Research And Realization Of Variable Step Size LMS Algorithm Based On Underwater Acoustic Communication

Adaptive equalization is widely used in modern communication to eliminate inter-symbol interference.Decision feedback equalizer(DFE)is a nonlinear adaptive equalizer which can meet the requirements of high-speed data transmission and achieve better noise immunity performance on the nonlinear distortion channel.Least mean square(LMS)is widely used in underwater acoustic communication which has the contradiction between convergence speed and residual error at fixed step,this paper proposed a variable step size algorithm based on residual error which turned out to be effective.Finally implement the algorithm on DSP in the way of SNR and the effect is remarkable.The main work and innovation of this paper are as follows:(1)The paper described the equalizer and some basic equalization algorithm in detail,and summarized the research theories and methods in the field of equalization technology in recent years systematically.Not only involved the fixed-step LMS algorithm,which has the problem that the convergence rate and the residual error are mutually constrained,but also referred variable step size LMS algorithm.And then,analyzed and compared the algorithm mentioned above.It turns out that the variable step algorithm are better than the fixed-step algorithm,not only on there resolve of the constrains problem,also on the equilibrium effect.(2)A variable step size algorithm based on residual error was proposed.The convergence speed and steady-state error of the existing variable-step algorithm are poor,especially when there is strong noise outside,the equilibrium effect become poorer.In face of the fact that the fixed-step algorithm has the contradiction between convergence speed and residual error,the paper proposed a variable step size algorithm based on residual error.Firstly,by establishing the relationship between the step factor and the error function,the variable step expression is obtained.The impact,which is produced by the dramatic changes of step size,will be weaken with the increase in the number of iterations.In this way,the effects of interference are weakened and the constraint is resolved.(3)Implement the new variable step algorithm on the DSP.It described the principle and realization process of every part in QPSK spread spectrum modulation.And then on the basis of the above,implementing the proposed algorithm——ReLMS by software radio technology.The experiment choose ADSP-BF533 chip which was produced byADI as its hardware platform and choose c language as development language,the result shows the realization of equalization algorithm and its superior performance.