The technology oftime-division multiple-access (TDMA) and code-division multiple-access (COMA) promises to significantly improve the capacity and operation ofexisting analog cellular networks. AT&T has been on the leading edge ofthese breakthroughs, developing trial and commercial systems for both oftheseadvanced digital radio technologies. This paper describes TDMA and COMA, theiradvantages, specific features, field testresults, and speculations ontheirfutures.

System availability is defined as the fraction of time that a system provides the service for which it is specified. For a system that can exist in only two states, such as "up" or "down," the

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