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 CDMA (Code Division Multiple Access) approach is a spread-spectrum system covered by IS-95. Promoted hard by patent holder Qualcom, CDMA claims an edge in performance over TDMA, particularly in the areas of number of users and multipath immunity. This system uses 1.23 MHz-wide channel sets, with a variable number of users on each carrier frequency. Access is by frequency and spreading code. The full vocoder rate is 8.55 kbits/sec, but voice activity detection and variable rate coding can cut the data rate to 1200 bits/sec. The effective data rate, determined empirically for simulated conversations, is 3700 bits/sec. Primary differences from a radio standpoint include more stringent dynamic range requirements which may translate into stricter linearity (intercept point) and noise floor requirements. The base station also uses OQPSK (offset pi/4 QPSK) modulation in the CDMA system.
Links to more information: