*Mobile TelecommunicationsCellular Wireless Networks

ReferencesMobile Communications by Jochen Schiller chapter 2 (section 2.8) and chapter 4 (sections 4.1.0 to 4.1.6)

Computer Networks by Andrew Tanenbaum edition 4 chapter 2 section 2.6

*Cellular Networks Wireless TransmissionCellular ConceptFrequency ReuseChannel AllocationCall SetupLocation ManagementCell HandoffsOptimizations: Power control, Cell capacityImplementations: AMPS, GSM, GPRS, 3G

*Cellular Wireless Networks First Generation (Analog voice)AMPS (Advanced Mobile Phone System)

Second Generation (Digital voice) D-AMPS ( Digital Advanced Mobile Phone System)GSM (Group Special Mobile)CDMA (Code Division Multiple Access)

Third Generation (Data and Digital Voice)IMT 2000 (International Mobile Telecommunications)W-CDMA (Wideband CDMA)UMTS (Universal Mobile Telecommunications System)CDMA 2000GPRS (General Packet Radio Service)

*Cellular Wireless Networks: Basic IdeaSingle hop wireless connectivity to the wired worldSpace divided into cellsA base station is responsible to communicate with hosts in its cell: Access point to the networkMobile hosts can change cells while communicatingHand-off occurs when a mobile host that has a call in progress starts communicating via a new base station

Factors for determining cell sizeNo. of users to be supportMultiplexing and transmission technologies

*Cellular Wireless Networks: Cellular ConceptLimited number of frequencies => limited channelsSingle high power antenna => limited number of usersSmaller cells => frequency reuse possible => more number of users

Base stations (BS): implement space division multiplexEach BS covers a certain transmission area (cell)Each BS is allocated a portion of the total number of channels availableCluster: group of nearby BSs that together use all available channelsMobile stations communicate only via the base stationFDMA, TDMA, CDMA may be used within a cell

As demand increases (more channels are needed)Number of base stations is increasedTransmitter power is decreased correspondingly to avoid interference

*Cellular Wireless Networks: Cellular Conceptcellpossible radio coverage of the cellidealized shape of the cellCell size:100 m in cities to 35 km on the country side (GSM) even less for higher frequenciesUmbrella cell: large cell that includes several smaller cellsAvoid frequent handoffs for fast moving trafficCell shapeHexagonal is useful for theoretical analysisPractical footprint (radio coverage area) is amorphousCell Frequenciesuse of several frequencies : 10 50 frequencies per cellnot the same frequency in adjoining cells to avoid interferencePower of base transceiver is controlled toAllow communications within cell on given frequencyLimit escaping power to adjacent cellsAllow re-use of frequencies in nearby cellsBS placement:Center-excited cell: BS near center of cell where omni-directional antenna is used Edge-excited cell: BSs on three of the six cell vertices where sectored directional antennas are used

*Frequency ReuseFrequency reuse only with a certain distance between the base stationsFrequency reuse patternN = number of cells in repetitious patternStandard model using cluster of 7Reuse factorEach cell in pattern uses unique band of frequencies (channels)

ExampleN cells all using same number of frequenciesK total number of frequencies used in systemsEach cell has K/N frequenciesAdvanced Mobile Phone Service (AMPS) K=395, N=7 giving 57 frequencies per cell on average

*Frequency ReuseCellular System with two frequency bands: 825 to 845 MHz for mobile station transmission and 870 to 890 MHz for base station transmission. A duplex circuit consists of one 30 kHz channel in each direction. The system reuse factor is N = 7

Consequentlyeach two-way channel is 60 kHz. number of channels available per cell cluster is K = 40*1000/60 = 666if number of clusters M = 7; hence the total number of simultaneous calls that can be supported by the system is 7* 666 = 4662each cell can use K/N channels (666/7=95 channels)

*Cellular System ArchitectureEach cell is served by a base station (BS)Each BS is connected to a mobile switching center (MSC) through fixed links

