GSM System Survey 88

8/3/2019 GSM System Survey 88

http://slidepdf.com/reader/full/gsm-system-survey-88 1/88



MOBILE TELEPHONY

• rs genera on sys ems were ana og w reasona yreliable networks but limited service offerings and did notpermit roaming between networks

• Second generation (2G) mobile systems are digital and bring

significant advantages in terms of service sophistication,capac y an qua y

• General Packet Radio Services (GPRS) is an example of a

technology enabling mobile use of the Internet• 2G systems uses incompatible radio technologies, on different

frequency spectrum, they cannot capture a real worldwidemass-market in the long-term

systems which will allow communication, information andentertainment services to be delivered via wireless terminals

• An example of a 3G system is Universal MobileTelecommunication Systems (UMTS).



HISTORY OF WIRELESS COMMUNICATION

1906 Reginald Fesseden successfully transmits human voice over radio. Until

then, radio communications consisted of transmissions of Morse Code

1915J. A. Fleming invents the vacuum tube making it possible to build mobile

radiosmp u e o u a on , wo-way mo e sys ems – a up ex

1935Invention of frequency modulation ( FM ) improved radio quality ,

re uires less ower to o erate

1981Ericsson launches the world's first cellular system in Saudi Arabia based

on the analog NMT 450 standard.

1991 The first digital cellular standard (GSM) is launched

1999 The GPRS packet data standard released.

2000First 3G test networks built in Monaco, Isle of Man and Sweden and first

3G licenses auctioned in UK.

2001 First successful test calls made on 3G networks

2002 TDMA networks in the Americas moving to GSM



GSM PHASES ( Services )•

• Voice telephony

• International roaming

• Basic fax/data services u to 9.6 kbits/s

• Call forwarding

• Call barring

• Short Message Service (SMS)

• Phase 2• Advice of charge

• Calling line identification

• Call waiting• Call hold

• Conference calling

• Closed user groups

• Additional data communications capabilities

• Phase 2+• Multiple service profiles

• Private numbering plans

• Access to Centrex services

• Interworkin with GSM 1800 GSM 1900 and the Di ital Enhanced Cordless Telecommunications(DECT) standard





The GSM network is divided into two Systems

AUCSSSwitching System

HLR EIRVLR

MSC

Othernetworks GMSC

BSSNMC and OMC

Base Station System

BSC

BTS

Signaling transmission

Call connections andsignaling transmission

MS

System Model



SS

ILRAUC

EIRGMSC

HLR

SMS-GMSC

MSC/

VLR

DTI

-



AUTHENTICATION CENTER (AUC)

e pr mary unc on o an s o prov e n orma on, w cis then used by an MSC/VLR to perform subscriber

authentication and to establish ci herin rocedures on theradio link between the network and MSs. The informationprovided is called a triplet and consists of:

• A non predictable Random number (RAND)• A Signed Response (SRES)• A ciphering Key (Kc)



AUTHENTICATION CENTER (AUC)

Authentication Procedure

1. RAND

3. SRES

MS2. MS calculates SRES using RAND + Ki

(SIM-card) through A3 and Kc using RAND+Ki

4. Compare SRES received fromMS with SRES in triplet. If they

..

MSC/VLR Mobile service Switching Center

MS Mobile StationRAND Random numberSRES Signed Response



Authentication in GSM

HLRAUCRandom number (RAND) Signal response (SRES)

RAND / SRES Code key (Ki)

MSC

RAND

SRES



Encryption in GSM

HLRAUCRandom number (RAND)

Encryption key (Kc)

Subscriber number (IMSI) Code key (Ki)

RAND / Kc

VLR

RAND MS

Kc

BSC11Kc

BTS



MOBILE SERVICES SWITCHING CENTER /

• Each MSC has a VLR, VLR Stores Temporarily data for,

IMSI

Mobile Station Roaming Number

oca o e a on en y

Temporary Mobile Station Identity

ocat on area o t e o e

Supplementary services



MOBILE SERVICES SWITCHING

• Switching and call routing

• Char in

• Service provisioning

• Communication with HLR

• Communication with the VLR

• Communication with other MSCs

• Control of connected BSCs



HOME LOCATION REGISTER (HLR)

• Stores the details of all subscribers in the network such as :

Location information : mobile station roaming number,

VLR, MSC International mobile subscriber identity (IMSI)

MS ISDN number

Service restrictions

Su lementar services

• There is only on HLR in the network ( it may consist of



GATEWAY MOBILE SERVICES

• Routes Traffic entering a mobile network to the correct

destination• ccess t e to n t e ocat on o t e requ re mo e

subscriber

• The operator may decide to assign more than one GMSC



EQUIPMENT IDENTITY

• It’s a Database store uni ue International Mobile E ui ment

Identity number (IMEI), it have 3 levels :

White-listed : Terminal is allowed to use the network.

