Code division multiple access (CDMA) is a form of multiplexing (not a modulation Scheme) and a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes.

It is to perform the multiplexing CDMA also refers to digital cellular telephoney systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, and W-CDMA by the International Telecommunication Union or ITU.  CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics.

Whereas GSM is a specification of an entire network infrastructure, the CDMA interface relates only to the air interface - the radio part technology. For example GSM specifies an infrastructure based on internationally approved standard while CDMA allows each operator to provide the network features as it finds suited. On the air interface, the signalling suite (GSM: ISDN SS7) work has been progressing to harmonise these.

CDMA is suited for data transfer, with bursty behaviour, and where delays can be accepted. It is used in WLAN - and the cell size here is 500 feet because of the high frequency (2.4 GHz) and low power. The suitability for data transfer is the reason for why W-CDMA seems to be "winning technology" for the data part of 3G mobile networks.

A number of different terms are used to refer to CDMA implementations. The original U.S. standard defined by QUALCOMM was known as IS-95, the IS referring to an Interim Standard of the Telecommunications Industry Association (TIA). IS-95 is often referred to as 2G or second generation cellular. The QUALCOMM brand name cdmaOne may also be used to refer to the 2G CDMA standard. The CDMA has been submitted for approval as a mobile air interface standard to the ITU International Telecommunication Union.

COVERAGE
When it comes to cell size, this depends on the power used by the handset, the terrain and frequency. Effective algorithms can reduce the noise, but will require that extra information is sent to validate the transfer. Hence the radio frequency and power of the handset determines the cell size. The long waves need less energy to travel far - so lower frequencies are used to make larger cell. The higher the frequency - the shorter will the coverage be. This is used by mobile networks to vary the size of the cell. In cities you need many small cells, and high frequencies allows sites to be placed closer, and more subscribers provided service.