The GSM System – Global System for Mobile Communications Magne Pettersen [email protected] (acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut

The GSM System – Global System for Mobile Communications

Magne [email protected]

(acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut Erik Walter, Anders Spilling)

mailto:[email protected]

Content

• Introduction• Network architecture • Fundamental functionality• Physical layer / radio interface• Radio planning• GSM in the future

Content


GSM status (end 2006)

• 2.18 billion connections in 212 countries

• 82 % market share globally

• An incredible industry success!

But, let us take a few steps back…

GSM – The idea of a common European mobile communications system

• 1982: A Nordic group is considering the next generation of mobile telephone. – NMT (Nordisk Mobil Telefon), the analogue first generation system has only just been started

• These ideas are presented to CEPT (European Conference of Postal and Telecommunications Administrations) in June 1982

• September 1982: The first meeting in CEPT GSM – Groupe Spécial Mobile

• In 1988 ETSI (European Telecommunications Standards Institute) is established and the work is continued under a new name: SMG – Special Mobile Group

GSM - Specifications

• Original specifications for the GSM system:– Good subjective voice quality– Low terminal and service cost– Support for international roaming– Support for handheld terminals– Support for new services– Spectrum efficient– Compatible with ISDN

GSM - Growth• 1991: First operational GSM network in Finland: Radiolinja• 1993: Tele-mobil (later: Telenor Mobil) and NetCom GSM open their

networks in Norway• 1998: GSM 1800 is deployed to increase capacity in cities and other

densely populated areas

0200400600800

1 0001 2001 4001 6001 8002 0002 2002 4002 6002 8003 0003 200

19

82

19

83

19

84

19

85

19

86

19

87

19

88

19

89

19

90

19

91

19

92

19

93

19

94

19

95

19

96

19

97

19

98

19

99

20

00 0 %

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

TM NMT TM GSM NetCom

9M

’01

GSM improvements – 2.5 G

• The need for data services increase:– In 1998-99 the HSCSD – High Speed Circuit Switched Data - is

standardised. Introduced in Norway 1. July 2001 (Telenor)– I 1999 packet switching using GPRS (General Packet Radio

Service) is standardised. Introduced in Norway 1. February 2001 (Telenor)

• Theoretical data rates up to 171 kbit/s

• "2.5 G" – EDGE – Enhanced Datarates for GSM Evolution– Standardised in 2001-2002– Introduced in September 2004 – deployment ongoing – Theoretical data rates up to 373 kbit/s

Some GSM terminals

Development.. Sony Ericsson W950i”the Walkman phone”

HTC P4350Pocket computer running Windows

Some more GSM terminals

Samsung BlackjackNokia N95with ”everything”, e.g. GPS built in

iPhone – Apple’sMobile phone initiative

Competing standards

• The ”CDMA family” of standards is the second largest group of mobile communications systems

– 340 million connections (November 2006)

• Standard developed in USA• Strongest standing in the Americas

• Also other

Content


High level network architecture (1/2)

SIM

ME:Mobile equipment

Services / Applications

Core Network(CN)

Ext.network

UE: User equipment

Access Network(AN)

High level network architecture (2/2)

• The network contains functionally of: User Equipment (UE), Access Network (AN), and Core Network (CN)

– User equipment: Interfaces the user, handles radio functionality

– Access network: Communication to and from the user equipment, handles all radio related functionality in the network

– Core network: Communication between access network and external networks, handles all switching and routing

• Services and applications lie above the network

GSM user equipment

• User equipment: Mobile equipment (ME) + SIM card

– Subscriber Identity Module (SIM) contains encryption key and personal data

– The user is uniquely identified through ”International Mobile Subscriber Identity” (IMSI)

– The mobile equipment is uniquely identified through ”International Mobile Equipment Identity” (IMEI)

