Compensation of transmitter nonlinearities using predistortion techniques: Case studies of envelope tracking amplifiers and radio-over-fibre links: Dissertation

This thesis studies compensation of nonlinear distortions introduced in the transmitters of wireless communication systems. In particular, adaptive predistortion of the envelope tracking (ET) amplifier and the radio-over-fibre (RoF) link is considered. The main goal is to develop compensation algorithms and architectures for predistortion. By providing a low attenuation and broadband solution, the RoF technology enables using cost-efficient and energy-efficient distributed antenna systems (DASs). Using highly efficient power amplifier (PA) structures, such as the ET amplifiers, energy efficiency of the transmitters is increased. Unfortunately, the RoF links and the ET amplifiers are inherently nonlinear. The results of this thesis indicate that these nonlinearities have to be taken into account when designing wireless communication systems. The nonlinearities can be compensated using the low complex adaptive predistortion techniques proposed in this thesis. In this thesis, a general architecture for the predistortion of the ET amplifier is proposed. Using the architecture, the performance of predistortion is demonstrated. In addition, new time misalignment compensation methods are studied. For the compensation of the RoF link, an extension to the conventional least mean square (LMS) algorithm for the memory polynomial-based predistortion is presented. In addition, different combinations of adaptive algorithms for predistortion are considered. From the compensation architecture point of view, the joint compensation of the RoF link and PA connected in series in the presence of nonlinear feedback is studied. Finally, feasibility of the adaptive predistortion concept is demonstrated by the measurements. The results presented in this thesis can be applied to any wireless communications systems. The performance studies demonstrate that distortions of the RoF link and the ET amplifier can be considerably diminished using a low-complexity design. To get even better results in terms of linearity and energy efficiency, the proposed predistortion techniques can be implemented together with other techniques, such as the peak-to-average power ratio (PAPR) reduction methods.