Performance of Multi-User Spatial Multiplexing with Measured Channel Data

The application of MIMO processing techniques in channels that are shared among multiple users is a relatively new problem that is increasingly important as MIMO transmission is put into practical use. In this chapter we specifically consider the multi-user downlink, where a base station with multiple antennas transmits simultaneously to more than one user. We begin with an overview of some of the multi-user MIMO transmission schemes that have been proposed up to this point, then demonstrate how they might be expected to perform by applying the algorithms to measurement data from indoor and outdoor propagation environments. Specifically, we compare the number of simultaneous users the channel will support for the two different environments, the amount of separation of the users necessary to achieve maximum throughput, and the quality of channel information available to the base station when the users are mobile. In both environments, full multi-user diversity is achieved at relatively short distances on the order of one meter. The total number of simultaneous users in outdoor environments is limited compared to uncorrelated channels due to the relatively sparse multipath structure of the channel. The distances at which channel information becomes too old to be useful to the transmitter appears to be similar for both types of channels.