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 an adaptive baseband equalizer to determine the magnitude of an interfering signal and inputs a cancelling signal to the co-polarized channel until power in the error signal derived from the adaptive baseband equalizer 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. The equalizer enhances the capability for transmitting two unique signals in the same frequency allocation on orthogonal polarizations. The equalizer operates even if the modulations and data rates on the two channels are different. 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 in the adaptive baseband equalizer. The error signal is the difference between the received baseband data and an estimate of the ideal transmitted data. A control logic circuit sets the: phase and amplitude for 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.