Some Recent Developments in Digital Feedback Communication Systems

This paper summarizes the results of investigations into two methods for increasing the reliability of digital communication systems: 1) by improvements in the decision system of the receiver, and 2) by the introduction of a feedback link for error correction between the receiver and the transmitter. The improved decision system provides for the withholding of decision in doubtful cases which are marked by the received signals falling in an ambiguous region called the null zone. The use of a null-zone decision region in conjunction with feedback is called decision feedback. The characteristics and performance of this system are discussed. In addition, the results of combining integration and coding with decision feedback, called respectively, cumulative and coded decision feedback, are presented. Finally, the behavior of decision feedback under conditions of multipath reception is noted. The performance of decision feedback systems is compared with that of unidirectional systems.