NLMS Adaptive Digital Filter with a Variable Step Size for ICS (Interference Cancellation System) RF Repeater

In the NLMS (normalized least mean square) adaptive filters, choosing a suitable step size is crucial as the stability and the steady-state convergence error of the adaptive filter heavily depends on the step size value. In this paper, we propose a new VSS (variable step size) control method for the NLMS adaptive filter applicable to on-frequency RF repeaters that operate under fading feedback multipath channel. In the proposed method, the step size is obtained by multiplying the two step sizes. The first one adapts fast based on the estimated RFR (received-to-feedback signal power ratio) value, while the second one is adjusted slow according to the estimated fading rate of the feedback multipath channel. Computer simulations show that by adopting the proposed VSS control method the RF repeater can track the time-varying feedback channel with a good convergence error while previous VSS control methods suffer from performance degradation.