Apparatus and method for digital predistortion initialization of high-power amplifiers

A method of reducing distortion in an electrical signal for wireless transmission, the method comprising:

• generating a multi-tone electrical signal comprising a first portion and a second portion, the first portion having at least a first frequency component and a second frequency component, and the second portion having a synchronization signal;
• transmitting the multi-tone electrical signal through a nonlinear channel to generate an output electrical signal at an output of the nonlinear channel, the nonlinear channel causing linear distortions and nonlinear distortions in the output electrical signal;
• detecting, in the output electrical signal, intermodulation distortion components, the first frequency component, and the second frequency component;
• using the synchronization signal to measure, in the output electrical signal, phases of the intermodulation distortion components, the first frequency component, and the second frequency component;
• and
• determining coefficients of a digital predistortion processor based on
• the intermodulation distortion components,
• the first frequency component,
• the second frequency component, and
• the phases of the intermodulation distortion components, the first frequency component, and the second frequency component, wherein
• the digital predistortion processor reduces the intermodulation distortion components in the output electrical signal, wherein the intermodulation distortions are based on the first frequency component and the second frequency component.

A transmitter, comprising:

• a waveform generator configured to generate an input electrical signal;
• a digital predistortion processor configured to receive the input electrical signal and apply thereto a nonlinear filter;
• a digital to analog converter configured to convert an output of the digital predistortion processor to an analog signal;
• a nonlinear channel comprising an amplifier that is configured to amplify the analog signal, the nonlinear channel causing nonlinear distortions and linear distortions to the analog signal;
• an analog to digital converter configured to convert a part of the analog signal to an output electrical signal;
• and
• processing circuitry configured to initialize values of coefficients of the nonlinear filter of the digital predistortion processor to pre-compensate for and thereby reduce the nonlinear distortions to the analog signal, the processing circuitry being configured to initialize the values of the coefficients of the first filter and the second filter by:
• receiving an original propagated electrical signal resulting from a multi-tone electrical signal input into the nonlinear channel, the original propagated electrical signal having second linear distortions, below a predetermined threshold, after processing by the nonlinear channel, the original propagated electrical signal further having nonlinear distortions caused by the nonlinear channel,
• determining a modified input electrical signal that pre-compensates for the nonlinear distortions, such that transmitting the modified input electrical signal through the nonlinear channel results in a modified output electrical signal having reduced nonlinear distortions relative to the nonlinear distortions in the original propagated electrical signal,
• using the original input electrical signal and the modified input-output electrical signal to determine nonlinear filter coefficients for the nonlinear filter, that reduces nonlinear distortions in an output electrical signal from the nonlinear channel when the nonlinear filter is applied to an input electrical signal, input into the nonlinear channel, to perform predistortion prior to the transmission of the input electrical signal through the nonlinear channel,
• wherein determining nonlinear coefficients for the nonlinear filter is based on the modified output electrical signal and the original propagated electrical signal having multiple simultaneous tones;
• and
• wherein the modified input electrical signal is determined using an iterative process that adapts the modified input electrical signal to reduce levels of intermodulation distortions in the modified output electrical signal, wherein the intermodulation distortions are based on the multiple simultaneous tones, wherein the iterative process comprises, for at least one intermodulation distortion component:
• estimating phase and amplitude of the at least one intermodulation distortion component in the modified output electrical signal;
• and
• determining the nonlinear coefficients based on the estimated phase and amplitude of the at least one intermodulation distortion component.