Capacity of multiuser diversity systems with adaptive transmission and different MIMO schemes

In this paper, we present a comprehensive capacity analysis of the downlink of multiuser diversity (MD) systems with adaptive transmission over Rayleigh fading channels. First, the exact capacity of the single-input single-output (SISO) systems with MD and adaptive transmission technique is derived. The optimal power allocation scheme for such a system is shown to be a water-filling algorithm. Next, we derive the exact closed-form capacity expressions for different multiple-input multiple-output (MIMO) schemes, including the selective combining (SC), maximum ratio combining (MRC) and space-time block codes (STBC). In order to avoid the Cumbersome numerical root finding techniques in solving the optimal cutoff SNR level below which the channel is not used, we also provide the approximate expressions for the cutoff level. For the MD MIMO systems, it is observed that the optimal power allocation strategy is to focus transmit power on a single transmit antenna (e.g. Tx-MRC/Rx-MRC scheme) or selecting the best transmit antennas (e.g. Tx-SC/Rx-MRC scheme). Copyright (C) 2007 John Wiley & Sons, Ltd.