– Both equipment and user uniquely identified

SIM

ME

SIM = Subscriber Identity Module

GSM Radio Access Network (GRAN)

cell

cell

cell

BTSBSC

Packet domain

Circuit domainBTS

BSC

Abis

A

Gb

Elements in GSM radio access network

• Base Transceiver Station (BTS):– The base station, radio access point. The coverage area of one

BTS is a cell

• Base Station Controller (BSC)– Controls a number of BTSs. Owns and controls the radio resources

within its domain

GRAN must handle interfaces towards both a packet switched (packet domain) and a circuit switched (circuit domain) part of the core network

Some base station equipment

Some more base station equipment

Typical macro cellTypical micro cell

Open interfaces access network

• The interfaces between network elements must be well defined to achieve open interfaces, i.e. different network elements can be delivered by different vendors

• Interfaces in GRAN:– Um: The air interface between the mobile equipment and the BTS

– Abis: Interface between BTS and BSC

– A: Interface between GRAN and circuit switched part of core network (CN).

– Gb: Interface between GRAN and packet switched part of the core network (CN)

GSM core network

MSC GMSC

HLR

GGSN

GRAN

External networks

PSTN/ISDN

IP network

Service platforms

A

Gb

SGSN

Elements in GSM core network

• MSC – Mobile Switching Centre– Switch in the circuit domain. Contains copy of service profile for all users currently in

the MSC coverage area (Visiting Location Register –VLR, not shown explicitly in figure)

• GMSC – Gateway MSC– Handles all traffic to and from GSM and external circuit switched networks, such as

PSTN, ISDN or other mobile networks

• HLR – Home Location Register– Database containing a master copy of all the mobile operator’s subscribers. There is

only one logical HLR per GSM network. HLR contains information about e.g. permitted services and permitted roaming networks

• SGSN – Serving GPRS Support Node and GGSN – Gateway GPRS Support Node have similar functionality as MSC / GMSC, but for the packet switched part of the network. GGSN handles connections to external IP networks

• Also open interfaces between network elements. Not discussed here.

Content


Fundamental functionality

• The following functions are described:– Circuit switched connectivity– Packet switched connectivity– Mobile messaging– Security– Roaming– Choice of network– Location update– Handover

Circuit Switched connectivity

ISDNMobile network

• Fixed connection and reserved resources while the communication lasts. – (Mobile) telephony– Circuit switched data, e.g. WAP, mobile office solutions using data cards etc.

• Transparent channel with defined performance

• Billing typically per time unit and dependant on transport data rate

• Standard GSM: up to 14.4 kbit/s (more using HSCSD - High Speed Circuit Switched Data)

Packet Switched connectivity

• Resources allocated only when data is transferred • Same ”path” through network can be maintained (but not necessarily)• Billing typically dependant on amount of data transferred (or fixed tarrifs)• GPRS: Theoretically up to 171 kbit/s, typically 40 – 50 kbit/s- 4 different quality classes for packet ”bearer services”:

Internet ordifferent IP networkMobile network

Background Typically automatic download of email, MMS

Interactive Typically web/WAP-browsing, MMS, games

Streaming ”Network radio”, video streaming, web TV

Conversational Voice, video conferencing

Mobile messaging formats

•SMS: Short Message Service– Text based service to transfer up to 160 characters per message (solutions

exist to connect messages into longer messages, and also to carry other types of content – ring tones, logos…)

•MMS: Multimedia Messaging Service– A service for multimedia content, such as text, picture, sound, video

•Both SMS and MMS are ”store and forward” services, i.e. messages are intermediately stored in the network

Security functions

• The purpose of security functions is to protect users and network against improper and illegal use:

– Verify that the user has a valid subscription

– Protect the user’s identity against tracking

– Protection against wiretapping on the radio connection

• The mechanisms in GSM are based on secure storage of information in the user’s SIM card

International network

Roaming (1/2)

• Circuit switched call to a mobile in a visiting network

Home network

Visiting network

ISDN (country A)

