mobile communication - unit iv – gsm fundamentals

Mobile CommunicationUnIT IV – GSM FunDAmEnTALS :: PART I, II & III

Sumit Kr. ChoubeyMarch 20, 2020

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Table of contents

1. Introduction

2. Global System for Mobile Communications

3. Characteristics of GSM standard

4. Services

5. Basic Radio Parameter

6. Application

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Introduction

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Mobile?

1The term “mobile” has historically been used to classify all radio ter-minal that could be moved during operation.

2use “mobile” to describe a radio terminal that is attached to a highspeed mobile platform

3 e.g., a cellular telephone in a fast moving vehicle

4use “portable” to describes a radio terminal that can be hand- heldand used by someone at walking speed

5 e.g., a walkie-talkie or cordless telephone inside a home

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History

1The first wireless networkswere developed in thepreindustrial age.

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These systems transmittedinformation over line-of-sight distances (laterextended by telescopes)using smoke signals, torchsignaling, flashing mirrors,signal flares, or semaphoreflags.

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History : Radio

• Early communication networks were replaced first by the telegraphnetwork (invented by Samuel Morse in 1838) and later by the telephone.

• In 1895, Marconi demonstrated the first radio transmission.• Very high transmission power (> 200 kW)

• Early radio systems transmitted analog signals.

• Todaymost radio systems transmit digital signals composed of binary bits.

• A digital radio can transmit a continuous bit stream or it can group thebits into packets.

• The latter type of radio is called a packet radio and is characterized bybursty transmissions

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History : ALOHANET

1The first network based onpacket radio, ALOHANET,was developedat the University of Hawaii in 1971.

2ALOHANET incorporated thefirst set of protocols for channel accessand routing in packet radio systems, and many of the underlyingprinciples in these protocols are still in use today.

3Lead to Ethernet and eventually wireless local area networks(WLAN).

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History : Pre-Cellular

1Themost successful application ofwire-less networking has been the cellulartelephone system.

21915: Wireless voice transmission (wire-less telephony) between New York andSan Francisco was first established byAT& T.

31946: First public mobile telephone ser-vice was introduced in 25 cities acrossthe US. The equipment was expensiveat $2,000.

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• These initial systems used a single central transmitter to cover an entiremetropolitan area.

• High-powered transmitter & Large tower• Inefficient!• FM push-to-talk

• 1976: (30 yrs after the introduction of the service in 1946),• the New York system (10M people) could only support 543 paying

customers.• 3,700 on the waiting list

• The mobile units weighed about 10 kilograms and put out a steady 20-25watts.

• The central transmitters that communicate with the mobile unitsbroadcast 200 to 250 watts.

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1The central station could reliably communicate with the mobileunits up to a radius of approximately 25 miles (50 km).

2Beyond that, up to a radius of 60 to 100 miles, the signal was tooweak for consistent service, but strong enough to interfere with anyother mobile radio system.

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History : 1G Cellular

• A solution to this capacity problem emerged during the 50’s and 60’swhen researchers at AT& T Bell Laboratories developed the cellularconcept.

• 1968: AT& T proposed the concept to the FCC

• Cellular systems exploit the fact that the power of a transmitted signalfalls off with distance.

• Thus, two users can operate on thesame frequency at spatially-separatelocations with minimal interferencebetween them.

• Frequency reuse

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History : 1G Cellular• Japan had the world’s first commercially available cellular

• Nippon Telegraph and Telephone (NTT) created a cellular test system forTokyo in 1975, with the result coming to market in 1979.

▶ Japan’s national monopoly telecoms operator/carrier

• The first trial in America of a complete, working cellular systemwas held inChicago in the late 1970’s.

• 1983: Advanced Mobile Phone System (AMPS)• First US cellular telephone system• Deployed in 1983 by Ameritech in Chicago, IL.• Worked well. (FM, FDMA)• May even have worked too well.

▶ Its satisfactory performance lowered the demand for a better system,allowing Europe to take the lead by creating a digital cellular system first.

• AMPS, RIP: 1983-2008▶ As of February 18, 2008, carriers in the United States were no longer required

to support AMPS.12

History : 2G Cellular

1The (1G) systems introduced in the 1980swere characterized by ana-log speech transmission. The (2G) of cellular systems, first deployedin the early 1990’s, were based on digital communications.

