brief notes of gsm 2003

8/7/2019 brief notes of gsm 2003

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GSM Fundamentals & RF

Different Standards Worldwide

GSM 900: The term GSM-900 is used for any GSM system which operates in any 900 MHz band.

P-GSM 900:P-GSM-900 band is the primary band for GSM-900 Frequency band for primary GSM-900 (P-GSM900) : 2 x25 MHz

890 915 MHz for MS to BTS (uplink) 935 960 MHz for BTS to MS (downlink)

E-GSM 900: In some countries, GSM-900 is allowed to operate in part or in all of the followingextension band. E-GSM-900 (Extended GSM-900) band includes the primary band(P-GSM-900)and the extension band:

880 890 MHz for MS to BTS (uplink) 925 935 MHz for BTS to MS (downlink)

R-GSM-900: R-GSM-900 (Railway GSM-900) band includes the primary band (P-GSM-900) and the

following extension band:876 890 MHz for MS to BTS (uplink) 921 935 MHz for BTS to MS (downlink)

GSM-1800: Frequency band: 2 x 75 MHz1710 1785 MHz for MS to BTs (uplink) 1805 1880 MHz for BTS to MS (downlink)

Industry Vs Technology Spread

Telecom Service Providers/OperatorsGSM

CDMABasic-WLLInternet ServicesLong Distance

VendorTelecom Consultancy

Analog Mobile Telephony

End of 1980s Analog Systems unable to meet continuing demandsSeverely confined spectrum allocations

Interference in multipath fading environmentIncompatibility among various analog systemsInability to substantially reduce the cost of mobile terminals and infrastructure required

Digital Mobile Telephony

Spectrum space - most limited and precious resourceSolution - further multiplex traffic (time domain)Can be realized with Digital Techniques only

Service Industry


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Service Provider is not a Equipment Manufacturer.The Service Provider has a license to operate in a geographical boundary (state/circle/

country).It buys equipment from OEM Suppliers (Vendors).

Installs & commissions the equipment thus making its own Network.Provides the desired service to its subscribers.

Vendor

Vendor is a Equipment Manufacturer.It supplies Product, Consultancy and TrainingsService provider has the option of taking the Consultancy and Training

Cellular Communication

A cellular system links Mobile subscribers to Public Telephone System or to another Mobilesubscribers.

It removes the fixed wiring used in a traditional telephone installation.Mobile subscriber is able to move around, perhaps can travel in a vehicle or on foot & still

make & receive call.

Advantage of Cellular Communication

MobilityFlexibility

ConvergenceGreater QOSNetwork ExpansionRevenue/Profit

The Cell

Cellular Radio involves dividing a large service area into regions called cells.Each cell has the equipment to switch, transmit and receive calls.Cells - Reduce the need of High powered transmissionCells - Conventionally regarded as being hexagonal, but in reality they are irregularly shaped.

Cell shape is determined by the nature of the surrounding area e.g. Hills , tall building etc.

Cell Size

Large Cells Small Cells35 Km Near about 1 KMRemote Areas Urban AreasHigh Transmission Power Low Transmission PowerFew subscribers Many Subscribers


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Coverage & Capacity

Coverage:Percentage of the geographical area covered by cellular service where mobiletelephony is available

Capacity :Number of calls that can be handled in a certain area within a certain period of time.

Capacity can also refer to the probability that users will be denied access to a system

due to the simple unavailability of radio channels.

Frequency Spectrum

Designation Abbreviation Frequencies Free-space Wavelengths

Very Low Frequency VLF 9 kHz - 30 kHz 33 km - 10 km

Low Frequency LF 30 kHz - 300 kHz 10 km - 1 km

Medium Frequency MF 300 kHz - 3 MHz 1 km - 100 m

High Frequency HF 3 MHz - 30 MHz 100 m - 10 m

Very High Frequency VHF 30 MHz - 300 MHz 10 m - 1 m

GSM - IN CELLULAR TELEPHONY

Each Cell in the Cellular Network consists of one or more RF carriers.An RF carrier is a pair of radio frequencies

One used in upward direction by MS - UplinkOther used in downward direction by BTS - DownlinkThe transmit and receive frequencies are separated by a gap of

45 MHz in GSM of 75 MHz in DCS.There are 124 carries in GSM Band. With each carrier carrying 7 timeslots,

only 124 x 7 = 868 calls can be made!Frequency Reuse is the solution

FREQUENCY & ARFCN:

Ful(n)=890.0+(0.2)*n MHz ul=890 to 915 MHzFdl(n)=Ful+45MHz dl=935 to 960 MHz

Frequency Reuse Pattern

Three types of frequency reuse patterns1. 7 Cell re use pattern2. 4 Cell re use pattern3. 3 Cell re use pattern

Multiple Access Methods


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Principal Of Sectorization


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`Omni Directional Cells (1 Antenna)120 degree Sectors (3 Antennas)60 Degree sectors (6 Antennas)

Each Sector in a Site has its own allocation of Radio CarriersAdvantage

By frequent reuse of frequency more capacity can be achieved

Frequency Hopping : Multipath Fading results in variations in signal strength whichis known as Rayleigh Fading.

