A Technique for Orthogonal Frequency Division Multiplexing Frequency Offset Correction

Abstract

This paper discusses the effects of frequency offset on the performance of orthogonal frequency division multiplexing, (OFDM) digital communicatious. The main problem with frequency offset is that it introduces interference among the maulpiplicity of carriers in the OFDM signal. It is shown, and confirmed by simulation, that to maintain signal-to-interference ratios 0f 20 dB or greater for the OFDM carriers, offset is limited to 4% or less of the intercarrier spacing. Next, the paper describes a technique to estimate frequency offset using a repeated data symbol. A maximum likelihood estimation (MLE) algorithm is derived and its performance computed and compared with simulation results. Since the intercarrier interference energy and signal energy both contribute coherently to the estimate, the algorithm generates extremely accurate estimates even when the offset is far too freat to demodulate the data values. Also, the estimation error depends only on total symbol energy so it is insensitive to channel spreading and frequency selective fading. A strategy is described for initial acquisition in the event of uncertainity in the initial offset that exceeds 1/2 the carrier spacing, the limit of the MLE algorithm. © 1994 IEEE