2The shift fromanalog to digital was drivenby its higher capacity andthe improved cost, speed, and power efficiency of digital hardware.

31991: US Digital Cellular (USDC – IS-54 > IS-136) Three times capacityof AMPS via digital modulation, speech coding, and TDMA

4While second generation cellular systems initially provided mainlyvoice services, these systems gradually evolved to support data ser-vices such as email, Internet access, and short messaging.

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Two important 2G systems

• GSM supports SMSs and user data at rates only up to 9.6 kb/s.• Security features including (for example) the encryption of data and• signaling messages on the path between the mobile phone and the BS.

▶ Subscriber identity module (SIM)▶ A smart card▶ Contain the subscriber’s personal details▶ Can be moved from one handset to another.

• IS-95B (cdmaOne) provides data rates in the range of 64 to 115 kb/s inincrements of 8 kb/s over a 1.25 MHz channel.

• Each cell uses a carrier with a bandwidth of 1.25MHz, which is divided into64 data and signalling channels by the use of orthogonal CDMA codes.

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GSMWorld Coverage Map

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3G Cellular

• Studies started even before the earliest 2G systems arrived on the market.

• International Mobile Telecommunications-2000 (IMT-2000)• A subgroup of the International Telecommunication Union (ITU)• Published a set of performance requirements of 3G (for both packet-

switched and circuit-switched data):▶ Aminimum data rate of 144 Kbps in the vehicular environment.▶ Aminimum data rate of 384 Kbps in the pedestrian environment▶ Aminimum data rate of 2 Mbps in the fixed indoor and picocell environment

• There are several wireless standards and systems that qualify as thirdgeneration (3G) systems.

• UMTS• CDMA2000

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4G Cellular

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4G is the fourth generation of broadband cellular network technol-ogy, succeeding 3G. A 4G systemmust provide capabilities definedby ITU in IMT Advanced. Potential and current applications includeamended mobile web access, IP telephony, gaming services, high-definition mobile TV, video conferencing, and 3D television.

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The first-release Long Term Evolution (LTE) standard was commer-cially deployed in Oslo, Norway, and Stockholm, Sweden in 2009,and has since been deployed throughout most parts of the world.It has, however, been debatedwhether first-release versions shouldbe considered 4G LTE.

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Global System for Mobile Communications

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The Global System for Mobile Communications (GSM) is a standarddeveloped by the European Telecommunications Standards Institute(ETSI) to describe the protocols for second-generation (2G) digital cellularnetworks used by mobile devices such as mobile phones and tablets.It was first deployed in Finland in December 1991. By the mid-2010s, itbecame a global standard for mobile communications achieving over90%market share, and operating in over 193 countries.

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Origin As a result of cooperation within the European Union, a spe-cial working group for mobile telephony was established,task was to specify the objectives and the standards of acommonmobile cellular telephony system.

• Scientific & industrial organizations from 17 countries signed theMoU, par-ticipated in the development of a new system

• As a result of common research and discussions a series of standards hasbeenestablishedcoveringall the required layers of theOSI referencemodel

• Most popular standard formobile phones in theworld. GSM900 /GSM 1800MHz are used in most parts of the world: Europe, Asia, Australia, MiddleEast, Africa

• GSM 850 / GSM 1900 MHz are used in the United States, Canada, Mexicoand most countries of S. America.

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• GSM networks operate in a number of different carrier frequency rangeswithmost 2G GSM networks operating in the 900MHz or 1800MHz bands.

• Where these bands were already allocated, the 850 MHz and 1900 MHzbands were used instead (for example in Canada and the United States).

• In rare cases the 400 and 450 MHz frequency bands are assigned in somecountries because they were previously used for first-generation systems.

• Currently the GSM stands for Global System for Mobile Communicationswhich stresses the fact that the application of GSM spans far beyond theEuropean continent.

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Features of GSM Module

• Improved spectrum efficiency

• International roaming

• Compatibility with integrated services digital network (ISDN)

• Support for new services.

