A Low Inter-Code Interference Walsh-Hadamard Code Division Multiplexing for Helicopter Satellite Communications

One of the most important problems in helicopter satellite communications is the periodic blockage of the received signal due to rotor blades. Walsh-Hadamard (WH) code division multiplexing (WHCDM) is promising in order to overcome the periodic blockage because it has an implicit time diversity effect. However, the inter-code interference (ICI) caused by the loss of orthogonality of WH codes reduces the time diversity effect. Although the combination of WHCDM with the explicit time diversity can mitigate the ICI, its configuration is rather complex and the spectral efficiency is degraded. In this paper, we propose a low ICI WHCDM without the explicit time diversity. In the proposed WHCDM, the second half set of WH codes is equivalently phase shifted by π/2 in order to reduce the ICI. The results of computer simulation confirm that the proposed one improves the bit error rate (BER) performance of the conventional one without the explicit time diversity in the periodic blockage channel. We also confirm that the proposed one, not degrading the spectral efficiency, achieves the same BER performance as the conventional one with the explicit time diversity.