Code Division Multiple Access - CDMA

The code division multiple access (CDMA) system is a 2^nd generation digital mobile radio communication system that provides for voice and medium-speed data communication services. Upgrading to CDMA capability from 1^st generation analog mobile systems allows CDMA service providers to add more customers for each radio tower (cell site), offer medium-speed data communication services, and to provide new high-value information services.

The CDMA system is composed of mobile devices (wireless telephones and data communication devices), radio towers (cell sites), and an interconnection system (switches and data routers). The IS-95 CDMA system (a specific standard for CDMA) uses three key types of radio channels; a wideband digital channel, a narrowband analog voice channel, and a narrowband analog control channel. The wideband digital radio channel provides for both multiple voice (audio) channels and control channels. The narrowband analog voice (audio) channels and analog control channel are slightly modified versions of the advanced mobile phone service (AMPS) radio channels.

There are three types of mobile devices that can operate on an IS-95 CDMA system; CDMA digital only; CDMA digital and AMPS analog, and AMPS analog only. The decision to include both analog and digital channels into the IS-95 CDMA system allowed for a gradual migration of AMPS analog customers to advanced digital services.

CDMA radio towers (cell sites) are composed of each the antenna system and radio equipment (base station). CDMA base stations contain one or more CDMA digital and/or AMPS analog radio channels. The base station converts radio signals from mobile devices into electrical signals that can be transferred to the cellular interconnection system (typically a mobile switching center). In a typical cellular system, each wide CDMA radio channel typically replaces two AMPS radio channels.

Cell sites are typically interconnected to each other through a central switching system called a mobile switching center (MSC). The MSC for a CDMA system is enhanced from analog switching systems as it allows simultaneous communication between a mobile device and two or more cell sites. This ability of simultaneously communicating through multiple cell sites during call transfer is called soft handoff.

This figure shows a simplified diagram of a CDMA system. This diagram shows that the CDMA system includes mobile communication devices (mobile stations) that communicate through a radio tower (base station) and a mobile switching center (MSC) or data routing networks to connect to other mobile telephones, public telephones, or to the Internet. This diagram shows that the CDMA system contains both 2^nd generation CDMA IS-95 voice radio channels and 1^st generation analog radio channels. CDMA devices include voice only (such as a mobile telephone), data only (such as a computer browsing the Internet), and multimedia devices (such as a personal digital assistant that has voice and data capability).

Code Division Multiple Access - CDMA System Diagram