Code Division Multiple Access (CDMA)

A digital, spread spectrum, packet-based access technique generally used in RF (Radio Frequency) radio systems. Perfected and commercialized by Qualcomm, CDMA is used in certain cellular phone systems and in some WLANs (Wireless Local Area Networks). CDMA organizes thedata to be transmitted into discrete packets of various lengths. Prior to being packetized, the data may be converted from analog to digital format by a vocoder, as would be the case in a voice transmission over a cellular network. Once in a digital format, the data may be compressed in order to reduce the raw number of bits to be transmitted and, therefore, make more efficient use of limited RF spectrum. In a CDMA-based cellular voice network, for example, each transmission comprises a stream of data packets, which stream is assigned a unique 10-bit code sequence known as a PN (Pseudo Node) sequence. That PN code is prepended (added to the front of) each packet in the packet stream, enabling each receiver to separate that specific transmission from both the inherent background noise and the other transmissions that share the RF channel. Thereby, multiple packets associated with multiple conversation can share the same spectrum, overlapping in both frequency and time, without mutual interference.

The major benefit of CDMA is increased capacity (up to 20 times analog cell service) through more efficient use of spectrum. CDMA also provides three features that improve system quality:
1. The "soft hand-off" feature ensures that a call is connected before handoff is completed, as the cellular phone moves from cell-to-cell; this reduces the probability of a dropped call

2. Variable rate vocoding allows speech bits to be transmitted at only the rates necessary for high quality, which conserves the battery power of the subscriber unit

3. Multipath signal processing techniques combines power for increased signal integrity
Additional benefits to the subscriber include increased talk times for portable units, more secure transmissions and special service options such as data, integrated voice and data, fax and tiered services.

Source: Newton's Telecom Dictionary 22nd Edition 2006