Frequency Modulation In Underwater Acoustic Communication Based On Mode Modulation

With the development of marine resources of modern society, the requirements of thedevelopment of underwater acoustic communication technologies are increasingly urgent inboth military and civilian areas. However, the underwater acoustic channel is one of themost complex channels among all communication channels. The signal frequency offset,which is caused by underwater acoustic environment, multipath interferences and Dopplereffects, affects the performance of underwater acoustic communication seriously. Becausethe attenuation of underwater acoustic communication signal increases as the frequencyincreases, the frequency tends to extend to the low frequency, which leads to the limitationof the available bandwidth of underwater acoustic communication.M-ary Chirp Linear Frequency Modulation (LMF) is an effective way anti multi-pathand fading in the remote underwater acoustic communication. However, it has strictrequirement of the orthogonality of the signal. This means the frequency offsets introducedby Doppler effect will strongly affect the demodulation result of the system. Moreover, therequirement of the orthogonality also means that each sub-band can only carry few bits.Therefore the m-ary chirp LMF signals can only achieve low data rate. To increase the datarate of the communication systems and take advantage of high anti-Doppler performance ofLFM in a limited frequency band, we consider using nonlinear FM signal in underwateracoustic communication.In phonetics, the absolute value of the fundamental frequency of tones of a languagedoes not make sense for tone perception. Instead, the shape and trend of the fundamentalfrequency curve can represent different tones of the same syllable of a language. Weborrowed the idea in this phenomenon and proposed a method of frequency modulation inunderwater acoustic communication based on pattern modulation.Our new method uses FM curves having different patterns in the same frequency band tomodulate the transmitted signals. By doing this, we can use the different patterns of FMcurves to represent different symbols that contain information data. The improvements of the linear frequency modulation method allow us to use other schemes other than thecoherent demodulation which has strict requirements on the orthogonality of the signals atthe receiver side of the system. Instead, we use an instantaneous frequency estimationmethod to reconstruct the frequency curves of the signals. Then we can match the receivedFM curves with the known ones by pattern recognition to recover the transmitted symbols.The simulation results show that the pattern FM, with the combination of instantaneousfrequency estimation and pattern recognition, can effectively combat Doppler interferenceof underwater acoustic communication, and obtain a higher data rate compared with theLMF algorithm。...