Feedback Communication Systems for Time-Varying Channels

Abstract

The growing use of centralized data processing systems has led to a demand for data transmission facilities from remote locations, mobile stations, and even from space. To meet this demand the possibilities of data transmission over radio channels have been widely investigated. A major problem encountered on such channels is that of fading. This thesis is concerned with the possibility of using feedback to combat the effects of fading on digital communication systems. A number of interesting techniques have appeared in the literature but these have usually been based on the assumption that the receiver obtains perfect knowledge of channel conditions. In the system described in the first part of this work, this assumption is removed by the simple expedient of devoting some transmitter energy to a channel estimation pilot-tone signal. Channel information thus derived by the receiver is fed back to the transmitter to control transmitter power and/or transmission rate. The resulting improvement in receiver error probabilities is calculated. It is shown that this system is superior in performance to dual and fourth-order diversity systems. In the second part, a feedback system is proposed which uses two level transmitted power, controlled by an intermediate decision fed back from the receiver. This is essentially a nonsequential decision feedback system based on Schalkwijk's generalised centre of gravity feedback technique. The system has been assessed analytically and by analog computer simulation to determine its superiority over corresponding feedback - less systems. Though not matching the variable rate system in performance, it has the advantage of simplicity and is therefore easily implementable. Both techniques described are especially applicable to low-datarate multi-tone modems for short distance HF channels where propagation delays are small in comparison with baud lengths. Performance comparisons with existing data transmission systems such as Janet and Kathryn are given.