An Accurate AFC Scheme for Walsh-Hadamard Code Division Multiplexing
Toshiharu Kojima, Yuki Kanamaru, Anna Himeno · 2019
This paper addresses automatic frequency control (AFC) in Walsh-Hadamard (WH) code division multiplexing (WHCDM). WHCDM is code division multiplexing employing WH codes as spreading codes. Because WHCDM has an implicit time diversity effect, it is promising for helicopter satellite communications, in which rotor blades periodically blocks the received signal. However, WHCDM is required to overcome the time-varying carrier frequency offset due to the Doppler shift caused by the maneuver of the helicopter. We propose an AFC scheme for WHCDM in this paper. The proposed scheme estimates the carrier frequency offset from the variance of the amplitude of the signal after demultiplexing. Results of computer simulation show that the proposed AFC scheme accurately tracks the time-varying frequency offset even in the very low carrier-to-noise power ratio environment.