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Didn't know what is CDMA? NOW YOU WILL KNOW!

Code division multiple access (CDMA) is a form of multiplexing and a method of multiple access to a physical medium such as a radio channel, where different users use the medium at the same time thanks to using different code sequences.

A number of 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 the Global System for Mobile Communications (GSM) standard is a specification of an entire network infrastructure, the CDMA interface relates only to the air interface—the radio part of the 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 features.

After a couple of revisions, IS-95 was superseded by the IS-2000 standard (CDMA2000).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.


Coverage and applications
The size of a cell depends on the power of the signal transmitted by the handset, the terrain and the radio frequency being used. Various algorithms can reduce the noise introduced by variations in terrain, but require extra information be sent to validate the transfer. Long wavelengths need less energy to travel a given distance than short wavelengths, so lower frequencies generally result in greater coverage. These characteristics are used by mobile network planners in determining the size and placement of the cells in the network. In cities, many small cells are needed; the use of high frequencies allows sites to be placed more-closely together, with more subscribers provided service. In rural areas with a lower density of subscribers, use of lower frequencies allows each site to provide broader coverage. (See also the Market situation section of GSM.)

Various companies use different variants of CDMA to provide fixed-line networks using wireless local loop (WLL) technology. Since they can plan with a specific number of subscribers per cell in mind, and these are all stationary, this application of CDMA can be found in most parts of the world.

CDMA is suited for data transfer with bursty behaviour and where delays can be expected. Therefore, it is used in Wireless LAN applications; the cell size here is 150 m (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 portion of third-generation (3G) mobile cellular networks.

 Want to know more? Visit : CDMA at Wikipedia

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