GSM_Networks CU Presentation

8/2/2019 GSM_Networks CU Presentation

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GSM Cellular System And GPRS

Technology

By

Seyhun Barbaros YABACI Ersin ADALDecember 2010


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Cellular Systems - Background

1G - First Generation NMT(Nordic Mobile Telephone) ,AMPS(Advanced Mobile Phone System)TACS (Total

Access Communications System)

Narrow Band Analog Systems,

Voice Services, no handover.

Now Obsolote

2G - Second Generation GSM, CDMA ONE, CDMA2000, PDC

%80 of Currently users are GSM users.

Digital, Coded Voice, Voice and Additional Services, SMS.

FDMA , TDMA, CDMA

3G - Third Generation

WCDMA UMTS, CDMA2000(1xEVDO) Voice, Video, Streaming, Data services

Wideband, WCDMA FDD and TDD

4G Fourth Generation

LTE, OFDMA, all IP system, Wideband.


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GSM Definition

GSM: Global System for Mobile Communication

A Narrow Band Digital Cellular System.

Standardized by ETSI(European Telecommunications

Standards Institute), www.etsi

.org

3GPP TS 45.005 - GSM/EDGE Radio Access Network; Radio

transmission and reception

3GPP TS 45.001 - Physical layer on the radio path; Generaldescription
http://www.etsi.org/http://www.etsi.org/http://www.etsi.org/http://www.etsi.org/http://www.etsi.org/http://www.etsi.org/


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GSM-UMTS Network Architecture


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Base Station System

The Base Station Controller (BSC)

Manages BSS operations as an intelligent point. Provides Mobile station connection handlingand Radio NW management.

Transcoding and Rate adaptation.

BTS transmission management and remote control.Base Transceiver Station (BTS)

Consisted of one or more GSM cells, performs radio releated functions, mamaged by BSC.

Has radio and transmission interfaces

MS (Mobile Station)

Consists of Mobile Terminal and SIM(Subsiscriber identitiy module). Mobile terminal providesradio access functions via air interface. SIM stores IMSI number that used for authenticationprocesses..


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Switching System Mobile Services Switching Centre (MSC): Setup, routing and

supervision of calls to and from mobile subscribers. Performs

suthentication processes.

Visitor Location Register (VLR): Contains non-permanent

information about the mobile subscribers in its location area.

Gateway Mobile Services Switching Centre (GMSC): Routes

incoming calls from other networks to the MSC.

Home Location Register (HLR): HLR holds information of all

subscribers of its PLMN. (Such as IMSI and location of

subscribers)

Authentication Center (AUC): Provides authentication and

encryption parameters for subscriber verification and call

confidientality.

Equipment Identity Register (EIR): Checks the mobile

equipment IMEI and prevents barred equpment usage.

Serving GPRS Support Node (SGSN): Center of packet switched

operations , performs similar functions as MSC for GPRS.

Gateway GPRS Support Node (GGSN):Gateway point to other

packet switching networks, functions as similar to GMSC.


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GSM Frequeny Bands

GSM is a narrow band, full duplex system.

Downlink: BTS transmit MS receive.

Uplink: MS transmit BTS receive.


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ARFCN

Fixed designation of ARFCN

Dynamically mapped ARFCN

Absolute Radio-Frequency Channel Number : A code that

specifies a pair of physical radio carriers and channels used fortransmission and reception on the air interface.


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P-GSM , DCS-1800

Duplex Distance: 45 Mhz , 95 Mhz

Carrier Seperation: 200 KHz

For GSM 900 , 124 channels

For GSM 1800 , channels

GSM bands in Trkiye


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FDMA and TDMA GSM system uses two radio access methods for air interface, these two system

combined to achieve better spectrum efficiency.

FDMA (Frequency division multiple access): For FDMA system frequency banddivided to number of channels by seperation distance 200 Khz.

TDMA (Time division multiple access): A TDMA frame consists of 8 Time Slot. Eachtime slot corresponds to a physical channel. Logical channels of GSM mappedon these physical channels.

The Modulation type of GSM is GMSK(Gaussian minimum-shift keying). GMSK is a

continuous-phase frequency-shift keying modulation scheme. This modulation

type sends one bit per symbol.


