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1、ECE DepartmentFlorida Institute of TechnologyECE 5221 Personal Communication SystemsIntroduction to GSM1ECE DepartmentFlorida Institute of TechnologyCourse OutlinePart 1: Introduction oHistorical overviewoElements of network architectureoElements of air interfacePart 2: Signal processing and network
2、 featuresoVoice processingoGSM Network featuresPart 3: Network designoCoverage planningoCapacity planningoMigration towards 3G and beyondThe GSM logo used on numerous handsets and by carries who wish to identify a GSM product 2ECE DepartmentFlorida Institute of TechnologyHistoryDriving Factors:Incom
3、patibility of the European analog cellular systemsReaching of capacity limitsCosts of the equipment1982, Conference of European Post and Telecommunications formed Group Speciale Mobile (GSM)1987, 15 operators from 13 countries signed Memorandum of Understanding (MoU)1991, Finlands operator Radiolini
4、a launched first GSM network in July 19911992, Massive deployment of GSM startedBy 2000 GSM became the most popular 2G technology worldwideGSM standard still evolving and enriched with new features and servicesGSM = Global System for Mobile communications (GSM: originally from Groupe Spcial Mobile)
5、3ECE DepartmentFlorida Institute of TechnologyDeployment worldwide930 networks in 222 countries and regionsMore than 3 billion subscribers worldwideMore than 80% worldwide market shareWorldwide map of GSM coverage (source )4ECE DepartmentFlorida Institute of TechnologyGSM in the USA1994, US FCC auct
6、ioned large blocks of spectrum in 1900MHzGSM started deployment in PCS band1995, American Personal Communications launched first GSM networkIn 2002, 850 band opened for GSM Currently there are 95M GSM subscribers Largest GSM operatorsATT T-MobileATT coverage mapT-Mobile coverage map5ECE DepartmentFl
7、orida Institute of TechnologyGSM StandardsDivided into 12 series Standardization efforts coordinated by ETSISpecifications available online free of chargeStandardization and public availability of specification - one of fundamental factors of GSM successSeriesSpecifications area01General02Service as
8、pects03Network aspects04MS-BS interface and protocol05Physical layer and radio path06Speech coding specification07Terminal adapter for MS08BS-MSC interface09Network internetworking10Service internetworking11Equipment and type approval specification12Operation and maintenanceGSM Standard6ECE Departme
9、ntFlorida Institute of TechnologyGSM Network LayoutGSM system layout is standardizedoStandardization involves:Elements of the networkCommunication InterfacesoStandard layout allows for the use of equipment from different suppliers7ECE DepartmentFlorida Institute of TechnologyGSM Components and Inter
10、facesNetwork has many functional componentsComponents are integrated through a network protocol MAPStandardized interfacesUm (air interface)A GERAN interfaceA-Bis (somewhat standardized)8ECE DepartmentFlorida Institute of TechnologyMobile Station (MS)Two functional partsoHW and SW specific for GSM r
11、adio interfaceoSubscriber Identity Module (SIM)SIM detaches user identity from the mobileoStores user informationoWithout SIM only emergency callsFunctional diagram of GSM mobileSIM cardMost popular GSM phone Nokia 1100 200M+ sold9ECE DepartmentFlorida Institute of TechnologyBase Transceiver Station
12、 (BTS)BTS is a set of transceivers (TX/RX).GSM BTS can host up to 16 TX/RX. In GSM one TX/RX is shared by 8 users.The main role of TX/RX is to provide conversion between traffic data on the network side and RF communication on the MS side.Depending on the application, it can be configured as macroce
13、ll, microcell, omni, sectored, etc. Typical BTS installationBTS antenna systemMacrocell BTS radio cabinet hosts TX/RXFemto-cell10ECE DepartmentFlorida Institute of TechnologyBSC plays a role of a small digital exchange.It can be connected to many BTSs and it offloads a great deal of processing from