Gray-listed : Terminal is under observation by the network

for possible problems. Black-listed : the terminal can’t use the network (e.g.

Stolen mobiles).



INTERWORKING LOCATION

• Makes inter-system roaming possible, meaning that a

subscriber can roam from a GSM 1900 network to an AMPSne wor

•possible in both directions between all GSM/AMPS/TDMA

networks



DATA TRANSMISSION

• The DTI is a hardware platform which implements

the GSM-defined InterWorking Function (IWF).

conversion.



TRANSCODER CONTROLLER (TRC)

• The purpose of a TRC is to multiplex network traffic

channels from multiple BSCs onto one 64 kbits/sPCM channel which reduces network transmission

costs. The TRC can be combined with the BSC or

exist as a stand-alone node.



ase a on ys em omponen s

BSC

BTS

Base Station System



• Allocates a channel for the duration of a call.

• Maintains a call :

Monitorin ualit .

Controlling the power transmitted by the BTS or MS.

Generatin a handover to another to another cell when

required. Antennas and feeder cables.

• Positioning the BTS is important for radio coverage.



BASE TRANSCEIVER STATION

(BTS)• The purpose of BTS is :

Provide radio access to mobile stations

Manage the radio access aspects to the system

• BTS Contains :

Radio transmitter / Receiver (TRX)

gna process ng an con ro equ pmen

Antennas and feeders cables

'product called RBS

• Ericsson offers a wide ran e of RBSs for use in GSMnetworks e.g. :

BS 2103

RBS 2106

RBS 2207



NETWORK MONITORING CENTERS

An OMC is a computerized monitoring center which is

connected to other network com onents

2. Network Management Center (NMC)

Centralized control of a network is done at a Network Management Center (NMC)



Mobile Station (MS)

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

.

MobileEquipment

SubscriberIdentityModule



Mobile Station (MS)

The MS is the equipment used to access the network.The MS consists of two independent parts:

• Subscriber Identity Module (SIM) card

•

A SIM card is an electronic smart card which stores

n ormat on a out t e su scr pt on. e s t e actuatelephone terminal.



SIM Subscriber Identit Module

The SIM stores three types of subscriber relatedn ormat on:

• Fixed data stored before the subscription is sold:e.g. IMSI,authentication key and securityalgorithms

• Temporary network data: e.g. the location area ofthe subscriber and forbidden PLMNs.

• Service data: e.g. language preference, advice of

charge.



GSM GEOGRAPHICAL NETWORK

Every telephone network needs a specific structure to route

subscriber

CELL A cell is the basic unit of a cellular system and is defined as the

area of radio coverage given by one BS antenna system. Each

(CGI).

LOCATION AREA (LA)

A Location Area LA is defined as a rou of cells The identit

of the current LA is stored in the VLR.



MSC Service Area

An MSC service area is made up of a number

LA1LA3

of LAs and represents the geographical part

of the network controlled by one MSC

cell 6LA5

LA4

VLR

cell 1 cell 5

cell 4 cell 2 cell 3

PLMN Service Area

A Public Land Mobile Network (PLMN) service area is the

entire set of cells served by one network operator and is defined

to its network



GSM SERVICE AREA

GSM Service Area

PLMN Service Area

(one per operator)

MSC Service Area

Location Area

Cell

The GSM service area is the entire geographical area in which a

subscriber can gain access to a GSM network.



"Hardware" view of a Sample

MSC Service Area 2

MSC Service Area 1

BSC 1C

BSC 2C

BSC 1BBSC 2B

MSC/VLR 2BSC 2A

HLRAUC

MSC/VLR 1PSTN

BSC 1A

EIRGMSC

ILR

LEGEND

MSC Boundary

BSC Boundary

PCM LinksBase Station



"Software" view of a Sample Network

LA 2-ACell 2-A-25

MSC Service Area 2

MSC Service Area 1

LA 1-B

-

LA 2-B

LA 2-C

LA 2-DMSC/VLR 2

HLR

EIR

AUC

GMSC

MSC/VLR 1PSTN

ILR

LEGEND

MSC Boundary

BSC Boundary

n s

Base Station



GSM FREQUENCY BANDS

Used forcoverage

United

Used forcoverage

other

Used for

Capacity

Used for

Capacity

state &Japan )

countries )

countries )

countries )



Frequency bands for the different

- ase networ s. a ueswithin brackets refer to E-GSM)

Network type Ericsson’sFrequency band

ImplementationsUL/DL

- -

GSM 1800 1710 - 1785/1805 -1880 MHz GSM 1800

GSM 1900 1850 - 1910/1930 -1990 MHz GSM 1900



KEY TERMS

The primary terms used to describe

call cases and situations involving an

are set out e ow.

