A combined OFDM/SDMA approach

Two major technical challenges in the design of future broadband wireless n etworks are the impairments of the propagation channel and the need for spe ctral efficiency. To mitigate the channel impairments, orthogonal frequency division multiplexing (OFDM) can be used, which transforms a frequency-sel ective channel in a set of frequency-flat channels. On the other hand, to a chieve higher spectral efficiency, space division multiple access (SDMA) ca n be used, which reuses bandwidth by multiplexing signals based on their sp atial signature. In this paper, we present a combined OFDM/SDMA approach th at couples the capabilities of the two techniques to tackle both challenges at once. We propose four algorithms, ranging from a low-complexity linear minimum mean squared error (MR ISE) solution to the optimal maximum likelih ood (ML) detector. By applying per-carrier successive interference cancella tion (pcSIC), initially proposed for DS-CDMA, and introducing selective sta te insertion (SI), we achieve a good tradeoff between performance and compl exity. A case study demonstrates that, compared to the MR ISE approach, our pcSIC-SI-OFDM/SDMA algorithm obtains a performance gain of 10 dB for a BER of 10(-3), while it is only three times more complex. On the other hand, i t is two orders of magnitude less complex than the ML approach, for a perfo rmance penalty of only 2 dB.