Loral Aerospace Corp.

Michael E Jacobson

Gary L Wagner

Francis James Smith

Richard W Koralek

Geoffrey S Waugh

Steven Andersen

James F Lebar

Chris Hinkle

James Berman

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.

Residual carrier adaptive cross-polarization equalizer

A system for processing inphase and quadrature data channels, such as a Costas loop QPSK demodulator, employs an additional feedback loop for adjustment of phase offset between carrier reference signals, this loop being in addition to the Costas error loop for control of frequency and phase of an oscillator which provides the regenerated carrier signal. The additional loop employs cross-channel products of demodulated inphase and quadrature data signals as does the Costas loop. The regenerated carrier is applied via an adjustable phase-offset unit to provide quadrature carrier reference signals to phase detectors of inphase and quadrature data channels. The phase offset unit includes a 90 degree hybrid circuit energized by the carrier signal at a main input port plus an adjustable fraction of the carrier power applied to an auxiliary input port. Adjustment of the relative magnitudes of the signals at the two input ports of the hybrid circuit in accordance with the sum of the cross-channel products provides a desired amount of phase offset between the two carrier reference signals in correspondence with the deviation from orthogonality of the components of the composite input signal. This removes crosstalk resulting from the deviation in orthogonality of the components of the composite input signal.

Quadrature compensation for orthogonal signal channels

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San Jose, CA, United States of America

Michael E Jacobson