What is CDMA

Code division multiple access (CDMA), formerly known as Interim Standard 95 (IS-95), is a second generation (2G) air interface modulation technique used in wireless communication, developed by Qualcomm and enhanced by Ericsson. It employs spread-spectrum technology, meaning the coded signal has a significantly higher bandwidth than the data being transmitted.

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In addition, CDMA also employs a coding scheme where a code is designated to each transmitter, as opposed to time division multiple access (TDMA) where access is divided by time, and frequency-division multiple access (FDMA) where access is divided by frequency. CDMA is characterized by its high capacity and small cell radius.

CDMA is an important feature of various communications and navigation systems such as the OmniTRACS satellite system for transportation logistics, and the Global Positioning System (GPS). The use of CDMA cell phone signals for reference clock purposes is advantageous because it works better inside buildings, so the need for mounting a GPS antenna outside a building becomes obsolete.

The following features comprise the CDMA standard: one unique pseudonoise code (PN) per user; greater capacity, robustness and better signal quality due to soft handoff, which is the technique of adding base stations to a connection to ascertain that the next base is ready over a user's navigation of the terrain, and also the method of moving a call from one base station that is about to experience congestion to one with better capacity; usage of a rake receiver due to greater diversity, which is a result of wide bandwidth; limited interference, although power control is required; and a near-far problem, where a transmitter experiences noise and signal degradation if there is another transmitter operating at a shorter distance to the receiver.

CDMA has roaming capabilities which are based on ANSI-41. Roaming capabilities refer to being able to use many services of the home system in other wireless systems. CDMA carriers had a series of add-on standards that enabled the global launch of international roaming.

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The original CDMA or IS-95 system has since been superseded by CDMA2000 (IS-2000, also known as "1xRTT" which stands for "1 times Radio Transmission Technology"), which was developed to meet the criteria included in the 3G specification IMT-2000.

Both CDMA and CDMA2000 operate on the 1.25 MHz carrier shared channel. Another technology is Evolution-Data Optimized (1xEV-DO), which provides higher packet data transmission rates which have managed to meet that required by the IMT-2000 standard.