Introduction to GSM

Cellular Mobile Communications-

III Introduction to GSM Dr. Nasir D. Gohar

AN INTRODUCTION TO GSM  Second Generation Cellular Mobile Radio Systems-Digital Cellular Telephony:

 Main Advantages of Digital Cellular Networks

Higher Spectrum Efficiency  Speech Encoding  Better Access Technologies: TDMA and CDMA

Higher System Capacity  Aggressive Frequency Reuse

Low Infra-Structure and Terminal Cost  Application of VLSI/VVLSI Chips  Many Traffic Channels per Carrier Channel [Transceivers are shared among several users)  Low S/I Ratio (SIR) Allowing Smarter Equipment

Better Integration with Digital PSTN [ISDN] New Services such as Data Communications, Fax, and SMS Better Privacy due to Encryption

T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM  Digital Cellular Standards Table-L13.1: Overview of Second Generation Standards Region Europe

USA

Japan

Standard

Developing Organization

Remarks

GSM

ETSI

Harmonized European Standard

DCS-1800

ETSI

GSM Varient for 1800 MHz Band

D-AMPS, D-AMPS 1900 TIA [IS-54B, IS-136]

Digital AMPS works in 800 MHZ and 1900 MHz Bands

IS-95 CDMA

TIA/QualComm[IS-95]

Downward Compatible with AMPS, later modified to work with 1900 MHz Band

PCS-1900

ETSI

GSM Vairent to work with 1900 MHz Band

PDC

NTT

Works both in 800 MHz and 1500 MHz

 Why So Many Standards? T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM  Digital Cellular Standards

Table-L13.2: Overview of Important Technical Parameters of Digital Cellular Standards Parametrs Standard Standard

Frequency band(Mhz)

Air Interface Forward Ch.

Reverse Ch.

Duplex Separation

Access Tech. Channel BW

Mod. Rate

Speech Encoding

Tx Power of MS

H/O

GSM/DCS-1800/PCS1900

900/1800 /1900

935-960/1805- 890-915/17101880/19301785/18501970 1890

45/95/80

TDMA, 8 Time Slots, Later 16 Time Slots

200 KHz

271 Kbps

LPC,13 kbps

2W/1W/1W

MAHO

D-AMPS

800/1900

869-894/1930- 824-849/18501970 1890

45/80

TDMA, 3 Slots

30 KHz

48.6 Kpbps

VSELP, 7.95 kpps

1.2 W

MAHO

0.2 W

Soft Handover

0.8 W

MAHO

IS-95 CDMA

800/1900

869-894/1930- 824-849/18501970 1890

45/80

CDMA

1.25 MHz

PDC

800-1500

810-826/1477- 940-956/14291489 + 1501- 1441 + 14531513 1465

130/48

TDMA, 3 Slots, later 6 Slots

25 KHz


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1.2 to 14.4 Kbps QSELP, 1.2 to Transmitted 8 kbps at 1.339 Mchips/s 42 kbps

VSELP, 11.2 kbps

4

AN INTRODUCTION TO GSM  Overview of GSM Milestones  1982  1986 made.

 1987    

1989 1992 1994 1996

GSM Group, set up by CEPT, started Investigation to reserve a Spectrum in 900 MHz band for Pan-European Cellular Digital Telephony Two 25 MHz frequency Blocks [890-915 MHz, 935-960 MHz] reserved by EC, July 01, 1991 set as Introduction date. Choice of Digital and Narrow band [Approx. 200 KHz] TDMA 13 Operators from 12 European Countries signed MOU to Support GSM ETSI established which took over GSM from CEPT Introduction of first Commercial GSM Network 51 Commercial Networks Established More Than 20 Million Subscribers in 191 Networks, More than Double of all Subscribers of other Digital Systems [D-AMPS,

PDC, IS-95 CDMA]  2006 More than 2 billion subscribers (30% of world population and 82% of cellular market) T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM  Implementation Phases of GSM  Phase-1: Most Essential Services [Voice, Data, SMS] Implemented. Mostly all Current GSM Systems belong to this Phase.  Phase-2: Half-Rate Speech Encoding, Doubling the System Capacity, Several Encryption Algorithms, Conference Call Facility[ up to 5 Participants] , Call Waiting Facility, Information on Call Charges, Calling or Called Part etc., Cell-Broadcast [Point to Multi-Point SMS]. Many of the Phase-2 Facilities have already been Implemented by some Manufacturers.

