Transmitter Diversity in SDMA based Asynchronous Multi-user 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. Space division multiple access OFDM may be an appropriate solution to design spectrally efficient and high data-rate communication systems. Moreover, communication systems using transmitter diversity can provide additional spatial diversity gains. However, increasing the number of transmitting antennas may induce another source of co-channel interference in asynchronous multi-user channels. In order to observe the performance of various transmitter diversity techniques under the asynchronous mode of transmission, 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) multiuser 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.