MSCMSCTo otherMSCsPSTNPSTNEach MSC is a local switching exchange that handlesSwitching of mobile user from one base station to anotherLocating the current cell of a mobile user (paging process)Interfacing with other MSCs Interfacing with PSTN (traditional telephone network)Visitor Location Register (VLR): database recording the visiting mobilessome channels in each cell is set aside for signalling information between BS and mobilesMobile-to-BS: location, call setup for outgoing, response to incomingBS-to-Mobile: cell identity, call setup for incoming, location updating

*Underlying technology for mobile phones, personal communication systems, wireless networking etc.Area divided into cellsEvery set of cells forms a Location AreaA Location Area is managed by an MSC

Profiles of mobile units that are visiting the location area are stored in a location area database called visitor location register (VLR)Profiles of all subscribers are stored in a network centralized database called Home Location Register (HLR) .Mobile stations communicate only via the base station

Cellular Wireless Networks: ArchitectureMobile Unit

*Cellular Wireless Networks: Mobility ManagementLocation Management: necessary when Mobile unit joins the network (location registration)mobile unit moves from one location area into another (location update)Mobile unit is called call delivery procedure that determines the current LA of a mobile unitPaging process to determine the current cell

Hand-off : necessary when mobile unit that has a call in progress moves from area of one BS into another. BS monitors the signal level of the mobileHandoff occurs if signal level falls below threshold

*Cellular Wireless Networks: Advantages and LimitationsAdvantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locallyProblemsfixed network needed for the base stationsinterference with other cellsMobility Managementhandover (changing from one cell to another) necessaryLocation ManagementLocation UpdateCall deliveryPagingLimited resources

*GSM Cellular Wireless Networksformerly: Groupe Spciale Mobile (founded 1982)now: Global System for Mobile CommunicationPan-European standard (ETSI, European Telecommunications Standardisation Institute)Hierarchical, complex system architecture comprising many entities, interfaces, acronyms EntitiesMS (mobile station)BS (base station)MSC (mobile switching center)LR (location register)GSM consists of three subsystemsRSS (radio subsystem): covers all radio aspectsNSS (network and switching subsystem): call forwarding, mobility management, switchingOSS (operation subsystem): management of the network

*GSM Architecturefixed networkBSCBSCMSCMSCGMSCVLRHLRNSS with OSS

RSSVLRSS7BSCBSCOMC, EIR, AUC

*GSM versus DAMPSGSM : 890 915 MHZ for uplinks and 935-960 MHz for downlinksDAMPS: 1850 1910 MHz for uplinks and 1930-1990MHz for downlinksIn both systems Frequency Division Multiplexing is is used with each mobile transmitting on one frequency and receiving on higher frequency.In Both systems, a single frequency pair is split by Time Division Multiplexing into time slots shared by multiple mobilesGSM Channels (200 kHz) are much wider than DAMPS channels(30 kHz)GSM channels is divided in 8 time slots while DAMPS channels is divided into 3 time slots. This gives GSM a much higher data rate per user than DAMPS.

*GSM Cellular Wireless NetworksGSM has 124 pairs of simplex channels.Each channels is 200 kHz wide and supports 8 separate connections.Theoretically 992 channels can be supported in each cell, but many of them are not available to avoid frequency conflicts with neighboring cells.

*GSM ChannelsControl channelsSetting up and maintaining callsEstablish relationship between mobile station and nearest BSGSM Manages three types of control channelsBroadcast Control Channel (BS to MS): Base Station continuously broadcasts its identity and some other info (channel status). Mobile stations monitor their signal to see when they have moved to a new cell.Dedicated Control Channel (bidirectional): used for location updating, registration, and call setup.Common Control Channel (BS to MS): Base station uses this channel to announce incoming calls (Paging process)Traffic channels (bidirectional)Carry voice and data

*System architecture: radio subsystem (RSS)RSS componentsBase Station Subsystem (BSS)Mobile Station (MS)Interfaces (Um, Abis, A) BSSNetwork comprises many BSSs. each BSS contains several BTSs and controlled by BSC. BSS performs all functions necessary to maintain radio connections to an MS, coding/decoding of voiceBase Transceiver Station (BTS) radio components including sender, receiver, antenna

