Low complexity OFDM MIMO system based on channel correlations

Orthogonal frequency division multiplexing (OFDM) is known for its efficiency in combating frequency selectivity in wireless channels. Multiple transmit and/or multiple receive (MIMO) antennas can be employed with OFDM to achieve higher diversity or capacity. In traditional OFDM MIMO systems, transmitter beamforming and receiver combining are done in the frequency domain, which requires one discrete Fourier transform (DFT) per antenna. As recently proposed, transmitter beam-forming and receiver combining can be done in time domain, and only one DFT is needed at either the transmitter or the receiver. System complexity can thus be greatly reduced at the cost of slight performance loss. We investigate the low-complexity OFDM MIMO system where the transmitter has partial side information, specifically, the knowledge of channel correlations. We show that in multiple input single output (MISO) case, time domain beamforming can be used to do two-dimensional eigen-beamforming. For MIMO case, we derive design criteria for the transmitter beamforming and receiver combining weighting vectors, and show some suboptimal solutions. Simulations show that the performance degradation of the low-complexity combining is small.