The main mobile telephone systems in the last twenty five years have been:

Analog and Digital Mobile Telephone Technologies

Mobile telephone systems are either analog or digital. In analog systems, voice messagesare transmitted as sound waves. When you speak into an analog mobiletelephone, your voice wave is linked to a radio wave and transmitted.In digital systems,voice messages are transmitted as a stream of zeroes and ones. When youspeak into a digital mobile telephone, your voice wave is convertedinto a binary pattern before being transmitted.

FDM, TDMA, and CDMA Mobile Telephone Technologies

Mobile telephone system all utilize some method to allow multipleusers to share the system concurrently. The three methods for doingthis are:

FDM	Frequency Division Multiplexing
TDMA	Time Division Multiple Access
CDMA	Code Division Multiple Access

In a FDM system, the available frequency is divided into channels.Each conversation is given a channel. When the system runs out ofchannels in a given area, no more telephone calls can be connected. In this way, FDM operates much like the channel button on your television set. The AMPS and NAMPS mobile telephone systems utilize FDM.

In a TDMA system, your encoded voice is digitized and then placed ona radio-frequency (RF) channel with other calls. This is accomplishedby allocating time slots to each call within the frequency. In theD-AMPS (Digital AMPS) system, each 30kHz carrier frequency is dividedinto three time slots. In the GSM and PCS systems, each 200kHz carrieris divided into eight time slots. The D-AMPS, D-AMPS 1900, GSM, PCS andiDEN systems all utilize TDMA.

In a CDMA system, your encoded voice is digitized and dividedinto packets. These packets are tagged with "codes." The packets thenmix with all of the other packets of traffic in the local CDMA networkas they are routed towards their destination. The receiving system onlyaccepts the packets with the codes destined for it.

Analog systems are FDM. Digital systems can utilize either TDMA or CDMA.

FDM systems typically allow one call per 10Khz or 30Khz of spectrum.Early TDMA systems tripled the capacity of FDM systems. Recent advancesin TDMA promise to provide forty times the carrying capacity of FDMsystems. CDMA promises to improve on the results of TDMA.

Early Mobile Telephone Technologies

Before there were cellulartelephone systems, there was MTS (Mobile Telephone Service) and IMTS(Improved Mobile Telephone Service). These early systems have ceasedoperations.

Other Mobile Telephone Systems

Many mobile telephone systems exist outside the United States,including NMT, TACS/ETACS, and JTACS. New systems are constantly indevelopment.