An MS can have one of thefollowing states:

• Idle: the MS is ON but a call is not

in progress

•

in progress

The following table defines key

terms used to describe GSM traffic

cases ere are no ra c cases n

detached mode):



MS REGISTRATION AND

ROAMING

• When the MS has just been powered on, the MS must register

with the network which will then update the MS's status to idle.

• When the user is outside of your service provider’s local

coverage area or ome ca ng area, t s user s n roam ng.

Roaming arrangements between service providers allow you to



ERICSSON'S GSM SYSTEM

FREQUENCY



• FREQUENCY

An MS communicates with a BTS b transmittin or receivin radio

waves, which consist of electromagnetic energy. The frequency of a radio

wave is the number of times the wave oscillates per second. Frequency is

measured in Hertz (Hz).

• Wavelength

.

electromagnetic waves can be described by a sinusoidal function,

which is characterized b wavelen th.

• BANDWIDTH

Bandwidth is the term used to describe the amount of frequency range

allocated to one application.

A channel is a frequency or set of frequencies which can be

allocated for the transmission.



Uplink and Downlink on a

a o anne



Duplex Distance

The use of full duplex requires that uplink and downlink

transmissions are separated in frequency by a minimum distance,

called duplex distance.

ou , up n an own n requenc es wou n er ere w

each other.

890 960915 935MHzUplink Downlink

Duplex Distance (45 MHz)

Bandwidth (2 x 25 MHz)



Carrier Se aration

This is the distance on the fre uenc band between channels

being transmitted in the same direction.

895.4 895.6MHz



Capacity and Frequency Re-use

• ac company w t a cense to operate a mo e networ sallocated a limited number of frequencies.

•

B

avoided.

G

A

C

G

AD

F

F

E

• Frequencies must be re-used many times at different

sufficient capacity.



ACCESS METHOD: TIME DIVISION

MULTIPLE ACCESS (TDMA)

,carrier at designated periods in time.

• These eriods of time are referred to as time slots.





ANALOG AND DIGITAL

TRANSMISSION•

An analog signal is a continuous waveform which changes in

represented.

• Digital Signals

or mo e sys ems, g a s gna s may e cons ere o e se s

of discrete waveforms.

Advantages Of Using Digital



Advantages Of Using Digital

, ,over distances. In analog,the only solution to this

distortion is also amplified. In digital, the signalcan be completely regenerated as new, withoutthe distortion.

Advantages of Digital (cont)



Advantages of Digital (cont)

Therefore, digital signals are more immunity to

noise.

In digital signals more signaling operations is

allowed like Encr tion Multi-media

messages….etc

Using Digital signals is more faster…

Disadvantages of Digital



Disadvantages of Digital

The problem with using digital signals to transfer

missing due to the technique of taking samples.

However, the more often the samples are taken, the

closer the resultin di ital values will be to a true

representation of the analog information.

Moreover, digital signals require more bandwith thananalog signals.



Path loss

Shadowing Rayleigh fading

Signal strength

Time Alignment

PATH LOSS



PATH LOSS

Path loss occurs when the received signal becomesweaker and weaker due to increasing distancee ween an , even ere are no o s ac es

between the

.

,before the problem becomes extreme, a new

SHADOWING



Shadowing occurs when there are physical obstaclesincluding hills and buildings between the BTS and theMS. The obstacles create a shadowing effect whichcan ecrease e rece ve s gna s reng .

CARRIER TO INTERFERENCE C/I



Reusing an identical carrier frequency in different- .

Co-channel interference is the relation between the

desired si nal C and the undesired re-used si nal I, both using the same carrier frequency.

CARRIER TO ADJACENT C/A



Some of the energy of the adjacent frequency will leak into theserving cell and cause interference. The relation between the

from the carrier 200 kHz away is called adjacent channelinterference or C/A.

MULTI PATH FADING



MULTI PATH FADING

transmission path to the MS or BTS, and therefore more

than one si nal is arrivin at thereceiver. This may be due to buildings or mountains,either close to or far from the receiving device.

Ra lei h fadin and time dis ersion are forms ofmultipath fading.

