In this paper, we propose a mechanism to achieve multi-user (MU) MIMO downlink spatial diversity gain for both frequency flat and selective fading channel. The MU interference is first cancelled by precoding the signal with the null space (basis) of other users' propagation space and then the diversity gain is achieved by using the dominant eigen transmission (DET) method on the composite space of the regular MIMO space and the MU interference null space. We prove both in theory and simulations that a diversity gain of an order equal to N k n k is achieved for MU MIMO, where N k is the # of Rx antennas at the MS and n k is the rank of the MU interference null space. When n k > 1 and N k = 1, DET reduces to transmit maximal ratio combining (TMRC). When n k = 1 and N k > 1, DET reduces to receive MRC (RMRC). Selection combining (SC) can also be applied to MU SIMO or MU MISO.
