2008 LK Communications Sans Fil

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Communications sans filWireless Communications

Professeur Kamoun Lotfi

Introduction to Wireless Networks

Wireless network characteristics

• Use of wireless medium=>freedom fromcables.

• Freedom from cables=>mobility.

• Rapid growth in last 15-20 years.

First wireless transmission: 1895 (Marconi)


Challenges

• Wireless medium unreliability.

• Spectrum use.

• Power management.

• Security.

• Location/routing.

• Interfacing with wired networks.• Health concerns.


• Evolution of Wireless Networks

1. Early mobile telephony (MTS, IMTS)

• 2. Analog cellular telephony (1G) systems

• 3. Digital cellular telephony (2G) systems

• 4. Cordless Telephony (CT) systems

• 5. Paging systems.




6. Wireless Local Area Networks

(WLANs)• Provide high-speed data within a relatively

small region (“Wireless Ethernet”).

• IEEE 802.11 standard: targets rates up to 54Mbps.

• The ETSI Hiperlan standards targets rates up to

155Mbps.


7. Wireless Asynchronous Transfer Mode

(WATM)• Aims to combine mobility (wireless networks) and

statistical multiplexing /QoS guarantees supportedby traditional ATM networks.

• Targets multimedia applications.

• Hiperlan 2: ATM compatible WLAN.


8. Personal Area Networks (PANs)• Target applications that demand very short-range

communications (few meters).

• Bluetooth standard.

• HomeRF standard.

• IEEE 802.15 Working Group.


9. Fixed Wireless Links• Wireless but not mobile!!

• Multipoint Multichannel Distribution System(MMDS).

• Local Multipoint Distribution System (LMDS).




•• 10. Satellite systems10. Satellite systems

•• Advantage: almost worldwide coverage.Advantage: almost worldwide coverage.

•• Various orbits (Low, medium,Various orbits (Low, medium,

•• geosynchronous, elliptical orbit systems).geosynchronous, elliptical orbit systems).

•• Iridium,Iridium, GlobalstarGlobalstar commercial systems.commercial systems.


11. Third generation (3G) systems• Target high-speed mobile data (up to 2 Mbps).

• Enhanced data Rates for GSM Evolution (EDGE)standard: speeds up to 473 Kbps.

• Cdma2000 standard: backwards compatible withcdmaOne (IS95), speeds up to 2 Mbps.

• Wideband CDMA (WCDMA): speeds up to 2Mbps.

Fundamentals of wireless networks

Wireless networks characteristics:

Increased BER (can be as high as 10-3).

Reasons for increased BER: atmosphericnoise. Multipath propagation, interference,etc.

Need for spectrum licensing.

Dynamic topologies – hidden terminals.

Energy limitations.


– Wireless channels are more prone to biterrors than wired ones. Apart from the higher

BER of wireless channels compared to wiredones, measurements also indicate adifference in the pattern of bit erroroccurrence: in contrast to the random natureof bit error occurrence in wired channels, biterrors over wireless channels occur in bursts.




Phenomena causing reception errors:

Free space path loss. Doppler shift: Due to station mobility.

Propagation mechanisms Reflection, diffraction, scattering)make signals travel over many different paths => Multipathpropagation.

Reflection (R), diffraction (D) and scattering (S).


Doppler Shift

• Doppler shift is caused when a signal transmitter andreceiver are moving relative to one another. In such asituation, the frequency of the received signal will not bethe same as that of the source. When they are movingtowards each other, the frequency of the received signalis higher than that of the source, and when they aremoving away form each other the frequency decreases.This phenomenon becomes important when developingmobile radio systems.


• Different spectrum bands=>different properties.

• Higher bands: less coverage, more energy.

• Lower bands: higher coverage, less energy.

The electromagnetic spectrum


• Multipath propagation=>rapid fluctuations of received signal power.

• AVG received signal power however has very small variations in the largescale and decreases only with significant distance increase.

• These fluctuations are due to the fact that echoes of the signal arrive with

different phases at the receiver and thus behaves like a noise signal.

Variation of signal level according to transmitter-receiver distance




General propagation model: P(r)=kr -n

r: distance n: path loss exponent (2 for free space, 2 ≤n ≤5 for real situations.

P(r): Received power at distance r .


Spectrum licensing procedures

Comparative bidding: Proposals of potential

operators are evaluated by nationalregulators. These specify to which spectrumis licensed.

Lottery: The luckiest candidate wins!

Auction: Candidate that makes the higher bidgets the license.


• Terminal mobility => Dynamic network topologies!

• Topologies: Not fully connected as well.

