Adaptive closed-loop frequency compensation for multicarrier transmissions

Due to the Doppler shifts and the inherent instabilities of local oscillators at the transmitter and the receiver, the residual frequency error leads to the orthogonality loss between subcarriers and degrades the overall OFDM system performance. In practical systems the instantaneous SNR is estimated at the receiver for the purpose of link adaptation, power control or QAM scaling, etc. A novel adaptive closed-loop frequency compensator is proposed in this paper by adapting the loop gain based on instantaneous channel condition such as SNR without requiring additional processing. The justification is that more reliable estimates could be obtained with higher SNR so that the higher loop gain should be defined for the corresponding frequency offset estimates. Link analysis illustrates that the proposed adaptive scheme can significantly improve the system performance.