Rayleigh Fading phenomenon is dependent on path difference and hence

frequency of reception.

A fast moving mobile may not experience severe effect of this fading since

the path difference is continuously changing.

A slow moving mobile ( or a halted mobile ) may experience severe deterioration in quality.

But, if the frequency of reception is changed when this problem occurs, could solve it.

The fading phenomenon is fast and almost continuos, this means the

frequency change should also be continuos.

This process of continuously changing frequency is known as Frequency Hopping Frequency Hopping is done in both Uplink and Downlink. Frequency is changed in every TDMA Frame. Mobile can Hop on maximum 64 frequencies. The sequence of Hopping can be Cyclic or Non-Cyclic. There are 63 Non-Cyclic Hopping sequences possible.

Different Hopping sequence can be used in the same cell. BCH Timeslot can never HOP, but the remaining Timeslots can very well hop.

Reduction in Average Interference:

With Frequency Hopping consistent interference will become bursty.

So even though, both the co-channel cells will be using the same set of ARFCN's forHopping, interference will not be continuos.

This is because, GSM cells are not Frame synchronized, and change in frequency isrelated to Frame nos.

If same HSN is used in two cells, then either the interference will be nil , or if a phasecorrelation exists then it will be continuos.

So the two cells shouldpreferably use different HSN's .

Sectorial cells ( controlled by the same BTS) can use same HSN, since the sectors don'tcome up at the same time.

Cells if they are synchronized, can use same HSN, if each cell has an offset of someTDMA frames.

Offset of TDMA frames is also required to avoid SACCH occurring at the same time in allsynchronized cells, as they kills away the objective of DTX.

Features of GSM:


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CompatibilityNoise RobustIncreased Capacity & FlexibilityUse of Standard Open Interfaces

Improved Security & ConfidentialityCleaner HandoversSubscriber IdentificationISDN CompatibilityEnhanced Range of Services

Handovers: There are 2 types Hard Handoff handover: it causes in Analog, TDMA & GSM

1. It is in the type of Break before Made

Soft Handoff Handover: it causes in CDMA2. It is in the type Make before break

Cleaner Handovers:The mobile measures up to 32 adjacent cells for

Signal Strength (Rx Level)Signal Quality (Rx Quality)updated every 480mS and sends to BTS

Sophisticated Handover based onRx LevelInterferenceRx QualityTiming AdvancePower Budget

GSM NETWORK ELEMENTS

GSM Network Components:

Mobile Station consists of two parts-Mobile Equipment (ME)Subscriber Identity Module (SIM)

ME Hardware e.g. Telephone, Fax Machine, Computer.SIM Smart Card which plugs into the ME.

ME (Classmark Information):


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Revision Level Phase of the GSM specs ME comply with.RF Power Capability Max power ME is able to Transmit.Ciphering Algorithm Used Presently A5

Phase 2 specifies Algorithms A5/0 to A5/7.

Frequency CapabilitySMS Capability

MobileEquipment:

Class Power O/p1 20 W2 8 W3 5 W4 2 W - - - t y p i c a l s e t t i n g s5 0.8 W

SIM(IMSI):IMSI(International Mobile Subscriber Identity)

Transmitted over Air Interface on initializationPermanently stored on SIM card15 digit Decimal

SIM (TMSI):Temporary Mobile Subscriber Identity

Periodically changed by the System Management on instances like location update

etc.Reason for use of TMSITo prevent a possible intruder from identifying GSM users, TMSI is used

ManagementAssignment, Administration & Updating is performed by VLR.

Transcoder:

Converts 64 Kbps PCM circuits from MSC to 16 Kbps BSS circuits.Each 30 channel 2 Mbps PCM link can carry 120 GSM -specified voice channels.

