Research On Single Input Multiple Output Long-range Underwater Acoustic OFDM Communication Technology In Deep Water

OFDM technology has high frequency band utilization and strong anti-multipath ability,which has been widely used in the field of underwater acoustic communication in recent years.Single Input Multiple Output(SIMO)communication system can improve the reliability and stability of the system by processing the received signals of multiple receiving array elements.In view of the characteristics of low SNR and serious multipath delay spread of long-range acoustic communication in deep water,this paper studies the multi-channel equalization technology suitable for deep water long range communication based on SIMO-OFDM system.Firstly,time-varying channel is modeled by BELLHOP simulation software and statistical model of underwater acoustic channel.In this paper,channel simulation is divided into large-scale fading and small-scale fading.In order to better adapt to the real deep water environment,the time-varying channel under the horizontal seabed and under the seamount in the seabed are studied,which lays the foundation for more reliable deep water long range communication.In view of the poor anti-interference ability of SISO communication system,this paper studies two multi-channel equalization methods based on spatial diversity: multi-channel equalization based on spatial diversity combining and multi-channel equalization based on VTRM.For spatial diversity combining multi-channel channel equalization,three combining methods are analyzed and compared: SC,MRC and EGC.Aiming at the problem that the equivalent channel of deep-water channel after VTRM processing still has serious multipath effect,the secondary channel equalization is carried out in this paper,that is,after estimating the equivalent channel,the zero-forcing equalization is carried out.The simulation and processing of sea experiments data prove the effectiveness of the two multi-channel equalization methods,and the use of diversity technology can greatly improve the reliability of the communication system.Beamforming is the most common method to improve the input SNR before decoding,however,for deep water long range communication systems,the bending of the sound line causes direct sound path and reflection path to reach the receiver at different angles.The traditional beamforming-based communication method only outputs the beam for the angle of arrival of the path with the strongest energy,and then performs subsequent demodulation.The single beam channel equalizer cannot obtain multipath diversity gain.To solve the above problems,this paper proposes a multi-channel frequency domain equalization method based on beam domain for underwater acoustic OFDM communication.In this method,the received signals of the hydrophone array are processed by beamforming to obtain the beam output of multiple arrival angles,and then the multi-beam frequency domain equalization is realized based on the MRC.Compared with the traditional method that only outputs the strongest beam in long range communication,this method can obtain additional multipath diversity gain;and compared with the traditional multi-channel MRC method,the beam forming processing can improve the input SNR before decoding,so as to achieve the optimal combined output.Simulation and experimental data processing prove the feasibility and effectiveness of the multi-beam MRC frequency-domain equalization method.