Joint TX-RX optimisation for MIMO-SDMA based on a null-space constraint

We address the problem of transmission pre- and post-filtering in multi-user MIMO communications using SDMA. In this set-up, multiple antennas are used at the base station and at the terminals. A double level of spatial multiplexing is used: the users are spatially multiplexed (SDMA) and each user receives spatially multiplexed bit streams (SDM). Multi-user interference is a potential performance limitation in this scheme. To avoid this, we propose a pre-filter with a null-space constraint that block-diagonalizes the channel matrix, resulting in zero multi-user interference. Each terminal then only has to eliminate its own inter-stream interference, which does not require information from the other users' channel. This scheme thus breaks down the optimisation schemes in independent, smaller per-user optimisation. It lends itself to numerous designs of which joint transmit-receive MMSE is discussed. The performance of this design scheme is assessed by simulations.