Doppler compensation for mobile OFDM systems with multiple receive antennas

In OFDM systems, mobility induced Doppler spreading distorts the orthogonality among the subcarriers, and results in intercarrier interference (ICI) which degrades the reception quality. In this paper, we investigate how ICI can be mitigated by using multiple receive antennas. In this context, by using Taylor series approximation of the time-varying channels, we derive minimum mean-squared error (MMSE) linear combining scheme for Doppler compensation. Motivated by lower complexity, we develop new combining schemes, i.e., Simple Doppler Compensating Combiner (SDCC) and approximated Zero Forcing (aZF) combiner, based on approximation of MMSE combining. We show that by new combining schemes, the Doppler tolerance of mobile OFDM systems can be increased significantly with respect to conventional maximum ratio combining (MRC) with a moderate increase in computational complexity. Furthermore, we show that the proposed linear combining schemes can be combined with existing single-antenna ICI mitigation methods, forming a Hybrid Doppler compensation method that obtains higher mobility tolerance.