14、MSCOne BSC connects to several tens to couple of hundred BTSSome of BSC responsibilities:oHandoff managementoMAHO managementoPower control oClock distributionoOperation and maintenanceTRAU is responsible for transcoding the user data from 16Kb/sec to standard ISDN rates of 64Kb/sec.It can physically
15、 reside on either BSC side or MSC side.If it resides on the MSC side, it provides substantial changes in the backhaul 4 users over a single T-1/E-1 TDMA channel.TRAU, BSC and BTSs form Base Station Subsystem (BSSBase Station Controller (BSC) and TRAUTypical BSCTRAU = Transcoding and Rate Adaptation
16、Unit11ECE DepartmentFlorida Institute of TechnologyResponsible for connecting the mobile to the landline sideGSM MSC is commonly designed as a regular ISDN switch with some added functionality for mobility supportGSM Network can have more than one MSCOne of the MSC has an added functionality for com
17、munication with public network Gateway MSC (GMSC)All calls from the “outside networks are routed through GMSCMobile Switching Center (MSC)12ECE DepartmentFlorida Institute of TechnologyRegistry HLR/VLRHLR Home Location RegistryDatabase for permanent or semi-permanent data associated with the userLog
18、ically, there is only one HLR per networkTypical information stored in HLR: International Mobile Service Identification Number (IMSI), service subscription information, supplementary services, current location of the subscriber, etc. HLR is usually implemented as an integral part of MSCVLR Visitor L
19、ocation registryTemporary database that keeps the information about the users within the service area of the MSCUsually there is one VLR per MSC The main task of the VLR is to reduce the number of queries to HLR. When the mobile, registers on the system its information is copied from HLR to VLRVLR i
20、s usually integrated with the switch13ECE DepartmentFlorida Institute of TechnologyAUC/EIRAUC Authentication centerIntegral part of HLRGSM specifies elaborate encryptionThree levelsoA5/1 USA + EuropeoA5/2 COCOM country listoNo encryption rest of the worldEIR Equipment Identity RegistryResponsible fo
21、r tracking equipment and eligibility for serviceMaintains three listsoWhite list approved mobile typesoBlack list barred mobile typesoGray list tracked mobile typesOver years many other vendor specific features added to the system 14ECE DepartmentFlorida Institute of TechnologyGSM Air Interface - Um
22、Interface between the MS and the GSM networkSubject to rigorous standardization processWe examine:oChannelization oMultiple access scheme oInterface organization:On the physical levelOn the logical level15ECE DepartmentFlorida Institute of TechnologyFrequency allocation For PCS-1900 bandoARFCNul = (
23、Fc-1850)/0.2+511; ARFCNdl = (Fc-1930)/0.2+511For GSM-850oARFCNul = (Fc-824)/0.2+127; ARFCNdl = (Fc-969)/0.2+127Mapping formulasGSM is FDD technology16ECE DepartmentFlorida Institute of TechnologyTDMA Access SchemeMultiple users operate on the same frequency, but not at the same time.Advantages of TD
24、MA:oRelatively low complexityoMAHOoDifferent user rates can be accommodatedoEasier integration with the landlineDisadvantages:oHigh sync overheadoGuard timesoHeavily affected by the multipath propagationTDMA = Time Division Multiple Access17ECE DepartmentFlorida Institute of TechnologyGSM as a TDMA
25、systemGSM is a combination of FDMA and TDMATDMA supports:oUp to 8 full rate usersoUp to 16 half rate usersGSM uses Frequency Division Duplexing18ECE DepartmentFlorida Institute of TechnologyGSM burstsData sent over one time slot = burstFive types: normal, frequency correction, synchronization, dummy
26、, accessFormat of a burst defied by its functionDL: normal, frequency correction, synchronization, dummyUL: normal, accessTime/Frequency/Amplitude diagram for GSM normal burst19ECE DepartmentFlorida Institute of TechnologyNormal BurstUsed to carry information on both control and traffic channelsMixt
