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GSM Fundamentals

GBSS Training Team


After the completion of this course, the trainees

should understand the following contents: Basic idea of GSM system Features of GSM The structure of the GSM system. Certain numbers that refer to BSS Handovers, frequency hopping and location

updates Propagation Mechanisms The idea of EDGE and GPRS


BSS Feature Description

BSS Signaling Analysis Manual

BSC Technical Manual


Chapter 1 GSM System OverviewChapter 1 GSM System Overview

Chapter 2 Features of GSMChapter 2 Features of GSM

Chapter 3 GSM Network StructureChapter 3 GSM Network Structure

Chapter 4 Chapter 4 Service Area and Number PlanningService Area and Number Planning

Chapter 5 Handover and Frequency hoppingChapter 5 Handover and Frequency hopping

Chapter 6 Location UpdateChapter 6 Location Update

Chapter 7 Propagation MechanismsChapter 7 Propagation Mechanisms

Chapter 8 GPRS & EDGE IntroductionChapter 8 GPRS & EDGE Introduction


GSM system overview The Global System for Mobile Communications (GSM) is a set of

recommendations and specifications for a digital cellular telephone

network (known as a Public Land Mobile Network, or PLMN).

These recommendations ensure the compatibility of equipment

from different GSM manufacturers, and interconnectivity between

different administrations, including operation across international

boundaries.

GSM networks are digital and can cater for high system capacities.

They are consistent with the world-wide digitization of the telephone

network, and are an extension of the Integrated Services Digital

Network (ISDN), using a digital radio interface between the cellular

network and the mobile subscriber equipment.


The GSM system is a frequency- and time-division cellular system,

each physical channel is characterized by a carrier frequency and a

time slot number

Cellular systems are designed to operate with groups of low-power

radios spread out over the geographical service area. Each group of

radios serve MSs presently located near them. The area served by

each group of radios is called a CELL

Uplink and downlink signals for one user are assigned different

frequencies, this kind of technique is called Frequency Division

Duplex (FDD)

Data for different users is conveyed in time intervals called slots ,

several slots make up a frame. This kind of technique is called Time

Division Multiple Access (TDMA)

GSM system overview


CELLULAR TELEPHONYCELLULAR TELEPHONY

A cellular telephone system links mobile subscribers into the public telephone system or to

another cellular subscriber.

Information between the mobile unit and the cellular network uses radio communication.

Hence the subscriber is able to move around and become fully mobile.

The service area in which mobile communication is to be provided is divided into regions

called cells.

Each cell has the equipment to transmit and receive calls from any subscriber located within

the borders of its radio coverage area.

Radio Mobile subscriberCell

GSM system overview


Standard Protocol for GSM take effect

System was named as Global System for Mobile Communication

GSM system began to provide service in Europe(2G)

Provide services for the whole world

Micro Cell Technique is used in GSM system

1989

1991

1992

1994

1996

GSM History and Overview


A certain radio coverage area formed by a set of transceivers that connected to a set of

antennas is called a CELL.

Macro Cell

In the beginning , High-Power BTSs are adopted to provide services. The BTS covers a

wider area , but its frequency utilization is not efficient. So , it can only provide a few

channels for subscribers.

Micro Cell

Later the Low-Power BTS joins the system for getting a better service area with high

capacity . At the same time it adopts the frequency reuse technique to improve the

efficiency of the frequency utilization and also the whole capacity of the network.

Macro Cell and Micro CellMacro Cell and Micro Cell

Cell Definitions


Multiple Access Technique allows many subscribers to

use the same communication medium.

There are three kinds of basic Multiple Access

Technique :

1) FDMA

2) TDMA and

3) CDMA.

GSM system adopt FDD-TDMA (FDMA and TDMA

together).

Multiple Access Technique


FDMA

FDMA uses different frequency channels

to accomplish communication.

The whole frequency spectrum available is

divided into many individual channels (for

transmitting and receiving) ， every

channel can support the traffic for one

subscriber or some control information.

Frequency

Time


TDMA accomplishes the

communication in different

timeslot.

A carrier is divided into channels

based on time. Different signals

occupy different timeslots in

certain sequence , that is , many

signals are transmitted on the same

frequency in different time.

Time

Frequency

TDMA


CDMA accomplishes the

communication in different code

sequences.

Special coding is adopted before

transmission, then different

information will lose nothing

after being mixed and

transmitted together on the same

frequency and at the same time.

Time

Frequency

CDMA


GSM 900GSM 900

Uplink

890 915 935 960MHz

Downlink

GSM systems use radio frequencies between 890-915 MHz for receive and between 935-960

MHz for transmit.

