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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

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Basic Idea

Single hop wireless connectivity to the wired world Space divided into cells

Abase station is responsible to communicate with hosts in its cell

Mobile hosts can change cells while communicating

Hand-offoccurs 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

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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)

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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

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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

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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

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Cellular System Architecture

Each cell is served by abase 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

PSTNPSTN

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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 ofvisiting mobiles

Interfacing with other MSCs

Interfacing with PSTN (traditional telephone network)

One channel in each cell is set aside forsignalling 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

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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 bycommunicating 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

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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 transmitsbeacon

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 mobiles call

Intersystem:

Mobile moves across areas controlled by different MSCs

Handled similar to mobile assisted case with additional HLR/VLR effort

Local call may become long-distance

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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