Fast adaptive polynomial I and Q predistorter with global optimisation

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Abstract
High spectral efficiency modulation methods such as quadrature amplitude modulation are of considerable interest for future mobile communication systems. These methods are highly sensitive to transmitter power amplifier nonlinearities. The linearity of a power amplifier can be improved by using an adaptive nonlinear predistorter. A modified version of a previously reported polynomial lineariser is presented which has a faster rate of convergence by an order of magnitude. Faster convergence is achieved using postdistorter polynomial functions of two variables. The price is 30% higher computation load for a predistorter having six coefficients. It is shown analytically that the gain and phase errors in the output demodulator used for adaptation of the predistorter simply result in a phase offset equal to the phase error. Adaptability of the proposed lineariser with changes in the power amplifier, predistorter quadrature modulator and the output demodulator are tested. Results show that the lineariser can adapt to these changes. The effect of the rotation of the demodulated output signal phase on convergence is studied. It is shown that the optimum value for this rotation is in the range of 15 to 70°. Spectrum-spreading results for different values of output back-off are included.