RF carriers are spaced every 200 kHz, allowing a total of 124 carriers for use.

An RF carrier is a pair of radio frequencies, one used in each direction.

Transmit and receive frequencies are always separated by 45 MHz.

Frequency Spectrum


UPLINK FREQUENCIES DOWNLINK FREQUENCIES

890 915 935 960880 925

UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 45MHZ

EGSM has 10MHz of bandwidth on both transmit and receive.

Receive bandwidth is from 880 MHz to 890 MHz.

Transmit bandwidth is from 925 MHz to 935 MHz.

Total RF carriers in EGSM is 50 more then GSM.

Extended GSM(EGSM)Extended GSM(EGSM)

Frequency Spectrum


Base Station Receive

1710 1785 1805 1880MHz

Base Station Transmit

DCS 1800DCS 1800

DCS1800 systems use radio frequencies between 1710-1785 MHz for receive and

between 1805-1880 MHz for transmit.

RF carriers are spaced every 200 kHz, allowing a total of 373 carriers.

Transmit and receive frequencies are always separated by 95 MHz.

Frequency Spectrum


Frequency Spectrum


INCREASED CAPACITYINCREASED CAPACITY

The GSM system provides a greater subscriber capacity than analogue systems.

GSM allows 25 kHz per user, that is, eight conversations per 200 kHz channel pair (a

pair comprising one transmit channel and one receive channel).

Digital channel coding and the modulation used makes the signal resistant to

interference from cells where the same frequencies are re-used (co-channel

interference).

This allows increased geographic reuse by permitting a reduction in the number of

cells in the reuse pattern.

Features of GSM


AUDIO QUALITYAUDIO QUALITY

Digital transmission of speech and high performance digital signal processors provide good

quality speech transmission.

Since GSM is a digital technology, the signals passed over a digital air interface can be protected

against errors by using better error detection and correction techniques.

In regions of interference or noise-limited operation the speech quality is noticeably better than

analogue.

USE OF STANDARDISED OPEN INTERFACES

Standard interfaces such as C7 and X25 are used throughout the system. Hence different

manufacturers can be selected for different parts of the PLMN.

There is a high flexibilty in where the Network components are situated.

Features of GSM


IMPROVED SECURITY AND CONFIDENTIALITYIMPROVED SECURITY AND CONFIDENTIALITY

GSM offers high speech and data confidentiality.

Subscriber authentication can be performed by the system to check if a

subscriber is a valid subscriber or not.

The GSM system provides for high degree of confidentiality for the

subscriber. Calls are encoded and ciphered when sent over air.

The mobile equipment can be identified independently from the mobile

subscriber. The mobile has a identity number hard coded into it when it is

manufactured. This number is stored in a standard database and whenever

a call is made the equipment can be checked to see if it has been reported

stolen.

Features of GSM


CLEANER HANDOVERSCLEANER HANDOVERS

GSM uses Mobile assisted handover technique.

The mobile itself carries out the signal strength and quality measurement of its

server and signal strength measurement of its neighbors.

This data is passed on the Network which then uses sophisticated algorithms to

determine the need of handover.

SUBSCRIBER IDENTIFICATIONSUBSCRIBER IDENTIFICATION

In a GSM system the mobile station and the subscriber are identified

separately.

The subscriber is identified by means of a smart card known as a SIM.

This enables the subscriber to use different mobile equipment while retaining

the same subscriber number.

Features of GSM


ENHANCED RANGE OF SERVICES

Speech services for normal telephony.

Short Message Service for point ot point transmission of text

message.

Cell broadcast for transmission of text message from the cell to all

MS in its coverage area. Message like traffic information or

advertising can be transmitted.

Fax and data services are provided. Data rates available are 2.4 Kb/s,

4.8 Kb/s and 9.6 Kb/s.

Supplementary services like number identification , call barring, call

forwarding, charging display etc can be provided.

Features of GSM


FREQUENCY REUSEFREQUENCY REUSE

There are total 124 carriers in GSM900 (additional 50 carriers are available in EGSM band).

Each carrier has 8 timeslots and if 7 can be used for traffic then a maximum of 868 ( 124 X 7 )

calls can be made. This is not enough and hence frequencies have to be reused.

The same RF carrier can be used for many conversations in several different cells at the same

time.

6

43

72

The radio carriers available are allocated according to a regular

pattern which repeats over the whole coverage area.

The pattern to be used depends on traffic requirement and spectrum

availability.

