The efficient use of transmission facilities often requires the sharing of resources by a number of users. Commonly used methods of shared media access are time division multiple access (TDMA), frequency-division multiple access (FDMA), code-division multiple access (CDMA), carrier-sense multiple access (CSMA), etc. We propose a different approach to designing an N-user multiple access system for the binary input binary adder channel. When all users are sending data, the total rate of information transfer is equal to the time-share rate. However, when fewer than N users are active, the unused channel capacity is used to achieve error control. The extent of error control is inversely proportional to T, the number of active users. We present in detail two coding schemes; the first based on cyclic block codes, particularly BCH codes and Reed-Solomon codes, and the second based on convolutional codes.

A multilevel approach to the design of DC-free line codes is presented. The different levels can be used for different purposes, for example, to control the maximum accumulated charge or to guarantee a certain minimum distance. The advantages of codes designed by this method over similar codes are the improved run-length/accumulated-charge parameters, higher transmission rate, and the systematic nature of the code construction. The multilevel structure allows the redundancy in the signal selection procedure to be allocated efficiently among the different levels. It also allows the use of suboptimal staged decoding procedures that have performance/complexity advantages over maximum-likelihood decoding.>