Wireless Sensor Networks Akyildiz/Vuran 1 PROTOCOL STACK Application Layer Transport Layer Network Layer Link Layer Physical Layer Power Management Plane

Wireless Sensor NetworksAkyildiz/Vuran

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PROTOCOL STACKPROTOCOL STACK

Application LayerApplication Layer

Transport LayerTransport Layer

Network LayerNetwork Layer

Link LayerLink Layer

Physical LayerPhysical Layer

Power M

anagement Plane

Mobility M

anagement Plane

Mobility M

anagement Plane

Task Managem

ent Plane


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Chapter 4:Chapter 4:Physical LayerPhysical Layer


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PHYSICAL LAYERPHYSICAL LAYER


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Source coding Source coding (data compression)(data compression) At the transmitter end, the information source is first At the transmitter end, the information source is first

encoded with a source encoderencoded with a source encoderExploit the information statistics Exploit the information statistics Represent the source with a fewer number of bits, Represent the source with a fewer number of bits, source codewordsource codeword

Performed at the Performed at the application layerapplication layer


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Channel coding Channel coding (error control coding)(error control coding) Source codeword is then encoded by the channel encoderSource codeword is then encoded by the channel encoder channel codeword channel codeword

Goal: address the wireless channel errors that affect the Goal: address the wireless channel errors that affect the transmitted informationtransmitted information


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Interleaving and modulationInterleaving and modulation The encoded channel codeword is then The encoded channel codeword is then interleavedinterleaved to to

combat the bursty errors combat the bursty errors Channel coding and the interleaving mechanism help the Channel coding and the interleaving mechanism help the

receiver either to receiver either to Identify bit errors to initiate retransmissionIdentify bit errors to initiate retransmissionCorrect a limited number of bits in case of errors. Correct a limited number of bits in case of errors.


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Interleaving and modulationInterleaving and modulation Then, an analog signal (or a set thereof) is Then, an analog signal (or a set thereof) is modulatedmodulated by the by the

digital information to create the waveform that will be sent digital information to create the waveform that will be sent over the channelover the channel

Finally, the waveforms are transmitted through the antenna Finally, the waveforms are transmitted through the antenna to the receiverto the receiver


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Wireless channel propagationWireless channel propagation The transmitted waveform travels through the channelThe transmitted waveform travels through the channel Meanwhile, the waveform is attenuated and distorted by Meanwhile, the waveform is attenuated and distorted by

several wireless channel effectsseveral wireless channel effects


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Information ProcessingInformation Processing


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Wireless channel propagationWireless channel propagation Attenuation:Attenuation: As the signal wave propagates As the signal wave propagates

through air, the signal strength is attenuated. through air, the signal strength is attenuated. Proportional to the distance traveled over Proportional to the distance traveled over

the airthe air Results in Results in path losspath loss for radio waves for radio waves

Reflection and refraction:Reflection and refraction: When a signal wave When a signal wave is incident at a boundary between two different is incident at a boundary between two different types of materialtypes of material a certain fraction of the wave bounces off a certain fraction of the wave bounces off

the surface, the surface, reflectionreflection.. a certain fraction of the wave propagates a certain fraction of the wave propagates

through the boundary, through the boundary, refractionrefraction. .


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Wireless channel propagationWireless channel propagation Diffraction:Diffraction: When signal wave When signal wave

propagates through sharp edges such propagates through sharp edges such as the tip of a mountain or a building, as the tip of a mountain or a building, the sharp edge acts as a the sharp edge acts as a sourcesource, , New waves are generatedNew waves are generated Signal strength is distributed to the Signal strength is distributed to the

new generated waves.new generated waves. Scattering:Scattering: In reality, no perfect In reality, no perfect

boundaries. When a signal wave is boundaries. When a signal wave is incident at a rough surface, it scatters incident at a rough surface, it scatters in different directionsin different directions


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Wireless Channel ModelWireless Channel Model Path-lossPath-loss Multi-path effectsMulti-path effects Channel errorsChannel errors Signals-to-bitsSignals-to-bits Bits-to-packetsBits-to-packets


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OverviewOverview Frequency bandsFrequency bands ModulationModulation Signal Distortion – Wireless Channel ErrorsSignal Distortion – Wireless Channel Errors From waves to bitsFrom waves to bits Channel modelsChannel models Transceiver designTransceiver design


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Wireless ChannelWireless Channel Wireless transmission Wireless transmission distorts distorts any transmitted signalany transmitted signal

WirelessWireless channelchannel describes these distortion effects describes these distortion effects Sources of distortionSources of distortion

