A Subset-Based Coherent FFH System

Fast frequency hopping (FFH) is a powerful antijamming method. Nevertheless, channel estimation in FFH is a difficult issue and even once was deemed impractical. FFH over fading channels usually adopts noncoherent schemes that suffer from combination loss and low spectral efficiency. To address these issues, in this letter, we propose a subset-based coherent FFH (S-CFFH) scheme. The proposed scheme partitions the hopping frequency set into a number of subsets. By controlling the size of hopping frequency subset, we ensure that each hopped frequency recurs within the channel coherence time, which thus enables efficient channel estimation and coherent schemes. We analyze the bit error ratio (BER) of S-CFFH/quaternary phase-shift keying in the presence of partial band noise jamming, highly frequency-selective Rayleigh fading, and channel estimation errors. We further study how to optimize the S-CFFH parameters. Closed-form expressions are provided for the BER and optimum pilot parameter. Simulation results are presented to corroborate our theoretical analysis. It is shown that S-CFFH significantly outperforms conventional noncoherent FFH in both BER and spectral efficiency.