Zero-forcing equalization for OFDM systems over doubly-selective fading channels using frequency domain redundancy

Rapid variations of doubly-selective fading channels lead to the loss of orthogonality among subcarriers, resulting in inter-carrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems. In this scenario, the one-tap frequency-domain equalizer is not applicable any more. Based on an ICI analysis in both time- and frequency-domain, we propose a novel zero-forcing equalizer (ZFE) for OFDM systems over doubly-selective channels using the existing frequency domain redundancy in many OFDM standards. To deal with the zero-forcing equalizer's disadvantage of noise amplification, which causes system performance degradation when there exist deep nulls in the channel's frequency response, we also provide the MMSE-based optimization of ZFE coefficients in the presence of channel noise. We show that ZFE matrix has a sparse structure, and thus a low computational complexity. Simulation results indicate that the proposed equalizer is capable of suppressing ICI effectively with a low complexity.