Some typical repeat patterns are 4*3,3*3, 7*3 etc.

The different Subscribers can use the same frequency in different

places.

The quality of communication must be ensured.

5

12

1

Features of GSM


4 site X 3 cells reuse

1

8

9

210

46

5 37

11

12

R

Frequency Reuse


Cell Types

Omni

1

120degree

1

23

Omni-directional CellOmni-directional Cell

120 Degree Cell120 Degree Cell


GSM /GPRS BSS

BTS

BSC

BTS

BSC

PCU

SS7

SMS system

PSTNISDN

Internet,Intranet

MSC/VLR GMSC

HLR/AUC

SGSN

CG BG

GGSN

GPRS Backbone

Other PLMN

MS

MS

OMC

GSM-GPRS Network Component


GSM /GPRS BSS

BTS

BSC

BTS

BSC

PCU

SS7

SMS system

PSTNISDN

Internet,Intranet

MSC/VLR GMSC

HLR/AUC

SGSN

CG BG

GGSN

GPRS backbone

Other PLMN

A

Gb

Gi

Gp

C/D/Gs

Gr/Gs/Gd/Ge Gc

Ga

Abis

Um

MS

MS

OMC

Interface Between Different Entities


Mobile Station—MS

An MS is used by a mobile subscriber to communicate with the mobile

network. Several types of MSs exist, each allowing the subscriber to make and

receive calls.

The range or coverage area of an MS depends on the output power of the

output. Different types of MSs have different output power capabilities and

consequently different ranges.

GSM MSs consist of

A mobile terminal

A Subscriber Identity Module (SIM)

In GSM the subscriber is separated from the mobile terminal. Each

subscriber’s information is stored as a “smart card” SIM. The SIM can be

plugged into any GSM mobile terminal. This brings the advantages of security

and portability of subscribers.


Mobiles are classified into five classes

according to their power rating.

CLASS POWER OUTPUT1 20W2 8W3 5W4 2W5 0.8W

SIM

Mobile Station—MS


BTSBTS

BSCBSC

TC/SMTC/SM

BSS

MSC

The Base Station Controller –

BSC

The Base Transceiver Station –

BTS

The Trans-coder – TC and Sub

multiplexer (SM)

Base Station Subsystem-BSS


The BSS is the fixed end of the radio interface that provides control and radio coverage

functions for one or more cells and their associated MSs.

It is the interface between the MS and the MSC.

The BSS comprises one or more Base Transceiver Stations (BTSs), each containing the radio

components that communicate with MSs in a given area, and a Base Site Controller (BSC)

which supports call processing functions and the interfaces to the MSC.

Digital radio techniques are used for the radio communications link, known as the Air Interface,

between the BSS and the MS.

The BSS consists of three basic Network Elements (NEs).

Transcoder (XCDR) or Remote transcoder (RXCDR) .

Base Station Controller (BSC).

Base Transceiver Stations (BTSs) assigned to the BSC.

Base Station Subsystem-BSS


The speech transcoder is the interface between the 64 kbit/s PCM channel in the land network and

the 13 kbit/s channel used on the Air Interface.

This reduces the amount of information carried on the Air Interface and hence, its bandwidth.

If the 64 kbits/s PCM is transmitted on the air interface without occupation, it would occupy an

excessive amount of radio bandwidth. This would use the available radio spectrum inefficiently.

The required bandwidth is therefore reduced by processing the 64 kbits/s PCM data so that the

amount of information required to transmit digitized voice falls to 13kb/s.

The XCDR can multiplex 4 traffic channels into a single 64 kbit/s timeslot. Thus a E1/T1 serial

link can carry 4 times as many channels.

This can reduce the number of E1/T1 leased lines required to connect remotely located equipment.

When the transcoder is between the MSC and the BSC it is called a remote transcoder (RXCDR).

Transcoder


30 Timeslots1 traffic channel / TS

64 Kbps / TS4 E1 lines = 30 X 4

=120 Timeslots

Each Timeslot =16 X 4 = 64 Kb/s

30 timeslots = 30 x 4=120 traffic channels

MSC XCDR BSC

0 1 2 3116

Transcoded information from four calls

Transcoder


The BSC network element provides the control for the BSS.

It controls and manages the associated BTSs, and interfaces with the Operations and

Maintenance Centre (OMC).

The purpose of the BSC is to perform a variety of functions. The following comprise the

functions provided by the BSC:

Controls the BTS components.-

Performs Call Processing.

Performs Operations and Maintenance (O & M).

Provides the O & M link (OML) between the BSS and the OMC.

