Cellular Wireless NetworksIntroductionCellular GeometriesFrequency ReuseSlide 5Increasing CapacityOperationsOperations – cont.Typical Call - 1Typical Call - 2HandoffHandoff: When to?First Generation (Analog Systems)AMPS - OperationSequence of EventsSecond Generation (TDMA/CDMA)StandardsGlobal System for Mobile (GSM) CommunicationsGSM System ArchitectureMobile StationGSM SubsystemsBase Station Subsystem (BSS)GSM InterfacesNSSDatabases in NSSOSSSecond Generation CDMA (IS-95)IS-95 (continued)Third Generation (3G) Wireless NetworksCellular Wireless NetworksIntroduction•Cellular technology is the underlying technology for most mobile wireless communications and data transfers•Main concept: Use of multiple low-power transceivers•An area is divided into cells, each of which are served by a base station, consisting of transmitter, receiver, and a control unit•Adjacent cells are assigned different frequencies to avoid interferences and crosstalk. Cells at sufficient distance from each other can use the same frequency bands•The cells are hexagonal in shape•Each cell has a base transceiver – the transmission power is carefully controlled to avoid interferences with neighboring cells•About 10-50 frequencies are assigned to each cellCellular Geometries•HandoutFrequency Reuse•How many cells must intervene between two cells using the same frequency?D: Minimum distance between centers of cells that use the same band (co-channels)R: Radius of a celld: distance between adjacent cells (d=3 R)N: Number of cells in a repetitious pattern (reuse factor)D/R = √(3N)or D/d = √NFrequency Reuse•HandoutIncreasing Capacity•Adding new channels: –add unused channels•Frequency borrowing: –borrowed from adjacent cells–Frequencies can be also assigned dynamically•Cell Splitting: –Original cells (about 6.5-13km in size) can be split to smaller cells (1.5 km minimum)–Powers of transmitters are lowered–Handoff becomes more frequent•Cell Sectoring: A cell is divided into number of wedge-shaped sectors–Each sector is assigned a separate subset of cells channel–Directional antennas at the base station are used to focus on each sectorOperationsBase StationMobile Telecom Switching Office (MTSO)Base Station Base StationOperations – cont.•Base stations (BS) includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell•Each BS is connected to an MTSO. –One MTSO serves a single or multiple BSs–MTSO assigns the voice channel to each call, performs handoff, and monitors the call for billing information•Channels between the mobile units and the base stations–Control channels: used to exchange information having to do with setting up and maintaining calls–Traffic channels: carry a voice or data connections between usersTypical Call - 1•Mobile Unit Initialization:–The unit scans and selects the strongest set-up control channel–A handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell–The scanning process is repeated periodically•Mobile-originated call:–The number of the called unit is sent on the preselected set-up channel to the BS–The BS sends the request to the MTSO•Paging:–The MTSO sends a paging message to certain BSs–Each BS transmit the paging signal on its own assigned set-up channelTypical Call - 2•Call accepted:–The called mobile unit recognizes its number on the set-up channel being monitored and responds to that BS, which send the response to the MTSO–MTSO sets up a circuit between calling and called BSs–MTSO selects an available traffic channel within each BS’s cell and notifies each BS, which in turn notifies its mobile unit.•Ongoing call: –While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO•Handoff:–If a mobile unit moves out of range of one cell and into the range of another during a connection, the traffic channel has to change to one assigned to the BS in the new cellHandoff•Handoff Initiation:–Network-initiated or mobile assisted•Performance Metrics:–Cell blocking probability–Call dropping probability–Call completion probability–Probability of unsuccessful handoff–Handoff blocking probability–Handoff probability–Rate of handoff–Interruption duration–Handoff delayHandoff: When to?•HandoutFirst Generation (Analog Systems)•Advanced Mobile Phone System (AMPS) – developed by AT&T (early 1980’s)•Uses FDD scheme•AMPS Parameters:–Base station transmission band = 869-894 MHz–Mobile unit transmission band = 824-849 MHz–Spacing between forward and reverse channel = 45 MHz–Channel bandwidth = 30 kHz–Number of full-duplex voice channels = 790–Number of full duplex control channels = 42–Data transmission rate = 10 kbps–Cell size = 2-20km radiusAMPS - Operation•Each AMPS-capable cellular telephone includes a numeric assignment module (NAM) in read-only memory•NAM contains telephone number (provided by service provider) and the serial number of the phone•When turned on, the phone transmits its serial number to MTSO•MTSO maintains a database to take care of authentication and billing issues.Sequence of Events•The subscriber dials a number•The MTSO authorizes and issues a message to the user’s cell phone indicating which traffic channel to use for sending and receiving•The MTSO sends a ringing signal to the called party. When the party answers, the MTSO establishes a circuit between the two parties and initiates billing information•When one party hangs up, the MTSO releases the circuit, frees the radio channels, and completes the billing informationSecond Generation (TDMA/CDMA)•Goal: higher quality signals, higher data rates, and greater capacity•Advances:–Digital traffic channels: second generation systems provide digital traffic channels, supports digital data; voice traffic is encoded in digital form before transmitting–Encryption–Error detection and correction–Channel access: allows multiple users per channel using TDMA and CDMA•Second Generation Cellular Telephone System:–GSM–Interim Standard 136 (IS-136)–IS-95Standards•GSM (Global System for Mobile Communications):–Introduced in 1990, TDMA-based–Transmission range: BS=935-960 MHz, MS = 890-915 MHz–Channel bandwidth = 200 kHz, 8 users/channel–Deployed widely in Europe,