ISDN (country B)

International network

Roaming (2/2)

• Mobile to mobile call in a visiting network – Effect referred to as ”tromboning”

Home network

Visiting network

ISDN (country A)

ISDN (country B)

Choice of network

• In GSM the following procedure is followed:– The latest used network is stored on the SIM– As long as a cell that fulfils the criteria is available from this

network, the mobile will not search for alternatives (the exception is national roaming, in which case the mobile will periodically search for the home network and connect when this becomes available)

– If the previously used network is not available, the mobile searches for alternative networks

– The mobiles can perform manually or automatic choice of network

Location Area / Routing Area (1/2)

• In GSM this is defined as follows:– Location area – LA is the area in which the network is ”searching” for a

registered mobile (not currently active) – for circuit switched services

– Routing area – RA: Similarly for packet switched service

HLR..IMSI >LAI,RAI..............

LA 1

LA 2

RA 1

RA 2

Location Area / Routing Area (2/2)

• The dependency between LA and RA is dependant on the practical realisation of the network. Normally they will be identical

• LA and RA contain a number of cells that can be reached from the MSC or SGSN

• LA and RA information for each mobile is stored in the HLR (in the home network)

• The mobile is responsible for updating the LA/RA information

Location update

• A location update is performed when:– The mobile is connecting to a cell and discovers that the LAI read is

different than the one stored in the mobile– The mobile has been turned on, but not used, for a pre-defined period of

time since the last location update (periodic location update)

• IMSI detach/attach:– An additional function where the mobile informs that it is turned on or off

(in the same LA), saves resources on the radio interface and leads to fater response on incoming calls

• Periodic detach– A network functionality where the network assumes that the mobile has

been turned off if periodic location update has not been performed and no other activity has been observed for a pre-defined amount of time

Handover

• To connect a call or communication session from one cell to another (or to a different channel in the same cell)

• Is normally performed because the signal level from the current cell is becoming to low, but can also be done for different reasons, such as too much traffic in a cell

Types of handover

• Intra cell (to another channel in the same cell) (1)

• Inter cell, intra BSC (2)• Inter BSC, intra MSC (3)• Inter MSC (4)

• In addition inter system handover can sometimes be performed, e.g. GSM to UMTS

– Complicated, special rules apply

• Type of handover has network implications, but the algorithms to decide handover are the same

Content


GSM radio interface – Main characteristics

• Frequency bands:– GSM 900:

• 890 – 915 MHz: Uplink (MS transmit) • 935 - 960 MHz: Downlink (MS receive)

– GSM 1800: • 1710 - 1885 MHz: Uplink• 1805 - 1880 MHz: Downlink

• Carrier bandwidth: 200 kHz• Channels / carrier: 8• Multiple access: TDMA / FDMA• Duplex: FDD• Gross bit rate pr carrier: 270,833 kbit/s • Modulation: GMSK • Spectrum efficiency: 1.35 bps/Hz

Radio parameters:

MS:• Sensitivity: -104 (-102) dBm• Typical – 106 dBm• Max. output power: 33 (30)

dBm

Numbers in parenthesis for GSM-1800

BTS:• Sensitivity: -104 (-104) dBm• Typical: – 107 dBm• Max. output power: 43 dBm

Channels in GSM900

890 MHz

45 MHz

960 MHz935 MHz

200 kHz

MS transmit MS receive

41 2 3 4 1241231

8

2345

67

1

8

2345

67

1231241 2 3 4

915 MHz

TDMA - principle

• GSM uses TDMA within each carrier• Each user occupies the entire carrier one time slot pr. time frame

– 8 slots per frame

GSM Channel structure

25 MHz124 carriers

577 sBurst period Time slot 1

Time slot 2

…..