27、ure of data and overheadGSM defines 8 training sequences assigned in color code modeBoth on the forward and reverse linkTotal of 114 encoded user information bitsTotal of 34 overhead bitsNormal burst20ECE DepartmentFlorida Institute of TechnologyFrequency Correction BurstSometimes referred to as the
28、 F-burstProvides mobile with precise reference to the frequency of the broadcast control channel Inserting the F-bursts on the control channel produces spectral peak 67.7 KHz above the central frequency of the carrierOnly on the forward linkSpectral characteristics of the control channel. The peak i
29、n the spectrum allows for easier MS network acquisition Format of the F-burstFixed sequence consists of all zerosFrequency correction burst21ECE DepartmentFlorida Institute of TechnologySynchronization BurstFacilitates the synchronization of the MS to the network at the base bandCommonly referred to
30、 as S-burstOnly on the forward link The same sync sequence is used in all GSM networksSynchronization burst22ECE DepartmentFlorida Institute of TechnologyDummy BurstSupports MAHOUsed to ensure constant power level of the broadcast control channelOnly on the forward link Dummy burst23ECE DepartmentFl
31、orida Institute of TechnologyAccess BurstUsed when the MS is accessing the systemShorter in length burst collision avoidanceExtended synchronization sequenceUsed only on the reverse linkGSM mobiles use slotted ALOHA to access the systemIn the case of collision a hashing algorithm is providedAccess b
32、urst24ECE DepartmentFlorida Institute of TechnologyGSM TDMA Hierarchical Organization25ECE DepartmentFlorida Institute of TechnologyGSM Time Division DuplexCommunication on the forward and reverse link does not happen simultaneouslyDelay of three slots between TX and RXTime division duplexing avoids
33、 RF duplexer at the RF stageoReduces the cost of mobileoSaves battery26ECE DepartmentFlorida Institute of TechnologyGSM Logical Channels27ECE DepartmentFlorida Institute of TechnologyTraffic channel carries speech and user data in both directionsoFull rate 33.85 Kb/secoHalf rate 16.93 Kb/secoFull ra
34、te uses 1 slot in every frameoHalf rate uses 1 slot in every other frame Data rates differ due to differences in Error Control CodingTraffic Channels (TCH)Full Rate TCH can carry: Voice (13 Kb/sec) Date at rates:-9.6 Kb/sec-4.8 Kb/sec-2.4 Kb/secHalf Rate TCH can carry: Voice (6.5 Kb/sec) Date at rat
35、es:-4.8 Kb/sec-2.4 Kb/sec28ECE DepartmentFlorida Institute of TechnologyControl ChannelsGSM Defines 3 types of Control Channels:1.Broadcast Channels (BCH)Broadcast information that helps mobile system acquisition, frame synchronization, etc. They advertise properties and services of the GSM network.
36、Forward link only2.Common Control Channels (CCCH)Facilitate establishment of the link between MS and systemBoth forward and reverse link3.Dedicated Control Channels (DCCH)Provide for exchange the control information when the call is in progressBoth forward and reverse in band signaling29ECE Departme
37、ntFlorida Institute of TechnologyBroadcast Channels (BCH)Three types of BCH:1.Synchronization channel (SCH)Provides a known sequence that helps mobile synchronization at the basebandCommunicates with S-burstBroadcasts Base Station Identity Code (BSIC)2.Frequency Correction channel (FCH)Helps mobile
38、tune its RF oscillatorCommunicates with F-burst3.Broadcast Control Channel (BCCH)Provides mobile with various information about network, its services, access parameters, neighbor list, etc.30ECE DepartmentFlorida Institute of TechnologyBroadcast Channels (BCH) contd.In general, the information sent
39、over BCCH can be grouped into four categories:1)Information about the network2)Information describing control channel structure3)Information defining the options available at the particular cell4)Access parametersSome BCCH messages31ECE DepartmentFlorida Institute of TechnologyCommon Control Channel
