A novel AFC scheme with time diversity combining for helicopter satellite communications

In helicopter satellite communications, the periodic blockage of received signal and the carrier frequency fluctuation are the most important problems. In this paper, we propose a novel automatic frequency controller (AFC) scheme with time diversity combining. The proposed AFC generates the frequency error signal based on the time diversity combined signal. The signal-to-noise power ratio (SNR) of the frequency error signal is therefore prevented from falling to zero even in the condition of signal blockage. The proposed AFC thus tracks and removes the carrier frequency fluctuation due to Doppler shift more exactly than the conventional one. Results of computer simulation show that the proposed AFC improves the SNR of the frequency error signal significantly. It is meant that the proposed AFC also improves both of the acquisition characteristics and jitter performance even in the severe environments of the helicopter satellite communications.