Optimal power control in multiple access fading channels with multiple antennas

This paper characterizes the optimal power control method for maximum sum capacity in a multiple access fading channel with multiple transmitter and receiver antennas when perfect channel side information is available at both the transmitters and the receiver. The profound benefit of multi-antenna diversity is demonstrated by a dimension counting argument. The optimal power allocation strategy in a system with n transmit antennas for each user and m receive antennas is a combination of successive cancellation and a TDMA-like scheme where in each time slot the rank of the transmit signals r_k for all users must satisfy Σ_kr_k(r_k+1)⩽m(m+1). Thus, the total number of users that are allowed to transmit simultaneously is constrained by the number of receiver antennas. Receiver diversity increases the total number of dimensions thus allowing more users to transmit at the same time. By contrast, transmitter diversity allows a single user to occupy multiple dimensions as to benefit its own transmission, thus having the effect of precluding simultaneous transmission by other users