Residual carrier adaptive cross-polarization equalizer

Abstract

An equalizer employed with a receiver for cancelling an interfering signal due to low cross-polarization isolation. The present equalizer provides a simple and inexpensive cross-polarization interference cancellation system. The equalizer uses an error signal generated in a residual carrier detector to determine the magnitude of an interfering signal and inputs a cancelling signal to the co-polarized channel until power in the error signal is minimized. In the equalizer, the determination of the magnitude of the interfering signal is performed at baseband (after demodulation but prior to data detection) and the cancellation is performed at RF, prior to dispersive microwave elements. In the receiver, co- and cross-polarized signals are received and amplified. A copy of the cross channel input signal is supplied to each receiver channel. An RF vector modulator is used to control the phase and amplitude of the cross-polarized signal prior to summation with a desired co-polarized signal. When the amplitude and the phase of the vector modulator are correctly set the interfering signal is cancelled. Control for the vector modulator is generated by minimizing the magnitude of the error signal produced by residual carrier detector. The residual carrier detector processes a recovered carrier signal derived from a demodulator in the co-polarization channel and an output of a downconverter of the cross-polarization channel to generate the error signal. The error signal is the difference between the received baseband data and an estimate of the ideal transmitted data. A control circuit sets the phase and amplitude of the vector modulator to minimize the magnitude of the error signal by stepping the setting of the vector modulator by one unit and sampling the power in the error signal. If the power is reduced the vector modulator is stepped again in the same direction. If the power is increased the vector modulator is stepped in the opposite direction.