MLSD Bounds and Receiver Designs for Clipped OFDM Channels

We derive tight closed-form approximations of symbol and bit error probability on maximum-likelihood sequence detection (MLSD) of Nyquist-rate and oversampled OFDM systems over the AWGN, flat and frequency-selective quasi-static Rayleigh fading channels. Contrary to common knowledge, we show that on a frequency-selective channel, clipping at the transmitter leads to achievable frequency-diversity and performance gain over unclipped systems with MLSD. We derive closed-form expressions of the diversity and the performance gain over the no-clipping case, as well as the optimal clipping ratio for a given system. We show that such diversity and performance gain are a result of both clipping and channel frequency-selectivity, and they do not occur on the AWGN and flat fading channels.We also present a new near-optimum detection algorithm that achieves the MLSD performance bound with a few iterations for the clipped OFDM systems with both high and low clipping levels on the AWGN and flat Rayleigh fading channels. For the frequency selective channel, we present a receiver design that utilizes the advantage of clipping to some extent. We consider rectangular QAM signaling in this paper.