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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
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
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
wireless transmission
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)
cellular concept
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
cellular concept1
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
cellular concept2
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

cellular system architecture

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
cellular system architecture1
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
call setup
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
hand offs
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
cellular implementations
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