H 1 Optimality of the LMS Algorithm 1

We show that the celebrated LMS (Least-Mean Squares) adaptive algorithm is H 1 optimal. The LMS algorithm has been long regarded as an approximate solution to either a stochastic or a deterministic least-squares problem, and it essentially amounts to updating the weight vector estimates along the direction of the instantaneous gradient of a quadratic cost function. In this paper we show that LMS can be regarded as the exact solution to a minimization problem in its own right. Namely, we establish that it is a minimax lter: it minimizes the maximum energy gain from the disturbances to the predicted errors, while the closely related so-called normalized LMS algorithm minimizes the maximum energy gain from the disturbances to the ltered errors. Moreover, since these algorithms are central H 1 lters, they minimize a certain exponential cost function and are thus also risk-sensitive optimal. We discuss the various implications of these results, and show how they provide theoretical justiication for the widely observed excellent robustness properties of the LMS lter. manuscript is submitted for publication with the understanding that the US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation thereon.