Research On Adaptive Modulation Code Scheme In Underwater Acoustic Communications

Underwater acoustic communication is one of the crucial supporting technologies in fields such as ocean exploration and resource extraction.However,the complex and everchanging underwater environment significantly impacts the reliability and effectiveness of underwater acoustic communication systems.Traditional communication methods with fixed parameters are inadequate to cope with the complex and dynamic underwater communication environment.Adaptive modulation and coding technology senses changes in communication conditions and dynamically adjusts parameters such as modulation order,coding method,and code rate,thereby improving the frequency band utilization while ensuring the reliability of the communication system.Due to the significant propagation delay in underwater sound propagation,the use of adaptive modulation and coding techniques introduces the challenge of outdated channel state information when the receiver needs to provide feedback to the transmitter about the communication quality.Nonetheless,adaptive modulation and coding techniques remain one of the viable options to address the complexities and variations of underwater acoustic communication environments.This paper focuses on exploring adaptive modulation and coding techniques in underwater acoustic communication.According to the characteristics of the underwater acoustic communication environment and the requirements of the adaptive coding and modulation method.Firstly,based on the target bit error rate criterion,the signal-to-noise ratio(SNR)switching thresholds of six typical modulation and coding modes are given.Secondly,in order to compensate the time-domain extension of the signal caused by the Doppler effect,on the basis of estimating the Doppler factor by the cubic correlation method,a method for estimating the Doppler factor based on the time-domain superposition of positive and negative swept chirp signals is proposed,resulting in a three-orders-of-magnitude improvement in estimation performance.Then,in order to ensure the reliable interaction of the modulation and coding schemes of the adaptive modulation and coding system,a communication frame structure was designed by combining linear frequency modulation signal,Chirp modulation and Pattern delay difference coding modulation,so as to realize the Doppler factor estimation and Compensation,interaction of modulation and coding schemes,channel estimation and equalization,information coding modulation and demodulation decoding.Moreover,this design reduced the additional communication overhead caused by feedback information loss.Finally,the effectiveness of the proposed method is verified by the simulation and experimental data analysis results.To address the need for perceiving changes in communication conditions in adaptive modulation and coding techniques,the received received SNR is utilized as a parameter to evaluate the channel quality.Building upon the designed communication frame structure,an orthogonal matching pursuit algorithm based on correlation peak search is proposed to estimate the channel state information.This algorithm takes advantage of the high autocorrelation property of linear frequency modulated signals,enabling the preliminary determination of sparsity and multipath delay range.The channel sparsity is estimated and used as prior information for the orthogonal matching pursuit algorithm,resulting in a significant reduction in computational complexity.Then,in order to address the issue of predicting the communication quality of the next period for adaptive modulation and coding techniques,the approach of using historical SNR sequences to predict SNR in the next period is adopted,and an adaptive triple exponential smoothing algorithm was proposed.This algorithm dynamically updates the smoothing coefficients based on real-time received SNR,allowing it to track changes in channel communication conditions.Finally,simulation and experimental data analysis results confirmed the effectiveness of the orthogonal matching pursuit algorithm based on correlation peak search and the adaptive triple exponential smoothing algorithm.