40、 (CCCH)Three types of CCCH:1.Random Access Channel (RACH)Used by mobile to initialize communicationMobiles use slotted ALOHAReverse link only2.Paging Channel (PCH)Used by the system to inform the mobile about an incoming callForward link onlyGSM Supports DRX3.Access Grant Channel (AGC)Used to send t
41、he response to the mobiles request for DCCHForward link only32ECE DepartmentFlorida Institute of TechnologyDedicated Control Channels (DCCH)Three types of DCCH:1.Stand Alone Dedicated Control Channel (SDCCH)Used to exchange overhead information when the call is not in progress2.Slow Associated Contr
42、ol Channel (SACCH)Used to exchange time delay tolerant overhead information when the call is in progress3.Fast Associated Control Channel (FACCH)Used to exchange time critical information when the call is in progressDCCHSDCCHSACCHFACCH33ECE DepartmentFlorida Institute of TechnologyLogical Channels -
43、 SummaryUL - UplinkDL - DownlinkChannelUL onlyDL onlyUL/DLPoint to pointBroadcastDedicatedSharedBCCHXXXFCCHXXXSCHXXXRACHXXXPCHXXXAGCHXXXSDDCHXXXSACCHXXXFACCHXXXTCHXXX34ECE DepartmentFlorida Institute of TechnologyTiming AdvanceMobiles randomly distributed in spaceTiming advance prevents burst collis
44、ion on the reverse linkMaximum advancement is 63 bits35ECE DepartmentFlorida Institute of TechnologySignal Processing From Voice to Radio WavesAs a digital TDMA technology GSM implements extensive signal processing36ECE DepartmentFlorida Institute of TechnologySampling and QuantizationSamplingoSampl
45、ing theorem specifies conditions for discretization of band limited analog signalsoVoice needs to be sampled at the sampling rate greater then 8000HzQuantization oDiscrete values assigned to continuous samplesoQuantization noiseoIn GSM, voice is sampled at 8 K samples/sec and quantized with 8192 lev
46、els (13 bit words)37ECE DepartmentFlorida Institute of TechnologySpeech Source EncodingSpeech coder reduces the data rate needed for voice signal representation GSM specifies operation of :oFull rate vocoder13Kb/secoHalf rate vocoderoEnhanced Full Rate (EFR)oAMR (Adaptive multi rate)AMR-FR (4.75-12.
47、2Kb/sec)AMR-HR (4.75-7.95Kb/sec)AMR rate - function of C/IVocoders enable efficient channel utilization38ECE DepartmentFlorida Institute of TechnologyPerformance comparison of some commercial vocodersMean Opinion Scores (MOS) - Voice Qualitysource IIR. The First Annual CDMA CongressLondon, Oct. 29-3
48、0, 199700.511.522.533.544.5Clean Speech20dB SNRBabble20dB SNRCar15dB SNRStreetMu-PCM8Kb/s EVRC(CDMA)13Kb/s CELP(CDMA)IS-136 ACELPGSM EFR39ECE DepartmentFlorida Institute of TechnologyError control coding (ECC) increases the robustness of the signalECC increases the overhead and reduces the efficienc
49、y of the communicationIn GSM, the ECC increases the overhead per user by 57%Channel Encoding 40ECE DepartmentFlorida Institute of TechnologyInterleaving In mobile communications, the errors are “burstyOptimal performance from ECC is obtained for uniform error distributionInterleaving increases the p
50、erformance of ECC in mobile environment41ECE DepartmentFlorida Institute of TechnologyModulation: GMSK (Gaussian MSK)GMSK has excellent spectral characteristicsoLow sidelobesoRobust to non- linearitiesPrice paid is in the increased Inter Symbol Interference (ISI)Simplified GMSK block diagramSpectral
51、 characteristics of GMSK42ECE DepartmentFlorida Institute of TechnologySequence used for equalizer trainingEqualizationNecessary due to the multipath propagation Needs to have :oFast convergenceoLow complexityTwo modes of operation1.Training2.EqualizationGSM equalizer capable of equalizing for two e
