FAST ADAPTIVE POLYNOMIAL-I AND Q-PREDISTORTER WITH GLOBAL OPTIMIZATION

High spectral efficiency modulation methods such as quadrature amplitu de modulation are of considerable interest for future mobile communica tion systems. These methods are highly sensitive to transmitter power amplifier nonlinearities. The linearity of a power amplifier can be im proved 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 converg ence is achieved using postdistorter polynomial functions of two varia bles. The price is 30% higher computation load for a predistorter havi ng six coefficients. It is shown analytically that the gain and phase errors in the output demodulator used for adaptation of the predistort er simply result in a phase offset equal to the phase error. Adaptabil ity of the proposed lineariser with changes in the power amplifier, pr edistorter quadrature modulator and the output demodulator are tested. Results show that the lineariser can adapt to these changes. The effe ct of the rotation of the demodulated output signal phase on convergen ce is studied. It is shown that the optimum value for this rotation is in the range of 15 to 70 degrees. Spectrum-spreading results for diff erent values of output back-off are included.