• SIM phonebook management

• Real time clock with alarmmanagement

• High-quality speech

• Uses encryption to make phone calls more secure

• Short message service (SMS)

• The security strategies standardized for the GSM systemmake it the mostsecure

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GSM Architecture

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GSM Architecture… I

The GSM network can be divided into three broad parts ::

1 The subscriber carries themobile station (MS)

2 The base station subsystem (BSS) controls the radio link with themobile station

3 The network subsystem performs the switching of calls betweenthe mobile users and other mobile and fixed network.

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Mobile Station

• Themobile station consists of the mobile equipment, i.e. the handset, anda smart card called the Subscriber Identity Module (SIM).The SIM providespersonal mobility

• The mobile equipment is uniquely identified by the International MobileEquipment Identity (IMEI).

• The SIM card contains the International Mobile Subscriber Identity (IMSI)used to identify the subscriber to the system

• The IMEI and the IMSI are independent, thereby allowing personalmobility.

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Base Station Subsystem

• The base station subsystem is composed of two parts, the base transceiverstation & the base station controller.

• These communicate across a standardised “Abis” interface, allowing oper-ation between components made by different suppliers.

• The base transceiver station houses the radio transceivers that define a cell& handles the radio-link protocols with the mobile station.

• The base station controller manages the radio resources for one or morebase transceiver stations. It is the connectionbetween theMSand theMSC.

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Network Subsystem

• The central component of the network subsystem is the mobile servicesswitching center.

• This acts like a normal switching node of the PSTN (Public Switched Tele-phone Network) or ISDN (Integrated Services Digital Network) and con-nects the mobile signal to these fixed networks.

• It additionally provides all the functionality needed to handle amobile sub-scriber, such as registration, authentication, location updating, handoversand call routing to a roaming subscriber.

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Radio Spectrum• Since radio spectrum is a limited resource shared by all users, a method

must be devised to divide up the bandwidth among asmany users as pos-sible. Themethod chosen by GSM is a combination of Time and FrequencyDivision Multiple Access (TDMA/FDMA).

• TheFDMApart involves thedivisionby frequency of the (maximum) 25MHzbandwidth into 124 carrier frequencies spaced 200kHz apart. One or morecarrier frequencies are assigned to each base station.

• Each of these carrier frequencies is then divided in time, using a TDMAscheme.

• The fundamental unit of time in this TDMA scheme is called a burst periodand it lasts 15/26 milliseconds (ms)

• Eight burst periods are grouped into a TDMA frame (120/26ms, or approx-imately 4.615ms), which forms the basic unit for the definition of logicalchannels. One physical channel is one burst period per TDMA frame.

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Additional components of the GSM I

• The area of GSM operation is divided into subareas managed by particularMobile Switching Centers(MSC)

• Each MSC connected with a respective VLR Database

• GSMsystem is administeredby aparticular operator is equippedwith threeother data base.

HLR The data base of mobile stations permanently registered inthe system by particular operator. The HLR(Home Locationreg) is the central database which stores the permanent pa-rameters of the users and information on their current loca-tion.

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Additional components of the GSM II

AUC Authentication Centre : The data base that allows checkingif the user with the assigned SIM card.

EIR Equipment Identification register : The database of serialnumbers of the mobile phones used in the system.

• In large systems there can be more than one HLR

• The individual user’s data is stored only in one of them

• The user’s record in the HLR contains his/her status, the Temporary MobileIdentificationNumber(TMSI) and the address of theVLR(visitor loc reg) reg-ister which is associated with the user’s current location area

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Additional components of the GSM III

VLR TheVLR is thedatabase consistingof recordsdescribingmo-bile stations currently registered in the service area of a par-ticular MSC

The data exchange between VLR and HLR allows identification of a currentlocation of a called user by reading information on his/her current locationarea in the HLR and routing the connection to that MSC.

