An amplifier predistort lineariser for future mobile radio basestations

The linearisation of a radio basestation amplifier using a third order and a fifth order predistortion scheme is studied. Adaptive predistortion using a third order predistorter improves the power level of the inter-modulation distortion (IMD), which results from amplitude and phase nonlinearities in the amplifier frequency response, by between 6.5 dB and 30 dB, as measured by a two-tone simulation test by exciting the input with a sinusoidal waveform. To keep the sampling rate, as low as possible for speeding up the computation time, the RF carrier signal is avoided since it would have no effect on the simulation results; hence frequency down-conversion to baseband is considered. The variation in improvement is due to changes in the input power level. The 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 between 17 dB and 45 dB if properly adjusted.