Module W.bas - cs.sjtu.edu.cn

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Wireless Network BasicsModule W.bas.2

Dr.M.Y.Wu@CSEShanghai Jiaotong University

Shanghai, China

Dr.W.Shu@ECEUniversity of New Mexico

Albuquerque, NM, USA

© by Dr.Wu@SJTU & Dr.Shu@UNM

W.bas.2-2

Wireless network basics

Frequency spectrumTransmission mediaRadio wavesSpectrum regulation

Radio channel basicsRadio propagation

Channel characteristics

End of W.bas.2


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Frequency SpectrumFrequency spectrum can be used for data transmission: 300 Hz – 300 THz (1012 Hz)

λ = c/f Frequency f; wave length λ; speed of light c ≅ 3x108m/s

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 μm3 THz

1 μm300 THz

visible light

VLF

LF MF

HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair


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Transmission media

Wired300 Hz – 300 KHz, twisted pair copper wires up to km

300 KHz – 300 MHz, coaxial cable or twisted pair for short distances

300 GHz – 300 THz, fiber optics

Wireless3 KHz – 300 GHz


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Antennas and wavelength

An antenna must be the order of magnitude of the signal’s wavelength in size to be effective

Expensive, huge in size

1 MHz signal, 300m antenna



Portable, practical for handheld devices

1 GHz, 30cm antenna

10 GHz, 3cm antenna

UHF ANTENNAS

GSM ANTENNA


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Radio wavesGeneral characteristics

Easy to generate, both indoor & outdoor useAble to pass through buildings & travel long distancesIts transmission is omni-directional, no need to physically align the transmitter & receiverFor lower frequency

Waves can pass through obstacles easilyPower falls with 1/d2, d is distance

For higher frequencyReflected by obstaclesMore prone to absorption by rain drops


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Radio wavesGround waves: VLF, LF, MF

Follow the curvature of the Earth, penetrate waterIn a range of a few hundred kmsCommon applications

AM (amplitude modulated), 520 –1605.5 KHzTime signal, 60 KHz, 2500 KHz

ApplicationsWavelengthFrequencyBand

AM broadcasting

1000 – 100m300KHz – 3MHz Medium frequency

MF

Marine communications

10 – 1Km30 – 300 KHzLow frequency

LF

Marine communications

100m – 10Km3 – 30 KHzVery low frequency

VLF


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Radio wavesSky waves: HF; Line-of-Sight: VHF

Absorbed by the atmosphere near the Earth’s surfaceParts of them radiates upward to the ionosphere in the upper atmosphere, and reflected back and forthCommon applications

FM (frequency modulated), 87.5 – 108 MHzSW (short wave radio), 5.9 – 26.1 MHz, transmission power up to 500 kWAnalog TV, 174 – 230 MHz; DAB, 223 – 230 MHz


FM broadcasting; Analog TV;Digital audio broadcasting

(DAB)

10 – 1m30 – 300MHz Very high frequency

VHF

Aircraft/ship communications; Shortwave broadcasting

100 – 10 m3 – 30 MHzHigh frequency

HF


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MicrowavesGeneral characteristics

Tend to travel in straight linesCan be narrowly focused, energy is concentrated into a small beam with higher SNR (signal-to-noise-ratio)Used for mobile phone & TV transmissionUsed for long-distance telephony, before replaced by fiber optics

Due to higher frequencyDo not pass through buildingsRequire LOS (line-of-sight) alignment between the transmitting and receiving antennasNeed repeaters


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MicrowavesMicrowaves: UH, SHF, EHF

Get attenuated by objects found in their pathCommon applications

Digital TV, 470 – 862 MHzCellular phones, 850, 890-960, 1710-1880 MHz Wireless LANs, 2.4, 5 GHz


Wireless local loop (WLL)

10 – 1mm30 – 300GHz Extremely high

frequency

EHF

Satellite communications;

ISM bands

10 – 1cm3 – 30GHzSuper high frequency

SHF

Cellular phones;Digital TV; ISM

bands

100 – 10cm300MHz – 3GHzUltra high frequency

UHF


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Infrared & visible lightInfrared & visible light

Direct link having light-like instead of radio-like propertyUsed for short-distance communicationsRelatively directional, cannot travel through obstacles, cannot penetrate through rain or thick fogCommon applications

TV controllers, unguided opticalIrDA (Infra red data association), 850-900 nm


Optical communications

770 – 330nm400 – 900THzVisible Lights

Consumer electronics

1mm – 770nm300GHz –400THz

Infrared


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Spectrum regulationITU = International Telecommunication Union

ITU-R: ITU Radio communication sectorSplit the world into 3 regions

Region 1: Europe, Middle East, AfricaRegion 2: North & south AmericaRegion 3: Far East, Australia, New Zealand

Spectrum allocationComparative bidding (beauty contest), abandonedLottery system, not workingAuctioning, currently generate huge revenues

Free bands:ISM (Industrial, Scientific, Medical) for unlimited usage

900 MHz, 2.4 GHz, 5 GHz


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Bands for cellular phones

W-CDMA1920-19802110-2170

W-CDMA1920-19802110-2190

GSM

903-909 (移动GSM)909-915 (联通 GSM)

