The world's first cellular networks introduced in the early 1990s, used analog2 radio transmission technologies. Within a few years, millions of subscribers signed up for service provided by the cellular networks, and demanded more and more airtime. As a result, dropped calls and network busy signals became common in many areas
During the early 1980s, analog cellular telephone systems were experiencing rapid growth in Europe Each country developed its own system, which was incompatible with everyone else's in equipment and operation.
So in 1982 the Conference of European Posts and Telegraphs (CEPT) formed a study group called the Group Spécial Mobile (GSM) to study and develop a pan-European public land mobile system.
Within a few years, millions of subscribers signed up for service provided by the cellular networks, and demanded more and more airtime. As a result, dropped calls and network busy signals became common in many areas.
To accommodate more calls within the limited amount of radio spectrum4 available, the industry developed a digital5 wireless technology called GSM (Global System for Mobile Communication). Today, most of the cellular phones in the world, outside the U.S., use GSM technology.
But just as GSM was being standardized, an even better solution for mobile communication6 was found in another technology called CDMA (Code Division Multiple Access). CDMA, a digital wireless technology, was pioneered and commercially developed by Qualcomm.7
The ITU (International Telecommunication Union) which manages the international allocation of radio spectrum allocated the bands 890-915MHz for the uplink (mobile station to base station )and 935-960Mhz for the down link(base station to mobile station )for mobile networks.
The transmission and reception frequency should be different by about 5% of the nominal RF to provide isolation
The FDMA involves the division by frequency of the 25MHz BW(915-890)into 124 carrier frequencies spaced 200KHz apart. One or more carrier frequencies are assigned to each Base station
The CDMA carriers require proprietary a handsets that are linked to one carrier only and are not card enabled .To upgrade a CDMA phone the carrier must deactivate the old phone then activate the new one. Thus rendering the old phone useless Depending on the protocol each call on a CDMA system is linked with a predefined code as per the protocols. this unique code is spread over the available frequencies
When we make a call the MSC dispatches the request to all the base stations . The MIN is the broadcasted as a paging message throughout the cellular system .the ,mobile receives the paging message and sends back an acknowledgement which is directed to the MSC which instructs the BS to move the call to an unused voice channel within the CELL .
the base station now signals the mobile to change frequencies to an unused FVC and RVC and a data message called ALERT is transmitted to instruct the mobile phone to ring.
Today, the battle between CDMA and GSM is muddled. Where at one point Europe clearly favored GSM and North America, CDMA, the distinct advantage of one over the other has blurred as major carriers like AT&T Wireless begin to support GSM, and recent trials even showed compatibility between the two technologies.
GSM still holds the upper hand however. There's the numerical advantage for one thing: 1Billion GSM users versus CDMA's 270million.