MMSE-Prewhitened-MLD Equalizer for MIMO DFT-Precoded-OFDMA

A low-complexity equalizer which uses a combination of space-frequency minimum-mean-square-error-estimation (MMSE) filter and a pre-whitened maximum likelihood detector (MLD) is proposed for discrete Fourier transform precoded orthogonal frequency division multiple accesses (DFT-precoded-OFDMA) systems employing multi-stream spatial multiplexing (SM). We show that this receiver behaves like an optimum MLD in channels with low frequency selectivity (flat fading) and the performance converges to that of MMSE in channels with high frequency selectivity. Further, we analytically characterize the performance of the zero-forcing (ZF) linear equalizer (LE) in an i.i.d. channel with infinite amount of frequency selectivity for the case when the number of receiver antennas N_r is greater than the number of transmitter antennas/streams N_t. The ZF-LE is shown to provide a per-stream post-processing signal-to-noiseratio (SNR) of N_r-N_t/N_tN₀ for N_r >; N_t. Additionally, simulation is used to compare the bit error rate (BER) performance of ZF and MMSE based receivers.