Generally, transcoder is collected with MSC so as to reduce the no.of 2Mbps A-links to

effectively use the BW

Base Station System (BSS):

BSCControls upto 40 BTSConveys information to/from BTSConnects terrestrial circuits & Air Interface ChannelsControls handovers between BTSs under itself


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BTSContains RF HardwareLimited control functionality 1- 6 carriers in a BTS Cabinet 7- 48 simultaneous calls per BTS

Network Switching System(NSS):

NSS (Network Switching System)MSC (Mobile Switching Centre)HLR (Home Location Register)VLR (Visitor Location Register)EIR (Equipment Identity Register)AUC (Authentication Centre)IWF (Interworking Function)EC (Echo Canceller)

GSM Network Component:

MSCCall SwitchingOperation & Management SupportInternetwork InterworkingCollects call billing data

Gateway MSCMSC which provides interface between PSTN & BSSs in the GSMNetwork.

Home Location Register (HLR):

Reference database for the Subscriber profiles-Subscriber ID (IMSI & MSISDN)Current VLR AddressSupplementary Services subscribedSupplementary Service InformationSubscriber Status (Registered/deregistered)Authentication Key and AUC functionalityAuthentication Key and AUC functionalityTMSIMSRN

Visitor Location Register (VLR):

Temporary Data,which exists as long as the subscribr is active in a particular coverage area.Contains the following-

Mobile Status (Busy/ Free/ No Answer/etc.)Location Area Identity (LAI)TMSIMSRN (Mobile Station Roaming Number


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Equipment Identity Register (EIR):

Contains Database for validating IMEIWhite List (valid ME)Black List (Stolen ME)

Grey List (Faulty ME)

Mobile Station Identities:

MSISDN : Human Identity used to call a Mobile Station MSRN : Mobile Station Roaming No

98 XXX12345

IMSI : Network Identity unique to a SIM TMSI : Identity unique in a LAI

404 XX 12345

IMEI: Serial number unique to every Mobile Station

6digits 2digits 6digits 1digit

Inter Working Function

Provides function to enable the GSM System tointerface with Public/Private Data NetworksThe basic feature of the IWF are

Rate ConversionProtocol adaptation

IWF incorporates Modem Bank.

e.g. GSM DTE ======== PSTN DTEIWF Analogue Modem

.Echo Canceller:

Echo is apparent only in Mobile - Land conversation & is generated at the 2 wire to 4 wireinterface.To avoid it, Echo Canceller (EC) is used.

Echo is irritating to MS SubscriberTotal Round Trip delay of 180 ms in the GSM systemEC is placed on the PSTN side of the SwitchCancellation up to 68 ms with EC

CC NDC SN

MCC MNC MSIN

TAC FAC SNR S


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Operation & Maintenance Centre:

Event & Alarm ManagementFault ManagementPerformance Management

Configuration ManagementSecurity Management

GSM Terrestrial Interfaces:

Broadly classified into two types of interfaces-Standard Interfaces

2 Mbps Trunks (E1)Signalling System No. 7 SS7 ( CCS7)X.25 (Packet Switched Mode)

GSM Interfaces

GSM Protocol Layers:

GSM protocols are basically divided into three layers:

Layer 1: Physical layer

Enables physical transmission (TDMA, FDMA, etc.)

Assessment of channel quality

Except on the air interface (GSM Rec. 04.04), PCM 30 or ISDN links are used

(GSM Rec. 08.54 on Abis interface and 08.04 on A to F interfaces).

Layer 2: Data link layer Multiplexing of one or more layer 2 connections on control/signaling channels

Error detection (based on HDLC)

Flow control

Transmission quality assurance

Routing

Layer 3: Network layer

Connection management (air interface)

Management of location data

Subscriber identification

Management of added services (SMS, call forwarding, conference calls etc.}

Basic Processes:AUTHENTICATIONCIPHERINGREGISTRATIONCALL ESTABLISHMENTHANDOVER / HANDOFFROAMING


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Ciphering:

Data protection is required on air interface.A specific key called Ciphering Key (Kc), is generated from RAND and A8 algorithm.A8 is on the SIM

Optical Fiber:

Different Possible CombinationsMono Mode Step Index 10 / 125mMono Mode Graded indexMulti Mode Step Index 100 / 300mMulti Mode Graded Index 75 / 130mMono Mode Graded Index would have been the best but fabrication not possible

140 Mbps OLTE , Mono Mode Step Index in our case

LOGICAL CHANNELS PHYSICAL CHANNELS:

Channels On Air Interface:

Physical ChannelPhysical channel is the medium over which the information is carried.