 Phase-2 + GPRS[General Packet radio Service] for Packet Switched Data Transmission to Support LAN and Internet Traffic. [Considerably Delayed due to Lack of Interest on part of Network required for Infra-Structure and MS T.S. Rappaport Ch 11 Operators due to Large NDG Investment Notes 6

AN INTRODUCTION TO GSM 2G EVOLUTION TO 3G


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AN INTRODUCTION TO GSM

GSM EVOLUTION


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AN INTRODUCTION TO GSM  GSM SYSTEM ARCHITECTURE  MS

MANAGEMENT SUBSYSTEM

SIM Card EIN/ESN

Fault Management

Cost Management

 Base Station Subsystem (BSS)

BTS BTS BTS

Several Dozens of BTS

BTS BTS BTS

 Network &Switching Subsystem

MTX [up to 1 M users] Call set up , all Other


Security Management

Performance Management

B S C

HLR = Home HLR Location Register

MTX

under one BSC Each BTS has 3-5 Carrier Channels

Functions

Config. Management

B S C

BTS = Base Transceiver Station BSC = base Station Controller

MS

BASE STATION SUBSYSTEM[BSS]

GMTX = Gateway MTX IWF = Inter Working Function

VLR = Visitor VLR Location Register

GMTX/ IWF

To Other Networks

NETWORK & SWITCHING SUBSYSTEM

•GMTX: An Interface to Other Networks •HLR: Keeps Record of System’s Own Users •VLR: Keeps Temporary Record of Visitors / Roamers [SID, ST,SVCES] •Au C/EIR: Manages the Sub. Authentication and Encryption Data NDG Notes

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AN INTRODUCTION TO GSM  GSM SYSTEM ARCHITECTURE-2 MANAGEMENT SUBSYSTEM

 Management Subsystem [Operation Subsystem (OSS)]

Fault Management

Cost Management

BTS BTS

Directly or Indirectly

Connected to all Other Subsystems Fault Management System Configuration Performance Management Cost Management Security Management


Config. Management

BTS

Security Management

Performance Management

B S C

HLR = Home HLR Location Register

MTX BTS BTS BTS

B S C

BTS = Base Transceiver Station BSC = base Station Controller

MS

BASE STATION SUBSYSTEM[BSS]

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GMTX = Gateway MTX IWF = Inter Working Function

VLR = Visitor VLR Location Register

GMTX/ IWF

To Other Networks

NETWORK & SWITCHING SUBSYSTEM

10

AN INTRODUCTION TO GSM  GSM SYSTEM ARCHITECTURE-3  GSM Interfaces

Radio Interface:

Describes Data Interchange between MS and BSS Abis Interface: Describes Data Communications between BTS and BSC, allows Various Manufacturers Equipment A Interface: Describes Data Interchange between BSS and NSS


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AN INTRODUCTION TO GSM  LOGICAL

CHANNELS


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Control Channels Control channels fall into three categories: Broadcast:: BCCH, FCCH, SCH  One way, from base to mobile

Common Control: RACH, AGCH, PCH  One way, some from base to mobile and some from mobile to the base

Dedicated: SDCCH, SACCG, FACCH  Two-way, stand-alone or embedded in the traffic channels

All signaling channels share one carrier in a cell the dedicated control channels may be transmitted on traffic carriers T.S. Rappaport Ch 11

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Broadcast Channels Frequency Correction Channel (FCCH) Carries information for frequency correction Synchronization Channel (SCH) Carries information for frame synchronization and for identification of the BTS

Broadcast Control Channel (BCCH) Broadcasts general information on the BTS Broadcasts cell-specific information, e.g. Control channel organization, frequency hopping sequences, cell identification, etc. T.S. Rappaport Ch 11

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Common Control Channels  Paging Channel (PCH) - downlink only

for paging purposes  Random Access Channel (RACH) - uplink only

used by any MS to request allocation of a signaling channel (SDCCH) a slotted Aloha protocol is used, so collisions among MSs may happen

 Access Grant Channel (AGCH) - downlink only

used to allocate a SDCCH or a TCH  Notification Channel (NCH) - downlink only

notify MS of voice group and voice broadcast call (ASCI feature)


