An Accurate and Wide-Range AFC for Modified Walsh-Hadamard Code Division Multiplexing

This paper proposes a novel automatic frequency control (AFC) for modified Walsh-Hadamard code division multiplexing (MWHCDM). While MWHCDM is suitable for helicopter satellite communications, where rotor blades periodically block the transmission channel, it is affected by a time-varying Doppler shift due to helicopter maneuvers. The conventional AFC has a problem where its pull-in range is significantly narrower than the fluctuation range of the Doppler shift. The proposed AFC utilizes the structure of MWHCDM signals, and its operating range covers the fluctuation range with enough margin. Computer simulation shows that the proposed AFC outperforms the conventional one on both operating/pull-in range and frequency estimation accuracy.