Önbozulum yolu ile güç yükselteci doğrusallaştırması

Power amplifiers are important elements in communication systems but they areinherently nonlinear. This nonlinearity shows itself in the form of amplitude andphase distortion. One way to get rid of this nonlinear behaviour is to applybackoff which means to operate the amplifier at an output power smaller thanits saturated output power. As the backoff is increased, the amplifier will behavemore linearly. But this will also reduce the efficiency of the amplifier, which isundesirable. This tradeoff between efficiency and linearity is solved by lineariza-tion techniques. By using linearization techniques, the amplifier can be operatednear to saturation with good efficiency and linearity.This thesis focuses on polar polynomial predistortion and polar look-up tablepredistortion, which are popular linearization techniques. A polar polynomialpredistorter and a polar look-up table predistorter are implemented and testedwith simulations in software. The implementation and testing is done by us-ing IT++ which is a C++ library of mathematical, signal processing, speechprocessing, and communications classes and functions. The testing of the predis-torters is done by using a baseband system model which consists of a 16-QAMmodulator, an upsampler, a raised cosine filter, the predistorter and a basebandiiibehavioural amplifier model. The performance of the predistorters is evaluatedin terms of adjacent channel power ratio, AM/AM & AM/PM responses andBER under AWGN. Simulation results show that the predistorters have goodperformance. In the simulations, the polar polynomial predistorter achieved 20dB reduction and the polar look-up table predistorter achieved 25 dB reductionin adjacent channel power ratio. The effect of polynomial order and table sizeon the performance of the predistorters is investigated. Furthermore, the effectof lowpass filtering on the performance of the predistorters is also investigatedby placing a lowpass filter after the predistorters in the system model. It is ob-served that as the ratio of the bandwidth of the lowpass filter to the bandwidthof the raised cosine filter decreases, the negative effect of the lowpass filter onthe performance of the predistorters increases.Keywords: polar polynomial predistortion, polar look-up table predis-tortion, linearization, AM/AM distortion, AM/PM distortion, ampli-fier nonlinearity.iv