Study And Implementation Of Passive Time Reversal Underwater Acoustic Communication Using Chirp Modulation

Time reversal communication is a research hotspot in underwater acoustic communications in resent years.As the passive time reversal communication requires only a one-way transmission,reducing the communication system’s need to send and receive,it has been more extensively studied.Chirp modulation can effectively be combined with conventional phase,frequency,amplitude modulation because of its unique characteristics which is pulse compression,thus it can transmit information at a lower power,so time reversal communication based on Chirp modulation technique is studied in this paper.The communication systerm includes Chirp modulation,passive time-reversing,joint estimate of carrier and symbol,adaptive equalization.First the characteristics of shallow water channel is given,then the time and frequency characteristics of chirp,pulse compression feature of chirp,modulation principle of chirp are introduced;then the a detailed analysis of chirp hybrid modulation which is based on the time domain overlapping is given.Then the theory of passive communication is analyzed.In the communication system the synchronization issue has an important impact on the performance and application of the communication,then several typical methods of carrier recovery and symbol timing are analyzed,then the joint estimate of loop carrier symbol which is a combination of sentence feedback loop and as early as late gate synchronization is adopted.in order to restrain the interference between codes further,the principle of adaptive equalization technology is analyzed.Lastly,the communication algorithm of the chirp modulation,passive time-reversing,joint estimate of carrier symbol,adaptive equalization technology are simulated by matlab.Software is implemented by a system with hardware of FPGA and DSP.Data exchange and system control between FPGA and DSP modules 、 FPGA and PC are carried out.Algorithm implementation is done according to the characteristics of FPGA and DSP.Pool and field experiments show that passive time reversal can effectively reduce the impact of multi-channel,the method binding decision feedback loop and the early-late gate can effectively track the small deviation,adaptive equalization can further suppress ISI with lower error rate and good constellation distribution.