52、qual multi paths separated by 16 microsecondsIntroduces overhead of about 18% 43ECE DepartmentFlorida Institute of TechnologyGSM Network FeaturesMobile Assist Handoff (MAHO)Discontinuous Transmission (DTX)Dynamic Power Control (DPC)Frequency Hopping (FH)Intercell Handoff 44ECE DepartmentFlorida Inst
53、itute of TechnologyMobile Assisted Handoff (MAHO)GSM Implements MAHOIn the process of evaluating handoff candidates, GSM systems evaluate measurements performed by both the MS and BTSThere are three types of measurements:1.Signal Strength Measurements2.Signal Quality Measurements3.Timing Advance Mea
54、surements45ECE DepartmentFlorida Institute of TechnologyMAHO - Signal Strength MeasurementsPerformed on uplink and downlinkReported as a quantized value RXLEV:RXLEV = RSLdBm + 110Minimum RXLEV: -110, MAX RXLEV = -47On the downlink, measurement performed for both serving cell and up to 32 neighbors U
55、p to 6 strongest neighbors are reported back to BTS through SACHH Example measurement report46ECE DepartmentFlorida Institute of TechnologyMAHO - Signal Strength MeasurementsMeasurements of the neighbors are performed on the BCCH channels not affected by the DTXMeasurements on the serving channel af
56、fected by the DTX. Perform over a subset of SACCH that guarantees transmission even in the case of active DTXBefore processing, the RXLEV measurements are filtered to prevent unnecessary handoffsExample RSL measurement47ECE DepartmentFlorida Institute of TechnologyMAHO Signal Quality MeasurementsPer
57、formed on uplink and downlinkOnly on the serving channelReported as a quantized value RXQUALFor a good quality call RXQUAL 24 dB will not be completed before the arrival of new commandoCommonly implemented as BSC feature. Many vendors are moving it at the BTS levelDPC for BTSoVendor specificoBased o
58、n MAHO reports52ECE DepartmentFlorida Institute of TechnologyHierarchical Cell Structure (HCS)Incorporates various cell sizes into layers of RF coverage Three common layers:1.Umbrella cells (HL = 0)2.Macrocells (HL = 1)3.Microcell (HL = 2)HCS provides a way to assign preference levels between the ce
59、llsVery effective way for capacity and interference management53ECE DepartmentFlorida Institute of TechnologyHandling of Fast Moving MobilesIf the mobile is moving at a high speed, it will spend a short time in the coverage area of the microcellTo prevent excessive handoffs, a temporal GSM introduce
60、s temporal penalty prevents immediate handoff initializationIf the duration of mobile stay within the coverage area is shorter than the temporal penalty, it will never initialize handoff54ECE DepartmentFlorida Institute of TechnologyFrequency Hopping (FH)FH - multiple carriers used over the course o
61、f radio transmissionoThere are two kinds of FH:1.Slow Hopping change of carrier frequency happens at the rate slower than the symbol rate2.Fast Hoping carrier frequency changes faster than the symbol rateoGSM implements slow FH SchemeoCarrier frequency is changed once per time slotoThere are two rea
62、sons for frequency hopping1.Frequency Diversity2.Interference avoidance55ECE DepartmentFlorida Institute of TechnologyFrequency Diversity of FHMobile environment is characterized with small scale fadingThe depth of signal fade is a function frequency If two signals are sufficiently separated in freq
63、uency domain they fade independentlyFrequency diversity gain diminishes for fast moving mobiles56ECE DepartmentFlorida Institute of TechnologyInterference Avoidance of FHFH averages interferenceAllows for tighter reuse of frequencies Increases the capacity of the system57ECE DepartmentFlorida Instit
64、ute of TechnologyBaseband FH in GSMEach radio operates on a fixed frequencyThe bursts are routed to individual radios in accordance to their hopping sequenceAdvantages of baseband hoppingNo need to “real time retune simpler radiosMore efficient combinersDisadvantage of baseband hoppingNumber of hopp