Logical ChannelLogical channels consists of the information carried over the Physical Channel


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LOGICAL CHANNELS:


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GSM (Logical)Channels:

Traffic Channel:

Time is divided into discrete periods called Timeslots

TCH carries payload data - speech, fax, data Connection may be:

- Circuit Switched - voice or data or - Packet Switched data TCH may be:

Full Rate (TCH/F)- one channel per user- 13 kb/s voice, 9.6 kb/s data or

Half Rate (TCH/H)- one channel shared between two users

Control (signaling) Channel:


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1.Broadcast Channels (BCH):

BCH channels are all downlink and are allocated to timeslot zero.Channels are:

FCCH: Frequency control channel sends the mobile a burst of all 0 bits which

allows it to fine tune to the downlink frequencyS CH: Synchronization channel sends the absolute value of theframe number

(FN), which is the internal clock of the BTS, together with the Base Station Identity

Code (BSIC)BCCH: Broadcast Control Channel sends radio resource management and control

messages, Location Area Code and so on.

Some messages go to all mobiles, others just to those that are in the idle state

2.Common Control Channels (CCCH):

CCCH contains all point to multi-point downlink channels (BTS to several MSs) and theuplink Random Access Channel:CBCH: Cell Broadcast Channel is an optional channel for general information such as road

traffic reports sent in the form of SMSPCH: Paging Channel sends paging signal to inform mobile of a callRACH: Random Access Channel is sent by the MS to request a channel from

the BTS or accept a handover to another BTS.A channel request is sent in response to a PCH message.

AGCH: Access Grant Channel allocates a dedicated channel(SDCCH) to the mobileNCH: Notification Channel informs MS about incoming group or broadcast calls

3.Dedicated Control Channels (DCCH):

SDCCH( Standalone Dedicated Control Channel )1. Uplink and Downlink2. Used for call setup, location update and SMS.

SACCH( Slow Associated Control Channel )1. Used on Uplink and Downlink only in dedicated mode.2. Uplink SACCH messages - Measurement reports.3. Downlink SACCH messages - control info.

FACCH( Fast Associated Control Channel )1. Uplink and Downlink.2. Associated with TCH only.

BURSTThe Time Slots are arranged in a sequence, conventionally numbered 0 to 7.Each repetition of this sequence is called a TDMA Frame.The information content carried in one time slot is called a burst


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BURST:

InformationMain Area where the Speech, Data or Control info is held

Guard Period

To enable the burst to hit the time slot (0.031ms)Stealing Flags

2 bits are set when TCH is to stolen by a FACCHTraining Sequence

For estimation of transfer characteristics of physical mediaTail Bits

Used to indicate beginning and end of the burst.

\GSM Burst & TDMA Frame:

Five Types of Burst:

Normal BurstTraffic & Control Channels Bi-directional

Frequency Correction BurstFCCH Downlink

Synchronization BurstSCH Downlink

Dummy BurstBCCH Carrier Downlink

Access BurstRACH Uplink


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Call Scenarios:

Mobile to Mobile Mobile to Land Land to MobileIntra-city Intra-city Intra-city

Inter-city Inter-city Inter-city

Data Services:

Data rates supported as of today are2.4 Kbps4.8 Kbps9.6 KbpsGPRS & EDGE implementation takes the data capability to higher level of the order of184 kbps and more

Customer..Expectation:

Good coverage where ever he goesGood qualityNo blockingValue added services

SMS

Voice mailMMSCall forward/call waitingData/internet at high data ratesprepaid

Basic Network Design Objectives:

The basic objectives of a wireless system are:

COVERAGE: provide sufficient cell sites to deliver RF coverage of the entire desired area.

BUILDING/VEHICLE PENETRATION: deliver sufficient signal levels to adequately penetrateBuildings and vehicles where appropriate.

TRAFFIC: ensure that no cell captures more traffic than it can handle at the desired grade ofService (i.e., blocking percentage)

PERFORMANCE: design, construct, and adjust the network to deliver reliable service free.from excessive origination and call delivery failures, dropped calls, quality impairments,

and service outages


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ECONOMICS: provide return on investment sufficient to support operating and capitalExpenses, expand the network to take advantage of growth opportunities, and

retire Costs of construction prior to depreciation of the network equipment

High Level Design:

Inputs Coverage objectives

Area coverage objectivesCoverage penetration objectivesMorphology data/clutter information

Terrain data and Vector maps

Traffic objectives Number of subscribers definedTraffic per subscriber defined

Desired grade of service definedCity regulations BTS Hardware specifications Link Budget Business and Logistical objectives

Capital budgetTiming: launch dataOperating revenue Vs. total costs

Output Cell database and traffic model Composite coverage plot

Equal power handoff boundaries plot

Background Issues Impacting System Design:

Site acquisitionAvailability of suitable candidate (building or land) Owner interestCost of leasingFrequency clearance (SACFA) Government authority approvalSpace constraints and other construction issues

Candidate Location line of sight to the objective Clutter typeTerrain variations Physical Blocking buildings, hoardingsWaterMumbai High end, high traffic areas are very close to water.

