signaling with pilots, the advantages of product superposition are demonstrated in low-SNR as well as high-SNR, and DoF optimality is established in a wider set of receiver antenna configurations. Two classes of decoders, with and without interference cancellation, are studied, and the effect of power allocation and partial CSI at the base station are investigated.;The second part of this dissertation investigates capacity limits of spectrum-sharing networks. Unlike point-to-point cognitive radio, where the constraint imposed by the primary rigidly curbs the secondary throughput, multiple secondary users have the potential to more efficiently harvest the spectrum and share it among themselves. Efficient methods are proposed and analyzed to exploit multiuser diversity in cognitive broadcast channels, cognitive multiple access channel (MAC) and cognitive relay channels. The optimal growth rate of the capacity of these channels is established, and the tradeoff between scaling the secondary throughput and reducing interference on the primary is highlighted and characterized. |
