| In modern military warfare,information warfare,as a new method of warfare,has a very important position in the entire war situation.Information reconnaissance is the key to victory in an information war.The quality of information reconnaissance determines whether it can effectively detect,receive,and even crack and interfere with enemy communications,and to a large extent determines the direction of the battle.In military communications,High frequency(HF)communication technologies that can realize long-distance non-line-of-sight communications,have low construction costs,strong anti-destructive performance,and good mobility are usually used.However,due to the harsh HF channel environment,the detection of HF multi-carrier signals Receiving faces a greater challenge.Because the structure of the detection signal is fixed,it is impossible to improve the communication quality by adjusting the signal structure,and it is usually necessary to adopt the method of diversity reception and combination to resist the influence of fading.In addition,the Orthogonal Frequency Division Multiplexing(OFDM)signal studied in this paper is more sensitive to synchronization errors,and synchronization errors have also become a major obstacle to the quality of the detected signals.Therefore,synchronization algorithms need to be studied.First,this article introduces the HF reconnaissance model,introduces the HF channel,analyzes its characteristics and fading characteristics,and establishes a mathematical model for it.Then introduced the HF multi-carrier system based on STANAG 4197 standard studied in this article,introduced its modulation principle and implementation mechanism,introduced its signal frame format,and gave its physical layer specific parameters;at the same time,the symbols were analyzed.The influence of synchronization error and carrier synchronization error on OFDM system performance.Next,this article studies the synchronization technology of the OFDM system.First,the time-frequency coarse synchronization algorithm is studied to control the error within a certain range;then the symbol timing synchronization algorithm is studied,and a symbol synchronization method based on power factor is proposed;finally,the carrier frequency synchronization technology is studied,and several Classical frequency offset estimation algorithms,discussed their advantages and disadvantages,and improved the Candan algorithm.An improved frequency offset estimation algorithm based on equidistant multitone square spectrum and multi-symbol frequency based on preprocessing are proposed.Partial estimation algorithm.The simulation results show that in the HF channel,the estimation accuracy under high signal-to-noise is higher.In addition,compared with traditional methods,the estimation performance under low signal-to-noise ratio is improved.Therefore,the performance of the proposed new algorithm is better.The range is also relatively large.Then,this paper studies the differential demodulation algorithm and diversity combining algorithm of HF multi-carrier systems.The differential demodulation algorithm mainly studies the traditional symbol-by-symbol differential demodulation algorithm and the decision feedback differential demodulation algorithm.Diversity merge algorithm mainly studies the traditional diversity merge algorithm and vector merge,etc.,and proposes a noncoherent vector merge method.Its implementation is simpler than the vector merge method.The simulation results show that the improved algorithm refines the weighting granularity.The algorithm has the best performance;finally,combined with the decision feedback differential detection and the improved vector merging algorithm,a multi-channel merging strategy of decision feedback differential detection combined with vector merging is designed.The simulation results show that the strategy is comparable to that of the combined encoding.Compared with the vector merging method,the performance is improved to a certain extent. |
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