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Dedicated Control Channels Stand Alone Dedicated Control Channel (SDCCH) used for call setup (authentication, signaling,, traffic channel assignment), location updates and SMS

Slow Associated Control Channel (SACCH) always coupled with a SDCCH or TCH for communicating measurement data and control parameters

Fast Associated Control Channel (FACCH) to respond to increased signaling demand, e.g.

during handover bandwidth (bit slots) are stolen from the associated TCH (traffic data are preempted)


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Traffic Channels GSM support two types of traffic channels full rate (TCH/F): 22.8 kbps half rate (TCH/H): 11.4 kbps Mapping to physical channel full rate traffic channel - 1 timeslot half rate traffic channel - 1 timeslot in alternating frames

Full rate channel may carry 13 kbps speech or data at 2.4, 4.8 or 9.6 kbps Half rate channel may carry 6.5 kbps speech or data at 2.4 or 9.6 kbps T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM Channel Usage & MS Terminating Calls


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AN INTRODUCTION TO GSM  GSM Call Setup Procedure  Locating the Subscriber HLR keeps record of the MSC area which the Subscriber last registered VLR keeps record of the Location Area[LA] in which Subscriber last registered Location Area: A Group of Neighboring Cells having the same LAC LAC: Each Cell in the Area Broadcasts this Code Each Mobile itself periodically registers itself with MSC  Paging and Random Access Procedure [RAP] GMSC interrogates HLR for MSC the Subscriber last registered Call is Switched to that MSC VLR tells about the LA the user last registered Ch MSC in all the Cells in the LA T.S. Rappaport 11 arranges a Paging NDG Notes 19

AN INTRODUCTION TO GSM  GSM Call Setup Procedure-2  Paging and Random Access Procedure[RAP]-Cont’d The Mobiles listen to the Paging Channels for their Number If received a Paging Message, the Mobile starts a RAP  It sends a Random Number [RN] + Brief Guide to describe the Purpose  The System [MSC] responds to it by repeating the RN and providing the decided Channel  The Mobile listens to this RN, if it is the same, it gets hold of the allocated Channel  Reconfirmation is done by the System by sending again the Mobile Number [in case some other Mobile has also sent the same RN at the same time]  If the Mobile hears its own number, it responds to the System and thus it gets connected to the incoming Call,  If it was a wrong number [other than its own number] it must leave that Channel.  In case, a Mobile listens no same RN from the System, it may repeat RAP at some time later randomly.


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AN INTRODUCTION TO GSM CALL PROCESSING & MOBILE TERMINATING CALLS


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AN INTRODUCTION TO GSM CALL PROCESSING & MOBILE ORIGINATING


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AN INTRODUCTION TO GSM  GSM Handover and Routing  Handover

The Mobile keeps updating MSC about the RSS levels from the

neighboring Cells. MSC, while looking at the quality of the existing link, decides whether a Handoff/ Handover is necessary or not. If yes, it asks the Candidate Cell to prepare the Channel [ GSM uses Fixed Channel Assignment] When Channel is allocated, it tells the Mobile to move to that Channel When Mobile has moved, it deactivates the old Channel

 Routing

GMSC interrogates HLR to locate the Subscriber by telling the MSC where the Mobile last registered. PROBLEM: Call originated from PSTN in CANADA for a US Mobile roaming in CANADA will result in two International Calls “TROMBONE PROBLEM” ???? [Don’t Worry, Be Happy, Other People will Take Care of this Problem] T.S. Rappaport Ch 11

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HANDOVER TYPES Intra-BSC Old and new BTSs are controlled by the same BSC The MSC is not involved Intra-MSC Old and new BTSs are attached to different BSCs The BSCs are attached to the same MSC Inter-MSC Handover to a new MSC Serving MSC becomes anchor MSC IMT (Inter Machine Trunk) T.S. Rappaport Ch 11 NDG Notes is

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AN INTRODUCTION TO GSM  GSM Numbering Scheme and Spectrum Efficiency  GSM Numbering Scheme Each GSM Mobile has at least three Numbers:  MSISDN [Mobile Station Integrated Services Digital Network] stored in SIM Card 

