Study On The Key Technologies Of Coherent Fast Frequency Hopping Systems

Fast frequency hopping(FFH) is an important anti-jamming communication scheme.With FFH, the symbol period is divided into a number of short hopping durations, during which the symbol is hopped onto different hopping frequencies with wide interval.Hence, FFH, which could efficiently combat jamming and fading, is widely used in antijamming communications.Nevertheless, due the signal properties above-mentioned, channel estimation in FFH is a difficult issue, and even once was deemed impractical. Consequently, FFH usually adopts non-coherent MFSK schemes due to its implementation simplicity. However, conventional non-coherent FFH/MFSK has shortcomings, including low spectral efficiency,non-coherent diversity combining loss, and insufficient in expandability. With the increasing demand of stronger anti-jamming capability and higher data rate in modern antijamming communications, we need to improve the FFH system, pursuing for better bit error probability(BER) performance, higher spectral efficiency, and better expandability.To address these issues, in this thesis, we propose the subset-based coherent FFH(S-CFFH) scheme. With S-CFFH, we provide the signal model, structure design, and the system parameter optimization. We analyze the BER performance of the single antenna S-CFFH system as well as the spatial modulated multiple antenna S-CFFH system in the presence of typical jamming circumstances. We propose and study a number of anti-jamming combining schemes, with closed-form BER results obtained. Finally, the analyses are verified by simulations.The major work and contributions of this thesis are summarized as follows,1) We propose the S-CFFH scheme, together with the structure design and the signal model. By solving the channel estimation problem in CFFH, the S-CFFH has high spectral efficiency and good expandability, without compromising in antijamming ability.2) In the absence of jamming, we analyze the BER performance and the system parameter optimization of the single antenna S-CFFH, which are verified by simulations. Considering the highly frequency-selective Rayleigh fading channels with perfect and imperfect channel state information(CSI), we analyze the BER of the maximal ratio combining(MRC) and the equal gain combining(EGC) for the S-CFFH/BPSK, with closed-form expressions obtained. Meanwhile, we study the Gaussian channel estimation error model for the S-CFFH system over highly frequency-selective Rayleigh fading channels. We derive the decomposition relation between the channel coefficient and its estimates, which could simplify the performance analysis. Then we derive the optimum pilot overhead for the S-CFFH/BPSK and the S-CFFH/QPSK with MRC and imperfect CSI.By numerically solving the differential equations, we obtain the relation curves between the optimum diversity order and the signal to noise ratio. Our analyses are collaborated by simulations, which show that, with BER = 1 × 10-3, as compared with the non-coherent FFH/BFSK and the FFH/4FSK, the S-CFFH/QPSK has a performance gain approximately up to 2 ～ 4.5 d B, and approximately a triple spectral efficiency.3) In the presence of partial band noise jamming(PBNJ) or multitone jamming(MTJ), we analyze the anti-jamming combining schemes and BER performance of the single antenna S-CFFH, which are verified by simulations. With perfect CSI, we first derive the BER of the S-CFFH/BPSK ML receiver in the presence of PBNJ or MTJ, with closed-form results obtained. Then, with imperfect CSI and PBNJ, we analyze the BER of the optimum and the suboptimum ML receivers, the optimum and the suboptimum weighted EGC(W-EGC, SW-EGC),MRC, and EGC for the S-CFFH/BPSK system, with a number of closed-form results obtained. Besides, we analyze the BER of the optimum and the suboptimum ML receiver for the S-CFFH/16 QAM. The performance of the S-CFFH ML receiver in the presence of MTJ is also studied. The analyses are collaborated by simulations, which show that, the jamming rejection ability can be listed as optimum ML > suboptimum ML > W-EGC > SW-EGC. Besides, as compared with the conventional non-coherent FFH, the S-CFFH shows significant performance improvement in terms of BER and spectral efficiency.4) We propose the multiple antenna S-CFFH system with spatial modulation(SM)and quadrature spatial modulation(QSM), respectively. We also provide the system structure design and the signal model. By combining the benefits of both the S-CFFH and the spatial modulation, the proposed multiple antenna S-CFFH schemes outperform the single antenna S-CFFH.5) In the presence of PBNJ, we analyze the performance of the S-CFFH/SM andthe S-CFFH/QSM, which are verified by simulations. With perfect and imperfect CSI, we analyze the BER performance of the proposed schemes with the optimum and the suboptimum ML receivers. We present closed-form expressions for the pairwise error probability(PEP) and asymptotically tight union bound for the BER. Our analyses are collaborated by simulations, which show that,with the same spectral efficiency, the S-CFFH/SM and the S-CFFH/QSM significantly outperform the S-CFFH in terms of BER, where the S-CFFH/QSM performs slightly better than the S-CFFH/SM. As compared with the non-coherent FFH/MFSK, the S-CFFH/SM and the S-CFFH/QSM show significant performance improvement in terms of both BER and spectral efficiency.