Approximated Maximum Likelihood Estimation of Carrier Frequency Offset in Practical OFDM Systems

This paper proposes a high-performance and relatively low-complexity approximated maximum likelihood estimator (MLE) for carrier frequency offset (CFO) in practical orthogonal frequency multiplexing (OFDM) systems, where the preamble has repetition structure in time domain, null subcarriers are allocated at two ends of the frequency spectrum, and the channel impulse response is assumably not longer than the cyclic prefix. We show the optimality of conventional MLEs do not hold in such systems, and derive a universal MLE and its modified Cramer-Rao lower bound in the general case. To reduce the complexity, we propose to cascade it with a low-complexity rough CFO estimator so that only a moderate number of maxima searches are needed to achieve satisfactory performance. Simulation with the IEEE 802.16 preamble symbol confirms the excellent performance of the proposed method.