lecture gsm

GSM System Standard

2

Introduction

Year Introduced:1990

Global System for Mobile (GSM) - a 2-G cellular system standard

Originally developed to serve as the pan-European cellular service

Aimed at specifying a common mobile communication system for Europe

Originally in the 900 MHz band

GSM (Groupe spe'cial mobile) : a committee of working group of the CEPT.

Later, GSM changed its name to the Global System for Mobile Communications

Standards by European Technical Standards Institute (ETSI)

Adopted by several non-European countries by the end of 1993

Technically equivalent offshoot - DCS 1800

3

GSM Variants

Variant Uplink

(MHz)

Downlink

(MHz)

Total

Bandwidth

GSM-400 451-458 and

479-486

461-468 and

489-496

Twice 14 MHz

GSM-900

(primary

band)

890-915 935-960 Twice 25 MHz

Extended

GSM-900

880-915 925-960 Twice 35 MHz

GSM-R 876-880 921-925 Twice 4 MHz

DCS-1800 1710-1785 1805-1880 Twice 75 MHz

PCS-1900 1850-1910 1930-1990 Twice 60 MHz

4

GSM System Architecture

5

Mobile Station (MS)

MS consists of following two components Mobile Equipment (ME)

Subscriber Identity Module (SIM)

Removable plastic card

Stores Network Specific Data such as list of carrier frequencies.

Stores International Mobile Subscriber Identity (IMSI) + ISDN

Stores Personal Identification Number (PIN) & Authentication Keys.

Also stores short messages, charging information, telephone book etc.

Allows separation of user mobility from equipment mobility

6

Base Transceiver Station (BTS)

One per cell

Consists of high speed transmitter and receiver

BTS antennas

Function of BTS

Provides two channels

Signalling and Data Channel

Performs error protection coding for the radio channel

7

Base Station Controller (BSC)

Controls multiple BTS

Functions:

Performs radio resource management Assigns and releases frequencies and time slots for all the MSs in

its area

Reallocation of frequencies among cells

Hand over protocol is executed here

Time and frequency synchronization signals to BTSs

Time Delay Measurement and notification of an MS to BTS

Power Management of BTS and MS

8

Mobile Switching Center (MSC)

Switching node of a PLMN

Allocation of radio resource (RR) Handover

Mobility of subscribers Location registration of subscriber

There can be several MSCs in a PLMN

Gateway MSC (GMSC) Connects mobile network to a fixed network

Entry point to a PLMN

Usually one per PLMN

Request routing information from the HLR and routes the connection to the local MSC

9

HLR/VLR

HLR - Home Location Register

For all users registered with the network, HLR keeps user profile

MSCs exchange information with HLR

When MS registers with a new GMSC, the HLR sends the user profile to the new MSC

VLR - Visitor Location Register

VLR is responsible for a group of location areas, typically associated with an MSC

10

Air Interface: MS to BTS

Uplink/Downlink of 25MHz

890 -915 MHz for Uplink (Reverse), 935 - 960 MHz for Downlink (Forward)

Combination of frequency division and time division multiplexing

FDMA No. of carriers = 25 MHz / 200 kHz = 125

Total 125 RF channels.

124 channels of 200 kHz +1 Guard band occupying RF channel BW.

TDMA Burst (8 time slots per RF channel)

Modulation used

Gaussian Minimum Shift Keying (GMSK), BT=0.3

11

Air Interface: Logical Channel

Two Types

Traffic Channels (TCHs)

Speech Channel

Data Channel

Signalling Channel (Control Channels)

Broadcast Channel (BCH)

Common Control Channel (CCCH)

Dedicated/Associated Control Channel

(DCCH/ACCH)

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1. Broadcast Channel (BCH)- BTS to MS

Broadcast Control Channel (BCCH)

Frequency Correction Channel (FCCH)

Synchronization Channel (SCH)

2. Common Control Channel (CCCH)

Paging Channel (PCH)

Random Access Channel (RACH)

MS to BTS

Uses Slotted Aloha

Access Grant Channel (AGCH)

Control Channels

13

3. DCCH (dedicated control channel)- Bidirectional point-to-point - main signaling channels

3 types 1. SDCCH (stand-alone dedicated control channel):

for service request, subscriber authentication, equipment validation, assignment to a traffic channel

2. Slow Associated Control Channel (SACCH) Always associated with either TCH or SDCCH Information

Optimal radio operation; Commands for synchronization, Transmitter power

control; Channel measurement Should always be active; as proof of existence of physical radio

connection

3. Fast Associated Control Channel (FACCH) for urgent signaling on a traffic channel, e.g, for handoff messages

14

Traffic Channels (TCHs)

Speech Channel

Full-Rate Speech Channel

User speech digitized at a raw data rate of 13 kbps.

With GSM channel coding added to the digitized speech, the full-rate speech channel carries 22.8 kbps.

Half-Rate Speech Channel

Designed for data rate of about 6.5 kbps.

With GSM channel coding added to the digitized speech, the half-rate speech channel will carry 11.4 kbps.

15

Data Channel

Full-Rate data Channel (TCHIFS)

Full-Rate Data Channel for 9600 bps (TCH/F9.6)



All produce 22.8 kbps after forward error correction coding

Half-Rate data Channel (TCHIFS)

Half-Rate Data Channel for 4800 bps (TCH/H4.8)

Half-Rate Data Channel for 2400 bps (TCH/H2.4)

Both produce 11.4 kbps after forward error correction coding

16

GSM Signal Processing

17

Frequency Hopping

Optionally, TDMA is combined with frequency

hopping to address problem of channel fading

TDMA bursts are transmitted in a pre-calculated sequence of

different frequencies (algorithm programmed in mobile

station)

If a TDMA burst happens to be in a deep fade, then next

burst most probably will not be so

Helps to make transmission quality more uniform among all

subscribers

Max. hopping rate:217.6 hops per second

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Roaming

VLR registers users roaming in its area

Recognizes mobile station is from another PLMN

If roaming is allowed, VLR finds the mobiles HLR in its

home PLMN

VLR constructs a global title from IMSI to allow signaling

from VLR to mobiles HLR via public telephone network

VLR generates a mobile subscriber roaming number

(MSRN) used to route incoming calls to mobile station

MSRN is sent to mobiles HLR

19

GSM handoffs

Intra-BSS: if old and new BTSs are attached to same

base station

MSC is not involved

Intra-MSC: if old and new BTSs are attached to

different base stations but within same MSC

Inter-MSC: if MSCs are changed

20

Upgrade in GSM: 2.5 G Standards

HSCSD - High Speed Circuit Switched Data GPRS - General Packet Radio Service EDGE - Enhanced Data Rates for GSM Evolution Provide solutions to increase the data rates over existing

GSM and IS-136 Networks.

HSCSD, GPRS & EDGE are collectively called 2.5G, However, EDGE is regarded by many as a 3G

technology.

lecture gsm

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