Provides the A Interface between the BSS and the MSC.

Manages the radio channels.

Transfers signaling information to and from MSs.

Base Station Controller-BSC


The BTS network element consists of the hardware

components, such as radios, interface modules and antenna

systems that provide the Air Interface between the BSS and the

MSs.

The BTS provides radio channels (RF carriers) for a specific

RF coverage area.

The radio channel is the communication link between the MSs

within an RF coverage area and the BSS.

The BTS also has a limited amount of control functionality

which reduces the amount of traffic between the BTS and BSC.

Base Tran receiver Station-BTS


MSC BSC BTS12

BTS1

BTS2

BTS4

BTS3BTS11

BTS13 BTS14

BTS5

BTS6

BTS7

BTS8

BTS9

BTS11

Open ended Daisy Chain

Daisy Chain with a fork. Fork has a return loop back to the chain

Star

Daisy Chain with a fork. Fork has a return loop back to the chain

BTS Connectivity


BTSBTS

BSCBSC

TC/SMTC/SM

BSS

MSC

Packet data switching

Bridge between SGSN

and BSC

Provide Pb and Gb

interface GPRS Backbone

PCUPCU SGSNSGSN

Packet Control Unit - PCU


Mobile-service Switching Center – MSC Home Location Register – HLR Visitor Location Register – VLR Equipment Identity Register – EIR Authentication Center – AUC Echo Cancellor – EC

AUCAUCHLRHLR

MSC/VLRMSC/VLR

PSTN

NSS

EIREIROMC

BSS

ECEC

Network Switching System


The Mobile services Switching Centre (MSC) co-ordinates the setting up of calls to

and from GSM users.

It is the telephone switching office for MS originated or terminated traffic and

provides the appropriate bearer services, teleservices and supplementary services.

It controls a number of Base Station Sites (BSSs) within a specified geographical

coverage area and gives the radio subsystem access to the subscriber and equipment

databases.

The MSC carries out several different functions depending on its position in the

network.

When the MSC provides the interface between PSTN and the BSS in the GSM

network it is called the Gateway MSC.

Some important functions carried out by MSC are Call processing including control

of data/voice call setup, inter BSS & inter MSC handovers, control of mobility

management, Operation & maintenance support including database management,

traffic metering and man machine interface & managing the interface between GSM

& PSTN N/W.

Mobile-service Switching Center - MSC


The HLR contains the master database of all subscribers in the PLMN.

This data is remotely accessed by the MSC´s and VLRs in the network. The data can also

be accessed by an MSC or a VLR in a different PLMN to allow inter-system and inter-

country roaming.

A PLMN may contain more than one HLR, in which case each HLR contains a portion of

the total subscriber database. There is only one database record per subscriber.

The subscribers data may be accessed by the IMSI or the MSISDN.

The parameters stored in HLR are

Subscribers ID (IMSI and MSISDN )

Current subscriber VLR.

Supplementary services subscribed to.

Supplementary services information (eg. Current forwarding address ).

Authentication key and AUC functionality.

TMSI and MSRN

Home Location Register - HLR


The Visited Location Register (VLR) is a local subscriber database, holding

details on those subscribers who enter the area of the network that it covers.

The details are held in the VLR until the subscriber moves into the area

serviced by another VLR.

The data includes most of the information stored at the HLR, as well as more

precise location and status information.

The additional data stored in VLR are

Mobile status ( Busy / Free / No answer etc. )

Location Area Identity ( LAI )

Temporary Mobile Subscribers Identity ( TMSI )

Mobile Station Roaming Number ( MSRN )

The VLR provides the system elements local to the subscriber, with basic

information on that subscriber, thus removing the need to access the HLR

every time subscriber information is required.

Visitor Location Register -VLR


The AUC is a processor system that

perform authentication function.

It is normally co-located with the HLR.

The authentication process usually takes

place each time the subscriber initializes

on the system.

Each subscriber is assigned an

authentication key (Ki) which is stored in

the SIM and at the AUC.

Authentication Centre - AUC


IMEI is Checked In White List

IMEI is Checked in Black/Grey List

If NOT found

EIR focus on the equipment , not the subscriber!!

White List: All Valid

assigned ID’s

Black List: Service

allowed but noted

Grey List: Service

denied

Equipment Identity Register - EIR


The OMC controls and monitors the Network elements within a region.

The OMC also monitors the quality of service being provided by the Network.

The following are the main functions performed by the OMC-R

The OMC allows network devices to be manually removed for or restored to

service. The status of network devices can be checked from the OMC and tests

and diagnostics invoked.