AttenuationAttenuation – Signal strength decreases with increasing distance – Signal strength decreases with increasing distance Reflection/refractionReflection/refraction – Signal bounces of a surface; enter material – Signal bounces of a surface; enter material DiffractionDiffraction – start “new wave” from a sharp edge – start “new wave” from a sharp edge ScatteringScattering – multiple reflections at rough surfaces – multiple reflections at rough surfaces


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AttenuationAttenuation Results in path lossResults in path loss Received signal strength is a function of the distance Received signal strength is a function of the distance d d

between sender and transmitterbetween sender and transmitter Friis free-space modelFriis free-space model

Signal strength at distance d relative to some reference Signal strength at distance d relative to some reference distance ddistance d00 < d for which strength is known < d for which strength is known

dd00 is is far-field distancefar-field distance, , depends on antenna technology depends on antenna technology


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AttenuationAttenuation Friis free-space modelFriis free-space model


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Non-line-of-sightNon-line-of-sight Because of reflection, scattering, …, radio communication Because of reflection, scattering, …, radio communication

is not limited to direct line of sight communicationis not limited to direct line of sight communicationEffects depend strongly on frequency, thus different Effects depend strongly on frequency, thus different

behavior at higher frequencies behavior at higher frequencies

Line-of-sight path

Non-line-of-sight path


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Non-line-of-sightNon-line-of-sight Different paths have different lengths = Different paths have different lengths =

propagation timepropagation timeResults in Results in delay spread delay spread of the wireless channelof the wireless channel

signal at receiver

LOS pulsesmultipathpulses


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Multi-pathMulti-path Brighter color = stronger signalBrighter color = stronger signal Simple (quadratic) free space attenuation formula is not Simple (quadratic) free space attenuation formula is not

sufficient to capture these effectssufficient to capture these effects


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To take into account stronger attenuation than only caused To take into account stronger attenuation than only caused by distance (e.g., walls, …), use a larger exponent by distance (e.g., walls, …), use a larger exponent > 2 > 2 is the is the path-loss exponent path-loss exponent

Rewrite in logarithmic form (in dB):Rewrite in logarithmic form (in dB):

Generalizing the Attenuation FormulaGeneralizing the Attenuation Formula


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Obstacles, multi-path, etc?Obstacles, multi-path, etc? Experiments show can be represented by a random variableExperiments show can be represented by a random variable

Equivalent to multiplying with a lognormal distributed r.v. Equivalent to multiplying with a lognormal distributed r.v. in metric units ! in metric units ! lognormal fadinglognormal fading

Generalizing the Attenuation FormulaGeneralizing the Attenuation Formula


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Log-normal Fading Channel modelLog-normal Fading Channel model

XdddPLPdP tr

00 log10)()(

TransmitTransmitPowerPower

ReceivedReceivedPowerPower Path lossPath loss

Path lossPath lossexponentexponent

Log-normalLog-normalShadow fadingShadow fading


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OverviewOverview Frequency bandsFrequency bands ModulationModulation Signal distortion – wireless channelsSignal distortion – wireless channels From waves to bitsFrom waves to bits Channel modelsChannel models Transceiver designTransceiver design


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Noise and interferenceNoise and interference So far: only a single transmitter assumedSo far: only a single transmitter assumed

Only disturbance: self-interference of a signal with multi-Only disturbance: self-interference of a signal with multi-path “copies” of itselfpath “copies” of itself

In reality, two further disturbancesIn reality, two further disturbancesNoiseNoise – due to effects in receiver electronics, depends on – due to effects in receiver electronics, depends on

temperaturetemperature InterferenceInterference from third partiesfrom third parties

Co-channel interferenceCo-channel interference: another sender uses the same spectrum: another sender uses the same spectrum Adjacent-channel interferenceAdjacent-channel interference: another sender uses some other part of the radio : another sender uses some other part of the radio

spectrum, but receiver filters are not good enough to fully suppress itspectrum, but receiver filters are not good enough to fully suppress it


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Symbols and bit errorsSymbols and bit errors Extracting symbols out of a distorted/corrupted wave form Extracting symbols out of a distorted/corrupted wave form

is fraught with errorsis fraught with errorsDepends essentially on strength of the received signal Depends essentially on strength of the received signal

compared to the corruption compared to the corruption Captured by Captured by signal to noise and interference ratio (SINR)signal to noise and interference ratio (SINR)


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Symbols and Bit ErrorsSymbols and Bit Errors For WSNFor WSN

Interference is usually low Interference is usually low MAC protocols MAC protocolsSINR ~ SNR SINR ~ SNR SNR = PSNR = Prr – P – Pn n (in dB)(in dB)

PPnn – Noise power (noise floor) – Noise power (noise floor)


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Noise FloorNoise Floor Changes with timeChanges with time Varies according to location Varies according to location

(indoor vs. outdoor)(indoor vs. outdoor) Even if received power is the Even if received power is the

same, SNR varies with time!same, SNR varies with time!