RAYLEIGH FADING



This occurs when a signal takes more than one path betweenthe MS and BTS antennas. In this case, the signal is notrece ve on a ne o s g t pat rect y rom t e x antenna.Rather, it is reflected off buildings, for example, and is received

from several different indirect paths. Rayleigh fading occurswhen the obstacles are close to the receiving antenna.

TIME DISPERSION



Time dispersion is another problem relating tomultiple paths to the Rx antenna of either an MS or.

Time dispersion causes Inter-Symbol Interference

each other making it difficult for the receiver todetermine which symbol is the correct one.

TIME ALIGNMENT



Each MS on a call is allocated a time slot on a TDMA frame.This is an amount of time during which the MS transmitsn ormat on to t e .

The information must also arrive at the BTS within that time

slot. The time ali nment roblem occurs when art of theinformation transmitted by an MS does not arrive within theallocated time slot. Instead, that part may arrive during the next

,

using that other time slot.



Duplex Maximum band, B, (MHz) to avoid IM order:

System distance,D, (MHz) IM3 IM5 IM7 IM9

DCS1800 95

.

47.5

.

31.7

.

23.8

.

19.0

PCS1900 80 40.0 26.7 20.0 16.0

. . . .

Definitions:

ors -case re a onsIM3 in Rx band if D/B < 2IM5 in Rx band if D/B < 3

3, 5, etc.= ntermo u at on pro ucts o 3r , 5t ,etc.,D = Duplex distance (MHz)

=7

IM9 in Rx band if D/B < 5

“Worst case” = IM from the lowest and

the highest frequencies in the allocated band

COMBINED SIGNAL LOSS



An illustration of what the signal strength may look like at theMS

Rx antenna when movin awa from the BTS Tx antenna is shown in the figure . The problems of path loss, shadowing andRayleigh fading are present for this transmission path.

Rx Signal Strength



At any one point from the Tx antenna, the receivedsi nal can look like the si nal in Fi ure below.

SOLUTIONS OF TRANSMISSION



PROBLEMS• Fadin

- Distance : Eventually leading to handover

- Shadowing : Make a better coverage of the cell or smaller cells

- Rayleigh : Antenna diversity (Space or Polarization Diversity) -Two antennas on the BS 5-6 m a art if S ace Diversit is used.

Repeated change of frequencies after each frame

• Interference

, , …

CHANNNEL CODING



In digital transmission, the quality of the transmitted

signal is often expressed in terms of how many of the.

Received bits 1 0 0 1 0 0 1 0 1 0

rrors =

BER

Channel coding is used to detect and correct errors in a

. .bits enable a channel decoder to determine whetherthe message has faulty bits, and to potentially correctthe faultybits.

INTERLEAVING



A process called interleaving is used to separate

transmitted in a nonconsecutive way.

Advantages of Interleaving



If interleavin is used If one block is lost intransmission, again there is a 25% BER overall.

However, this time the 25% is spread over the entireset of message blocks, giving a 25% BER for each.This is more manageable and there is a greater

errors.

ANTENNA DIVERSITY



Antenna diversity increases the received signalstren th b takin advanta e of the natural ro ertiesof radio waves.

SPACE DIVERSITY POLARIZATION DIVERSITY

SPACE DIVERSITY



POLARIZATION DIVERSITY



With polarization diversity the two space diversityan ennae are rep ace y one ua po ar ze an enna.

This antenna has normal size but contains two

.types are vertical/horizontal arrays and arrays in ±45degree slant orientation.

For most a lications, the difference between thediversity gain for space diversity and polarizationdiversity is negligible, but polarization diversity reduces

t e space requ re or t e antenna.

FREQUENCY HOPPING

R l i h f di i f d d Thi



Rayleigh fading is frequency dependent. This means

that the fading dips occur at different places for. ,

possible for the BTS and MS to hop from frequency to

fre uenc durin a call. The fre uenc ho in of theBTS and MS is synchronized.

Types of frequency hopping

Base-band F.H Synthesizer F.H

hopping between hopping fromfrequencies on

differentfrequency to

Frequency onransce vers n a ce . e same ransce ver

in a cell.

Frequency Hopping (cont)



TIMING ADVANCE



Timing advance is a solution specifically designed to counteractthe

pro em o me a gnmen . wor s y ns ruc ng e m s-a gneMS to transmit its burst earlier or later than it normally would.

In GSM, the timin advance information relates to bit times. Thus,an MS may be instructed to commence its transmission a certainnumber of bit times earlier or later, related to previous position, to

.



