Adaptive mode switching in the MIMO broadcast channel

This paper proposes an adaptive mode switching between single user and multi-user multiple input multiple output (MIMO) transmissions for Gaussian MIMO broadcast channels with partial channel state information at the transmitter. With full channel state information, it is known that multi-user transmission outperforms all other policies. With alternate channel state information available at the transmitter, it is not true that one transmission strategy always dominates the other. In particular, under some practical side information assumptions, there is instead a transition point - when the number of users is below this point, single user transmission outperforms multi-user transmission, and the reverse is true when the number of users exceeds this critical value. In this paper, we determine this mode switching point both analytically and numerically for a MIMO channel with transmit side correlation.