Power amplifier linearization by predistortion

Abstract

Power amplifiers are important elements in communication systems but they are inherently nonlinear. This nonlinearity shows itself in the form of amplitude and phase distortion. One way to get rid of this nonlinear behaviour is to apply backoff which means to operate the amplifier at an output power smaller than its saturated output power. As the backoff is increased, the amplifier will behave more linearly. But this will also reduce the efficiency of the amplifier, which is undesirable. This tradeoff between efficiency and linearity is solved by linearization techniques. By using linearization techniques, the amplifier can be operated near to saturation with good efficiency and linearity. This thesis focuses on polar polynomial predistortion and polar look-up table predistortion, which are popular linearization techniques. A polar polynomial predistorter and a polar look-up table predistorter are implemented and tested with simulations in software. The implementation and testing is done by using IT++ which is a C++ library of mathematical, signal processing, speech processing, and communications classes and functions. The testing of the predistorters is done by using a baseband system model which consists of a 16-QAM modulator, an upsampler, a raised cosine filter, the predistorter and a baseband behavioural amplifier model. The performance of the predistorters is evaluated in terms of adjacent channel power ratio, AM/AM & AM/PM responses and BER under AWGN. Simulation results show that the predistorters have good performance. In the simulations, the polar polynomial predistorter achieved 20 dB reduction and the polar look-up table predistorter achieved 25 dB reduction in adjacent channel power ratio. The effect of polynomial order and table size on the performance of the predistorters is investigated. Furthermore, the effect of lowpass filtering on the performance of the predistorters is also investigated by placing a lowpass filter after the predistorters in the system model. It is observed that as the ratio of the bandwidth of the lowpass filter to the bandwidth of the raised cosine filter decreases, the negative effect of the lowpass filter on the performance of the predistorters increases.