Base Station Controller (BSC)switching between BTSs, controlling BTSs, managing of network resources, mapping of radio channels (Um) onto terrestrial channels (A interface)UmAbisABSSradio subsystemnetwork and switching subsystemMSMSBTSBSCMSCBTSBTSBSCBTSMSCRSS comprises the cellular mobile network up to the switching centersSS7

*Tasks of a BSS are distributed over BSC and BTSBTS comprises radio specific functionsBSC is the switching center for radio channelsBase Transceiver Station and Base Station Controller

Functions

BTS

BSC

Management of radio channels

X

Frequency hopping (FH)

X

X

Management of terrestrial channels

X

Mapping of terrestrial onto radio channels

X

Channel coding and decoding

X

Rate adaptation

X

Encryption and decryption

X

X

Paging

X

X

Uplink signal measurements

X

Traffic measurement

X

Authentication

X

Location registry, location update

X

Handover management

X

*Network and switching subsystemnetwork subsystemMSCMSCfixed partner networksSS7EIRHLRVLRISDN PSTNMobile Services Switching Center (MSC) controls all connections via a separated network to/from a mobile terminal within the domain of the MSC - several BSCs can belong to a MSCDatabases (important: scalability, high capacity, low delay)Home Location Register (HLR) central master database containing user data, permanent and semi-permanent data of all subscribers assigned to the HLR Visitor Location Register (VLR) local database for a subset of user data, including data about all user currently in the domain of the VLR.NSS is the main component of the public mobile network GSM switching, mobility management, interconnection to other networks, system control

*Mobile Services Switching CenterThe MSC (mobile switching center) plays a central role in GSMswitching functionsadditional functions for mobility supportmanagement of network resourcesinterworking functions via Gateway MSC (GMSC)integration of several databasesFunctions of a MSCspecific functions for paging and call forwardingtermination of SS7 (signaling system no. 7)mobility specific signalinglocation registration and forwarding of location informationprovision of new services (fax, data calls)support of short message service (SMS)generation and forwarding of accounting and billing information

*Operation subsystemThe OSS (Operation Subsystem) enables centralized operation, management, and maintenance of all GSM subsystemsComponentsAuthentication Center (AUC)generates user specific authentication parameters on request of a VLR authentication parameters used for authentication of mobile terminals and encryption of user data on the air interface within the GSM system Equipment Identity Register (EIR)registers GSM mobile stations and user rightsstolen or malfunctioning mobile stations can be locked and sometimes even localizedOperation and Maintenance Center (OMC)different control capabilities for the radio subsystem and the network subsystem

*GSM protocol layers for signalingCMMMRRMMLAPDmradioLAPDmradioLAPDPCMRR BTSMCMLAPDPCMRRBTSM 16/64 kbit/sUmAbisASS7PCMSS7PCM64 kbit/s /2.048 Mbit/sMSBTSBSCMSCBSSAPBSSAPCM: Call ManagementMM: Mobility ManagementRR : Radio ResourceLAPD: Link Access Procedure for D-ChannelBTSM: BTS ManagementBSSAP: BSS Application PartPCM: Pulse Code Modulation

* Signaling System #7

*Localization and CallingOne GSM feature: Automatic, worldwide localization of usersSystem always knows where a user currently is.HLR contains information about current locationOnly the location area, not the precise geographical locationWhen a mobile station moves into the range of new VLR (new location area), GSM performs location updateNew VLR informs the HLR about location changeHLR sends all user data needed to new VLR.Roamingchanging VLRs with uninterrupted availability of all services.Roaming can take place within:The network of one providerBetween two providers in one countryBetween different providers in different countries

*Localization and Calling : Location Update/RegistrationThe MS moves into a new LA; therefore, the new LA sends a location update request to the HLRThe HLR has stored the location information for the mobile station; therefore, it requests location cancellation from the previous location area.The previous LA deletes the temporary mobile station record and sends an acknowledgment to the HLR.The HLR acknowledges the location update to the new LA which in its turn starts providing service to the station.

