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Introduction to GSM, 2nd ed.

Introduction to GSM Book

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Time Division Multiple Access (TDMA)

Time division multiple access - TDMA - is a process of sharing a single radio channel by dividing the channel into time slots that are shared between simultaneous users of the radio channel. When a mobile radio communicates with a TDMA system, it is assigned a specific time position on the radio channel. By allowing several users to use different time positions (time slots) on a single radio channel, TDMA systems increase their ability to serve multiple users with a limited number of radio channels.

The GSM system uses radio channel pairs. One RF channel for the downlink (from the system to the mobile) and one RF channel for the uplink (from the mobile to the system). The frequency separation of channel pairs varies (45 MHz at 800 MHz to 90 MHz at 1.9GHz) with the system. In general, the higher the frequency, the larger the frequency difference between downlink and uplink frequencies.

GSM uses time division multiplexing (TDM) to share one modulated carrier frequency radio waveform among 8 (full rate) to 16 (half rate) conversations. Therefore, documents related to GSM are careful to distinguish between a radio carrier and a communication channel.

This figure shows how time division duplex (TDD) communications uses a signal channel or frequency to provide simultaneous two-way communications between devices by time-sharing. When using TDD, one device transmits (device 1), the other device listens (device 2) for a short period time. After the transmission is complete, the devices reverse their role so device 1 becomes a receiver and device 2 becomes a transmitter. The process continually repeats itself so data appears to flow in both directions simultaneously.

 

Introduction to GSM Book

more details

Introduction to GSM, 2nd ed.

This book explains the basic components, technologies used, and operation of GSM systems. You will discover why mobile telephone service providers have upgraded from 1st generation analog systems to more efficient and feature rich 2nd generation GSM systems. You will also discover how 2nd generation systems are gradually evolving into 3rd generation broadband multimedia systems.

$19.99 Printed, $16.99 eBook