Carrier phase synchronization of OFDM systems over frequency-selective channels via the EM algorithm

A computationally efficient algorithm is presented for maximum likelihood (ML) carrier phase estimation of OFDM systems employing M-PSK modulation scheme for transmission over frequency selective channels with additive Gaussian noise, based on the EM algorithm. A nondata-aided (NDA) carrier phase synchronizer is considered which maximizes the low SNR limit of the likelihood function averaged over the M-PSK signal constellation. For this, an EM algorithm is derived which estimates the phase rotations of each subcarriers iteratively and which converges to the true ML estimation of the unknown phases. The algorithm is applied to the QPSK modulated OFDM systems and it is demonstrated by simulation that the phase error variances of estimated subcarrier phase rotations do not depend on the number of subcarriers. It is concluded that for SNR values greater than 10 dB the convergence is achieved in first iteration and for SNR values less than 10 dB, at most in three iterations. It is also concluded that the convergence is independent of the initial starting points