GSM, TDMA, CDMA1850-19101930-1990

GSM890-915, 935-9601710-17851805-1880

PDC810-826940-9561429-14651477-1513

AMPS, TDMA, CDMA824-849869-894

NMT453-457463-467

JapanChinaUSAEurope


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Bands for WLANs

HiperLan2IEEE 802.11a5150-5350

IEEE 802.11a5150-53505725-5825

HiperLan2IEEE 802.11a5150-53505470-5725

IEEE 802.112400-2497

IEEE 802.11IEEE 802.11bIEEE 802.11g902-9282400-2483

IEEE 802.112400-2483

JapanChinaUSAEurope


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Frequency spectrumTransmission media

Radio waves

Spectrum regulation

Radio channel basicsRadio propagationChannel characteristics

End of W.bas.2


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Channel: radio propagationTransmission range

communication possiblelow error rate

Detection rangedetection of the signal possibleno communication possible

Interference rangesignal may not be detected signal adds to the background noise

distance

sender

transmission

detection

interference


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Channel: radio propagation

ReflectionObject: very large compared to its wavelengthA phase shift of 180 degree between the incident & the reflected rays

DiffractionObject: impenetrableThe wave bends at the edge of the object, propagating in different directionsLower frequency waves diffracting moreCause the wave to reach places where no LoS

ScatteringObject: small compared to its wavelengthGets scattered into several weaker outgoing signals


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Channel: radio propagationMulti-path propagation

Signals traveling along different paths arrive at the receiver at different times delay spread

Typical values for delay spread: 3 μs in cityGSM can tolerate up to 16 μs, almost 5 km path difference

signal at sendersignal at receiver

LOS pulsesmultipathpulses


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Channel: path lossThe ratio = Preceive / Ptransmit over a given path

In generalFunction of radio frequency Function of propagation distanceDepends on the nature of the terrain

Free space propagation modelAssume a direct-path signalNo atmospheric attenuation, no multi-path

Preceive = Ptransmit Gt Gr ( λ/(4πd))2

Gt and Gr are the transmitter and receiver antenna gains, respectively


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Channel: path lossThe ratio = Preceive / Ptransmit over a given path

Function of radio frequency & propagation distanceTwo-path model

Capture multiple pathsA line-of-sight pathAnother path from reflection (or refraction)

Preceive = Ptransmit Gt Gr ( ht hr/d2)2

ht and hr are the heights of the transmitter and the receiver, respectively

Isotropic antenna modelAntennas power is the same in all directions

Preceive = Ptransmit Gt Gr (λ/(4π)2 ⋅ (1/d γ)γ is the propagation coefficient, varies 2 (free-space propagation) and 5 (strong attenuation)


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Channel: fadingFading = the fluctuations in signal strength when received at the receiverTwo types of fading:

Fast fading / small-scale fadingDue to the interference between multiple copies of the same signal arriving at slightly different times (caused by reflection, etc)

Slow fading / large-scale fadingBetween the transmitter and receiver, there exist objects that partially absorb the signal

The receiver is inside a building (the wall acts as an object in between)


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Channel: interferenceTwo types of interference:

Adjacent channel interferenceSignals in nearby frequencies have components outside their allocated rangesCan be avoided by introducing guard bands

Co-channel interference (or narrow-band interference)Due to other nearby systems using the same transmission frequency (frequency reuse)Can be reduced with the use of

Multi-user detection (for CDMA)Directional antennasDynamic channel allocation


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Channel: mobileDoppler shift = the change/shift in the frequency of the received signal

when the transmitter and the receiver are mobile with respect toeach other

fd = v / λMoving toward each other, the frequency is higherMoving away from each other, the frequency is lower

Limitation of different wireless systemsGSM: < 250km/hAMPS: < 100km/h

Common speed of moving objectsAircraft: 900 km/hTrain: 350 km/hAutomobile: 100 km/h


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Channel: rate constraintsTwo important constraints that determine the max rate of data transmission

Nyquist’s theorem in a noiseless channelBaud rate = number of times per second the signal changes its value/voltageBaud rate ≠ bit rate

Since each signal value can convey multiple bitsExample, the voltage values are 0, 1, 2, 3, used to convey two bits (00, 01, 10, 11)

C = 2 · B · log2 L bits/secC: the max channel data rate B: channel bandwidth in Hz (baud rate)L: the number of discrete signal levels used


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Channel: rate constraintsTwo important constraints that determine the max rate of data transmission

Shannon’s theorem in a noise channelC = B · log2 (1 + (S/N)) bits/sec

S: signal power N: noise powerC: the max channel data rate B: channel bandwidth in Hz (baud rate)SNR = 10 log10 (S/N), ratio in decibels


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Channel: rate constraintsExample 1: if a wireless communication link has the SNR = 20 dB, and the RF bandwidth is 30 KHz. Determine the maximum theoretical data rate can be transmitted.

Use Shannon’s theorem in a noise channelC = B · log2 (1 + (S/N)) bits/sec

S/N = 20 dB = 100 C = 30000 log2 (1+100) = 199.75 kbps

Example 2: what is the theoretical max data rate that can be supported in a 200KHz channel for SNR = 10 dB

C = 200000 log2 (1+10) = 691.9 kbps

The GSM data rate is 270 kbps, which is only about 40% of the theoretical limit for 10dB SNR condition.


W.bas.2-27


Frequency spectrumTransmission media

Radio waves

Spectrum regulation

Radio channel basicsRadio propagation

Channel characteristics

End of W.bas.2

Module W.bas - cs.sjtu.edu.cn

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