Adaptive carrier frequency synchronization using virtual subcarriers in OFDM systems

Abstract

The inherent immunity of OFDM to multipath limitations comes with the price of increased sensitivity to inaccuracies in the reference frequency. A carrier offset at the OFDM receiver causes losses in subcarrier orthogonality, and thus introduces inter-channel interference (ICI) which degrades the system performance. In mobile communication technology, adaptive algorithms are mostly chosen in real time signal processing because of their high speed to acquire and track changes in parameters. Unfortunately, the existing adaptive methods are not suitable in doubly selective fading channels. In this paper, an adaptive carrier frequency synchronization algorithm is proposed for applications under double dispersive fading channels. The algorithm takes advantage of the known structure of the subspace of OFDM signals. It is shown that the mean square errors of frequency offset estimates are much lower than those of existing adaptive and some non-adaptive algorithms. Simulations also show that it takes only one symbol for the proposed algorithm to track the changes in the channel, leading to high accuracy in the compensation. Since no matrix inversion is involved in the algorithm, excellent transient behaviors with very low complexity are obtained. The proposed algorithm is robust and outperforms the existing adaptive algorithms particularly in doubly dispersive channels.