A hybrid Walsh-Hadamard code division multiplexing for helicopter satellite communications

In helicopter satellite communications, periodic channel blockage due to rotor blades and Doppler shift caused by helicopter maneuvers are major issues. Code division multiplexing employing Walsh-Hadamard code (WHCDM) is a promising solution for the periodic blockage. However, automatic frequency control (AFC) removing Doppler shift limits the multiplexing factor of WHCDM to less than its maximum value because of its frequency discrimination performance. This limitation reduces the spectral efficiency in the conventional WHCDM. In this paper, we propose a novel WHCDM achieving the maximum spectral efficiency with the multiplexing factor of its maximum value minus one. In the proposed WHCDM, one Walsh-Hadamard code selected by information data is excluded from the transmitted signal. Results of computer simulation show that the bit error rate performance of the proposed WHCDM is slightly better than that of the conventional one in the higher E_b/N₀ region.