Detection and measurement of radio frequency feedback for an on-frequency repeater

Abstract

The radio frequency feedback (RFF) occurs when the insulation is insufficient between the antennas of an on-frequency repeater, increasing digital transmission errors. In addition, a strong RFF could compromise system stability of the on-frequency repeater because of the growing power in the closed-loop. Automatic gain control is widely used by the on-frequency repeater to regulate the power, this solution being generally used with echo cancellation processes. Most of echo cancellation techniques are based on digital processing such as adaptive filters whose the effectiveness and the algorithm speed are depending on the signal frequency, the bandwidth and the closed-loop parameters. This paper describes a solution of RFF estimation and detection regardless of the receiving signal modulation. By using the frequency scanning and the analysis of the power spectral density peaks in the system, this solution is reliable whatever are the values of the gain-margin and the loop-delay. Simulations and experimental implementation using field-programmable gate array validate the solution. In addition, an example of applications is given in the context of the interference cancellation.