Nonlinearity in Broadband Wireless Communication Systems

Licentiate thesis, 2003

In the implementation of a digital communication system, there often exist imperfections that are not considered in the primary design. One such imperfection is the nonlinearity of the front-end devices. In most cases the physics of the nonlinearity is not completely known, and therefore the analysis of the system is very difficult, if not impossible. In this case models based on the observations from the device behavior has to be built and used in the study of the system. This thesis focuses on modeling and analysis of a system with nonlinearity. The topics covered here are modeling the nonlinearities, the impact of nonlinearity on a digital communication link, compensation of the nonlinearity effects, and measures to evaluate the susceptibility of signals to nonlinearity. In the modeling part, a model for a bandpass nonlinearity which is based on measurements on several harmonics of the output is presented. Unlike linear systems, in nonlinear systems a baseband equivalent does not contain all the information about the bandpass system. In this case the original bandpass model has to be used in some analysis. The model is then used to evaluate the performance of a broadband wireless communication system in the presence of nonlinearity. Two major classes of modulation for broadband wireless transmission are studied and the effect of nonlinearity on them is addressed. We also present and compare several methods of compensating the nonlinearity of the RF front-end of a wireless system. The methods include baseband and RF compensation methods. The design procedure for each one is discussed and the performance of the compensated system is evaluated. Finally we study signal measures related to nonlinearities, such as PAPR (Peak-to-Average-Power-Ratio), with respect to their ability to correctly predict the impact of a nonlinearity on an OFDM (Orthogonal Frequency Division Multiplexing) system. We also propose new measures for signal distortion in nonlinear systems.