Implementation Of Low Rate Underwater Acoustic Digital Speech Communication System

Oceans have become the focus again due to the continuous deepening and development of marine research,resource exploitation and conservation.As the only way to achieve long-distance communication in underwater environment,underwater acoustic communication technology plays an important role in the ocean exploration.However,it is very difficult to achieve stability voice communication since underwater acoustic channels are characterized by strong multipath,narrow bandwidth,strong noise,and random time-space-frequency change.Speech coding technology can reduce greatly the transmission bit rate and needed bandwidth of the communication by deleting the redundant information in the speech signal.Frequency hopping technology is suitable for long-distance communication in underwater environment due to its excellent performance of eliminating multipath interference.By combination of the speech coding and the frequency hopping technology,this paper designs and realizes a low rate underwater acoustic digital speech communication system.The main works of this paper are as follows:1.Physical and transmission characteristics and their influences on speech communication in underwater acoustic channel have been analyzed.Two different shallow underwater acoustic channel models are simulated by using BELLHOP model.2.The basic principles of speech compression coding and frequency hopping communication are introduced.A low bit rate underwater speech frequency hopping communication method is proposed by combination of the speech compression coding and the frequency hopping technology.A design scheme of wide interval combined frequency hopping sequence is proposed for the system to anti strong multipath interference.Simulations show that the frequency-hopping sequence can effectively reduce the bit error rate,and improve the confidentiality of the system.3.A digital low-rate underwater speech communication system prototype is designed based on the OMAP-L138 DSP chip.The system consists of software and hardware modules.Hardware module includes DSP core,preamplifier,filter,power amplifier,matching circuit,vocoder and electric power management,etc.Software module achieves signal's modulation,demodulation,hopping and dehopping and synchronization.To achieve robust communication in long-distance underwater environments,the system uses 4-FSK modulation and FFT algorithm to demodulate in the receiver.4.Speech communication experiments were conducted in pools and the Wuyuan gulf of Xiamen port,respectively.The maximum distance is about 1.0 km,and the bit error rate is about 10-2.The synthetized speech in the receiver holds relatively high degree of intelligibility,clarity and naturalness.