Time slot 8

= 4.615 ms

=Physical channel

TDMA frame

• Logical channels built up of physical channels

– Control channels– Traffic channels

• Logical channels divided between:

– Dedicated channels– Common channels

GSM traffic channels

• Traffic channels (TCH) are used to carry voice or data– Typically uses one time slot per frame

– Gross data rate per TCH: 22 kbps• Effective data rate lower because of forward error correction

Training sequence

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

BP0 BP1 BP2 BP3 BP4 BP5 BP6 BP7

3 57 1 26 1 57 3 8.25

26 frame length: 120 ms

TDMA frame length: 4.6 ms

Data bit Data bitNormal burst

Some GSM control channels

BCCH Broadcast Control CHannel – Continuously transmitted from the BTS. Contains information about cell identity, frequency etc.

FCCH

SCH

Frequency Correction CHannel / Synchronisation CHannel – Used to correct/synchronise the frequency (FCCH) + time synchronise to the frame structure. Each cell has a FCCH and a SCH

RACH Random Access CHannel – Used by the mobile to send a request to the network for access. This is a slotted Aloha channel, no pre-allocation possible

AGCH Access Grant CHannel – Used by the network to inform the mobile that access has been granted and information about which channel to use

PCH Paging CHannel – Used by the network to notify users about incoming calls.

Error correction coding in GSM

• The different channels in GSM require different degree of protection, and therefore have different Forward Error Correction (FEC) schemes

• However, three types of techniques are often combined:

– Block coding, well suited to detect and correct bursts of error – Convolutional coding, high performance but not optimal for

bursts of errors– Interleaving, spreading neighbouring bits out, to decorrelate

the relative position

Block coding

• GSM uses two types of block codes:– Fire code 224 / 184 (control channels only)

• k = 184• t = 20

– Parity codes (only error detection, e.g. RACH)

• No block codes used on data channels

Convolutional coding

• When choosing depth (register length) in a convolutional code there is a trade-off between complexity and performance

– GSM uses a register length of 5

• Example of GSM ½ rate convolutional code shown in figure (used e.g. on a number of traffic channels)

Interleaving

• “Whitening process", optimising the conditions for the convolutional coder• Fundamentally important that the interleaving spreads the bit errors out• Interleaving depth improves performance, but also increases delay• GSM: Interleaving depth 4 – 19• Figure shows example with interleaving depth 4

– Write in vertically, read out horizontally – On reception, do the reverse process

Forward error correction - Overview

Outer Block Coding

InterleavingEncryption, modulation

Inner Convolutional

coding

Information bits from

source (voice, data)

Information bits to

receiver

Radio channel + noise

Demod., ch. equalising, decryption

DenterleavingConvolutional

decodingBlock

decoding

Forward error correction

Modulation

• Assuming that everyone is familiar with digital modulation :-)

• Considerations upon choosing modulations scheme:– Spectrum efficiency – Out of band emission (rapid drop off desired to limit adjacent

channel interference)– Constant envelope desired for low cost amplifiers, e.g. in

handheld equipment

• Always a trade off

• In GSM: GMSK – Gaussian Minimum Shift Keying is used

GMSK (1/2)

• Leftmost figure show spectrum for MSK, QPSK and BPSK• Rightmost figure shows envelope for different ”QPSK type”

modulation schemes– MSK has constant envelope, relatively low sidelobes

GMSK (2/2)

• GMSK further reduces sidelobes by using a Gaussian filter– Cost: introduces inter-symbol-interference (ISI)

• Figures show time and frequency response– GSM uses BT = 0.3

Channel equaliser

• Because of reflections, diffractions etc. in the radio channel, time dispersion is often experienced

– Transmitted signal arriving at the receiver from various directions over a multiplicity of paths

– Broadening of transmitted pulse, inter symbol interference (ISI)

