Interference cancellation for mobile communication radio repeaters

Abstract

When a radio repeater using the same frequency for signal transmission and reception has a gain that is larger than the isolation between its transmit and receive antennas, it oscillates due to feedback interference signals. To study the prevention of feedback oscillation of a radio repeater in this work, we first formulate a feedback oscillation model and then derive a time domain interference cancellation (ICAN) model for a radio repeater. Using the ICAN model, we show that the stability and the signal quality of the repeater depend on the repeater’s gain and time delay, the propagation delay on feedback paths, feedback channel characteristics, and the capability of the feedback channel estimation algorithm. It is also shown that the stability condition of the repeater does not guarantee the quality of the repeater’s output signal. To guarantee stability and signal quality for a radio repeater, we propose an ICAN scheme, where an adaptive filter based on an iterative algorithm is used to adaptively estimate a feedback channel response, generate estimated feedback signals using the estimated feedback channel response, and cancel the feedback interference signals using the estimated feedback signals. Using an efficient simulator, which was implemented to speed up the design and performance verification of the ICAN scheme for radio repeaters, we confirm that the stability and the signal quality of a radio repeater depend on various factors noted above. In addition, through a frequency domain ICAN model derived from the time domain ICAN model, it is found that if the proposed ICAN scheme is applied to a wideband radio repeater that relays a wideband signal such as a WCDMA signal with multiple bands, the output spectrum of the repeater gets deteriorated due to insufficient cancellation of the feedback signals. Therefore, to prevent the output spectrum of the wideband ICAN repeater from being deteriorated, a wideband ICAN scheme with a parallel structure is prop...