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Wireless Networks

Wireless networks

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Lecture Outline

• Wireless Networks Background

• Overview of Cellular Networks

• Overview of Satellite Networks

• Overview of Wireless Local Area Networks

• Other Wireless Networks

Based on the book: “Wireless communication and networks”

by © William Stallings, 2002 Prentice Hall

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Communication Frequency Spectrum

• Electromagnetic spectrum and applications (Tanenbaum 2003)

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• Aspects of Wireless Networks

– mobility and convenient deployment

– scarce frequency spectrum

– wireless implications such as transmission problems (e.g. interference, path loss, fading), security, battery, installation, health

• Wireless networks

– Cellular: GSM, PCS, IMT 2000

– Satellite: IRIDIUM, Globalstar

– WLAN: IEEE 802.11, HiperLAN

– Ad-Hoc, PAN, HAN

Wireless Networks Background

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• Extensive evolution and fast deployment

• First-Generation Mobile Phones: analog voice

(e.g. AMPS, NMT)

• Second-Generation Mobile Phones: digital voice

and some data (e.g. GSM, IS-95)

• Third-Generation Mobile Phones: digital voice

and data (e.g. 3G)

Wireless Cellular Networks

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• Mobile Terminals and Base Stations

• Communication area divided in hexagonal cells

• Cell dimensions from hundreds of meters till

tens of kilometers (e.g. GSM: 100m to 35 Km)

• Each cell served by a base station formed by a

transceiver and a control unit

• Each cell allocated a frequency band for

communication

• Communication from MS to BS -> reverse link

• Communication from BS to MS -> forward link

Principles of cellular networks [1]

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Principles of cellular networks [2]

• Frequency reuse: use the same frequency

spectrum in different set of cells

• Cells that reuse the same frequency must

be distant enough for avoiding

interference

• Transmission power control

• Migration of a mobile station from one

cell to another with continuance of

communication -> handoff

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Methods for increase capacity in cellular networks

• Adding new channels

• Frequency borrowing: congested cells use frequencies taken from adjacent cells

• Cell splitting:

– due to initial network design

– high-used cells are divided in smaller cells and frequencies are reallocated

• Cell sectoring:

– cells divided in sectors (e.g. 3, 6 sectors)

– each sector has allocated its own set of channels

– base stations use directional antennas for covering sectors

• Microcells and picocells: very small cells

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Cellular systems - general architecture

• General cellular system:– Mobile Station (MS)

– Base Station (BS)

– Mobile telecommunication switching office (MTSO)

• Communication between mobile station and base station use:– Control channels: exchange control data for calls management

– Traffic channels: data or voice connections between users

– Dominant switching mode: circuit-switch

MS

MS

BS

BS

BS

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Cellular systems operation example [1](a) Mobile station initialization (b) Mobile-originated call

(c) Paging (d) Call accepted

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(e) Ongoing call (f) Handoff

Cellular systems operation example [2]

• Other operations:

– call blocking

– call termination

– call drop

– calls to/from fixed and remote mobile subscriber

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Aspects of Cellular Networks [1]

• Radiowave propagation: – signal strength

– fading

– diverse propagation patterns

• Handoff: assigning a MS to a BS other than the current one when MS move from one cell toward other cell

• Different handoff parameters: cell blocking probability, call dropping, call completion, handoff success, handoff blocking, ...

• Handoff strategies:– relative signal strength

– relative signal strength with threshold

– relative signal strength with hysteresis

– relative signal strength with hysteresis and threshold

– prediction techniques

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Aspects of Cellular Networks [2]

• Power control requirements:

– reduce interference

– increase battery life

– overcome transmission conditions

– equalize received power (SS)

– other...

