Transmit and Multiuser Diversity Techniques in Wireless Communications

Recently, the demand for high-rate wireless data service has increased enormously. To meet this growing demand, many new techniques have been developed in wireless communications. Among them, techniques based on transmit and multiuser diversity have attracted much attention due to their ability to provide reliable high data rate services. This thesis is aimed at developing and analyzing advanced communication techniques making use of transmit and multiuser diversity. In this thesis, we consider a cellular network where data transmissions are controlled and coordinated at the base station. We study a variety of special cases of this cellular network including point-to-point communication over the multiple-input and single-output channel, multiuser communication in the downlink from the base station to multiple mobile users, and cognitive communication in an unlicensed secondary network. In the first part of this thesis, we consider the case that a single user is scheduled and there are multiple antennas at the base station. We investigate the communication reliability in terms of the symbol error rate. In particular, we propose several techniques relying on the transmit diversity including different combinations of transmit antenna selection, space-time coding and power allocation. In the second part of this thesis, we study the opportunistic scheduling scheme and determine the potential performance improvement provided by multiuser diversity in terms of the symbol error rate (SER). Two scaling laws on the asymptotic behavior are derived for this system. In the first regime of a large average signal-to-noise ratio (SNR) denoted by A and a fixed number of users K, a SER asymptotically proportional to A^{-K} can be achieved. In the second regime of a large number of users K and a fixed average SNR value A, the SER asymptotically decreases as fast as K^{-A}. In the third part of this thesis, we focus on the underlay cognitive radio network and study a cognitive broadcast scenario where one secondary transmitter and several secondary receivers share the same spectral resource with the primary users. We investigate the opportunistic scheduling scheme at the secondary transmitter which takes advantage of the multiuser diversity and meanwhile controls the interference at the primary receiver. The sum capacity of such a system is computed and scaling laws of the sum capacity are derived for two asymptotic regimes. In the first asymptotic regime of a large number of secondary receivers (SRs) K and a fixed number of primary receivers (PRs) U, we show that the ergodic sum capacity C scales as lnln K. In the second asymptotic regime, where we have a large number of U and a fixed number of K, we show that C decreases as a function of ln U and asymptotically converges to a certain finite limit. Further, we analyze the symbol error rate performance of this cognitive scenario. Our study shows that the opportunistic scheduling scheme exploiting the multiuser diversity can significantly enhance the performance of the secondary network while limiting the interference caused to the primary system.