SIM Card holds Subscriber ID[MSISDN, IMSI], some Extra Memory to store phone numbers and Encryption Algorithms

 IMSI[International Mobile Subscriber Identity] not known to the User, HLR does translation between MSISDN to IMSI and vice versa.  ESN or IMEI:Permanently stored/wired in the Mobile Station

 Spectrum Efficiency

TDMA, 200 KHz Channel BW, 8-Time Slots per Carrier Channel Radio Spectrum = 2 x 25 MHz bands can support 125[124] Duplex

Carrier Channels and 1000 Traffic Channels [1000 Simultaneous Calls]. A typical GSM System Cluster Size = 12, so each Cell have Approx. 10 Carrier Channels and a Capacity of 80 Simultaneous Call [Traffic Channels] T.S. Rappaport Ch 11 NDG Notes 25 In case of half-rate Coder, Spectrum Efficiency will be Doubled.

AN INTRODUCTION TO GSM  Technical Realization of GSM  GSM Speech Communications

User Speech is Digitized using Adaptive PCM Speech Encoding using Linear Predictive Coding [LPC] User data flow = 13 kbps [ 6.5 kbps in case of half-rate coding] Adding an overhead of 9.8 kbps[for error detection, error correction,

and synchronization], we get an over all user data flow of 22.8 kbps. This user data flow is subdivided into short data blocks each of 456 bits, which is divided into 8 sub-blocks, each of 57 bits, 2 such subblocks are used to make a burst [to fit into a time slot of 0.58 ms]. See frame structure. These bursts from a user are interleaved over 8 time slots spread over 8 frames. TDMA/FDMA Mechanism  Each Carrier Channels carries data burst of eight (8) users, each in its allocated time slot in the frame.  Time slots are transmitted on several Channels [Carrier Channel or Frequency]  Each Carrier Channel transmits the data of 8 users at 271 kbps.


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AN INTRODUCTION TO GSM  Technical Realization of GSM [ Cont’d]  GSM Speech Communications-2

GSM Carrier Frequencies are numbered 1-124 Channels[Duplex] Forward Channel = 935.2 MHz + (n-1) * 200 KHz Reverse Channel = 890.2 MHz + (n-1) * 200 KHz

Due to slightly more than 200 KHz bandwidth of the modulated

signal, Consecutive Frequencies are not used in the same System Also, 1 and 124 Channels are generally not used. The Spectrum of 124 Channels is generally not allotted to one Service Provider. Separation between two Duplex paired Channels is 45 MHz. Each user’s bursts are transmitted every 4.6ms apart. Frequency

97 96 95 94 93 92

1

2

3

4

5

6

7

8

1

2

3

5

6

7

8

1

2

3

4

5

6

7

8

Time

Frame [4.6ms]


4

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SPEECH PROCESSING


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SPEECH CODING

SPEECH ENCODER


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SPEECH DATA PROCESSING


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CHANNEL ENCODING


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AN INTRODUCTION TO GSM INTERLEAVING


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AN INTRODUCTION TO GSM VOICE TRANSMISSION PATH


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AN INTRODUCTION TO GSM  Technical Realization of GSM [ Cont’d]  GSM Framing Structure Hyper Frame 3 Hrs 28 Min

2048

1

1 1

51 26

One Super Frame Occurs when when the Speech and Control Channel restart at the same time.

Super Frame 6.12 Sec Multi-Frame 120 ms

Multi-Frame 235 ms 26

1

26 x Frames 4.61 ms

Speech Traffic

Control Traffic

Each Frame is sub-divided into 8 time-slots 0.58 ms

8

1

51

1

Preamble 3

57 bits User Speech Data

26 bits

57 bits User Speech Data

Time Slot 0.58 ms

3

GuardTime Block = 456 Bits 1

1


81

2

. . 8 sub-blocks 57 bits each

8

81 81 81 8 1 81 8 x Frames 4.61 ms, Slot-2 of each Frame is Allocated to the User

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Blocked User Signal Inter-Leaved into 8 Time Slots over 8 Frames

8 1

8

34

AN INTRODUCTION TO GSM  TIME-SLOT STRUCTURE


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AN INTRODUCTION TO GSM  FRAME HIERARCHY


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AN INTRODUCTION TO GSM  BURST


