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Cellular Networks

Cellular Networks . Wireless Transmission Cellular Concept Frequency Reuse Channel Allocation Call Setup Location Management Cell Handoffs Optimizations: Power control, Cell capacity Implementations: AMPS, GSM, GPRS, 3G… . Basic Idea. Single hop wireless connectivity to the wired world

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Cellular Networks

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  1. Cellular Networks • Wireless Transmission • Cellular Concept • Frequency Reuse • Channel Allocation • Call Setup • Location Management • Cell Handoffs • Optimizations: Power control, Cell capacity • Implementations: AMPS, GSM, GPRS, 3G…

  2. Basic Idea • Single hop wireless connectivity to the wired world • Space divided into cells • A base station is responsible to communicate with hosts in its cell • Mobile hosts can change cells while communicating • Hand-off occurs when a mobile host starts communicating via a new base station • Factors for determining cell size • No. of users to be support • Multiplexing and transmission technologies • …

  3. Wireless Transmission • Communication Frequencies • Frequencies in the VHF – SHF range are used • Regulation bodies • Antennas • Theoretically: equal radiation in all directions • Reality: directive effects, sectorized antennas • Signal Propagation • Classification: Analog/Digital, Periodic/Aperiodic • Parameters: Amplitude, Frequency and Phase shift • Modulation Techniques • Amplitude, Frequency, Phase • Multiplexing Mechanisms • Space (SDM), Frequency (FDM), Time (TDM), Code (CDM)

  4. Cellular Concept • Limited number of frequencies => limited channels • Single high power antenna => limited number of users • Smaller cells => frequency reuse possible => more number of users • Base stations (BS): implement space division multiplex • Each BS covers a certain transmission area (cell) • Each BS is allocated a portion of the total number of channels available • Cluster: group of nearby BSs that together use all available channels • Mobile stations communicate only via the base station • FDMA, TDMA, CDMA may be used within a cell • As demand increases (more channels are needed) • Number of base stations is increased • Transmitter power is decreased correspondingly to avoid interference

  5. Cellular Concept • Cell size: • 100 m in cities to 35 km on the country side (GSM) • even less for higher frequencies • Umbrella cell: large cell that includes several smaller cells • Avoid frequent handoffs for fast moving traffic • Cell shape: • Hexagonal is useful for theoretical analysis • Practical footprint (radio coverage area) is amorphous • BS placement: • Center-excited cell: BS near center of cell • omni-directional antenna • Edge-excited cell: BSs on three of the six cell vertices • sectored directional antennas

  6. Cellular Concept • Advantages: • higher capacity, higher number of users • less transmission power needed • more robust, decentralized • base station deals with interference, transmission area etc. locally • Problems: • fixed network needed for the base stations • handover (changing from one cell to another) necessary • interference with other cells: co-channel, adjacent-channel • Important Issues: • Cell sizing • Frequency reuse planning • Channel allocation strategies Bottom line: Attempt to maximize availability of channels in an area

  7. MSC MSC HLR VLR HLR VLR To other MSCs PSTN PSTN Cellular System Architecture • Each cell is served by a base station (BS) • Each BS is connected to a mobile switching center (MSC) through fixed links • Each MSC is connected to other MSCs and PSTN

  8. Cellular System Architecture • Each MSC is a local switching exchange that handles • Switching of mobile user from one base station to another • Locating the current cell of a mobile user • Home Location Register (HLR): database recording the current location of each mobile that belongs to the MSC • Visitor Location Register (VLR): database recording the cell of “visiting” mobiles • Interfacing with other MSCs • Interfacing with PSTN (traditional telephone network) • One channel in each cell is set aside for signalling information between BS and mobiles • Mobile-to-BS: location, call setup for outgoing, response to incoming • BS-to-Mobile: cell identity, call setup for incoming, location updating

  9. Call Setup • Outgoing call setup: • User keys in the number and presses send (no dial tone) • Mobile transmits access request on uplink signaling channel • If network can process the call, BS sends a channel allocation message • Network proceeds to setup the connection • Network activity: • MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs • Source MSC initiates a call setup message to MSC covering target area • Incoming call setup: • Target MSC (covering current location of mobile) initiates a paging msg • BSs forward the paging message on downlink channel in coverage area • If mobile is on (monitoring the signaling channel), it responds to BS • BS sends a channel allocation message and informs MSC • Network activity: • Network completes the two halves of the connection

  10. Hand-Offs Hand-off necessary when mobile moves from area of one BS into another • BS initiated: • BS monitors the signal level of the mobile • Handoff occurs if signal level falls below threshold • Increases load on BS • Monitor signal level of each mobile • Determine target BS for handoff • Mobile assisted: • Each BS periodically transmits beacon • Mobile, on hearing stronger beacon from a new BS, sends it a greeting • changes routing tables to make new BS its default gateway • sends new BS identity of the old BS • New BS acknowledges the greeting and begins to route mobile’s call • Intersystem: • Mobile moves across areas controlled by different MSC’s • Handled similar to mobile assisted case with additional HLR/VLR effort • Local call may become long-distance

  11. Cellular Implementations • First-generation: Analog cellular systems (450-900 MHz) • Frequency shift keying for signaling • FDMA for spectrum sharing • NMT (Europe), AMPS (US) • Second-generation: Digital cellular systems (900, 1800 MHz) • TDMA/CDMA for spectrum sharing • Circuit switching • GSM (Europe), IS-136 (US), PDC (Japan) • 2.5G: Packet switching extensions • Digital: GSM to GPRS • Analog: AMPS to CDPD • 3G: • High speed, data and Internet services • IMT-2000

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