On the beamforming weight of the MIMO/SDMA system under channel uncertainty

We consider beamforming methods for the downlink of space division multiple access (SDMA) systems when there are channel information errors. A single antenna at the mobile is generally assumed in SDMA systems, but we consider multi-input and multi-output (MIMO) systems which adopt multiple antennas at both transmitter and receiver. If we know the exact channel information of each user, cochannel interference among users can be perfectly removed by the zero-forcing nulling method in MIMO/SDMA systems. However, the channel estimate always contains estimation errors in real systems, which can cause significant performance degradation. To cope with channel information errors, we develop two beamforming weights based on null-space constraints. The first one is designed to minimize the transmit power while not influencing the desired receive signal. The interfering effect from channel information errors is reduced since the transmit power of cochannel interfering users is minimized. The second one is designed to minimize the power of the received interference signal. It computes the beamforming weight which has the minimum expectation of interference power with knowledge of the variance of channel estimation errors. The proposed beamforming weights are tested by computer simulations.