NADC (North American Digital Cellular) is a collection of standards covering digital cellular systems in the United States. It describes a typical cellular system operating in the 900 MHz band consisting of mobile units, base stations, and infrastructure links that rely on frequency re-use due to territory partitioning into "cells", and that can be used for either voice or data transmission. Digital systems are seen as the answer to the lack of capacity frequently encountered in metropolitan areas by the present widely installed analog cellular (AMPS) system. A primary (and differentiating) requirement of NADC is that it must be backwards compatible with the existing analog system.
There are two primary competing approaches to digital cellular, TDMA (IS-54) and CDMA (IS-95).
The TDMA (Time Division Multiple Access) approach covered in IS-54 was a the first digital system standardized in North America. It uses 30 kHz channels, three users per channel using TDMA, and has a vocoder rate of 8 kbits/s. E-TDMA or Extended TDMA uses the same 30 kHz channels as TDMA, but has 6 users per channel. The vocoder rate is cut to 4 kbits/sec, and the channels are dynamically assigned based on voice activity detection. E-TDMA is being proposed as a follow-on to TDMA. The radio portions of TDMA and E-TDMA are the same.
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