• Open-loop power control (a)

– depends on mobile station

• Closed-loop power control (b)

– depends on base station(a) Open-loop power control

(b) Closed-loop power control

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Aspects of Cellular Networks [3] - Access Methods• Frequency Division Multiple Access ->FDMA

– two (frequency) channels assigned per user, one for forward and one for reverse

– used in first generation cellular (e.g. AMPS)

• Time Division Multiple Access ->TDMA

– each physical channel divided in logical subchannels

– two logical channels assigned for user, for forward and reverse links

– transmission in repetitive sequence of frames divided in time slots

– each time slot position forms a logical channel that is assigned to the user

– used in second generation cellular (e.g. GSM)

• Code Division Multiple Access -> CDMA

– direct-sequence spread spectrum transmission -> use a chipping code for data

– two logical channels per user

– used in second and third generation cellular

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Second Generation Cellular Networks - Example

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GSM Cellular Network

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First, Second, Third Generation Cellular Networks

• More in 2G than 1G: – 2G have digital traffic channels, 1G pure analog– encryption – error detection and correction– dynamic channel access -> users share dynamically channels

• 3G capabilities:– voice quality comparable with switched telephone network– up to 384 kbps data rate outdoor– support for up to 2.048 Mbps indoor– symmetrical/asymmetrical data transmission rates– support for circuit switched and packet switched data services– support for wide variety of equipment– efficient spectrum usage– Internet interface– flexibility

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Satellite Communication

• Communication between earth stations and satellites

• Uplink-> earth station to satellite

• Downlink-> satellite to earth station

• Satellite categorization:

– Coverage area: global, regional, national

– Service type: fixed service satellite (FSS), broadcast service satellite (BSS), mobile service satellite (MSS)

– General usage: commercial, military, amateur, experimental

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Satellite Networks Configurations

• Point-to-point link

• Broadcast link

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Satellite Communication and Wireless Terrestrial Communication

• Advantages of satellite communication:– extensive area of coverage

– relative slowly variant conditions for communication between satellites

– transmission cost independent of distance

– support for broadcast, multicast and point-to-point communication

– high bandwidth and high data rates

– high quality of transmission

• Drawbacks of satellite communication :– expensive installation

– transmission delay

– needed terminals power

– needed number of satellites for global coverage

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Satellite and Orbits

• GEO - geostationary orbit satellites

• LEO - low earth orbit satellites

• MEO - medium earth orbit satellites

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GEO and LEO

• GEO characteristics:– no problems with the frequency change due to satellite movement

– simplified tracking of satellite

– very large area coverage (e.g. 3 satellite for almost whole Earth)

– weak signal and extensive delay (e.g. 0.5 s) due to long distance

– polar region poorly served

• LEO characteristics:– small coverage area-> big number of satellites

– satellite visibility cca. 20 minutes

– frequency changes due to satellite movement

– significant atmospheric drag

– small latency

– high data rates, up to few Mbps for Big LEOs

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Frequency Bands for Satellite Communication

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Issues in Satellite Communication

• Problems:

– Distance between earth station antenna and satellite antenna

– Downlink: terrestrial distance between earth antenna and the “aim point” of the satellite

– Atmospheric attenuation: oxygen, water, higher frequencies

• Capacity Allocation Strategies:

– frequency division multiple access (FDMA)

– time division multiple access (TDMA)

– code division multiple access (CDMA)

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Satellite Communication Examples

IRIDIUM LEO Satellite System

TDMA Satellite Transmission

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Wireless Local Area Networks (WLAN)

• General considerations:– limited utilization till last decade but extensive development lately

– usually in spaces where wired networks were difficult or not appropriate to deploy

– may increase reliability

– cost effective

– different standards

– different transmission medium used

• Some WLAN implications:– transmission problems: connection, multipath propagation, path loss and radio

signal interference

– network security

– system interoperability

– installation issues and health risks

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WLAN Applications [1]

• LAN Extension: – wireless extensions to fixed LANs

– stations in large open areas

– single or multiple cells

• Cross-Building Interconnect: – connect nearby building

– usually point-to-point communication

– connected devices: usually routers and bridges

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WLAN Applications [2]

• Nomadic Access: – connect mobile terminals with a LAN hub• Ad-hoc Networking: – spontaneous established temporary networks

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WLAN Requirements [1]

• Throughput: efficient use of the transmission medium

• Number of nodes: large number of nodes may be needed

• Connection to backbone LAN: usually a connection with a

wired networks is needed

• Service area: typical 100 to 300 m

• Battery life time: efficient management of mobile station

battery

• Transmission robustness and security: WLAN may be

interference prone and may be eavesdropped

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• Collocated network operations: more than one WLAN in the

same area

• License free operation: user oriented approach

• Handoff and roaming: moving between cells and even

networks may be needed

• Dynamic configuration: addition, deletion and reallocation

of end systems without affecting the network functionality

WLAN Requirements [2]

