Nonlinearity in Broadband Wireless Communication Systems
Licentiatavhandling, 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.
baseband predistortion
single carrier modulation
orthogonal frequency division multiplexing (OFDM)
bandpass nonlinearity
multicarrier modulation
nonlinearity distortion
high power amplifier
RF predistortion
peak to average power ratio (PAPR)
baseband nonlinearity