Downlink transmit beamforming in multi-user MIMO systems

The primary issue in downlink beamforming for wireless communications is how to balance the need for high received signal power for each user against the interference produced by the signal at other points in the network. In this paper, we describe several approaches to this problem: channel inversion, regularized channel inversion, vector modulo pre-coding, channel block diagonalization, and coordinated transmit/receive beamforming. While the basic idea behind these algorithms is the same, namely the use of channel information at the transmitter to predict and then counteract the interference produced at each node in the network, each of the algorithms is based on achieving a different performance objective. We compare the various goals of the above algorithms, and detail their respective advantages and disadvantages in terms of computational complexity, required transmit power, network throughput, and assumed receiver capabilities. The results of several simulation studies are presented to quantify these comparisons.