# 8.4 Code Division Multiple Access (CDMA)

## CDMA systems

Traditionally Code Division Multiple Access (CDMA) systems have been used almost exclusively by the military as a means of operating covert radio communications in the presence of high levels of interference. In recent years, the interference immunity of CDMA for multi-user communications, together with its very good spectral efficiency characteristics, has been seen to offer distinct advantages for public cellular-type communications. |

There are two very distinct types of CDMA system,classified as direct sequence CDMA and frequency hopping CDMA. Both of these systems involve transmission bandwidths that are many times that required by an individual user, with the energy of each user's signal spread with time throughout this wide channel. Consequently these techniques are often referred to as spread spectrum systems. One of the most prominent proponents of CDMA in the cellular market is Andrew Viterbi, and his book on the subject (Viterbi, 1997) is well worth reading.