Code division multiple access (CDMA)

December 31, 2007 | By archives | Filed in: Blogging.

CDMACode division multiple access (CDMA) describes a communication channel access principle that employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code). In communications technology, there are only three domains that can allow multiplexing to be implemented for more efficient use of the available channel bandwidth and these domains are known as time, frequency and space.


CDMA divides the access in signal space. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of “spread-spectrum” signaling, since the modulated coded signal has a much higher bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.

Interestingly, CDMA is based on a patent granted in 1942 to two people, one of which was world famous actress Hedy Lamarr. Lamarr, probably best known for doing one of the first nude scenes in a major motion picture, worked with a partner-composer George Antheil-on a system that would make radio controlled torpedoes more difficult to detect through an early version of frequency hopping. Their system was inspired by the mechanical rolls that make self playing pianos work.

CDMA is also the current name for the cellular technology originally known as IS-95. Developed by Qualcomm and enhanced by Ericsson, CDMA also refers to digital cellular telephony systems that use this multiple access scheme, as pioneered by QUALCOMM, and W-CDMA by the International Telecommunication Union (ITU), which is used in GSM’s UMTS. is characterized by high capacity and small cell radius.

CDMA has been used in many communications and navigation systems, including the Global Positioning System and the OmniTRACS satellite system for transportation logistics.
The terms are used to refer to CDMA implementations. The original US standard defined by QUALCOMM was known as IS-95, where IS refers to an Interim Standard of the US Telecommunications Industry Association. IS-95 is often referred to as the second generation (2G) cellular, or as cdmaOne (the QUALCOMM brand name). CDMA has been submitted for approval as a mobile air interface standard to the International Telecommunication Union (ITU).

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

After some revisions, IS-95 was superseded by the IS-2000 standard (CDMA2000). This standard was introduced to meet some of the criteria laid out in the IMT-2000 specification for third generation (3G) cellular. It is also called 1xRTT which means “1 times Radio Transmission Technology” because IS-2000 uses the same 1.25 MHz carrier shared channel as the original IS-95 standard. A related scheme, called 3xRTT, uses three 1.25 MHz carriers for a 3.75 MHz bandwidth that would allow higher data burst rates for an individual user, but the 3xRTT scheme has not been commercially deployed.

More recently, QUALCOMM has led the creation of a new CDMA-based technology called Evolution-Data Optimized (1xEV-DO, or IS-856), which provides the higher packet data transmission rates required by IMT-2000 and desired by wireless network operators.
This CDMA system is frequently confused with a similar but incompatible technology called Wideband Code Division Multiple Access (W-CDMA) which is the basis of the W-CDMA air interface. The W-CDMA air interface is used in the global 3G standard UMTS and the Japanese 3G standard FOMA, by NTT DoCoMo and Vodafone; however, the CDMA family of US national standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of ITU standards.

Another important application of code division multiplexing — predating and distinct from CDMA — is the Global Positioning System (GPS). The QUALCOMM CDMA system includes very accurate time signals (usually referenced to a GPS receiver in the cell base station), so cell phone CDMA-based clocks are an increasingly popular type of radio clock for use in computer networks. The main advantage of using CDMA cell phone signals for reference clock purposes is that they work better inside buildings, thus often eliminating the need to mount a GPS antenna outside a building.


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