Underwater Acoustic Communication Based On FH/MFSK

As a main approach of realizing the integration of underwater information perception and information interaction, underwater acoustic communication not only has high military use value, but also plays a vital role in the underwater information transmission of civilian field. Underwater acoustic channel, especially the one in shallow sea, turns to be the most difficult wireless channel till now, for it is a complex channel with variable time-space-frequency characteristics. It is mainly confronted by strong multipath effect, high signal fading amplitude, serious noise marine environment, low-carrier frequency and limited available bandwidth. How to overcome multipath interference, time-frequency and Doppler spread and time varying fading are major problems that should be solved for the underwater sound communication technology.With good compatibility with narrow band communication system, frequency hopping communication has a great capacity of anti-interference and anti-fading, which makes it perfectly suitable for the communication in shallow sea. Because there are strong multipath effects, serious interference and fading in underwater acoustic channel of shallow sea. Here both traditional frequency hopping combined with MFSK technology and differential frequency hopping were utilized for communication in this paper. We made simulation and experiment to compare the performance of four modulation systems (regular MFSK, SFH/MFSK, FFH/MFSK and DFH).This thesis introduced the underwater acoustic channel in shallow sea underwater acoustic channel of shallow sea. Modulation and demodulation technology of MFSK were summarized, and analysis was carried out for the theoretical feasibility of FFT demodulation technology. With regard to regular frequency hopping communication further studies were conducted for the generating and synchronizing method of frequency hopping sequence. With regard to differential frequency hopping further studies were conducted for the differential frequency hopping constructor and measurement technique. The symbol-by-symbol detection technology of differential communication was optimized.In this paper a new G function applied in the differential underwater acoustic communication was proposed. This function has good randomness, and the frequency interval is larger.With Hamming distance as the hard decision algorithm of distance measurement and energy spectrum value (Euclidean distance) soft-decision were used as the demodulation of differential modulation.In this thesis, their simulation and performance analysis were conducted on the basis of multipath underwater acoustic channel model, and then we made similar simulation and analysis in underwater acoustic channel model under simulated lake environment.The experiments of FH/MFSK underwater acoustic communication system conducted in Moganshan Lake were introduced in this thesis. The communication experiments for four systems with the distances of100m,150m and200m were carried out, and then data was acquired for post demodulation. The results show that DFH on field applications has better performance.