| This dissertation investigates the enormous potential of multiuser system design with multiple antennas for wireless communications. Roughly, we answer the following questions: What number of antennas is needed for each communication entity to achieve a given spectral efficiency? In what situations is the channel information necessary? In what situations is a sophisticated multiple access scheme required?; First, the channel capacity of multiuser wireless systems with multiple antennas is considered, and the effects of multiple antennas and multiple users are identified. The characteristics of the ergodic sum capacity of multiple access channels are investigated, and they are shown to generalize to deterministic and outage capacities in many situations. In particular, any outage capacity is shown to converge to the ergodic capacity with increasing space diversity. Our study shows that a multiuser communication system design, which allows many users to simultaneously transmit or receive in a common bandwidth, is a fascinating way of achieving a very high spectral efficiency. While the cost of user terminals can be minimized, the robustness to the channel correlations can be greatly increased with multiuser system designs.; To demonstrate the viability of the information-theoretic results, an uncoded multiuser system, which is designed according to the developed criterion, is proposed. The proposed system requires multiple antennas only at the base station, and a single antenna per user suffices to achieve high data rates for both uplink and downlink. Furthermore, the proposed system works well in correlated channel environments, because the multiuser diversity is fully utilized.; To develop a deeper understanding on the interactions among multiple users, an analytical tool based on convex optimization is developed for multiple access channels. An interesting result on the optimality of beam-forming is derived as an example of utilizing the analytical tool. Based on the beam-forming result, a suboptimal transmit scheme is derived for a system with partial channel information. Through simulations, the transmit scheme is shown to achieve a sum rate very close to the sum capacity only with partial channel information at the transmitters. Furthermore, the suboptimal scheme is shown to be mildly sensitive to the channel estimation error. |
