Performance Analysis of CPSK Transimission through Nonlinear Channels

Abstract

<p>Virtually all satellite repeaters use a traveling-wave tube (TWT) as their main power amplifier. Because on-board power is a limited commodity, it is highly desirable that the TWT be operated as efficiently as possible, namely in or near saturation where it is highly nonlinear. These nonlinear effects manifest themselves as an amplitude compression (AM/AM conversion) effect and an amplitude dependent phase modulation (AM/PM conversion) effect. In this thesis a number of investigations have been made in relation to the TWT nonlinearities and their effect on the performance of communication systems.</p> <p>A novel quadrature model of the TWT has been developed. This model is most useful in that it is analytic and requires the choice of only four parameters to obtain an excellent fit to the TWT characteristics.</p> <p>An optimal bandpass nonlinear transfer characteristic that maximizes its output signal to interference power ratio has also been derived. By making use of this optimal transfer characteristic and the quadrature model of the tube, a computer-aided design procedure has been described for obtaining a predistortion compensation network for the TWT. This network consists of a simple arrangement of attenuators and power-law devices and has been shown, by computer simulation, to yield about 1 dB improvement in system performance for the case when only a single carrier is present in the TWT.</p> <p>In the case when a single sample detection and majority logic decision circuit is assumed at the receiver, it has been possible to derive analytical expression for the probability of error of M-ary CPSK signals transmitted through a piecewise-linear envelope limiting repeater. An infinite series expression for the bit error rate of binary CPSK transmission through an actual TWT channel has also been derived.</p> <p>A performance analysis of a correlation receiver with a linear integrate and dump circuit has been carried out for the case of binary CPSK transmission through a bandpass nonlinearity exhibiting AM/PM conversion.</p> <p>For the case of purely amplitude-limiting channels, an optimal (maximum-likelihood) receiver structure and its approximate performance has also been investigated.</p>