Performance of asynchronous multi-user MIMO OFDM systems

In this paper, the effect of transmitter diversity on asynchronous co-channel interference suppression and equalization is studied in multi-user multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In an asynchronous mode of data transmission, independent decoding without user cooperation is a more appropriate approach. Thus, various suboptimal transmitter diversity techniques such as space-time block coding and beamforming are employed, and their performances are evaluated. First, we develop space-time minimum mean squared error (MMSE) receivers for both space-time block coded OFDM and beamforming based MIMO OFDM. The average signal-to-interference and noise ratio (SINR) resulting from MMSE combining indicates that the bit-error rate (BER) performance of space-time block coded (STBC) multi-user OFDM is highly dependent on the amount of timing offset. Overall, simulation results demonstrate that closed-loop MIMO systems need to be employed to obtain substantial diversity gains in asynchronous multi-user OFDM channels with random timing offset. Moreover, receiver diversity is an important factor to the performance of a large system.