ICI mitigation with CFO compensation for OFDM in mobile-to-mobile channel

In mobile-to-mobile channels, the Doppler shift is affected by mobile terminal velocities of the transmitter and the receiver end. Hence, the Doppler frequency becomes very high if both terminals move very fast. In this condition, the channel will change rapidly and cause large inter-carrier-interference (ICI) in OFDM system. A channel estimation technique using piece-wise linear approximation has previously been developed for ICI mitigation technique employing frequency-domain-equalizer (FEQ). Unfortunately, this channel estimation technique is limited by normalized maximum Doppler shift of 0.2, below which a negligible error results. In this paper, the carrier-frequency offset (CFO) compensation technique is proposed to improve this ICI mitigation technique, so that linear channel estimation still works well although normalized maximum Doppler shift is larger than 0.2. The CFO compensation frequency f₀ that should be added depends on transmitter Doppler shift f_d and has an optimal value, i.e. f₀ = 0.55f_d for mobile-to-fixed channels and f₀ = 0.6f_d1 for mobile-to-mobile channels. The channel estimation using linear approximation technique can be applied with negligible error up to normalized Doppler spread f_d = 0.4 (100 % higher than systems without CFO compensation) and the sum f_dT = f_d1 + f_d2 higher than 0.4 for mobile-to-mobile channels that is inversely proportional to the ratio a = f_d2 / f_d1. Furthermore, assuming f_d1 ≥ f_d2 for mobile-to-mobile channel, the BER performance increases significantly if CFO compensation is applied.