• “Hidden” and “exposed” terminal problems (due to notfully connected topologies).

(a) (b)

Terminal scenarios: (a) “Hidden” and (b) “exposed”


Analog and digital data transmission

Digital advantages:

– Transmission reliability (coding)

– Efficient spectrum use (less errors,compression)

– Security (encryption)




Modulation techniques for wireless systems

• Main Analog Modulation Techniques are:Amplitude Modulation (AM), and FrequencyModulation (FM).

• AM: superimposes the analog signal x(t) on the carrierc(t).

• FM: the information signal alters the frequency of thecarrier.

( ) ( )s t x t f t ( ) ( ) c o s= +1 2 π

s t A f x t d t t

( ) c o s ( )= +

∫ 2 π


Modulation techniques for wireless systems

• Digital techniques: Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK).

• ASK: carrier presence =>binary 1, carrierabsence=>binary 0.

• FSK: freq. fc-k=>binary 0, freq. fc+k=> binary 1. (BFSK)

• Higher level FSK techniques also exist.

( )

=

0,0

1,2cos)(

binary for

binary for ft At s

π

( )( )( )( )

s t A f k t f o r b i n a r y

A f k t f o r b i n a r y( )

c o s ,

c o s ,=

+

−

2 1

2 0

π

π


• BPSK: phase offset π =>binary 1, no phase offset=>binary 0.

• Higher level PSK techniques also exist.

• Differential PSK:binary 1 =>change in the phase of thecarrier, a binary 0 => carrier having the same phasewith that of the previous binary symbol.

• π /4-PSK.

( )

( )

s t A ft for binary

A ft for binary

( )cos ,

cos ,

=

+

2 1

2 0

π π

π


Multiple access for wireless systems

• Frequency Division Multiple Access (FDMA): Eachuser uses a different frequency channel.




Time Division Multiple Access (TDMA): EachTime Division Multiple Access (TDMA): Each

user uses the channel on a different time (timeuser uses the channel on a different time (timeslots).slots).


Code Division Multiple Access (CDMA): EachCode Division Multiple Access (CDMA): Each

user uses a unique code.user uses a unique code.


•• ALOHA: Stations transmit when ready.ALOHA: Stations transmit when ready.PerformancePerformance T(G)T(G) for loadfor load G G packets/slot:packets/slot:

•• Slotted ALOHA: Transmissions occur only atSlotted ALOHA: Transmissions occur only atbeginnings of slots. Performance:beginnings of slots. Performance:

•• Carrier Sense Multiple Access (CSMA): StationsCarrier Sense Multiple Access (CSMA): Stationsdefer when they detect active transmissions.defer when they detect active transmissions.Better performance than ALOHA. Used in IEEEBetter performance than ALOHA. Used in IEEE802.11,802.11, HiperlanHiperlan 1.1.

G

GeGT 2

)(−

=

T G G es

G( ) =

−


•• Polling protocols: A central BS grants toPolling protocols: A central BS grants to MSsMSs permissionpermission

to transmit.to transmit.

•• Examples: Randomly Addressed Polling (RAP), GroupExamples: Randomly Addressed Polling (RAP), Group

RAP (GRAP), TDMARAP (GRAP), TDMA--based RAP (TRAP), Learningbased RAP (TRAP), Learning

AutomataAutomata--based Polling (LEAP). Webased Polling (LEAP). We proposoedproposoed TDMATDMA

and LEAP.and LEAP.




Performance increasing techniques forwireless networks

• Antenna (spatial) diversity:

– Utilizes more than one (most of the times two)antennas spaced sufficiently apart from eachother.

– Combats multipath in NLOS situations, lessefficient in LOS situations.


• Multiantenna transmission/reception:

smart antennas. – not fixed but rather change in order to adapt

to the conditions of the wireless channel.

Use of smart antennas incellular systems

Possible placements of arrayelements in smart antennas.


• Coding

– Parity check.

– Hamming code. – Cyclic Redundancy Check (CRC).

– Convolutional coding.

A convolutional coder with r =1/3 and K =4


• Other performance increasing techniques: – Equalization: predicts the ISI (Inter-Symbol

Interference) that will be encountered by atransmission and accordingly modify the signal to betransmitted so as the signal reaching the receiver willrepresent the information the transmitter wants tosend.

– Power control: performance enhancement andenergy preservation by varying of transmissionpower.

– Multisubcarrier modulation: The channel bandwidthis divided into N subbands. The data stream isdivided into N interleaved substreams, which

modulate the carrier of each subband.




• The cellular concept

Simple cellular architecture


• Frequency reuse: Neighboring cells use

different frequencies

Frequency reuse scheme having a cluster size of 3.


