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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. 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
MSC MSC
HLR
VLR
HLR
VLR
To other
MSCs
PSTN
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