A New Local Polynomial Modeling Based Variable Forgetting Factor and Variable Regularized PAST Algorithm for Subspace Tracking

Abstract

This paper proposes a new local polynomial modeling based variable forgetting factor (VFF) and variable regularized (VR) projection approximation subspace tracking (PAST) algorithm, which is based on a novel VR-VFF recursive least squares (RLS) algorithm with multiple outputs. The subspace to be estimated is modeled as a local polynomial model so that a new locally optimal forgetting factor (LOFF) can be obtained by minimizing the resulting mean square deviation of the RLS algorithm after using the projection approximation. An $l_2$ -regularization term is also incorporated to the LOFF-PAST algorithm to reduce the estimation variance of the subspace during signal fading. The proposed LOFF-VR-PAST algorithm can be implemented by the conventional RLS algorithm as well as the numerically more stable QR decomposition. Applications of the proposed algorithms to subspace-based direction-of-arrival estimation under stationary and nonstationary environments are presented to validate their effectiveness. Simulation results show that the proposed algorithms offer improved performance over the conventional PAST algorithm and a comparable performance to the Kalman filter with variable measurement subspace tracking algorithm, which requires a considerably higher arithmetic complexity. The new LOFF-VR-RLS algorithm may also be applicable to other RLS problems involving multiple outputs.