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AN INTRODUCTION TO GSM TYPES OF BURST

Five different types of bursts Normal burst Traffic and control payload Frequency correction burst All zeroes sequence Synchronization burst A special fixed sequence Random access burst Extended guard period of 68.25 bitts (252 μs) Dummy burst T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM BURST STRUCTURES


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AN INTRODUCTION TO GSM  Technical Realization of GSM [ Cont’d]  GSM Modulation Scheme Gaussian Minimum Shift Keying [GMSK]

(For Detail see Text Book)

Ch-5 of the  No Speech, No Transmission  Saves Energy in MS, Reduces Ave. Interference  Comfort Noise added for the Listener’s Pleasure.

 GSM MS Power Classes Class 1: 20 W Not (yet) Available Class 2: 8 W Car Phone Class 3: 5 W Class 4: 2 W Normal Pocket Phone Class 5: 0.8 W Limited Coverage Phone [Urban Areas Only]  Maximum Cell Size: Depends on Max. Permitted Delay, 35 km.  Maximum Mobile Speed: 250 km/hr


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AN INTRODUCTION TO GSM  Technical Realization of GSM [ Cont’d]  GSM SMS Individual Messages:  up to 160 Characters long can be sent and received by MS  SMS uses Signaling Channel, thus, it can be received during current Communication Session  Max. rate 600 bps

Cell Broadcast:  up to 93 Character long message can be sent to all users in a given area [Cells].  Uses Communication Channel, so, can’t be received during current Communication Session  Neither Addressed Nor Encrypted

Applications of SMS: .Network Operator Messages, Third

Party Messages, Public Interest Information Messages SMS Cost: Differs from Operator to Operator and depends on type of Application T.S. Rappaport Ch 11

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AN INTRODUCTION TO GSM  Technical Realization of GSM [ Cont’d]

 GSM Security Aspects GSM provides security about the identity of MS Guards against Eavesdropping  Implementation User Authentication [A3 Algorithm]: 128 bit Secret Number, Ki, assigned to each User, stored in SIM Card as well as AuC, is used to create SRES using Ki and 128 bit RAND number from the network, if SRES of MS matches with SRES of the T.S. Rappaport Ch 11 the MS is NDG Notes network,

42


Data Encryption [A8+A5 Algorithm]: Frame # + (RAND+Ki(Alg-A8))(Alg-A5) -> 144 bit Code Train ⊗ 1 44 bit user Data Train -> [Network] ⊗ Frame # + (RAND+Ki(AlgA8))(Alg-A5) ->Original Message


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AN INTRODUCTION TO GSM  Technical Realization of GSM [Cont’d]  GSM Inter-Connection with Other Networks: PSTN/ISDN for Voice  GSM Speech data is Digital and Compressed  Converted into audio of 3.1 KHz BW

PSTN/ISDN for Data  For Data Interconnection, Modem Pools are used in GSM Network. Many Telephone Modems at GMSC/IWF supporting all the important telephone modem standards V.21[0.3 kbps, AS], V.22[1.2 kbps, AS and S], V.32 [4.8 kbps, S] etc.

PSDN (Basic PAD or Dedicated PAD) for Data  300 bps to 9600 bps on AS/S links


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NEW DEVELOPMENTS IN GSM  Frequency Shortage and Extended GSM Band  In some European countries, up to half GSM band is occupied by Interim Systems; NMT, ETACS etc.  Additional frequencies adjacent to GSM band is recommended by ERO[1996], new terminals for this extended band are not available in bulk

 New Encoding Techniques  Half-Rate Speech Encoding

Instead of 13 kbps only 6.5 kbps will be required for the same speech quality Standardized in early 1995, but, only few manufacturers have implemented  lack of interest on part of system suppliers  fear of investment decrease in network expansion [uCell]

 Enhanced Full Rate [EFR]:Developed by GSM and DCS-1800 Suppliers

Same 13 kbps speed, but, considerable improvement in speech

quality T.S. Rappaport Ch 11 NDG Notes More Immunity to transmission errors

45

NEW DEVELOPMENTS IN GSM  Any Cellular Phone may have many Speech Coders,

including FR/HR/EFR, it must be able to switch, and at least support FR for roaming purpose.