Analog to Digital Conversion



One of the primary functions of an MS is to convert the

transmission using a digital signal.

e convers on s per orme y us ng a process ca e

Pulse Code Modulation (PCM). PCM involves three main

• Sampling

• Coding

Sampling



Sampling involves measuring the analog signal atspecific time intervals.

The sampling theory states that:

To reproduce an analog signal without distortion, the signal

must be sampled with at least twice the frequency of the highest frequency component in the analog signal.

Quantization



The next step is to give each sample a value. For this,

is measured and approximated to one of a finite set of

values.

Coding



Coding involves converting the quantized values intobinary. Every value is represented by a binary code of

13 bits (213 = 8192). For example, a quantized value of2,157 would have a bit pattern of 0100001101101:

Summary For A/D Conversion



The result from the rocess of A/D conversion is 8 000samples per second of 13 bits each. This is a bit rate of104 kbits/s.

When it is considered that 8 subscribers use one radiochannel, the overall bit rate would be 8 x 104 kbits/s = 832

kbits/s. Recalling the general rule of 1 bit per Hertz, this bit

subscribers.

The bit rate must be reduced somehow - this is achieved.

SEGMENTATION



The key to reducing the bit rate is to send information about thespeech instead of the speech itself.

Segmentation: Given that the speech organs are relatively slow in

ada tin to chan es, the filter arameters re resentin the s eechorgans are approximately constant during 20 ms. For this reason,when coding speech in GSM, a block of 20 ms is coded into one set

. ,per second instead of the 8,000 used by A/D conversion.

SPEECH CODING



Instead of using 13 bits per sample as in A/Dconversion, GSM speech coding uses 260 bits.This calculates as 50 x 260 = 13 kbits/s.

acceptable for mobile telephony and- .

Types Of Speech Coding



Many types of speech coders are available. Some offer betterspeech

quality, at the expense of a higher bit rate (waveform coders).Others use lower bit rates, at the expense of lower speech quality

vo-coders . The h brid coder which GSM uses rovides oodspeech quality with a

relatively low bit rate, at the expense of speech coder complexity.

Summary of Segmentation



The GSM speech coder produces a bit rate of 13 kbits/sper subscriber. When it is considered that 8 subscribersuse one radio channel, the overall bit rate would be 8 x 13

s s = s s. s compares avora y w e832 kbits/s from A/D conversion.

However, speech coding does not consider the problems

path. The next stages in the transmission process,

channel coding and interleaving, help to overcome

these problems.

CHANNEL CODING



Channel coding in GSM uses the 260 bits from speechco ng as nput to c anne co ng an outputs 456

encoded bits.

INTERLEAVING

First level of interleaving



g

The channel coder provides 456 bits for every 20 ms of speech

w c are nter eave n e g t oc s o ts s own e ow.

NORMAL BURST



In a normal burst there is s ace for two of thesespeech blocks

Thus, if one burst transmission is lost, there is a 25%BER for the entire 20 ms of speech (2/8 = 25%).

2nd LEVEL OF INTERLEAVING



If only one level of interleaving is used, a loss of this burstresults in a total loss of 25%. This is too much for thechannel decoder to correct. A second level of interleavingcan be introduced to further reduce the possible BER to

12.5%.

Instead of sending two blocks of 57 bits from the same 20

ms of speech within one burst, a block from one 20 ms.

INTERLEAVING (cont)



CIPHERING/ENCRYPTION



The purpose of ciphering is to encode the burst so

that it cannot be interpreted by any other device than

the intended receiver. The ciphering algorithm inGSM is called the A5 algorithm. It does not add bits

burst, meaning that the input and output to the

per 20 ms.

BURST FORMATTING

The process of burst formatting is to add 26 training



sequence bits to the basic speech/data (57+57=114 bits)e ng sen . onsequen y s ncreases e urs rom

to 148 bits, thus increasing the transmission rate on the

,on the radio path.

Each time slot on a TDMA frame is 0.577 ms long. Thisrovides enou h time for 156.25 bits to be transmitted buta burst only contains 148 bits. The rest of the space, 8.25bit times, is empty and is called the Guard Period (GP).

The output of burst formatting is a burst of 156.25 bits (oneburst) or 625 bits(four bursts) for 20 ms sample. The

transmission bit rate for GSM

MODULATION TECHNIQUE



carrier frequency. As previously explained, GSMuses the GMSK modulation techni ue. The bits aremodulated onto a carrier frequency and transmittedover the air.

GSM Transmission Process



Documents

GSM System Survey 88