This work studies the Minimum Output Variance Least Mean Square Estimator (MOV -LMS) when the output of the adaptive filter is constrained by a saturation-type nonlinearity. This situation is typical in several adaptive modeling and control systems where the associated hardware and transducers have a finite power handling capability. Analytical expressions are obtained for the behaviors of the mean weight vector and for mean square error for Gaussian inputs and slow learning. The optimum leakage factor is detenn ined, which provides an unbiased solution to the associated nonlinear mean square estimation. problem. Monte Carlo simulations show excellent agreemen t between model and simulations for transient and steady-state conditions. The results in this paper demonstrate that the MOV-LMS algorithm with the optimized leakage factor has a superior steady-state performance than the LMS algorithm in the studied nonlinear environment
I, INTRODUCTIONSeveral adaptive modeling and control systems present saturation-type nonlinearities in the adaptive filter path [1-6]. Such nonlinearities can severely impair the adaptive algorithm performance in minimizing a cost function, usually the mean square output error (MSE) [5][6][7].Among the various existing adaptive solutions, the family of stochastic gradient-based algorithms (the LMS family) is a common choice due to its simplicity and robustness. Several works have studied the behavior of the LMS and its family in modeling and control applications, such as active noise control (ANC) [I]. A common assumption, for simplicity of analysis, is that the system is purely linear. However, it is well known that the associated hardware (power amplifiers, piezoelectric transducers and loudspeakers) is an important source of nonlinearities [2][3][4]. In order to avoid nonlinear distortions, the system is frequently overdesigned to keep signal power sufficiently low compared to the saturation levels.A recent work [5] studied the behavior of the LMS algorithm subjected to a nonlinear influence at the adaptive filter output. It was demonstrated that the mean converged weights corres pond to a biased solution with respect to the minimum of the MSE surface. This bias is a multiplicative scalar, which is a function of the system's degree of nonlinearity.The results obtained in [5] agreed with the observations of several authors in ANC area [2-4,6]. The possibility of quantifying the nonlinear effects on the performance surface motivated the search for new algorithms that could be capable of overcoming the poor steady-state performance of the LMS algorithm in a nonlinear environment. Such a solution would allow the use of cheaper amplifiers and transducers without a significant performance loss, therefore reducing implementation costs.Reference [7] proposed a new stochastic gradient-based algorithm, referred to here as the Nonlinear True-LMS algorithm. This algorithm is capable of reaching the minimum of the MSE surface, since the system's nonlinearity is incorporated in the weight update equ...