Dropping Users in a Multi-Antenna Broadcast Channel

We analyze the sensitivity of the capacity of a multi-antenna multi-user system to the number of users being served simultaneously. We show analytically that the capacity loss of serving fewer users at low SNR is much smaller than at high SNR. The advantages of serving fewer users are many: multi-user algorithms often have better performance, lower complexity, and reduced channel-knowledge requirements. We provide guidelines on how many users to serve to get nearcapacity performance with low complexity. For example, we show that the performance gain in some algorithms more than compensates for the loss in capacity in an eight-antenna eight-user system when only four users are served, and we are now approximately 2dB from capacity at very low SNR.