Recursive maximum likelihood estimation of time-varying carrier frequency offset for orthogonal frequency-division multiplexing systems
A recursive maximum likelihood carrier frequency offset (CFO) estimator is proposed in this work, where redundancy information contained in the cyclic prefix of multiple consecutive orthogonal frequency-division multiplexing (OFDM) symbols is exploited in an efficient recursive fashion. Because the estimator is based on multiple OFDM symbols, the time-varying CFO must be considered. We investigate the effect of time-varying CFO on the performance of the estimator and the trade-off between fast tracking ability and low estimation variance. We show that, without channel noise, the mean squared error (MSE) of estimation due to CFO estimation variation increases approximately quadratically with n, where n is the number of OFDM symbols used for CFO estimation (estimation window size), whereas the MSE due to channel noise decreases proportionally to 1/n (approximately) if the CFO is constant. A closed-form expression of the optimal estimation window size (approximately) is derived by minimizing the MSE caused by both time-varying CFO and channel noise. For wireless systems with time-varying rate of change for CFO, the proposed estimator can be implemented adaptively. In addition, typical optimal estimation window sizes for WiMAX, DVB-SH and MediaFLO systems are evaluated as an example. Copyright (c) 2011 John Wiley & Sons, Ltd.
cfo ofdm time-varying channels ml estimation