The alarms generated by the Network elements are reported and logged at the

OMC. The OMC-R Engineer can monitor and analyse these alarms and take

appropriate action like informing the maintenance personal.

The OMC keeps on collecting and accumulating traffic statistics from the

network elements for analysis.

Software loads can be downloaded to network elements or uploaded to the

OMC.

Operation and maintenance Centre for Radio – OMC-R


OS

MMI

DB

Event/AlarmManagement

SecurityManagement

ConfigurationManagement

Performance Management

Fault Management

OMC Functional Architecture


Different Interfaces

inGSM


The terrestrial interfaces comprises all the connections between the GSM system

entities ,apart from the Um or air interface.

The terrestrial interfaces transport the traffic across the system and allows the

passage of thousands of data messages to make the system function.

The standard interfaces used are

2 Mb/s

Signaling System (C7 or SS7)

Packet Switched Data

Abis using the LAPD protocol (Link Access Procedure D )

Terrestrial Interface


Each interface specified in GSM has a name associated with it.

Interface Names

NAME INTERFACE

Um MS ----- BTS

Abis BTS ----- BSC

Ater BSC ----- TRC

A MSC ------ BSC

B MSC ------ VLR

C MSC ------ HLR

D VLR ----- HLR

E MSC ------ MSC

F MSC ------ EIR

G VLR ------ VLR

H HLR ------ AUC


This interface carries the traffic from the PSTN to the MSC, between MSC’s, from

the MSC to the BSC’s and from the BSC’s to the BTS’s.

It represents the physical layer in the OSI model.

Each 2 Mb/s link provides 30 traffic channels available to carry speech ,data or

control information.

Typical Configuration

TS 0 TS 1-15 TS 16 TS 17 - 31

TS 0 - Frame allignment/ Error checking/ Signalling/ AlarmsTS 1-15 , 17-31 - TrafficTS 16 - Siganlling

2 Mbits/s Trunk 30-Channel PCM


PLMN service area

......

Service Area

MSC service area...

Location area...

cell

PLMN service area PLMN service area

MSC service area...

Location area...

cell

Service Area


Location Area Identification

The LAI is the international code for a location area.

MCC: Mobile Country Code ， It consists of 3 digits . For example: The MCC of China is "460"MNC: Mobile Network Code ， It consists of 2 digits . For example: The MNC of China Mobile is "00"LAC: Location Area Code ， It is a two bytes hex code. The value 0000 and FFFF is invalid. For example: 460-00-0011

MCC MNC LAC

LAI


CGI

The CGI is a unique international identification for a cell

The format is LAI+CI

LAI: Location Area Identification

CI: Cell Identity. This code uses two bytes hex code to identify

the cells within an LAI.

For example : 460-00-0011-0001

CGI: Cell Global Identification


BSIC

NCC: PLMN network color code. It comprises 3 bit. It allows various neighboring PLMNs to be

distinguished.BCC: BTS color code. It comprises 3 bit, used to distinguish different cells assigned the same

frequency!

NCC BCC

BSIC

BSIC （ Base Station Identification Color Code)


CC: Country Code. For example: The CC of China is "86".NDC: National Destination Code. For example: The NDC of China

Telecom is 139, 138, 137, 136, 135.SN: Subscriber Number. Format:H0 H1 H2 H3 ABCDExample: 86-139-0666-1234

MSISDN

CC NDC SN

National (significant)Mobile number

Mobile station internationalISDN number


MCC: Mobile Country Code ， It consists of 3 digits . For example: The MCC of China is "460"。MNC: Mobile Network Code ， It consists of 2 digits . For example: The MNC of China Telecom is "00"。MSIN: Mobile Subscriber Identification Number. H1H2H3 S ABCDEF For example: 666-9777001NMSI: National Mobile Subscriber Identification ， MNC and MSIN

form it together. For Example of IMSI : 460-00-666-9777001

Not more than 15 digits

3 digits 2 digits

IMSI

MCC MNC MSIN

NMSI

IMSI


TMSI

The TMSI is assigned only after successful subscriber authentication.

The VLR controls the allocation of new TMSI numbers and notifies them to the HLR.

TMSI is used to ensure that the identity of the mobile subscriber on the air interface is kept secret.

The TMSI consists of 4 bytes( 8 HEX numbers) and determined by the operator.

