Overview

While GSM was the second-generation digital cellular system that was introduced within Europe initially, another development took place in the USA. There were different requirements, and as a result the system, although digital and using Time-Division Multiple Access (TDMA) techniques, is very different.

The FCC had indicated that no new frequency allocations were to be made to accommodate a new cellular system. Therefore, with a very heavy investment in AMPS technology which had been deployed over vast areas of the USA, there was a need to ensure that any digital system that was developed was as compatible as possible with AMPS. In view of this, the system was designed to use the existing AMPS channels so that it would allow a smooth transition from AMPS to TDMA. During the changeover, both AMPS and TDMA could co-exist in the same area.

Although using the same 30-kHz channel spacing as adopted by AMPS, TDMA provides a threefold increase in voice traffic by splitting the voice channel using digitized voice, and using time slots for three voice channels. In addition to the improvement in capacity, the system has also been made secure against casual listening, cloning and other forms of fraud.

The system, which is primarily used only within North America, is known by a variety of names. Today it is most commonly called just TDMA, in view of its time-division multiple access technology. However, outside the USA it is often called NA or North American TDMA, for obvious reasons. It is...