An Improved AFC Scheme for Modified Walsh-Hadamard Code Division Multiplexing

Walsh-Hadamard code division multiplexing (WHCDM) is attractive to helicopter satellite communications due to its capability to overcome periodic channel blockage with rotor blades. Modified WHCDM (MWHCDM) is more attractive in terms of spectral efficiency. However, MWHCDM is relatively less robust to carrier frequency offset than WHCDM. This paper addresses automatic frequency control (AFC) for MWHCDM. We pay attention to the structure of the MWHCDM signal and propose an improved AFC scheme. The proposed scheme makes it possible to update the frequency error signal twice per symbol. Results of computer simulation show that MWHCDM gets more robust to carrier frequency offset with the proposed scheme.