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How Cellular Phones Works

There are two basic types of cellular phones: analog and digital. By far, the phone that most people (at least in the US) have been exposed to is the analog cellular phone. However, the digital phone is growing in terms of numbers in service, and any discussion of how cellular phones work would not be complete without covering all the bases.

Before we talk about how cellular phones work in general, let's talk about the difference between the types of cellular phones. We'll cover the main types, and then move on to how the cellular system works in general. This page is intended as a general overview, so we'll try not to get too technical here.

Analog & Digital Systems

AMPS: Advanced Mobile Phone Service.

AMPS is your plain vanilla analog cellular system. Voice signals are transmitted using an FM transmitter, just like a standard two way radio or music on your car FM radio would be. Signaling for call setup is done with digital signaling, but call supervision functions (on hook, off hook, hook flash, etc.) are done with various signaling tones. An important variation of this system is NAMPS, developed by Motorola. Similar to AMPS, but uses a narrower bandwidth channel and low speed digital signaling for call supervision.

TDMA: Time Division Multiple Access.

TDMA is one of the digital standards. The voice is digitized (much like a CD, but with much lower audio quality) and the resulting data is sent in bursts that are timed in such a way so as not to interfere with other stations using the same channel. An important variation of this system is GSM, formerly known as Groupe Special Mobile (French) but increasingly called Global System for Mobile telephones. GSM is a TDMA system, but also has frequency hopping and encryption features. While TDMA is primarily a North American standard, GSM, which originated in Europe, is rapidly being deployed worldwide.

CDMA: Code Division Multiple Access.

CDMA is a form of spread spectrum transmission, where the digitized voice is combined with a special code that allows several users to share the same portion of the radio spectrum. The different codes allow the various signals to be sorted out at the receiving end. The current CDMA standard was developed by Qualcomm in San Diego.

The major advantage of digital service is increased capacity. Digital services can allow cellular carriers to increase the number of subscribers in a given situation by many times, depending on the system used and the individual circumstances. Other benefits include enhanced privacy from the addition of encryption, and reduction of cellular fraud.

Cellular Transmission

Cellular telephones get their name from the way the system is set up. Instead of one large base station covering a wide area, cellular systems are divided into many small coverage base station areas called "cells." As a subscriber moves from one cell to the next, the system "hands off" the call to the new cell from the old one. For example, as a mobile unit moves from downtown Los Angeles to Beverly Hills, he may get passed on (handed off) from the downtown LA cell to a Wilshire Blvd. cell, and then on to a West LA cell, and then to a Beverly Hills cell. The hand off is accomplished by sending a special signal to the mobile unit, which then switches to the new cell.

Why use multiple cells like this? The main reason is frequency re-use. The same channel can be used in more than one cell, as long as the cells don't overlap in their coverage area. This produces a much greater efficiency in channel use, allowing more calls in the system. If a single wide area base station were to be used, 100 channels could support 100 simultaneous calls. If we take that same 100 channels, and divide them up among 100 different cell sites, re-using channels as appropriate, we can support thousands of simultaneous calls; a substantial improvement!

It is the job of the Mobile Telephone Switching Office (MTSO) to make all the connections. The MTSO is the bridge between the Public Switched Telephone Network (PSTN) and the cell sites that ultimately make the wireless connection to the subscriber's cellular phone. The MTSO not only makes the connections, but also controls all of the cell sites, and manages all of the mobiles via a control channel.

The control channel is used for several functions. Mobiles register with it, so that the system knows where to find them. The system calls mobiles with it, and mobiles initiate calls with it. Once the call is set up, then the mobile moves off to the specific voice channel (or time slot, or code) designated for that call by the system.

Simple, eh? Not hardly, but the cellular system has had many years of refinement to work on getting it right... and it does get it right, at least most of the time...

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