 Data Communications over GSM

General Packet Radio Service[GPRS] offers packet-switched data

communications suitable for LAN and Internet Applications[PVC] Requires GPRS enabled Handsets and Changes at BSS[addition of Packet Control Unit, PCU] Option of Upgrading of BTS to support Enhanced Data rate for GSM Evolution, EDGE [8-PSK system]

 Multi-band Terminals

Cellular phones to support more than frequency bands [GSM and DSC-1800 or GSM and PCS-1900 or GSM/DCS/PCS or many more combinations]

 GSM Moving into 3G Mode

GSM extends itself into 3G as Universal Mobile Telecomm. Services [UMTS]

 Radio Interface will use WCDMA technology [UMTS Terrestrial Radio Access, UTRA] in two different modes: FDD [two different frequencies for uplink and downlink] or TDD[same frequency for both uplink and T.S. Rappaport Ch 11 NDG Notes 46 downlink but time-shared]

The Market Share of GSM Suppliers [1/97]  Networking & Switching Subsystems [NSS]  Ericsson [48 %], Siemens [21 %], Nokia [14 %], and Alcatel [10%] = 93 %  Others (Lucent, Motorola, Nortel, etc] = 7%

 Basestation Sub-System [BSS]  Nokia [22%], Motorola [13%], Alcatel [10%] and Ericsson [7%] = 52%  Others [Italtel, Lucent, Matra, Philips etc] = 48%

 GSM / DCS-1800 / PCS-1900 Mobile Terminals  Ericsson [25 %], Nokia [24%], MOTOROLA [20 %], SIEMENS [9%] =78 %  Others [Alcatel, Panasonic, Nortel, etc.] = 22 %

 Due to Licensing Problems, Unfair and Restricted T.S. Rappaport Ch 11 NDG Notes Competition in GSM Markets

47

GSM and Health Problems  GSM Terminals Cause Interference with  Hearing Aid Devices and  Pacemakers [Instruments used to regulate the heart functionality of Heart Patients], and  Some Research show that GSM phones cause Brain Tumors *  Inherent in GSM TDMA setup to generate strongly pulsating transmission signals [Continuously Tx is switched on and off that generates LF signals 217, 434, 651 Hz]  * Extensive Research required to prove the validity of GSM Cellular Phones being the cause of Brain Tumors or Cancer.


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GSM Derivative Systems: DCS-1800 & PCS-1900

 Digital Cellular System[DCS]-1800  Originally started [in 1990] as a separate system, but, later on became just a GSM variant  Main modifications were made only in Air Interface  Developed [by ETSI] particularly for densely populated urban areas  1.7 - 2.3 GHZ band [ 2 x 75 MHz spectrum, 1710- 1785 + 18051888 MHz]  Duplex separation is 95 MHz, Channel BW is 200 KHz, 374 duplex channels  Much smaller cells [cells within a building], lower power BTS and MS as compared to GSM  handoff problems are much cleverly settled using hierarchical Cell structure  Max Cell size 8 km with Class 1 MS [1W]  Max Cell size with Class II[0.25W] even smaller [0.5 -4/5 km]  International as well as National roaming is possible [Country to T.S. Rappaport Ch 11 Network to Network, NDG Notes 49 Country, and Intra-Network]

GSM Derivative Systems: DCS-1800 & PCS1900  Personal Communication Services [PCS]-1900      

A GSM variant to adapt to US Market Frequency band 1900 MHz [1850-1890, 1930-1970 MHz] 2 x 40 MHz bands with Duplex Separation of 80 MHz Channel BW is 200 KHz, 200 Duplex Channels TDMA 8 time-slots EFR speech encoding is getting more Interest from US Service Providers


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GSM Facing Challenges  Higher Costs incurred on Network Capacity Increase  Business Market Saturation  More Money on Advertisement and Subsidies/Customer Discounts

 Interconnection Costs [to PSTN/ISDN] are very high  Leased Line Costs to Interconnect own Infra-structural elements are very high  High License Costs

 With Implementation of De-regulation policies this

will CHANGE. T.S. Rappaport Ch 11

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REFERENCES

www.utdallas.edu/~nhutnn/cs6v81/LECTURE http://www.gsmworld.com Dr. Veselin Rakocevic http://www.staff.city.ac.uk/~veselin/Wireless Comms


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Introduction to GSM

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