Performance study of OFDMA vs. OFDM/SDMA

We first consider the OFDM/SDMA system which admits several OFDM users simultaneously by reusing the same frequency band. With the linear minimum mean squared error (MMSE) receiver, it has been shown that such a system can improve the system capacity significantly compared to the single antenna OFDM system. However, the number of required linear filters is huge. Thus, in this paper, we propose two reduced-complexity MMSE receivers for OFDM/SDMA system; namely, interpolated MMSE and partial MMSE. We demonstrate that both methods perform much better than the previously proposed reduced-complexity receiver. Other than OFDM/SDMA, an alternative to support multiple users with high spectrum efficiency is to use orthogonal frequency division multiple access (OFDMA) with high-level modulation. With both theoretical analysis and simulation, we find that the relative BER performance of these two systems depends on the receiver design of OFDM/SDMA. In particular, when the MMSE receiver is used, OFDM/SDMA is usually worse than or comparable to OFDMA. However, the former has the potential to outperform the latter because when the optimal (and the most complicated) ML detector is used, it is superior to the latter. On the other hand, OFDMA has the advantage of a simple receiver structure.