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Logical channels

GSM LOGICAL CHANNELS

Control Channels

Broadcast Channels(BCH)

FCCH SCH BCCH

Common Control Channels(CCCH)

PCH RACH AGCH

Dedicated Channels (DCH)

SDCCH SACH FACH

TrafficChannels

TCH

FULLRATE HALFRATE

Uplink

Uplink -Downlink

Downlink

FCCH : Frequency Correction Channel

SCH: Synchronization Channel

BCCH: Broadcast Control Channel

PCH: Paging Channel

RACH: Random Access Channel

AGCH: Access Grant Channel

SDCCH: Stand Alone Dedicated Control Channel

SACCH: Slow Associated Control Channel

FACCH: Fast Associated Control Channel


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Mapping of Logical Channels Onto

Physical Channels Several logical channels can share the same physical channel or Time Slot.

On TS 0 (on one carrier per cell, the BCCH-carrier) the broadcast channels

and the common control channels are multiplexed.

Dedicated control channels and traffic channels mapped onto TS 1 to TS 7.

1 TDMA frame = 8 Time Slot = 4,615 ms


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Registering The Mobile Network

GSM network consists of many neighbor cells that creates continious air interface. In order to

communicate to the network, a mobile station must choose best serving cell in its location.

1. When a MS powered on, it firstly measures the BCCH carriers signal strengths of its

PLMN(Public Land Mobile Network) cells. If it can measure more than one allowed BCCH

carriers, makes a decision on basically measured signal strengths to camp on a cell.

2. MS receives system information from BCCH carrier than registers itself to the GSM network.

3. MS request an SDCCH channel via RACH. Then sends an IMSI attach message via SDCCH to

the VLR.

4. VLR analyzes IMSI and changes its flag on database to attached. Then sends a message to

MS that attaching process completed.


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Call setup Mechanism Calls From MS

1. Dedicated Channel Request from MS via RACH channel.2. Getting dedicated channel information from AGCH channel.

3. MS service(call) request via SDCCH channel

4. Authentication performed

5. Ciphering may be initiated

6. MSC recevies setup message from MS

7. A PCM TS allocated between MSC and BSC. BSC assigns a TCH for call. Traffic

control system sets up to connection to target subscriber.8. The ringing tone sent to MS.

9. When subscriber B answers, network sends a connect message to the MS, MS

completes the call setup


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Call setup Mechanism Calls to MS

1. When a call is made from another network(i.e PSTN) the exchange analysis the

number, the call routed to the GMSC in the PLMN .2. GMSC finds the HLR that MSISDN (Mobile Subscriber Integrated Services Digital

Network Number) registered in and gets the MSC/VLR information which called

MS registered. HLR finds out IMSI( International Mobile Subscriber Identity)

number of MS via MSISDN.

3. By rquest of HLR, VLR generates a roaming number , this number forwarded to

GMSC via HLR.

4. GMSC routes the call to appropriate MSC.5. MSC sends a paging message to BSC, and BSC pages for location area of called MS.

6. BSC sends paging message to BTS of appropriate location area.

7. BTS sends paging message via air interface with IMSI or TIMSI of mobile.

8. MS detects its identity on paging message and sends a request for SDCCH channel

allocation. MSC starts authentication and ciphering process.

9. BSC orders to BTS activate TCH and release the SDCCH. MS ordered to tune in TCH

channel . A ringing tone created in MSC and sent to calling number.


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Cellular System

GSM network consists of many neighbour cells that creates continious air interface.

Each cell has its coverage area and users can change their serving cell both in active and

idle mode.


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SEYHN1

SEYHN2

SEYHN3

ELEKT1

ELEKT2

ELEKT3

INDOR1

BARBA1

BARBA2

BARBA3

BARBA4

YABAC1

YABAC2

YABAC3

AHMET1

AHMET2

AHMET3

KEREM1

KEREM2

KEREM3

EMIR1EMIR2

ERSIN1

ERSIN2

ERSIN3

Cellular System Example Locations , Urban Areas


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ADATA1

ADATA2

ADATA3

GEKOY1

GEKOY2

GEKOY3

CAMDA1

CAMDA2

CAMDA3

KARTA1

KARTA2

KARTA3

KARTA4

TUZLA1

TUZLA2

TUZLA3

TUZCA1

TUZCA2

YOLKS1

YOLKS2

YOLKS3

CELMI1

CELMI2

CELMI3

Cellular System Example Locations , Rural Areas


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Frequency Planning - Reuse of Available

Frequencies

Since the capacity of a single cell is limited by available number of frequencies,the needing coverage area divided to many cells. Each cell uses a different set of

frequencies of its neighbours to prevent interference.