The ad-hoc and semi ad-hoc concepts

• Ad-hoc: a distributed network of peer nodes.• Ad-hoc characteristics:

– Distributed operation. – Dynamic topology. – Multi-hop communications. – Changing link qualities. – Dependence on battery life.

• Semi ad-hoc: dual mode (centralized, ad-

hoc).


Wireless services: Circuit and packet-mode

• Circuit switching: connection is assigned adedicated sequence of links between nodes.

• Packet switching: each packet can follow adifferent route inside the switched network inorder to reach its destination.



Standarisation Standarisation

IEEE 802.18, the Radio Regulatory Technical Advisory Group (RR-TAG), is responsible for actively monitoring ongoing radio regulatory

activities, at both the national and international levels.

IEEE 802 currently has 6 Working Groups with projects on standardsfor radio-based systems – IEEE 802.11 (WLAN) – IEEE 802.15 (WPAN) – IEEE 802.16 (WMAN) – IEEE802.20 (Wireless Mobility) – IEEE 802.21 (Handoff/Interoperability Between Networks) – IEEE 802.22 (WRAN)

www.ieee802.org/18

Current Activities

• IEEE P802.11mb, Accumulated maintenance changes• IEEE P802.11n, Enhancements for Higher Effective Throughput• IEEE P802.11p, Wireless Access in Vehicular Environments

• IEEE P802.11r, Fast BSS-Transition• IEEE P802.11s, ESS Mesh Networking• IEEE P802.11u, Wireless Interworking with External Networks• IEEE P802.11v, Wireless Network Management• IEEE P802.11w, Protected Management Frames• IEEE P802.11y, 3650-3700 MHz Operation in USA• IEEE P802.11.2, Recommended Practice for the Evaluation of

802.11 Wireless Performance• IEEE P802.15.3c, High Rate Personal Area Networks (WPANs)• IEEE 802.15.4d, Alternative Physical Layer Extension to support the

Japanese 950MHz Band

Current activities

• IEEE P802.15.5, Recommended practices for mesh topologycapability in Wireless Personal Area Networks (WPANs)

• IEEE 802.15, Medical Body Area Network Study Group

• IEEE 802.15, Alternative PHY for China Study Group• IEEE P802.16g, Management Plane Procedures and Services• IEEE P802.16h, Improved Coexistence Mechanisms for License-

Exempt Operation• IEEE P802.16i, Mobile Management Information Base• IEEE P802.16j, Multihop Relay Specification• IEEE P802.16m, Advanced Air Interface• IEEE P802.16, Revision• IEEE 802.16, Maintenance Corrigendum 2• IEEE P802.17b, Spatially Aware Sublayer



Current Activities

• IEEE P802.19, Methods for assessing coexistence of wirelessnetworks

• IEEE P802.20, Air Interface for Mobile Broadband Wireless AccessSystems Supporting Vehicular Mobility - Physical and Media AccessControl Layer Specification

• IEEE P802.21, Media Independent Handover Services• IEEE P802.22, Cognitive Wireless RAN Medium Access Control

(MAC) and Physical Layer (PHY) specifications: Policies andprocedures for operation in the TV Bands

• IEEE P802.22.1, Enhance harmful interference protection for lowpower licensed devices operating in TV Broadcast Bands

• IEEE P802.22.2, Installation and Deployment of IEEE 802.22

Systems

IEEE 802.11Wireless LAN Working Group

The IEEE 802.11 working group standardizes

wireless local area networks (WLANs). Currentactivities include

– Increasing speed of operation – Mesh networking – Interfaces for national and regional frequency bands – Application to vehicular environments – Enhancements to managability – Enhanced capabilities for audio/video applications

www.ieee802.org/11

WiFi ou IEEE 802.11

IEEE 802.15Wireless Personal Area Network Working Group

The IEEE 802.15 WPAN Working Group developsstandards for wireless personal area networks.

802.15 has a number of active projects as listedbelow: – Increasing speed of operation

– Lower cost and lower power interfaces

– Medical body area network

– Mesh networking

– Interfaces for national and regional frequency bands

www.ieee802.org/15



IEEE 802.16Broadband Wireless Access Standards Working Group

The IEEE 802.16 Working Group on Broadband WirelessAccess Standards develops standards andrecommended practices to support the development anddeployment of broadband Wireless Metropolitan AreaNetworks.

– Fixed and mobile enhancements

– Multi-hop relays

– Conformance and co-existence

– Managability

www.ieee802.org/16

2008 LK Communications Sans Fil

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