ADAPTIVE PREDISTORTION LINEARISER USING POLYNOMIAL FUNCTIONS

Linear modulation methods are highly sensitive to high power amplifier nonlinearities. An adaptive predistortion lineariser using polynomial functions is described. An important problem faced in optimisation of nonliner circuits such as predistortion linearisers is that of conver gence into a local minimum. This results from the nonquadratic shape o f the objective functions. A solution to this problem is presented usi ng a postdistorter with similar polynomial functions as the predistort er. Sample of the signals at the power amplifier output and predistort er input are demodulated. These demodulated signals are used for estim ation of the postdistorter polynomial coefficients. The objective func tions used in the estimation are quadratic functions of the coefficien ts being estimated resulting in a rapid convergence to the global mini mum. The coefficients of the predistorter polynomials are set from tho se of the postdistorter. Computer simulation results for the proposed lineariser are presented. These results show 50 dB spectrum spreading improvement. Further, unlike previously reported linearisers, the prop osed lineariser is insensitive to the demodulator gain and phase imper fections. The performance of the proposed lineariser structured with f ifth order polynomial functions is also compared with that for earlier polynomial type linearisers.