• The MSC which co works with the VLR currently contains the data on thecalled user

• The MSCs are connected with each other

• OneormoreMSCs are calledGatewayMobile SwitchingCenter(GMSC) playthe role of a gate to the external networks such as PSTN, ISDN and packetdata networks

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Additional components of the GSM IV• EachMSC controls at least one Base Station System(BSS) which consists of

BSC and a certain number of BTS or BSThe main task of MSC is to coordinate the call set-up between two mobileGSM users or between a GSM user and a user of an external network such asPSTN, ISDN. The task is realized by performing the following functions :

1 Calling the user, setting up and maintaining a connection

2Dynamic network resource management in the system area man-aged by that MSC

3Rerouting of the connection to a new cell served by a BSC differ-ent from the controller which supervised the cell which the MS hasbeen operating so far

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Additional components of the GSM V

• Information exchange between MSC and BSS is normalized by definingthe so called A interface

• Abis interface standardizes the data exchange between the base stationcontroller and its tranceivers

• A interface deals with network and switching aspects

The functions Performed by the MSC,HLR, and VLR withmanagement of fixed links with network management,controlling and encryption of user data and signalling infor-mation with management resulting from the necessity ofMS authentication and location updating caused by MS ge-ographicalmovement andwithmanagement due to callingthe users

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Additional components of the GSM VI

The Abis interface is associated with the information ex-change related to the radio transmission, such as the distri-bution of radio channels, connection supervising, the queu-ing ofmessages before their transmission, carrier frequencyhopping control etc.

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The operation and maintenance Centre

• The operation andmaintenance Centre (OMC) supervises the operation ofparticular GSM system blocks.

• It is connected with all the switching blocks of the GSM system and per-formsmanagement functions such as tariff accounting, trafficmonitoring,management in case of failures of particular network blocks.

• The most important task of OMC is HLR management

• In case of large networks, there are more than one OMC and the wholenetwork is managed by Network Management Centre(NMC).

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Characteristics of GSM standard

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Modulation : GMSK · · · I

The GSM uses Gaussian Minimum Shift Keying (GMSK)modulation method.

• GaussianMinimumShift Keying (GMSK) is a form of continuous-phase FSK(CPFSK)

That Mean The phase is changed between symbols to provide a con-stant envelope.

So GMSK allows efficient class C non-linear amplifiers to beused.

• The RF bandwidth is controlled by the Gaussian low-pass filter.

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Modulation : GMSK · · · II

The degreeof filtering

It is expressed bymultiplying the filter 3dBbandwidth (B) bythe bit period of the transmission(T).

B× T = 0.3

Data rate in Gaussian Minimum Shift Keying (GMSK)modulation method is 270.8 kbps.

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Generating a GMSKWaveform · · · I

GMSK as implemented by quadrature signal processing at basebandfollowed by a quadrature modulator.

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Generating a GMSKWaveform · · · II

Modulationof GMSK

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Generating a GMSKWaveform · · · III

PulseShaping

• Input : Binary pulse train (+1/-1)

• Each binary pulse goes through a LPF with a Gaussianimpulse response

• This process results in a train of Gaussian shapedpulses

Summingand Inte-gration

• The pulses are summed together (left)

• The signal is integrated over time to obtain acontinuous waveform which captures the bittransition information (right)

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Generating a GMSKWaveform · · · IV

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Generating a GMSKWaveform · · · VI& Q Signals • The resulting waveform is divided into In-Phase and

Quadrature components• In-phase : Left• Quadrature : Right

• The two signal components are then up-converted tothe carrier frequency

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Generating a GMSKWaveform · · · VI

Advantage • High spectral efficiency.

• Reducing sideband power.

• Excellent power efficiency due to constant envelope.

• reducing the interference between signal carriers inadjacent frequency channels.

• Self synchronizing capability

Disadvantage • causes (inter symbol interference[ISI])

• Higher power level than QPSK.

• Requiring more complex channel equalization

• algorithms such as an adaptive equalizer.

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GMSK Demodulator

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Multiple Access Techniques

• GSM devised a combination of TDMA/FDMA as the method to divide thebandwidth among the users.

• FDMA part divides the frequency of the total 25 MHz bandwidth into 124carrier frequencies of 200 kHz bandwidth.

• Each BS is assigned with one or multiple frequencies, and each of thisfrequency is divided into eight time slots using a TDMA scheme.

• Each of these slots are used for both transmission as well as reception ofdata.

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Characteristics of GSM standard …

Channelspacing

Channel spacing indicates the spacing between adjacentcarrier frequencies. For GSM, it is 200 kHz.

FrequencyBand

• The uplink frequency range specified for GSM is 935-960 MHz (basic 900 MHz band only).

