| Underwater acoustic（UWA）channel has the characteristics of limited available frequency band,serious multipath effect,Doppler effect and channel structure changing with time.These characteristics make the research of UWA communication more complex than radio communication.To achieve robust UWA communication performance,spread spectrum communication system is usually used.Direct sequence spread spectrum（DSSS）is a high reliability communication technique commonly used in UWA communication.However,due to the limited underwater bandwidth,the problem of low utilization of the DSSS band becomes more and more prominent.How to make full use of the restricted bandwidth in the UWA channel containing many random factors and flexibly adjust the system design in the face of different undersea communication scenarios on the premise of ensuring the robustness of UWA spread spectrum communication is a challenging problem.In this paper,M-ary spread spectrum（MSS）is used instead of DSSS to improve the spectral efficiency and energy efficiency of the system.The key techniques such as MSS energy detection,double differential coding,Doppler estimation and time-domain equalization based on decision feedback are theoretically analyzed and simulated.On this basis,mobile UWA communication under small Doppler,variable speed UWA communication and deepsea long range mobile UWA communication are studied,which are verified by trials in Harbin,Weihai and Sanya.Aiming at the requirements of real-time processing of received data and low complexity of system hardware in the UWA communication scene with fixed end or uniform relative speed less than 3m/s,a scheme of double differential M-ary spread spectrum（DD-MSS）UWA communication system is proposed in this paper.DD coding is used to eliminate the signal phase and frequency errors caused by a small Doppler,and MSS phase modulation mode is used to make full utilization of the weak cross-correlation and sharp autocorrelation characteristics of different spread spectrum sequences,so that each spreading symbol can carry more data.On the premise of ensuring the communication reliability of the spread spectrum system,the communication rate of the DSSS system is doubled.The application of carrier synchronization and code synchronization makes the system realize reliable communication without Doppler estimation,and compensation.The experiment of Songhua River UWA communication verifies that the DD-MSS has a lower bit error rate when the communication rate is higher than that of the DSSS.Aiming at the communication scenario with obvious Doppler time variation in the UWA channel and the requirements of Doppler estimation,tracking and symbol synchronization of the received signal in a shorter time interval,an optimized MSS UWA communication system with a symbol-by-symbol Doppler estimation and synchronization is proposed in this paper.Using the characteristics of large time bandwidth product and Doppler sensitivity of the spread spectrum signal,the correlation search is carried out in two dimensions of Doppler domain and spread spectrum code domain with symbol interval,and the Doppler estimation and de-spreading process of current symbols are realized at the same time.In the simulation and experiment,it is found that the orthogonal MSS system has a strong Doppler tolerance ability,and the low Doppler estimation accuracy does not affect the communication performance.To reduce the huge amount of related operations,this paper proposes three optimization steps,namely,expanding the Doppler search step,heuristic search,and the assumption based on Markov process.Through these optimizations,the amount of receiving and processing operation of the symbol-by-symbol Doppler estimation is reduced to about 1.24%,which is only twice as much as that of de-spreading in a conventional MSS system.In Weihai mobile communication experiment,this method realizes the bit error rate of 4km communication distance less than 10-3 with the maximum acceleration of 47.2m/s2.In addition,this paper proposes a method to improve the accuracy of Doppler estimation,constructs the objective function based on the correlation between the local reference signal and the received signal,and uses the particle swarm optimization algorithm to find the maximum value of the cost function in the reduced search range.This method is an improvement of constructing the goal function by minimizing the error of reference signal and standard signal by the least square method.The simulation results show that this method has a better adaptability to multipath and large Doppler.Aiming at the suppression requirements of serious inter chip interference and inter symbol interference caused by multipath effect in the communication scenario of long time delay and non minimum phase UWA channel,this paper presents two time-domain equalization techniques suitable for the MSS UWA communication system according to the different time variation of the channel.When the channel time variation is weak,the decision feedback equalizer（DFE）coefficients are updated at the spread spectrum symbol interval.To reduce the error propagation probability caused by incorrect feedback decisions,this paper proposes the DSSS bidirectional DFE technique.Which takes an advantage of the low correlation of the DFE output noise in the normal mode and the time reversal mode to improve the output signal-to-noise ratio（SNR）of equalization processing at the cost of doubling the amount of calculations.In the time varied UWA channel,the MSS hypothetical feedback equalization is studied,and the MSS communication method hybrid CHF and energy detection is proposed.The appropriate processing method is optimized in different noise environments,effectively expanding the application scope of the MSS system in the deep sea long range UWA channel.In the 103km deep-sea long-range UWA spread spectrum communication experiment conducted in Sanya,the MSS hypothesis feedback equalization realizes the error-free decoding of a small amount of data at the relative motion speed of 2.4m/s.The effectiveness of the proposed method is verified. |
PDF Full Text Request