CDMA (code division multiple access)

A form of digital phone service that assigns a code to all speech bits, sends a scrambled transmission of the encoded speech over the air and reassembles the speech to its original form at the other end. (Cingular)

A technology for digital transmission of radio signals between, for example, a mobile telephone and a radio base station. In CDMA, a frequency is divided into a number of codes. (Ericsson)

CDMA separates communications by code. Voice is broken into digitised bits, and groups of bits are tagged with a code. Each code is associated with a single call in the network. Groups of bits from one call are randomly transmitted along with those of other calls. Then they are reassembled in the correct order to complete the conversation. (Motorola)

Code Division Multiple Access (CDMA) is one of several digital wireless transmission methods in which signals are encoded using a pseudo-random sequence - which corresponds to a different communication channel - that the receiver also knows and can use to decode the received signal. CDMA is one of several "spread spectrum" techniques. CDMA offers improvements over analogue transmission in the areas of reduced call dropping, battery power conservation, more secure transmission and increased service options. (Nokia)

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 to perform the multiplexing. CDMA also refers to digital cellular telephony systems that make use of this multiple access scheme, such as those pioneered by Qualcomm, or W-CDMA.

CDMA is a military technology first used during World War II by English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.

CDMA has since been used in many communications systems, including the Global Positioning System (GPS) and in the OmniTRACS satellite system for transportation logistics. The latter system was designed and built by Qualcomm, and became the seed which helped Qualcomm engineers to invent Soft Handoff and fast power control, the necessary technologies that made CDMA practical and efficient for terrestrial cellular communications.

Usage in mobile telephony

A number of different terms are used to refer to CDMA implementations. The original standard spearheaded 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.

After a couple of revisions, IS-95 was superseded by the IS-2000 standard. This standard was introduced to meet some of the criteria laid out in the IMT-2000 specification for 3G, or third generation, cellular. It is also referred to as 1xRTT which simply means "1 times Radio Transmission Technology" and indicates that IS-2000 uses the same 1.25-MHz 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 1xEV-DO, or IS-856, which provides the higher packet data transmission rates required by IMT-2000 and desired by wireless network operators.

The QUALCOMM CDMA system includes highly 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 on the outside of a building.

Also frequently confused with CDMA is W-CDMA. The CDMA technique is used as the principle of the W-CDMA air interface, and 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 standards (including cdmaOne and CDMA2000) are not compatible with the W-CDMA family of standards.

Another important application of CDMA—predating and entirely distinct from CDMA cellular—is the Global Positioning System, GPS.

Coverage

As CDMA is newer than GSM, it may not be available in some parts of the world. However, as the signal can be transmitted over greater distances, it may give reception in more remote or rural areas where a GSM phone does not pick up a signal.

CDMA features

* Narrowband message signal multiplied by wideband spreading signal or pseudonoise code
* Each user has his own pseudonoise (PN) code
* Soft capacity limit: system performance degrades for all users as number of users increases
* Cell frequency reuse: no frequency planning needed
* Soft handoff increases capacity
* Near-far problem
* Interference limited: power control is required
* Wide bandwidth induces diversity: rake receiver is used

(Wikipedia)