An Adaptive Nulling System for Digital Microwave Radio

This paper describes the development and operation of an adaptive nulling system designed to enhance the reception of quadrature-amplitude-modulated (QAM) signals which have been corrupted by cochannel and/or multipath interference. More specifically, an adaptive antenna preprocessor to an existing digital demodulator is described. The adaptive antenna preprocessor is used to spatially null undesired directional interference by linearly combining the signals received by two antennas. A 64-tap adaptive equalizer contained within the digital demodulator is used to remove adjacent channel and intersymbol interference (ISI). Incorporated within the demodulator are gradient-search adaptive algorithms that automatically adjust both the equalizer and combiner weights to maximize the SINR of the received signal. These adaptive algorithms do not require knowledge of antenna calibration nor prearranged training signals. Instead "blind" adaptation is used during initial acquisition and decision-directed adaptation is used during tracking.

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