Spectrally weighted frequency domain equalizer for mobile OFDM

Orthogonal FDM (OFDM) systems are inherently sensitive to fast fading channels. In this paper, tracking techniques applicable for the frequency domain equalization (FDE) of a fast fading (mobile) OFDM system are proposed. The fact that OFDM systems are over parameterized in the frequency domain (compared to the time domain where the maximum impulse response possible is designed to be less than cyclic prefix) is exploited to derive the spectrally weighted frequency domain equalizer (SWFDE). An error weighted channel tracking and estimation technique is proposed which exploits the diversity due to constrained impulse response length. The proposed SWFDE is particularly useful for narrowband OFDM systems and also implicitly reflects guard tone positions in deriving the weights. For medium to high normalized Doppler fading the SWFDE technique gives a 2 dB gain in error rate performance when compared with a conventional least squares FDE.