*callingstationGMSCHLRVLRBSSBSSBSSMSCMS123456789101112131610101111111415173: GMSC identifies the HLR for the subscriber and signals the call setup to HLR4, 5: The HLR checks whether the number exists and requests Mobile Subscriber Roaming Number (MSRN) from current VLR6: HLR forwards responsible MSC to GMSC7: GMSC forwards the call to current MSC8, 9: MSC gets current status of MS10, 11: If MS is available, MSC initiates paging of MS12, 13: MS answers14, 15: security checks16, 17: set up connectionMobile Terminated Call-Reference Model1: user dials the phone number of GSM subscriber. PSTN notices that the number belongs to a user in the GSM network2: PSTN forwards the call to Gateway MSC

*HLRMSC/VLR(1)Calling MU(2)(3)(4)(5)Called MUConnexion EstablishementMobile Originated/Terminated Call-Real ImplementationA call request to a MS arrives to an originating LA. The originating LA sends a message to the MSs HLR through SS7 signaling system.The HLR sends a routing request message to the MSs current LA.The MSs current LA allocates a Routing Number for the call and returns it to the HLR. The HLR relays it to the originating LA which uses it to route the call to the called LA. The call starts after a communication is established between the originating LA and the called LA.

*Handover or HandoffCellular system require handover procedure, as single cells do not cover the whole service area.The smaller the cell size and the faster movement of a mobile station through the cells, the more handoffs of ongoing calls are required.Handoff should not cause a call drop or service interruption Some Reasons for a handover (Standard identified more than 40 reasons):Mobile station moves from one cell to another cellMobile station, that has a call in progress, moves out of range of a BTS The received signal level decreases continuously until it falls below the minimal requirements. This will diminish the quality of the radio link. Load balancingMSC or BSC may decide that traffic in one cell is too high and shift some MS to another cells with a lower load.

*Handoff ScenariosMSCMSCInter-cell, intra-BSC handoverMS moves from one cell to another but stays within the control of the same BSC. The BSC performs the handover by assigning a new radio channel in the new cell and releasing the old one.Inter MSC handoverHandover between two cells belonging to different MSCsInter-BSC, intra-MSC handoverMS moves between two cells controlled by different BSCs. This handover has to be controlled by the MSC.

*Handoff TypesMSCMSCSoft HandoffThe MS is acquired by the new BS before the previous one sign off.There is no loss of continuity.Downside: MS needs to be able to tune to two frequencies at the same time (The old channel and the nee one)Hard HandoffOld BS drops the MS before the new BS acquires it. If the new BS is unable to acquire it (e.g., because there is no available channel), the call is disconnected abruptly.The user tend to notice this, but it is inevitable occasionally with the current design

*Handoff Decisionreceive levelBTSoldreceive levelBTSoldMSMSHO_MARGINBTSoldBTSnewHandoff decision depends on receive levelHandoff decision does not depend on the actual value of the received signal level, but on the average value.BSC collects all values from BTS and MS and calculates the average value then compare it to HO_MARGIN (threshold)

*Intra MSC Handoff ProcedureHO accessBTSoldBSCnewmeasurementresultBSColdLink establishmentMSCMSmeasurementreportHO decisionHO requiredBTSnewHO requestresource allocationch. activationch. activation ackHO request ackHO commandHO commandHO commandHO completeHO completeclear commandclear commandclear completeclear completeBased on collected values, old BSC decide to perform a handover by issuing HO_required to MSCMSC asks the new BSC to allocate the resources needed for the handover from the new BSC. If resources are available, new BSC activates a physical channel at the new BTS for the MS.New BTS acknowledges the successful channel activation. New BSC acknowledges the handoff request. MSC issues a handover command that is forwarded to MS. MS breaks its old radio link and access the new BTS and establishes the linkOnce the handoff is completed, resources at the old BSC and BTS should be released