Time-Domain and Frequency-Domain Per-Tone Equalization for OFDM over Doubly-Selective Channel

In this paper, we propose new time-domain and per-tone frequency-domain equalization techniques for OFDM transmission over timeand frequencyselective channels. We present one mixed timeand frequency-domain equalizer and one per-tone frequency-domain equalizer. The mixed timeand frequency-domain equalizer (MTFEQ) consists of a one-tap time-varying (TV) time-domain equalizer (TEQ), which converts the doubly-selective channel into a purely frequency-selective channel, followed by a one-tap frequency-domain equalizer (FEQ), which then equalizes the resulting frequencyselective channel in the frequency-domain. The per-tone frequency-domain equalizer (FPTEQ) is then obtained by transferring the TEQ operation to the frequency-domain. While the one-tap TEQ of the MTFEQ optimizes the performance on all subcarriers in a joint fashion, the FPTEQ optimizes the performance on each subcarrier separately. This results into a significant performance improvement of the FPTEQ over the MTFEQ, at the cost of a slight increase in complexity. Through computer simulations we show that the MTFEQ suffers from an early and high error floor, while the FPTEQ approaches the performance of the MMSE equalizer for OFDM over purely frequency-selective channels. An important feature of the proposed techniques is that no bandwidth expansion or redundancy insertion is required except for the cyclic prefix. keywords-Wireless communications, OFDM, Equalization.