Makes RF design much more challengingDeviation from desired location impacts surrounding site locations

Design considerations of Network (GSM/CDMA) :


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Understand geographical area as per license agreementDefine coverage expectations in terms

On road coverageIn-building coverage (different penetration margins)

Capacity considerations busy hour per subscriber call attempts and minutes of use (Erlangs)1 Erlang is 1 call of 1 hour durationDecide number of sites based on coverage capacity requirementPropagation tools used for this analysisFinalize exact site locations after field surveyInitiate candidate identification processSite acquisition/antenna positioningModify existing design if site location changes

GSM Planning Steps:

Various steps are listed belowCW surveyModel TuningNominal PlanningRF site SurveysRealized PlanningFrequency Planning

ImplementationOptimizationDrive TestingPerformance Analysis

Nominal Planning:

It consists of planning a set of sites on planning tool so as to predict the coverage of the target

area

Tool needs to be made intelligent so as to predict the coverage as close as possible to actual

coverageCoverage plots are based on customer intension of providing indoor and outdoor coverage

Clutter TypesClutter types

Dense UrbanUrbanSub UrbanRural


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WaterVegetationIndustrialForest

RF surveys:

Each nominal has a search ring defined by the RF PlannerCandidates needs to be identified as close as possible to the nominal within the search ringHeight, orientations & antenna placement at siteare the key RF parameter which are based uponthecoverage requirement in the areaMajor obstructions and clutter type in various directions to be observed on RF surveyEquipment required for RF Survey

GPSDigital CameraBinoculars

Magnetic CompassThere might be 3 or more candidates surveys for one siteEach candidate would have an RF survey form and panoramic associated with it

Drive Testing

Drive testing is an important activity to get statistics & graphs on coverage, quality & capacityin the downlink direction

Drive test setup DT tool, Engineering Handset, GPS, accessories Call in 2 modes

Dedicated while the mobile is on callIdle while the mobile is idle

Important parameters observed during drive testing

Coverage Rx level (Full & Sub)

Quality RxQual & SQI

Handover, Dropped call, Neighbor list, TA

What is Performance Optimization?

The words performance optimization mean different things to different people, viewed fromthe perspective of their own jobs

System Performance Optimization includes many different smaller processes at many points

during a systems life

recognizing and resolving system-design-related issues (cant build a crucial site, too

much overlap/soft handoff, coverage holes, etc.)


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cluster testing and cell integration to ensure that new base station hardware worksand that call processing is normal

fine-tuning system parameters to wring out the best possible call performanceidentifying causes of specific problems and customer complaints, and fixing them

carefully watching system traffic growth and the problems it causes - implementing

short-term fixes to ease hot spots, and recognizing problems before they becomecritical

Optimization:

Optimisation is an ongoing process of analysing network performance against Quality of Service

targets

Performance:

Measurements of network performance cover: Traffic in erlangs

TCH and SDCCH Grade of Service (Congestion) Call success rate Handover failure Coverage area Coverage quality Subscriber base and growth

Key Performance Indicators (KPI) are measurable dynamic parameters that help to target areas ofconcernKPIs:

Appropriate KPIs to use depend on:

The nature of the network

Data sources available

Measurement tools available

Ability of engineering team

Cost of network infrastructure Sources of data include:

Surveyed data - from drive tests

Network statistics - from OMC

Field engineer reports

Radio Interface Optimization:

Transmission TimingPower ControlVAD Voice Activity Detector and DTXMultipath FadingEqualizationDiversity


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Frequency HoppingAntenna Parameters ( Height, Azimuth, Tilts )

Benchmarking:Surveyed data from test-mobile measurements can be used to benchmark system

performance against that of a competitor Problems that may be identified from surveyed data: Poor coverage Unexpected interference Missing handover definitions Installation problems at BTS

Test-mobile measurements should include: continuous calls to test coverage repetitive short calls to test call-success Overview

Antenna Tilts:


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brief notes of gsm 2003