Study On The Key Anti-Jamming Technologies Of Frequency Hopping Spread Spectrum Systems

Wireless communications have been applied to every aspect of our life with the development of modern communication technologies, which makes the problem of interference more critical. Especially for the frequency hopping (FH) systems, the application scenario of them is very complicated. Developing anti-jamming technologies of FH systems is a valuable research subject. In this thesis, this topic was discussed from the following two aspects:interference detection and suppression.The object of interference detection under radio silence is to select high quality channels or to provide a basis for selecting an optimal anti-jamming communication system. However, to the best of our knowledge, there are few papers to discuss this problem. In this thesis, we put forward several solutions for the interference detection under radio silence based on the ideas of spectrum sensing of cognitive radio. According to the characteristicof interference detection, multiple band correlation detection for interference was proposed.The interference detection under radio silence can be looked as active interference detection, In contrast, the interference detection for frequency hopping systems can be looked as passive interference detection. In this thesis, a wideband FH receiver was proposed, based on which, wideband energy detection for interference was discussed. In this method, estimation of the noise power is unnecessary, avoiding the performance degradation caused by the estimation error. Wideband energy detection is a simple algorithm, but it is not an optimum detection structure. For a more optimal one, interference detection based on generalized likelihood ratio test (GLRT) was proposed. Simulation results show that the performance of interference detection based on GLRT is much better than wideband energy detection. Furthermore, it can be also applied to the interference detection under radio silence. Except for the detection of the interference, the power of the interference, the power of the noise and the jamming ratio can be also estimated, which provide side information to suppress the interference.The suppression of interference for FH systems was emphasized on the suppression algorithms for fast frequency hopping (FFH) in recent years. In this thesis, different diversity combining methods were summarized and the selection diversity combining was proposed. The noise-normalized receiver for FFH systems is a suboptimum receiver among different diversity combining receivers. The performance of noise-normalized FFH M-ary orthogonal frequency-shift-keying (MFSK) receiver over fading channels with partial-band noise jamming (PBNJ) was analyzed. Instead of numerical evaluation, a closed-form error probability expression was given based on the theory of complex variables. Another problem appears in the performance of FFH receivers with PBNJ is that the existing performance analysis method can only be used for block hopping, but can not be used for independent tone hopping. In this paper, the performance of FFH binary orthogonal frequency-shift-keying (BFSK) independent tone hopping receiver with PBNJ was analyzed.It is well know that there is a worst case PBNJ. It is stand from the jammer's viewpoint to look at the problem. It is shown that there is an optimum diversity order, if we stand from the messager's viewpoint to look at the problem, there is a best case performance at the receiver. The best case performance was discussed in this thesis.In the aspect of interference suppression for different frequency hopping (DFH) systems, an improved G function was proposed. The free distance of the new G function is no longer confined to the frequency number, which improves the performance of DFH systems. Furthermore, the noise-normalized combining receiver for DFH system was proposed based on the idea of FFH diversity combining. The performance of it was analyzed. It is shown that the anti-jamming capacity of DFH system is enhanced.Anti-jamming technologies for FFH system were systematically studied in this thesis. The results of this thesis will be very helpful to the theory and practice of FFH system.