Limiting Performance of Conventional and Widely Linear DFT-precoded-OFDM Receivers in Wideband Frequency Selective Channels

This paper describes the limiting behavior of linear and decision feedback equalizers (DFEs) in single/multiple antenna systems employing real/complex-valued modulation alphabets. The wideband frequency selective channel is modeled using a Rayleigh fading channel model with infinite number of time domain channel taps. Using this model, we show that the considered equalizers offer a fixed post signal-to-noise-ratio (post-SNR) at the equalizer output that is close to the matched filter bound (MFB). General expressions for the post-SNR are obtained for zero-forcing (ZF) based conventional receivers as well as for the case of receivers employing widely linear (WL) processing. Simulation is used to study the bit error rate (BER) performance of both MMSE and ZF based receivers. Results show that the considered receivers advantageously exploit the rich frequency selective channel to mitigate both fading and inter-symbol-interference (ISI) while offering a performance comparable to the MFB

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-noise-ratio (SNR) of N r-N t/N tN 0 for N r > N t. Additionally, simulation is used to compare the bit error rate (BER) performance of ZF and MMSE based receiver