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: Cell shape: BS placement:
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: Problems: Important Issues:
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
HLR
VLR
To other
MSCs
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: Network activity: Incoming 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 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 ( 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