A HIGH-CAPACITY TERRESTRIAL DIGITAL MICROWAVE RADIO EMPLOYING ADAPTIVE TECHNIQUES

Abstract

<p>A prototype 90 Mbit/s, 4.5 bits/cycle digital microwave radio is constructed which uses 49QPRS modulation. Feedback is used to linearize the transmitter power amplifier and in the receiver an adaptive decision feedback equalizer (DFE) is used to counter the effects of multipath distortion. The modulation envelope at the output of the cascade of a quadrature amplitude modulator and a microwave GaAsFET power amplifier is linearized with respect to the baseband signal, by feedback of the coherently demodulated microwave output. The power amplifier was operated at the 0.5dB compression point with only a very small increase in the receiver bit error rate (BER). To be practical, the feedback loop must have small physical size so that the loop transit time is small relative to the modulation symbol period. The receiver sampling phase and DFE tap-weights are jointly adapted to minimize the mean squared error and peak error respectively, of the DFE output. A one bit quantized estimated gradient algorithm is used to digitally adjust the adapted parameters. In the presence of a two-ray multipath, causing a transmission zero at the mid-band frequency of the receiver, the adaptive receiver can operate with only a small increase in the BER. In addition, the adaptive receiver can track easily between the minimum phase and non-minimum phase regions which are separated by the transmission zero. A recursive matrix equation is derived to describe the joint steepest descent adjustment of the equalizer tap-weights and sampling phase. The eigenvalues of the adjustment matrix of that equation are used to explain the observed behaviour of the constructed system.</p>