Cooperative transmission for the downlink of multiuser mimo cellular networks

Abstract

In this thesis, we propose a distributed transmission scheme for the downlink of a multiuser system. The base-stations (BSs) cooperate with each other with limited, local message-passing to find the optimum beamforming vectors, where there are individual signal-to-interference-plus-noise-ratio (SINR) targets for each user. Majority of the previous work on this problem assumed a total power constraint on the BSs. However, since each transmit antenna is limited by the amount of power it can transmit due to the limited linear region of the power amplifiers, a more realistic constraint is to place a limit on the per-antenna power. In a recent work, Yu and Lan proposed an iterative algorithm for computing the optimum beamforming vectors minimizing the power margin over all antennas under individual SINR and per-antenna power constraints. However, from a system designer point of view, it may be more desirable to minimize the total transmit power rather than minimizing the power margin, especially when the system is not symmetric. Reformulating the transmitter optimization problem to minimize the total transmit power subject to individual SINR constraints on the users and per-antenna power constraints on the base stations, the algorithm proposed by Yu and Lan is modified. Performance of the modified algorithm is compared with the existing methods for various cellular array scenarios. The modified algorithm requires inversion of a matrix, which cannot be implemented fully distributively using limited information exchange between BSs. By approximating the matrix as tridiagonal, a suboptimal distributed algorithm for computing the beamforming vectors in a cooperative system is obtained. The proposed distributed algorithm is shown to achieve near optimal performance when the target SINRs and the size of the array are small.