CDMA is a digital technology for transmitting data. It is a general technology utilized through various standards. CDMA has no limit on capacity but the base station will only connect users upon determining that the call quality would fall bellow a predetermined limit.
The term is often used to refer to one specific family of technologies – IS-95 (often referred to cdmaOne) and CDMA2000. Networks using this technology operate in the 800 and 1900 MHz frequency bands and are primarily used in the Americas and Asia.
Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme (where each transmitter is assigned a code).
CDMA optimizes the use of available bandwidth as it transmits over the entire frequency range and does not limit the user’s frequency range.
CDMA allows several users to share a band of frequencies without undue interference between the users. It is used as the access method in many mobile phone standards. IS-95, also called “cdmaOne”, and its 3G evolution CDMA2000, are often simply referred to as “CDMA”, but UMTS, the 3G standard used by GSM carriers, also uses “wideband CDMA”, or W-CDMA, as well as TD-CDMA and TD-SCDMA, as its radio technologies.
It can be also used as a channel or medium access technoloy, like ALOHA for example or as a permanent pilot/signalling channel to allow users to synchronize their local oscillators to a common system frequency, thereby also estimating the channel parameters permanently.
In these schemes, the message is modulated on a longer spreading sequence, consisting of several chips (0es and 1es). Due to their very advantegous auto- and crosscorrelation characteristics, these spreading sequences are also used for radar applications since many decades, where they are called Barker-Codes (with a very short sequence length of typically 8 to 32).
For space based communication applications, it is also used since many decades, due to the large pathloss and Doppler-shift caused by the satellite motion. Typically in those applications, neither FDMA nor TDMA is used as a single modulation, due to this effect. CDMA is often used with BPSK in its simplest form, but can be combined with any modulation scheme like in advanced cases QAM or OFDM, which typically makes it very robust and efficient (and equipping them with accurate ranging capabilities, which is difficult without CDMA). Other schemes use subcarriers based on binary offset carrier (BOC), which is inspired by Manchester codes and enable a larger gap between the virtual center frequency and the subcarriers, which is not the case for OFDM subcarriers.
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