92-93نیمسال دوّم یار افشین هّمت

دانشکده مهندسی کامپیوتر

(40-873شبکه های بی سیم )مقدمه

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References

A. Kumar, D. Manjunath, and J. Kuri, Wireless Networking, Morgan Kaufmann Publishers, 2008.

C.M. Cordeiro and D.P. Agrawal, Ad Hoc and Sensor Networks: Theory and Applications, 2nd Ed,

World Scientific, 2011.

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Contents

Background Wireless communication: concepts, techniques,

and models Application models and performance issues Cellular FDM-TDMA Cellular CDMA Random Access and Wireless LANs (WLANs) Mobile Ad-hoc Networks (MANETs) Wireless Mesh Networks (WMNs) Wireless Sensor Networks ( WSNs)

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Wired Vs. Wireless Communications

Wired WirelessEach cable is a different channel One media shared by all

Low signal attenuation High signal attenuation

No interference High interferencenoise; co-channel interference; adjacent channel interference

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Advantages• Sometimes it is impractical to lay cables• User mobility• Cost

Limitations• Bandwidth• Fidelity• Power• Security

Why Wireless?

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Propagation Principle

electricfield

magneticfield

propagation direction

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Electromagnetic Spectrum

Propagation characteristics are different in each frequency band.

UV

1 MHz1 kHz 1 GHz 1 THz 1 PHz 1 EHz

infrared visible

X raysGamma rays

LF HF VHF UHF SHF EHFMF

AM radio

S/W ra

dioFM

radio

TV TV cellu

lar

902 – 928 Mhz2.4 – 2.4835 Ghz

5.725 – 5.785 Ghz

ISM band

30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz

10km 1km 100m 10m 1m 10cm 1cm 100mm

3GHz

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Unlicensed Radio Spectrum (ISM: Industrial, Science, Medicine)

902 Mhz

928 Mhz

26 Mhz 83.5 Mhz 125 Mhz

2.4 Ghz2.4835 Ghz

5.725 Ghz5.850 Ghz

cordless phonesbaby monitorsWaveLan

802.11bBluetoothMicrowave oven

802.11a

33cm 12cm 5cm

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Propagation Mechanisms

Non Line-of-Sight

Reflection

λ << D

Diffraction

λ D

Scattering

λ >> D

S DLine-of-Sight

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Propagation in the “Real World”

a wave can be absorbed

reflect

penetrate

bend

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Path-loss Models

Path-Loss Exponent Depends on environment:

L(d) = L(d0)(d/d0)n

Free space n = 2Urban area cellular n = 2.7 to 3.5Shadowed urban cell n = 3 to 5In building LOS n = 1.6 to 1.8Obstructed in building n = 4 to 6Obstructed in factories n = 2 to 3

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Networking as Resource Allocation

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Resource Allocation

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Wirelineo Static bit-carrier infrastructureo High quality digital transmission over copper or optical

mediao Bit pipes with a certain bit rate and very small bit error

rateo Dynamically reconfigured based on traffic demands

Wirelesso Point-to-point Line-of-sight (same as wireline or higher bit rate)o Time-varying channel impairmentso Adaptable PHY layer

Wireless Networking

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Our view:All the mechanisms, procedures, or algorithms for efficient sharing of a portion of the radio spectrum so that all instances of communication between the various devices obtain their desired Quality of Service (QoS).

Wireless Networks

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Fixed Networks

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Point to point Long distance transmission High gain antennas Tall masts Higher bit rate and also higher bit error rate than wireline

Mobile and Ad-hoc Networks

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Access Networks Mesh Network

Mobile Networks: Circuit Multiplexing

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GSM (2G)o Narrowbando FDM-TDMAo High SINR o Careful frequency planning to avoid co-

channel interferenceo Call admission control

GSM-GPRS (2.5G) Combining TDM Time slots GSM-EDGE (2.75G) Combining TDM Time slots

and higher order modulation schemes

Mobile Networks: Centralized Statistical Multiplexing (1)

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CDMA (IS-95)o Widebando CDMA (Spread spectrum)o Correlation receiverso No frequency planningo Interference limitedo Call admission control

WCDMA (CDMA-2000) o Most widely adapted standard for 3G

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WiMAX (IEEE 802.16 series)o Wireless access to Interneto Fixed subscriber stationso OFDMAo TDD (uplink & downlink)

o Specifications now have been extended to include broadband access to mobile users.

Mobile Networks: Centralized Statistical Multiplexing (2)

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WLAN (IEEE 802.11 series)o Wireless access to Interneto Limited mobilityo Statistical TDMAo Few Mbps (over 100s of meter) up to 100Mbps (over a few meters)o MIMO-OFDM (enhancement)

Mobile Networks: Distributed Statistical Multiplexing

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No infra-structure Multi-hop communication Point-to-point store and forward traffic

Miniature devices for nodes Low power, low bit rate digital radio transceiver, and small battery

Ad hoc Networks: Internet Access and Sensor Networks

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General: Transport of the user’s bits over the shared radio spectrum

Neighbor discovery, association and topology formation, routing

Transmission scheduling (cross layer)

Only in ad hoc sensor networks: Location determination Distributed computation

Technical Elements