Project PDA

(Code Division Multiple Access). A spread spectrum air interface technology used in some digital cellular, personal communications services and other wireless networks. It is a digital wireless technology that uses a spread spectrum technique to scatter a radio signal across a wide range of frequencies. CDMA is a 2G technology. WCDMA, a 3G technology, is based on CDMA.

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.

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.

After a couple of revisions, IS-95 was superseded by the IS-2000 standard.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

This CDMA system is frequently confused with a similar but incompatible technology called Wideband Code Division Multiple Access (W-CDMA) which forms 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 International Telecommunication Union (ITU) standards.

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

CDMA has multiple variants, including CDMA 1X, cdma2000, CDMA2000 1X, CDMA2000 1xEV-DO and cdmaOne.

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
• Extended battery life because of effective power control.

Optical CDMA

Recencty, CDMA has been also investigated to be employed in optical communication networks, especially in local access network (LAN). Current optical CDMA (O-CDMA) technques fall into two categories, coherent and inchoerent O-CDMA. In incoherent O-CDMA, unipolar codes (0, 1) are used to modulate the power of optical signals. In coherent O-CDMA, bipolar (-1, 1) or multilevel codes are used to modulate the field of optical signals.