Code Division Multiple Access

Code division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply CDMA), which use CDMA as an underlying channel access method.

One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called Multiple Access. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signalling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

An analogy to the problem of multiple access is a room (channel) in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different languages (code division). CDMA is analogous to the last example where people speaking the same language can understand each other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can communicate. Technology code division multiple access channels has long been known. In the USSR, the first work devoted to this subject was published in 1935 by professor D.V. Aggeev in the "CDMA". It was shown that the use of linear methods, there are three types of signal separation: the frequency, time and compensatory. Technology of CDMA was used in 1957, the young military radio engineer Leonid Kupriyanovich in Moscow, made the experimental model of wearable automatic mobile phone, called him as LK-1, with base station. LK-1 has 3 kg weight, 20-30 km operating distance, and 20-30 hours of battery life ("Nauka i zhizn", 8, 1957, p. 49, "Yuniy technik", 7, 1957, p. 43-44). The base station, in accordance with author's description, could serve several customers. In 1958, Kupriyanovich made the new experimental "pocket" model of mobile phone. This phone has 0,5 kg weight. To serve more customers, Kupriyanovich proposed the device, named him as correllator. ("Nauka i zhizn", 10, 1958, p.66, "Technika-molodezhi", 2, 1959, 18-19) In 1958 USSR also started the developing of "Altay" national civil mobile phone service for cars, based on Soviet MRT-1327 standard. The main developers of Altay system were VNIIS (Voronezh Science Research Institute of Communications)and GSPI (State Specialized Project Institute). In 1963 this service started in Moscow and in 1970 Altay service used in USSR for 30 cities.