An experimental facility for sequential decoding Technical report no. 450
Charles W. Niessen · NASA Technical Reports Server (NASA) · 1965
Sequential decoding is one of the few practical methods known for communicating over a noisy channel which, for interesting rates, attains the error-correction capability predicted by Shannon's coding theorem.Since analytical investigations a r e limited by the difficulty of the mathematics involved, experimental studies into the behavior of sequential decoding a r e necessary.This report describes the system design and implementation of a facility for the experimental study of sequential decoding that may be used at M. I. T. by graduate researchers in communications theory.Flexibility and ease of use a r e the primary requirements of this system.Thorough investigation of the characteristics of sequential decoding and likely problems to be studied led to a system based upon the Project MAC PDP-6 computer.The design reflects constraints imposed by time, cost, equipment availability, and the anticipated class of users.A portable data-acquisition system, consisting of a digital tape recorder and analog-to-digital conversion equipment, is provided to make available to the computer the outputs of experimental demodulation equipment.The experimenter can decode the acquired data sequentially in accordance with an algorithm specified and easily written by him in a version of Fortran modified for this purpose.The modified Fortran contains statements for the use of special subroutines provided, such as a convolutional coder.The program is run withirl a monitor system which handles most inputoutput automatically and provides for man-machine interaction with the program.The monitor also collects statistics on the decoding process to aid the user in evaluating his algorithm.All sequential decoding algorithms may ultimately be described as tree search algorithms in which it is desired to find the "best" path through a tree.A display of the paths searched by the algorithm has therefore been made the principal tool for the manmachine interaction.The user watches this display and controls the running of the algorithm via a light pen and commands typed to the monitor.The system has been successfully implemented and tested, and experimental results a r e described.