A combined OFDM/SDMA approach

Two major technical challenges in the design of future broadband wireless networks are the impairments of the propagation channel and the need for spectral efficiency. To mitigate the channel impairments, orthogonal frequency division multiplexing (OFDM) can be used, which transforms a frequency-selective channel in a set of frequency-flat channels. On the other hand, to achieve higher spectral efficiency, space division multiple access (SDMA) can be used, which reuses bandwidth by multiplexing signals based on their spatial signature. In this paper, we present a combined OFDM/SDMA approach that 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 (MMSE) solution to the optimal maximum likelihood (ML) detector. By applying per-carrier successive interference cancellation (pcSIC), initially proposed for DS-CDMA, and introducing selective state insertion (SI), we achieve a good tradeoff between performance and complexity. A case study demonstrates that, compared to the MMSE approach, our pcSIC-SI-OFDM/SDMA algorithm obtains a performance gain of 10 dB for a BER of 10/sup -3/, while it is only three times more complex. On the other hand, it is two orders of magnitude less complex than the ML approach, for a performance penalty of only 2 dB.