Study Of Chirp Underwater Acoustic Spread-spectrum Communication Technology

The transmission properties of underwater acoustic channel is very complex.Time-varying,frequency-varying,space-varying are its typical characteristics. Intersymbol interference caused by multipath and Doppler effect are the main two factors that influence the performance of underwater acoustic communication. It is necessary to improve the performance of underwater acoustic communication under the condition of low SNR multipath interference and Doppler. Spread spectrum communication is a good way to solve the noise and multipath interference. The bandwidth of Chirp signal expands as time changing.And Chirp signal has the better performance of spread spectrum. And it is a big wide bandwidth signal, so signal gain is high. And it has a big tolerance to resist the Doppler Effect.Because of the Chirp signal itself has good advantages of spread spectrum communication, so it is suited to the study of underwater acoustic spread spectrum communication. Chirp signal has good matching correlation and the fractional Fourier domain signal good processing properties. So this paper study on these two points. Paper firstly studies the binary keying on Chirp spread spectrum communication system which has two kinds of demodulation De - MF and De - FRET and compares their performance. Then this paper studies in relative motion at a constant speed and variable speed relative motion Doppler estimation and compensation method. Finally in the time domain and frequency domain respectively puts forward the equalization algorithm of channel estimation multipath interference. The algorithm is tested its feasibility by Matlab simulation and sea trial. Paper main research works are as follows:1. This paper introduces the purpose and significance to carry out research in underwater acoustic spread spectrum communication, as well as the development present situation at home and abroad.2. Study the Doppler frequency shift, symbol synchronization error and phase error which influence on De- MF and De - FRFT. Comparing two demodulation method under the noise interference and multipath interference of the decoding performance. Binary keying in the Chirp spread spectrum communication system based on quaternary spread spectrum communication system is put forward,and compare both advantages and disadvantages. The simulation results show that the quaternary improves communications speed at the same time and increases the BER of the system.3. In relative motion at a constant speed and variable speed under the condition of relative motion Doppler estimation method is studied. The simulation results show that packet data length estimating Doppler method is suitable for relative motion at a constant speed,but for variable speed estimate Doppler is not applicable. Thus proposed sliding track Doppler estimation method can effectively solve the problem.4. In order to solve the multipath interference problem in underwater acoustic communication, to study the frequency domain forced zero equalization and the fractional Fourier domain forced zero equalization and the fractional Fourier domain MMSE criterion equalization, and the time reversal mirror technology equalization. FRFT-MMSE is the best method in these four equalizations.5. Equalization algorithm and Doppler estimation method are tested practical feasibility by sea trial.