– Frequency selective fading

• Must be counteracted by using some sort of equalisation

• GSM uses a Maximum Likelihood Sequence estimator (MLSE)• MLSE looks conceptually like shown in the figure below • The impulse response of the radio channel is calculated• A Viterbi algorithm is used to estimate the most likely (Maximum Likelihood - ML)

symbol sequence• MLSE is an optimal technique in terms of removing ISI, but the complexity

increases exponentially with the length of the channel response• GSM uses a MLSE which operates over 5 bit periods (approx. 16 s)

Maximum likelihood sequence estimator

Power control

• GSM uses power control, adjusting transmit power level in accordance with path loss

• Advantages: – Reduces interference– Reduces power consumption

• Can also be used on downlink

• Manner of operation, GSM:– The system (BSC) measures bit error rate (BER)– Transmit power adjusted up or down according to target value– Step size 2 dB– Maximum update interval: 60 ms

Power control - Example

Content


Fundamentals

• Planning and deploying a GSM network is from an operator’s point of view a question of:

– Build as few sites as possible, while maintaining required coverage and capacity

– Trade off

Coverage limited and capacity limited

• A network can be either– Coverage limited:

• The radio coverage decides the BTS density

• Typically rural areas, large cells, high masts

• Macrocells

– Capacity limited:• The traffic decides the BTS

density

• Typically urban areas, small cells, low BTS position

• Microcells

Frequency reuse

• Frequencies can not be reused in every cell due to co-channel interference (CCI)

• A cell cluster uses all the operator’s frequencies (A, B, C, E, F, G, H in Figure)

• Co channel interference level decided by – Cell clustre size, and thereby Frequency

reuse distance (D in Figure)

– Propagation properties

– Can be reduced by different techniques: • Sectorisation• Cell splitting

• Typical cell cluster size in GSM: 7

Coverage map example

• Unfortunately cell coverage is normally neither hexagonal or circular

• Figure shows coverage example from a city centre

• Complicates radio planning

Hierarchical cell structures

• In a GSM system it is common that cells of different sizes co-exist in that same area:

– Picocells, microcells, macrocells

• This is called hierarchical cell structure

• Can make handover (cell change) complicated. Often different types of users are reserved for one cell type, e.g.:

– Users walking indoors on picocell, users walking outdoor on microcell, users driving use macrocell

Radio planning tools

• Radio planning is most often performed assisted by an automated process using a computer

• Underlying functionality– Digital maps– Propagation modelling– System parameters and system performance– Traffic assumptions and theory

• Often theoretical computer based modelling can be tuned by real life data

– Propagation measurements – Live network traffic data

Example – Astrix

Content


GSM development

• GPRS and EDGE has introduced packet data and support for higher data rates into GSM

• UMTS is a 3G technology building on GSM core network, which is “backwards compatible” with GSM

– GSM-UMTS handover supported– Almost all UMTS terminals are also GSM terminals

• HSDPA / HSUPA (High Speed Downlink/Uplink Packet Access) supports real mobile broadband

GSM

EDGE

UMTS(WCDMA)

HSDPA / HSUPA

1999 2002 2006/2007

GPRS

171 kbit/s

384 kbit/s

2 Mbit/s 14.4 Mbit/s

2G 2.5G 2.75G 3G 3.5G

Trends (1) – Convergence

• Mobile communications system become more broadband

• At the same time computer network solutions start to support mobility (e.g. WiFi, WiMAX)

– Mobile goes broadband and broadband goes mobile?– Everything comes together?

Trends (2) – Horizontal integration

• The same services become available on different platforms and on different devices

• IP is the glue• Will mobile circuit switch disappear?

Fix

ed

line

Sat

elli

te

GS

M

3G

WL

AN

WiM

AX

Service 1 Service 2 Service n

IP

Fix

ed

line

Sat

elli

te

GS

M

3G

WL

AN

WiM

AX

Service 2 Service n

IP

Thank you for your attention!

Documents

The GSM System – Global System for Mobile Communications Magne Pettersen [email protected] (acknowledgements: Per Hjalmar Lehne, Rune Harald Rækken, Knut