Study On Anti-jamming And Anti-interception Differential Frequency Hopping System

Shortwave communication is one of the important ways to achieve over the horizon communication. As the shortwave channel is often corrupted by some poor properties such as multi-path fading and serious time-varying and the available bandwidth is relatively small, to achieve high data transmission speed has long been a hot concern in military communications. Differential frequency hopping (DFH) technology offers a new way of improving the rate of short-wave communication. Researches on anti-jamming and anti-interception technologies for DFH system are still inadequate. DFH is a wideband asynchronously receiving frequency hopping scheme and power detection is performed over all the frequencies of the hopping frequency set, therefore partial-band jamming may reduce the performance of DFH system severely. Signal detection and decoding algorithms proposed nowadays have their individual limitations. In terms of anti-interception, it is shown by related researches that DFH is not a communication scheme with low probability of interception (LPI). Any third party interception receivers can obtain the frequency transition matrix through the intercepted frequency sequence and jamming signal generated with the same frequency transition matrix may seriously damage the performance of DFH system. Studies on methods of improving the anti-interception performance of DFH system are necessary therefore. In addition, high data transmission speed capability of DFH system makes it a potential for shortwave burst communication, but anti-jamming design must be considered seriously for the special characteristics and requirements of DFH and burst communication.This paper focuses on studies on anti-jamming and anti-interception technologies used for DFH system. The main research contents and conclusions are listed as follows.(1) A novel anti partial band jamming (PBJ) DFH receiver based on noise normalization combining (NNC) is proposed. Symbol error rate (SER) of NNC-DFH receiver is derived with multiple fading conditions considered including non-fading, Rayleigh fading and the more general Nakagami-m fading. The inevitable non-ideal noise power estimation and its influence on system performance are also considered. It is shown that when jammed by non-worstcase jamming, NNC-DFH receiver is superior to linear combining (LC) receiver and product combining (PC) receiver except for the special condition when channel fading is weak and the jamming power is dispersed and the superiority increases with the concentration of the jamming power. When suffering worstcase jamming, NNC-DFH receiver is always superior to LC and PC DFH receiver over the studied fading conditions for any jamming bandwidth ratio. It is shown that when noise power estimation error is not great (0.1<λ<3,λis the estimation error factor), NNC-DFH receiver is superior to LC and PC DFH receiver when corrupted by worstcase PBJ. System with noise power a little lower estimated (0.2<λ< 0.6) outperforms that with ideal noise power estimation.(2) Anti-interception design criterions and the evaluation methods of vary anti-interception performance indexes are studied first. G-functions proposed by literates are classified and anti-interception performances of DFH systems applying different G-function are studied. After proving that normal DFH system can not achieve good two-dimensional continuity performance, two methods of improving anti-interception performance of DFH system are proposed including time varying G-Function (TVG) method and FH/DFH method which combines traditional frequency hopping with DFH. For TVG, an optimization criterion is provided and it is proved that DFH system using TVG method has good two-dimensional continuity performance in long observing period sense. For FH/DFH, after description of system model, one-dimensional uniformity and two-dimensional continuity performance is analyzed theoretically. Theoretical and simulation results validate the effectiveness of these two proposed methods in improving the anti-interception performance of DFH system. Comparing with TVG method, FH/DFH method is superior in two-dimensional continuity and randomness performance with the cost of inceread complexity of implementation.(3) To reduce the signal radiation time as much as possible, adaptive frequency set optimization are not applied in burst communication although which is often used in normal DFH system. On the other hand, wideband property should be reserved to enhance the communication concealment, but which will make many interferer signals unavoidable. An anti-jamming method based on multi antenna diversity is used in burst DFH communication system, which is suitable for the special application scene where there are single transmitter and multiple receivers. SERs of this system (jammed or not) are deduced in multi-dimensional addition style, which is easy to be evaluated. It is shown by these analyses that even when half of the frequencies are jammed with jamming power equal to the instantaneous signal power, a 10-4 SER can be obtained with the proposed system with diversity order not less than 4. This is a new approach of designing of shortwave burst communication system.