TMSI: Temporary Mobile Subscriber Identification)


IMEI

TAC FAC SNR SP

IMEI

IMEI: International Mobile Station Equipment Identification

TAC: Type approval code, 6 bit, determined by the type approval centerFAC: Final assembly code, 2 bit, It is determined by the manufacturer.SNR: Serial number, 6 bits, It is issued by the manufacturer of the MS. SP: 1 bit , Not used.

Check the IMEI in your MS : *#06#

TAC: Type approval code, 6 bit, determined by the type approval centerFAC: Final assembly code, 2 bit, It is determined by the manufacturer.SNR: Serial number, 6 bits, It is issued by the manufacturer of the MS. SP: 1 bit , Not used.

Check the IMEI in your MS : *#06#


The GSM handover process uses a mobile assisted technique for accurate and

fast handovers, in order to:

▪ Maintain the user connection link quality.

▪ Manage traffic distribution

The overall handover process is implemented in the MS,BSS & MSC.

Measurement of radio subsystem downlink performance and signal strengths

received from surrounding cells, is made in the MS.

These measurements are sent to the BSS for assessment.

The BSS measures the uplink performance for the MS being served and also

assesses the signal strength of interference on its idle traffic channels.

Initial assessment of the measurements in conjunction with defined thresholds

and handover strategy may be performed in the BSS. Assessment requiring

measurement results from other BSS or other information resident in the MSC,

may be perform. in the MSC.

Handover


The MS assists the handover decision process by performing

certain measurements.

When the MS is engaged in a speech conversation, a portion

of the TDMA frame is idle while the rest of the frame is used

for uplink (BTS receive) and downlink (BTS transmit)

timeslots.

During the idle time period of the frame, the MS changes

radio channel frequency and monitors and measures the

signal level of the six best neighbor cells.

Measurements which feed the handover decision algorithm

are made at both ends of the radio link.

Handover


At the MS end, measurements are continuously signalled, via the

associated control channel, to the BSS where the decision for handover

is ultimately made.

MS measurements include:

Serving cell downlink quality (bit error rate (BER) estimate.

Serving cell downlink received signal level, and six best neighbor

cells downlink received signal level.

The MS also decodes the Base Station ID Code (BSIC) from the six

best neighbor cells, and reports the BSICs and the measurement

information to the BSS.

MS ENDMS END

Handover


The BTS measures the uplink link quality, received signal level, and MS

to BTS site distance.

The MS RF transmit output power budget is also considered in the

handover decision.

If the MS can be served by a neighbor cell at a lower power, the

handover is recommended.

From a system perspective, handover may be considered due to loading

or congestion conditions. In this case, the MSC or BSC tries to balance

channel usage among cells.

BTS ENDBTS END

Handover


Handover takes place in the same cell from one timeslot to another timeslot of the same carrier

or different carriers( but the same cell).

Intra-cell handover is triggered only if the cause is interference.

Intra-cell handover can be enabled or disabled in a cell.

Intra-Cell Handover

BSC

BTSCall is handed

from timeslot 3 to timeslot 5

0 1 2 3 4 5 6 7

Handover Types


Handover takes place between different cell which are controlled by the same BSC.

Intra-BSC Handover

BSC1

BTS1Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled

by the same BSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7


Handover takes place between different cell which are controlled by the different BSC.

Inter-BSC Handover

BSC1

BTS1Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled

by the different BSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7BSC2

MSC

BTS2


Handover takes place between different cell which are controlled by the different BSC and each

BSC is controlled by different MSC.

Inter-MSC Handover

BSS1

BTS1

Call is handed from timeslot 3 of cell1 to timeslot 1 of cell2 . Both the cells are controlled

by the different BSC, each BSCbeing controlled by different MSC.

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7BSS2

MSC1

BTS2

MSC2


MOTOROLA

aaaa

1

4

7

2 3

5 6

8 9

0* #

63250/0/0/

Information is transmittedby different frequencies atdifferent timeslot

Voice

ModulatedRF signal

f1

f2

f3

f4

f1

Frequency Hopping


Introduction to Frequency Hopping

Can be used to improve the quality of the network

Also can be used to increase the capacity of the Network

thereby reducing the number of sites required for

CAPACITY.

The way it works:

Each burst is transmitted on a different frequency

Both mobile and base station follow the same hopping

sequence


Introduction to Frequency Hopping Fading

Causes quality deterioration

Is frequency dependent

FH diversifies the impact of fading and improves quality.

The immunity to fading increases by exploiting its frequency selectivity, because using

different frequencies the probability of being continuously affected by fading is reduced, so

the transmission link quality is improved.

This improvement is much more noticeable for slow moving mobiles.


