Maximum-likelihood carrier estimation in orthogonal frequency division multiplexing systems

Abstract

The octagonal frequency division multiplexing (OFDM) phics. signaling is proven to be an elective way to overcome the effects of fading channel and multi-path by dividing the frequency selective fading channel into a number of sub-channels corresponding to the OFDM sub-carrier frequencies. However, OFDM systems are more sensitive to carrier frequency offset than single carrier systems are. In both single carrier and multi-carrier systems, phase rotation is a major issue that affects the performance of the system. So the problem of carrier synchronization in the presence of frequency and phase offset is considered. A new frequency and phase offset estimation scheme is proposed for M-ary signaling based on the maximum likelihood principle. The performance of these estimators are assessed by both analysis and simulation. Analytical expressions are developed for the variance of the frequency and phase offset. Analysis of the likelihood function shows that the frequency offset estimator can be implemented in both acquisition and tracking mode. The performance of each kind of estimator is observed through simulation. A closed-loop OFDM system is realized which uses these different synchronization algorithms to estimate and correct carrier Frequency and phase offset for both QAM and MPSK signaling . The performance of the complete system is evaluated in terms of symbol error rate versus signal-to-noise ratio. Simulation results agree with the analytical predictions.