• The downlink frequency band 890 - 915 MHz (basic900 MHz band only).

SpeechCoding

• For speech coding or processing, GSM uses LinearPredictive Coding (LPC).

• The speech is encoded at 13 kbps.

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Services · · · I

Telephoneservices

• These include emergency calling and facsimile.

• GSM also supports Videotex and Teletex.

BearerServices

• Data services or Bearer Services are used through aGSM phone to receive and send data is the essentialbuilding block leading to widespread mobile Internetaccess and mobile data transfer.

• GSM currently has a data transfer rate of 9.6k Newdevelopments that will push up data transfer rates forGSM users are HSCSD (high speed circuit switcheddata) and GPRS (general packet radio service) are nowavailable.

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Services · · · II

Supplementary services : These are digital in nature and include call diversion,closed user groups and caller identification.

• Supplementary services also include short messaging service (SMS) whichallows GSM subscribers and base station to transmit alphanumeric pagesof limited length

• SMS provides cell broadcast also can be used for safety and advisory ap-plications such as the broadcast of highway or weather information to allGSM subscribers.

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Services · · · III• Some of the supplementary services are :

1Conferencing : It allows amobile subscriber to establish amul-tiparty conversation, i.e., a simultaneous conversation betweenthree or more subscribers to setup a conference call.

2Call Waiting : This service notifies amobile subscriber of an in-coming call during a conversation. The subscriber can answer,reject, or ignore the incoming call.

3 Call Hold : This service allows a subscriber to put an incomingcall on hold and resume after a while.

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Services · · · IV

4 Call Forwarding : Call Forwarding is used to divert calls fromthe original recipient to another number

5Call Barring : Call Barring is useful to restrict certain types ofoutgoing calls such as ISD or stop incoming calls from unde-sired numbers.

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Basic Radio Parameter · · · I

• Two 25 MHz wide bands

• Uplink 890-915 MHz

• Downlink 935-960 MHz

• Frequency Division Duplex (FDD) is used – separate frequency band foropposite direction of communication

• Both bands divided into 124 frequency intervals of 200 kHz

• For each carrier time is divided into 8 slots

• Multiple access is realised by assigning the connection a particular carrierfrequency and a selected time slot

• Thus GSM uses TDMA/FDMA scheme (Time/Frequency Division MultipleAccess)

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Basic Radio Parameter · · · II

• In TDMA/FDMA a physical channel is a sequence of time slots (denoted bythe assigned slot no.) which are placed on a selected carrier

• Physical channels are arranged in pairs – one physical channel in each di-rection – they are marked with the same time slot no and they frequencydiffers by 45 MHz

• The time slots numbering in the downlink direction is delayed by 3

• Thusmobile station never transmits and receives signals to and fromBS atthe same time.

• This reduces requirement for RF and DSP blocks Computation power canbe shared between transmitter and receiver

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Frequency and time structure of GSM channels · · · I

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Frequency and time structure of GSM channels · · · II

• Presents the frequencies and time slot assignment in the GSM system

• Numbering of the time slots is delayed by three in downlink as comparedwith uplink transmission.

• Due to that, a mobile station realizing a connection to a base station in theassigned time slot never transmits and receives the signals at the sametime

• This way, an electromagnetic feedback between amobile station transmit-ter and receiver is avoided.

• Thus, the computational power can be shared between the transmitterand the receiver.

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Logical Channel Description

• Channels arranged to organize the information exchange.

• In each cell a mobile station finds a particular carrier on which in its zerotime slot the system information important for all the mobile stations lo-cated within that cell is transmitted.

• In turn, on the associated carrier always shifted down by 45 MHz,

• In the zero time slot the mobile stations can declare their need for infor-mation exchange with the system or even their need to set up a call.

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Application

• Smart home, smart office & Transportation

• Medical field

• Location tracker

Access con-trol devices

Access control devices can communicate with servers andsecurity staff through SMSmessaging.

Transactionterminals

POS terminals can use SMS messaging to confirm trans-actions from central servers with GSM technology. Centralserver can be anywhere in the world.

Supply ChainManagement

A server with GSM capability is required to manage the ap-plication

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I’d love to hear your thoughts, issues, questions and comments.

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mobile communication - unit iv – gsm fundamentals

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