Richard Neil Braithwaite

Adaptive Echo Cancellation for an On-Frequency RF Repeater using a Weighted Power Spectrum

Adaptive echo cancellation based on a weighted power spectrum is discussed for use in an on-frequency RF repeater with a digital IF stage. Radiated coupling between the input and output antennas cause echoes to be transmitted by a repeater. Echo cancellation, a technique used to reduce the effect, is often based on the auto-correlation of the repeater signal and is effective when the width of the auto-correlation of the input signal is less than the loop delay of the echo path. Narrow bandwidth signals such as EDGE cause problems because the auto-correlations of the input signal and echo overlap. However, in this paper it is shown that the repeater signal can be filtered to reduce the width of the autocorrelation by adjusting the power of all carriers within the input signal to the same power density. A method is presented for partitioning the power spectrum into disjoint bands approximating carrier bandwidths and computing a weighting function that normalizes the carriers. This is done without affecting the ripples within the power spectrum that are needed to detect the echoes. Results based on computer simulation models are provided.

An Improved Doherty Amplifier Using Cascaded Digital Predistortion and Digital Gate Voltage Enhancement

This paper describes algorithms used to improve linearity, efficiency, and peak power of a Doherty amplifier whose auxiliary transistor gate voltage is adjusted digitally as a function of the signal envelope. Two predistortion stages are used to compensate for high-order memoryless nonlinearities and low-order memory effects. The digital gate voltage waveform is a sigmoid function of the predistorted signals envelope with two voltage limits corresponding to class C and AB bias levels. The instantaneous gate voltage is controlled by two adjustable parameters: a breakpoint and a slope. The parameters are adjusted to reduce the variance of the AM-AM curve of the power amplifier. The enhancement of the gate voltage at higher signal envelope levels increases the peak power and enhances improves the efficiency of the Doherty amplifier structure. The cascaded digital predistortion ensures sufficient linearity to keep the ACPR2 below - 55 dBc.