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1,3,5,7,9,11

22,24,28,16

2,4,6,18,22

10,12,14,16

18,22,24,28,30

2,4,6,8

INDOR1

BARBA1

BARBA2

BARBA3

BARBA4

3,5,7,9,11

2,4,6,8,10

10,12,14,,32

AHMET1

AHMET2

AHMET3

KEREM1

KEREM2

KEREM3

EMIR1EMIR2

ERSIN1

ERSIN2

ERSIN3

Frequency Planning Example


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Traffic Dimensioning

Cellular system capacity depends on;

Number of available channels

Target Grade of Service (GOS = Blocking Probability) level of Subcribers

Traffic expressed in Erlang;

E= h , where=Call arrival rate, h=Average call duration.

Erlang B formula gives;

How many traffic can be handled for desired GOS(Blocking Probability), for

available traffic channels.

Assuming an average subscriber creates 0,050 Erlang traffic, a cell with 4 traffic

channel at GOS 0.2 , can serve 1/0,050= 20 subscriber.

Erlang B Table


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Handover

In a cellular system the ongoing calls of the moving users must be carried between theneighbour cells to ensure the communication continuity. This process called handover.

The handover decision is a result of locating algorithm and given by network and

specifically on BSC. The signal strength measurements of serving and neighbor cells andnetwork parameter settings are used as inputs for locating algorithm.

The reasons that produces handover decision;

Decreasing field strength of the serving cell prior to neighbouring cell/cells.

Decreasing signal quality of serving cell.

Exceeding allowed timing advance limits for serving cell.

Parameter settings that favoured another cell.


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TEMS Output example


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GPRS(General Packet Radio Service) GSM is actually designed for carrying simultaneous duplex speech data, it is

based on the circuit switched system. GPRS is an extenison of GSM

architecture. To carry data packets on air interface we have to reserve some traffic channels

for this purpose. This data channels called PDCHs (packet data channels).

Packet data is coded in radio blocks which consists of four consecutive radioburst. A Radio block includes 456 bits. The number of information bits isdepend on Coding Schema(CS-1 to CS-4).

At the limit values, coding of GPRS System can carry maximum 20 kbit/s foreach time slot. But these are theoratical values, heavily depended upon radioconditions. To achieve more data rate, GSM terminals(e.g. MS) can use morethan one time slot at the same time.


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EGPRS (Enhanced GPRS)

To achieve higher data rates in same TS and frequency limitations, there is another

technology in GSM named EDGE. The main improvement on EDGE is modulation

scheme. Edge uses 8 PSK modulation, which allows transmitting 3 bits per symbol

versus 1 bit of the GMSK.

Coding Scheme Modulation Method Max data rate MCS family Max Data rate(Kbps)

MCS-9 8 PSK 59.2 A 236.8

MCS-8 8 PSK 54.4 A 217.6

MCS-7 8 PSK 44.8 B 179.2

MCS-6 8 PSK 29.6 A 118.4

MCS-5 8 PSK 22.4 B 89.6

MCS-4 GMSK 17.6 C 70.4

MCS-3 GMSK 14.8 A 59.2

MCS-2 GMSK 11.2 B 44.8

MCS-1 GMSK 8.8 C 35.2


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Antennas


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In order to restrain the serving area or provide better signal strength for highly

located antennas, antenna tilting is used.

Antennas can be tilted electrically or mechanically. Electrical tilting is more

effective and does not degenerates antenna pattern.

Antenna Downtilting


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To reduce the negative effect of fading dips and improve the receiver

performance pair of antennas used at receiver side. This is called diversity. Thereis two types of diversity used in GSM.

Space Diversity; Vertically polarized antennas placed in 12-18 (wavelength)

Polarization Diversity; Cross polarized antennas used. No space seperation

used.

Antenna Diversity


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Base Station Examples


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Suggested Reference Books

HEINE G. , 1998, GSM Networks, Protocols, Terminology and

Implementation, Artech House

MISHRA A. R. , 2004, Cellular Network Planning and

Optimization, John Wiley & Sons Ltd

HALONEN T. , ROMERO J. And MELERO J. , 2003, GSM, GPRS

and EDGE performance, John Wiley & Sons Ltd.


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Thanks

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