Maximum likelihood frequency detection for precoded OFDM

Abstract

In recent years, orthogonal frequency division multiplexing (OFDM) has become very popular, particularly in the field of wireless communications. Precoded OFDM (POFDM) provides additional advantages, including greater immunity to the effects of multipath channels. However, OFDM is more sensitive to frequency errors at the receiver than single carrier transmission. In this paper, we introduce a maximum likelihood frequency detector (MLFD) for POFDM, in an AWGN channel. The performance of two different unitary precoders, namely the discrete Fourier transform (DFT) and the discrete Hadamard transform (DHT) is investigated and the characteristic curves (CC's) for these schemes are presented. Existing MLFD schemes for OFDM assume QAM input symbols. However, the input symbols to the OFDM system after precoding are approximately Gaussian distributed, due to the central limit theorem. It is shown that the CC for POFDM utilising the DHT as a precoder is much more useful than that for POFDM utilising a DFT precoder and is in fact similar to the CC for conventional OFDM. Frequency acquisition and tracking will be a more robust process with DHT precoding than with DFT precoding.