Increased immunity to Fading

In a cellular urban environment in most cases multipath propagation will be

present and, as a consequence of that, important short term variations in the

received level are frequent . This is called Raleigh fading which results in

quality degradation because some of the information will be corrupted.

For a fast moving mobile, the fading situation can be avoided from one burst

to another because it also depends on the position of the mobile so the

problem is not so serious.

For a stationary one the reception may be permanently affected resulting in

a very bad quality, even a drop call.

Once the information is received by the mobile or the base station, the only

way to cope with the disturbance produced by the fading (errors in the

information bits) are the decoding and de-interleaving processes, with an

effectiveness limited by the number of errors they have to deal with.


Increased immunity to Fading

Interference

A result of frequency reuse & irregular terrain and sites.

FH diversifies the impact of interference and improves quality

The situation of permanent interference coming from neighbour

cells transmitting the same or adjacent frequencies is avoided

using Frequency Hopping because the calls will spend the time

moving through different frequencies not equally affected by

interfering signals. This effect is called Interference Averaging.


Types of Hopping

Base Band Hopping (BBH)Base Band Hopping (BBH)

The radio units transmit always the same frequency.

Number of frequencies for hopping = Number of carriers

The radio units are always transmitting a fixed frequency and

frequency hopping is performed by moving the information for

every call among the available radio units in a cell on a per

burst basis.

In reception the call is always processed by the same radio unit

(the one where the call started).

The number of frequencies to hop over is limited by the

number of radio units equipped in the cell.

The BCCH carrier can hop in timeslots 1 to 7.


Types of HoppingSynthesiser Frequency Hopping (SFH)Synthesiser Frequency Hopping (SFH)

The radio units change (retune) the frequency every

burst.

The call always stays in the same radio unit.

Number of frequencies for hopping > Number of

carriers.

The radio units can hop over a range of different

frequencies.

Hybrid combiners are required in the base station

(Cavity Combiners can not be used with SFH).

The BCCH carrier can never hop.


MSC should always know the location of the MS so that it can contact it by sending pages whenever required.

The mobile keeps on informing the MSC about its current location area or whenever it changes from one LA to another.

This process of informing the MSC is known as location updating.

The new LA is updated in the VLR.

LAI = MCC + MNC + LAC

MCC MNC LAC

3 digits 1-2 digits Max 16 bits

MCC = Mobile country code.

MNC = Mobile Network Code.

LAC = Location area code. Identifies a location area within a GSM PLMN network. The maximum length of LAC is 16 bits. Thus 65536 different LA can be defined in one GSM PLMN.

Location Update


Normal location update

Periodic location update

IMSI attach

Normal Location UpdateNormal Location Update

Mobile powers on and is idle.

Reads the LAI broadcast on the BCCH.

Compares with the last stored LAI and if it is different does a location update.

Location Update Types


IMSI AttachIMSI Attach Saves the network from paging a MS which is not active in the system.

When MS is turned off or SIM is removed the MS sends a detach signal to the Network. It is marked as detached.

When the MS is powered again it reads the current LAI and if it is same does a location update type IMSI attach.

Attach/detach flag is broadcast on the BCCH sys info.

Periodic Location UpdatePeriodic Location Update Many times the MS enters non-coverage zone.

The MSC will keep on paging the MS thus wasting precious resources.

To avoid this the MS has to inform the MSC about its current LAI in a set period of time.

This time ranges from 0 to 255 deci-hours.

Periodic location timer value is broadcast on BCCH sys info messages.


ReflectionReflection

Occurs when a wave impinges upon a smooth surface.

Dimensions of the surface are large relative to .

Reflections occur from the surface of the earth and from buildings and walls.

Diffraction (Shadowing)Diffraction (Shadowing)

Occurs when the path is blocked by an object with large dimensions relative to

and sharp irregularities (edges).

Secondary “wavelets” propagate into the shadowed region.

Diffraction gives rise to bending of waves around the obstacle.

ScatteringScattering

Occurs when a wave impinges upon an object with dimensions on the order of

or less, causing the reflected energy to spread out or“scatter” in many directions.

Small objects such as street lights, signs, & leaves cause scattering.

Propagation Mechanisms


Multiple Waves Create “Multipath”

Due to propagation mechanisms, multiple waves arrive at the receiver

Sometimes this includes a direct Line-of-Sight (LOS) signal

Multipath


Multipath Propagation

Multipath propagation causes large and rapid fluctuations in a signal

These fluctuations are not the same as the propagation path loss.

Multipath causes three major things

Rapid changes in signal strength over a short distance or time.

