Mobile Computing

Cellular Concepts

Cellular Networks • Wireless Transmission• Cellular Concept• Frequency Reuse• Channel Allocation• Call Setup• Cell Handoffs• Location Management

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

Wireless Transmission

• Communication Frequencies– Frequencies in the VHF range are used– Regulation bodies

• Antennas– Theoretically: equal radiation in all directions– Reality: directive effects, sectorized antennas

Wireless Transmission

• 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

• Mobile hosts can change cells while communicating– Hand-off occurs when a mobile host changes base

station• Factors for determining cell size

– No. of users to be supported– Multiplexing and transmission technologies

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• As demand increases (more channels needed)

– Number of base stations is increased– Transmitter power is decreased

correspondingly to avoid interference

Cellular Concept

• 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, using FDMA, TDMA, CDMA…

Cellular Concept

• Cell size:– 100 m in cities to 35 km on the country side

(GSM) – even less for higher frequencies

Cellular Concept

• 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 cell vertices• sectored directional antennas

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

Cellular Concept

• Problems:– fixed network needed for the base stations– handover necessary– interference with 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

Geometric Representation

• Cells are commonly represented by hexagons.

• Why hexagon? • How about circle?• How about square, or triangle?

Hexagonal Cells

Channel Reuse

• The total number of channels are divided into K groups.– K is called reuse factor or cluster size.

• Each cell is assigned one of the groups.• The same group can be reused by two different

cells provided that they are sufficiently far apart.

Example:

K = 7

Coordinate System

Use (i,j) to denote a particular cell.

Example:

Cell A is represented by (2,1).

A

R D

Distance Formula

RK

RjijiD

3

)(3 22

22 jijiK

Note: i and j are integers

where

Reuse factor

Cellular System Architecture

MSC MSC

HLR

VLR

HLR

VLR

To otherMSCs

PSTNPSTN

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 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 and PSTN

Cellular System Architecture

• 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

• 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

Call Setup (contd)

• 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

Call Setup (contd)• 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

Hand-Offs

Mobile moves from 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

Hand-Offs (contd)• 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

Hand-Offs

• Intersystem: (Roaming)– 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

• 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)

Cellular Implementations

• 2.5G: Packet switching extensions– Digital: GSM to GPRS– Analog: AMPS to CDPD

• 3G:– High speed, data and Internet services– IMT-2000