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WLAN Technologies [1]

Categorized according with the used transmission technology:

• Infrared (IR) LAN :

– IR does not penetrate walls

– limited to a single room

• Spread Spectrum LAN:

– use spread spectrum

– usually operate in ISM band

• Narrowband microwave LAN:

– operate at microwave frequency (e.g. above 1 GHz)

– can use ISM or licentiate frequency spectrum

– do not use spread spectrum

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WLAN Technologies [2]

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Infrared (IR) WLAN

• Directed Beam IR (a)

– point-to-point links

– range depend on power and wave focus

• Diffused (c)– all transmitter focus at a point on ceiling

– IR radiation is retransmitted (reradiate)

– reradiated IR waves are received by all

stations in the area

• Omnidirectional IR (b)

– single base station in LOS of all stations

– the base station acts as a repeater

(a)

(b)

(c)

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Infrared (IR) WLAN vs. Microwave WLAN

• Strengths

– virtual unlimited spectrum

– unregulated spectrum

– simple equipment needed -> inexpensive

– reflected by light-colored objects

– does not penetrate walls: more secure against eavesdropping and does not

introduce interference

• Drawbacks:

– sunlight, indoor lighting and other ambient radiation perceived as noise

– high-power transmitter required -> may introduce health problem (i.e. eye

safety) and power consumption problems

– limited range

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Spread Spectrum and Narrowband WLAN

• Spread Spectrum WLAN – usually multi-cell with different frequencies

– hub or peer-to-peer topology in a cell

– hub topology: the hub may provide access control and repeater operations and is

usually connected to a backbone network

– peer-to-peer topology: use ad-hoc connectivity without any hub

– usually unlicensed spectrum

– interference prone

• Narrowband WLAN – usually use narrow microwave band for transmission

– unlicensed as well as licensed frequency spectrum

– does not use spread spectrum

– licensed -> interference-free

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Other Wireless Networks - Cordless Systems• Characteristics– Residential: single base station with support for voice and data– Office: single base station for small offices, multiple base stations deployed using a cellular configuration for larger offices– Telepoint: a base station set up in a public place (e.g. an airport)– Small range for hand set -> low power– Inexpensive base station and hand set -> simple technical approaches– Limited frequency flexibility -> must work in different places

• Cordless Systems Example: DECT– Band: 1.88 - 1.9 GHz; Bandwidth: 20 MHz; Number of channels 120 – Access Method : TDMA/FDMA– Data rate: 1.152 Mbps; Speech rate: 32 kbps– Mean power: 10 mW– Maximum cell radius: 30 to 100 m– Provide handoff

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Other Wireless Networks - Wireless Local Loops (WLL)

• Narrowband WLL - replacement for telephony services

• Broadband WLL - high-speed voice and data services

• Usually use milimetric waves (e.g. above 10 GHz)• Advantages: cost, installation time, selective installation• Propagation problems: free space loss, rainfall attenuation, atmospheric and gaseous absorption, mutipath losses, vegetation effects• Multichannel Multipoint Distribution Service: MMDS (ex. 2.67-2.68 GHz)

• Local Multipoint Distribution Service: LMDS, standardized as IEEE 802.16(ex. 27.5-28.3 GHz)

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Other Wireless Networks - Personal Area Networks (PAN) and Home Area Networks (HAN)

HAN • Broadband smart house with intelligent appliances• HANs over phone lines, powerline and wireless HANs: diverse combination possible • Examples: HomeRF, Home Audio Video interoperability HAVi• Control Networks: low-speed powerline networks – specify protocols that are used via the power line– examples: LonWorks, X10, CEBus

PAN• Network serving a single person or a small group• Usually accommodate diverse mobile devices• Provide support for virtual docking station, peripheral sharing• Examples: Bluetooth, Infrared Data Association (IrDA), HomeRF

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Lecture Summary

• Overview of wireless networks and frequency spectrum for wireless

communication

• Cellular networks, cellular principles, channel access, different

generations of cellular networks

• Satellite communication, GEO, LEO, MEO

• WLAN, infrared, spread spectrum, microwave

• Other wireless networks: WLL, HAN, PAN, ad-hoc