GSM—Global System for Mobile

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How does GSM handle multiple users

The 1G cellular systems used FDMA. The first cellular standard adopting TDMA was GSM, a 2G system using digital technology.

First deployment in 1992.

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GSM is a TDMA/FDMA/FDD System

A total of 124 carrier frequencies are available in the 25 MHz band in each direction (forward or reverse). This is FDMA.

Each carrier frequency, occupying a frequency band of 200 kHz, has 8 time slots for 8 users. This is TDMA (TDMA overlaid on FDMA).

The forward (downlink) and reverse (uplink) uses two different frequency bands: 935-960 MHz for downlink and 890-915 for uplink. This is FDD (Frequency Division Duplex).

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Mobile station

Mobile Equipment (ME): the phone including transmitter, receiver, etc.

Subscriber Identity Module (SIM): a smart card that stores the subscriber’s ID number, the networks the subscriber is authorized to use, encryption keys, etc.

Thus a subscriber needs only carry his or her SIM to use a wide variety of subscriber devices in many countries. The SIMs roam, not the phones. (The US is different from most other countries. In the US, service providers lock the phones because they subsidize them.)

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Base Station Subsystem (BSS)

Base Transceiver Station (BTS) for a single cell: an antenna, a transceiver (transmitter & receiver), and a link to a base station controller

A GSM cell can have a radius of 100m-35km.Base Station Controller (BSC) may control

multiple BTS units and hence multiple cells. It reserves frequencies, manages the handoff of a mobile unit from one cell to another within the BSS.

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Network Subsystem (NS)

Provides the link between the cellular network and the public switched telecommunications networks (PSTN).

The central element of the NS is the Mobile Switching Center (MSC), which Determines whether the subscriber is associated with the

switching center Stores locations of the subscribers currently in the region

covered by the switching center Stores authentication and encryption keys for all the

subscribers in both the home and visitor location registers Keeps track of the type of equipment that exists at the mobile

stations

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GSM Frames and Data Rate

Each time slot has 114 bits of dataEach frame has 8 time slots, one for a userEach multiframe (duration=120ms) has 24 data

framesData rate:

114 bits/slot 24 slots/multiframe 22.8

120 ms/multiframekbps

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Speech Coding

Speech coding is used to compress the data (reduce the data sent). This will increase the speed of transmission.

It uses previous data samples to predict current sample.

Analog voice is transformed into 13 kbps digital signal.

Adding error-detection and error-correction coding brings the transmission rate to 22.8 kbps.

Data rate: 9.6, 4.8 or 2.4 kbps.

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Frequency Hopping

A slow frequency hopping: 216 hop/sTo compensate multipath (frequency

selective) fadingTo reduce the cochannel interference