Downlink multiuser MIMO methods for spatial diversity gain over a frequency flat or selective fading channel

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_kn_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.