What is CDMA? Detail definition of CDMA Technology.

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. 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.

Bandwidth of CDMA

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