Minimum probability of error demodulation for multipath OFDM-SDMA systems

Orthogonal frequency division multiplexing (OFDM) is attractive for wireless communication because it simplifies the problems posed by the frequency and time selective nature of a wireless channel. This simplification is achieved by converting a frequency selective channel into a frequency flat one. On the other hand, space division multiple access (SDMA) leads to spectral efficiency. These two techniques, when combined, can therefore achieve reliable wireless communication at high data rates. The resulting system however suffers from co-channel interference. In this paper we develop two linear demodulators that effectively combat this interference. The demodulators are based on the principle of minimizing the probability of error in linear demodulation of the received symbols. The first demodulator, minimum probability of error (MPOE), minimizes the probability of error of each transmitting source separately. The second demodulator, minimum joint probability of error (MJPOE), minimizes the joint error probability of the sources. The demodulators are computed iteratively by using the method of gradient descent. Both demodulators show considerable improvement in BER compared to the MMSE demodulator and enjoy fast convergence. At a BER of 10-3, the MJPOE shows a gain of over 7 dB SNR over the MMSE demodulator and 10 dB SNR over the training based adaptive RLS demodulator. For the MPOE demodulator, the corresponding gains are 5.5 and 8.5 dB SNR.