Random frequency modulation due to Doppler Shifts on different

multipath signals.

Time dispersion caused by multipath delays

These are called “fading effects

Multipath propagation results in small-scale fading.

Multipath


The communication between the base station and mobile station in mobile systems is mostly non-LOS.

The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.

The mobile station is also moving in different directions at different speeds.

The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.

This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and Raleigh and Rican fading, which degrades the performance of the RF channel.

What is Fading?

Sig

nal

Po

wer

(d

Bm

)

Large scale fading component

Small scale fadingcomponent


This phenomenon is due to multipath propagation of the signal.

The Raleigh fading is applicable to obstructed propagation paths.

All the signals are NLOS signals and there is no dominant direct path.

Signals from all paths have comparable signal strengths.

The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.

Rayleigh fading


This phenomenon is due to multipath propagation of the signal.

In this case there is a partially scattered field.

One dominant signal.

Others are weaker.

Ricean fading


Receive diversity provides an effective technique for both overcoming the impact of

fading across the radio channel and increasing the received signal to interference ratio.

The former is achieved by ensuring “uncorrelated” (i.e. low enough correlated) fading

between antenna branches i.e. not all antennas experience fades at the same time.

Diversity

Building

Building

Building


In a typical cellular radio environment, the communication

between the cell site and mobile is not by a direct radio path

but via many paths.

The direct path between the transmitter and the receiver is

obstructed by buildings and other objects.

Hence the signal that arrives at the receiver is either by

reflection from the flat sides of buildings or by diffraction

around man made or natural obstructions.

When various incoming radio waves arrive at the receiver

antenna, they combine constructively or destructively, which

leads to a rapid variation in signal strength.

The signal fluctuations are known as ‘multi-path fading’.

Need of Diversity


Time diversity

Coding, interleaving

Frequency diversity

Frequency hopping

Space diversity

Multiple antennas

Polarization diversity

Dual-polarized antennas

Multi-path diversity

Equalizer

t

f

Kind of Diversity


Interference is the sum of all signal contributions that are neither noise not the wanted signal.

Interference


Interference is a major limiting factor in the performance of cellular systems.

It causes degradation of signal quality.

It introduces bit errors in the received signal.

Bit errors are partly recoverable by means of channel coding and error correction mechanisms.

The interference situation is not reciprocal in the uplink and downlink direction.

Mobile stations and base stations are exposed to different interference situation.

Effects of Interference

Source of Interference

Another mobile in the same cell.

A call in progress in the neighboring cell.

Other base stations operating on the same frequency.

Any non-cellular system which leaks energy into the cellular frequency band.


TYPES OF INTERFERENCETYPES OF INTERFERENCE

There are two types of system generated interference

Co-channel interference

Adjacent channel interference

Co-Channel InterferenceCo-Channel Interference

This type of interference is the due to frequency reuse , i.e. several cells use the same set of frequency.

These cells are called co-channel cells.

Co-channel interference cannot be combated by increasing the power of the transmitter. This is because an increase in carrier transmit power increases the interference to neighboring co-channel cells.

To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance to provide sufficient isolation due to propagation or reduce the footprint of the cell.


Adjacent-Channel InterferenceAdjacent-Channel Interference

Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent channel interference.

Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak into the passband.

Adjacent channel interference can be minimized through careful filtering and channel assignments.

By keeping the frequency separation between each channel in a given cell as large as possible , the adjacent interference may be reduced considerably.


General packet radio service-GPRS The first phase of GSM network architecture enhancements

that allow mobiles to connect to IP or X.25 networks.

Characteristics of GPRS:

Packet-switched

Data rate: 9Kbps ~ 150Kbps

New functionalities: point-to-point data transferring,

routing, logical link management, radio resource

management

Modulation: GMSK


Characteristics

Uses 200kHz carrier/multi-slot operation, time slot

structure

Modulation: 8PSK(8-phase Shift Keying)

modulation(3bits per modulated symbol) <->

apposed to the 1-bit per symbol GMSK in

GSM/GPRS

Data rates: 384Kbps

Enhanced Data Rates for GSM Evolution


57.6 kbps

115 kbps

384 kbps

2 Mbps

GSM

HSCSD

GPRS

EDGE

IMT-2000

9.6 kbps

2G

2.5G

3G

GSM Development Evolution


The basic idea of GSM

The features of GSM

The structure of GSM

Certain service area and numbers

Handover, Frequency hopping and

Location Updates.

Propagation Mechanisms

Evolution of GSM

www.huawei.com

Thank You

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