Study On The Key Technology Of Underwater Acoustic DFH Communication

In recent years, along with the need for the national defense and development of marine resources, underwater acoustic communications has attracted people's attention, the hotspot of the current study focused on communication speed, range and quality. However, due to the openness of underwater acoustic communication channel, the acoustic communication information can easily be captured or jamming by the enemy, and therefore to ensure the rate of communication, at the same time how to improve the communication of concealment, reliability, confidentiality and timeliness, which are the issues of importance the underwater acoustic military communications facing.Differential frequency hopping technology has a strong anti-multipath, anti-jamming capability and high-speed data transmission capacity, a certain processing gain, easily compatible to narrow-band systems, etc. so in recent years it has been widely used in the wireless short-wave frequency-hopping radio domain. Based on the Harbin Engineering University Research Fund " The Research of Underwater Acoustic Differentail Frequency Hopping Technology " (HEUFTO8026), some problems on differential frequency hopping technology in the application of acoustic communication are studied in this thesis.The main contents of this thesis are as follows:1. Using the research method of convolutional codes and Markov chain, a concrete test is given which introduced the distance performance verification as an addition based on randomicity and uniformity test of frequency-hopping sequence, through in-depth analysis of the performance of the frequency transition function of DFH system. A new frequency transition function is propounded according to the acoustic channel characteristics, whose validity is validated by contrasted it with several other frequency transition functions in common use in performance test and simulation.2. The performance equivalent conditions between FFT detection and non-coherent detection of DFH signal are given in the thesis. The DFH signal detection technology based on FFT algorithm was researched, the performance of which is analyzed in AWGN channel. Combined the characteristics of acoustic channel and the constraint relation of frequency transition function, an improved algorithm on viterbi hard decision is presented, which is simulated in acoustic multipath channel, its performance was validated by the results of lake experimentation.3. The structure of Turbo code and algorithm for Turbo decoding are studied, a structure of parallel and serial concatenated convolutional code combining with differential frequency hopping is presented,and proposing an improved MAP algorithm and SOVA algorithm. The performance of new Turbo-DFH in underwater acoustic multi-path channel is analysised and compared with conventional DFH, the simulation results show that Turbo-DFH has better performance than conventional DFH system.4. The doppler estimation and compensation technology in mobile underwater acoustic DFH communication are studied. Analysising the effect on demodulation algorithm for DFH signal based on the FFT method,combining with characteristic of underwater acoustic DFH communication and presenting an improved scheme based on the packet length measurement and frequency estimation method.A new method in time-frequency domain is proposed,which estimates doppler especially large doppler using Fractional Fourier Transform, Finally performance simulation of underwater acoustic DFH system based on these three estimation and compensation schemes is given.