Two-Tone and Nine-Tone Excitations in Future Adaptive PredistortedLinearised Basestation Amplifiers of Cellular Radio

Abstract

Linearisation of a radio basestation amplifier using a third order, a fifth order and a seventh order predistortion scheme is studied. Adaptive predistortion using a third order predistorter improved the amplifier intermodulation distortion (IMD) by between 6.4 dB and 33 dB from the third IMD power level (i.e., maximum power amplitude variation of the IMD), provided by the raw amplifier, as measured by a two tone test. The variation in improvement is due to changes in the input power level. Improvement increases as the input power is backed off from the 1 dB compression point. A fifth order predistorter will give an IMD improvement of 8.7 dB and 48.4 dB if properly adjusted. A seventh order predistorter (with the fifth complex coefficient fixed to zero) will show the best performance, particularly throughout a window (approximately from 1.5 dB to 3 dB away from the 1 dB compression point). The IMD improvement throughout this window within the high input range measures between 9.1 dB to 21.7 dB over that achieved with a fifth order predistorter. For a nine-tone test (with uniformly distributed random phase), a third order predistorter will hardly improve the worst IMD power level, provided by the raw amplifier. A fifth order predistorter, if properly adapted will give an IMD improvement of between 8.8 dB and 41.5 dB from the worst IMD level while a seventh order predistorter will give a close performance to that of a fifth order one. Fixing the predistorter coefficients enables a comparative study of the IMD power level improvement between 5th order fixed and adaptive predistorters, for a nine-tone excitation. A 5th order predistorter with its coefficients fixed to those values obtained for optimum IMD level at a backed-off power input value of 8.5 dB from the 1 dB compression point is found to maintain an IMD power variation better than 55 dB from fundamental power level, throughout the input range, up to the backed-off value.