65、ing frequencies limited by the number of radios58ECE DepartmentFlorida Institute of TechnologySynthesized FH in GSMEach radio is hopping in an independent wayRadios retune “real timeAdvantages of synthesized hopping:Set of the hopping frequencies can be assigned in an arbitrary wayDisadvantage of sy
66、nthesized hopping:Need for expensive and lossy combiners59ECE DepartmentFlorida Institute of TechnologyFH Algorithms Random HoppingImplemented in a pseudo random wayUses one of 63 available pseudorandom sequencesThe actual frequency is obtained as a modulo operation with number of available frequenc
67、ies in allocation list (FH group)Cyclic HoppingoFrequencies are used in the consecutive orderoIf the radio is performing cyclic FH the order of frequencies in the sequence goes from the lowest ARFCN to the highest ARFCN60ECE DepartmentFlorida Institute of TechnologyIntracell HandoffHigh Interference
68、 Measurement indicates:oPoor RXQUALoGood RXLEVThere is high probability that the call will improve with the handoff to different carrier within the same cellTo avoid unnecessary handoffs, system introduces maximum number of intercell handoffs61ECE DepartmentFlorida Institute of TechnologyGSM RF Plan
69、ning / DesignLink Budget and Nominal Cell Radius CalculationReceiver SensitivityRequired C/I ratioMobile Transmit PowerExamples of Link BudgetCalculation of a Nominal Cell RadiusFrequency Planning and Reuse StrategiesFrequency Planning Using Regular SchemesAutomatic Frequency PlanningCapacity of GSM
70、 Networks62ECE DepartmentFlorida Institute of TechnologyMigration:1.High speed circuits switched data (HSCSD)2.Packet switched data (GPRS,EDGE)3.Integrated packet services possibly under different access scheme (UMTS)GSM Migration Towards 3G63ECE DepartmentFlorida Institute of TechnologyGSM 2+ Data
71、Services This data rate can be used for:Short messagesFax services E-mail, etc.Circuit switched data servicesNot suitable for InternetToo slowToo costly (user would pay for the “circuit even if there is no traffic exchanged64ECE DepartmentFlorida Institute of TechnologyHigh Speed Circuit Switched Da
72、ta (HSCSD)HSCSD is using existing GSM organization to provide data services of a somewhat higher data rates It can combine several existing traffic channels into a single connection, i.e., it allows for mobiles multislot operationHSCSD can be implemented through software upgrades on existing network
73、s and no hardware upgrades are needed Seems to be less accepted by the service providers 65ECE DepartmentFlorida Institute of TechnologyGPRS is another new transmission capability for GSM that will be especially developed to accommodate for high-bandwidth data traffic GPRS will handle rates from 14.
74、4Kbps using just one TDMA slot, and up to 115Kbps and higher using all eight time slotsIt introduces packet switching - can accommodate the data traffic characteristicsGeneral Packed Radio Data (GPRS)66ECE DepartmentFlorida Institute of TechnologyGPRS Network architectureNew type of node: GPRS Servi
75、ce Node (GSN)67ECE DepartmentFlorida Institute of TechnologyGPRS Call routingRouting is performed “parallel to the GSM network68ECE DepartmentFlorida Institute of TechnologyPacket switchedUpgrades the modulation schemeoFrom GMSK to 8-PSKoMaximum speed 59 Kb/sec per time slot, 473.6 Kb/sec for all 8
76、time slotsoVariable data rate depending on the channel conditionsDefines several different classes of service and mobile terminalsEnhanced Data GSM Environment (EDGE)EDGE enabled data mobile69ECE DepartmentFlorida Institute of TechnologyPractically achievable data ratesTheoretical rates are constrai
77、ned by mobile power and processing capabilitiesMost mobiles support less than the maximum allowed by standardPractically achievable data rates70ECE DepartmentFlorida Institute of TechnologyUMTS 3G cellular serviceProvides data rates up to 2Mb/secPossibly standardized as W-CDMAUniversal Mobile Telephone Service (UMTS)Outline of UMTS (WCDMA) network71