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Bit Error Rate pb – Probability that a received bit will be in error

1 sent 0 received pb is proportional to SNR (channel quality)

Exact relation depends on modulation scheme


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Channel ModelsChannel Models


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Channel ModelsChannel Models Main goal: Stochastically capture the behavior of a wireless Main goal: Stochastically capture the behavior of a wireless

channelchannelMain options: model the SNR or directly the bit errorsMain options: model the SNR or directly the bit errors

Simplest modelSimplest modelTransmission power and attenuation constantTransmission power and attenuation constantNoise an uncorrelated Gaussian variable Noise an uncorrelated Gaussian variable

Additive White Gaussian Noise Additive White Gaussian Noise model, results in constant SNRmodel, results in constant SNR


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Channel ModelsChannel Models Non-line-of-sight pathNon-line-of-sight path

Amplitude of resulting signal has a Amplitude of resulting signal has a Rayleigh Rayleigh distribution distribution ((Rayleigh fading)Rayleigh fading)

One dominant One dominant line-of-sightline-of-sight plus many indirect paths plus many indirect pathsSignal has a Signal has a Rice Rice distribution (distribution (Rice fadingRice fading))


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Channel Model for WSNChannel Model for WSN Typical WSN propertiesTypical WSN properties

Low power communicationLow power communicationSmall transmission rangeSmall transmission range Implies small delay spread (nanoseconds, compared to Implies small delay spread (nanoseconds, compared to

micro/milliseconds for symbol duration) micro/milliseconds for symbol duration) !! Frequency-non-selective fading, low to negligible inter- Frequency-non-selective fading, low to negligible inter-

symbol interferencesymbol interference Coherence bandwidth often > 50 MHz Coherence bandwidth often > 50 MHz


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Channel Model for WSNChannel Model for WSN Some example measurementsSome example measurements

path loss exponentpath loss exponentShadowing variance Shadowing variance 22


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Channel Model for WSNChannel Model for WSNMarco Zuniga, Bhaskar Krishnamachari, "Marco Zuniga, Bhaskar Krishnamachari, "An Analysis of Unreliability and Asymmetry in Low-Power An Analysis of Unreliability and Asymmetry in Low-Power Wireless LinksWireless Links", ACM Transactions on Sensor Networks, Vol 3, No. 2, June 2007. (Conference version: ", ACM Transactions on Sensor Networks, Vol 3, No. 2, June 2007. (Conference version: ""Analyzing the Transitional Region in Low Power Wireless LinksAnalyzing the Transitional Region in Low Power Wireless Links", IEEE SECON 2004)", IEEE SECON 2004)

Log-normal fading channel best characterizes WSN channels

Empirical evaluations for Mica2

XdddPLPdP tr

00 log10)()(


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PRR – Packet reception rate (1-pb)k

Transitional region for packet reception Not too good, not too bad


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PRR significantly varies in the transitional region

d = 20m PRR = [0,1]

We cannot operate solely in the connected region Communication distance too

short


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Channel Models – Digital Channel Models – Digital Directly model the resulting bit error behavior (pDirectly model the resulting bit error behavior (pbb)) Each bit is erroneous with constant probability, independent of the Each bit is erroneous with constant probability, independent of the

other bitsother bits Binary symmetric channel (BSC)Binary symmetric channel (BSC)

Capture fading models’ property that channel is in different states! Capture fading models’ property that channel is in different states! Markov models – states with different BERsMarkov models – states with different BERs

Example: Gilbert-Elliot model with “bad” and “good” channel states and high/low bit Example: Gilbert-Elliot model with “bad” and “good” channel states and high/low bit error rateserror rates

good bad

pgb

pbg

pgg pbb


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Channel Models – Digital Channel Models – Digital Fractal channel models describe number of (in-)correct bits Fractal channel models describe number of (in-)correct bits

in a row by a heavy-tailed distributionin a row by a heavy-tailed distributionBurst errors (bit errors are NOT independent)Burst errors (bit errors are NOT independent)


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Wireless Communication BasicsWireless Communication Basics


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Wireless Communication BasicsWireless Communication Basics

Channel encoder(FEC)

Inter-leaver

Modula-tor

Demo-dulator

Deinter-leaver

Channel decoder

Channel

Tx antenna

Rx antenna

Source symbols Channel symbols Channel symbols Digital waveform

Channel symbols Channel symbolsSource symbols Digital waveform

Information source

Informationsink


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Wireless Communication BasicsWireless Communication Basics Frequency bandsFrequency bands ModulationModulation Signal distortion – wireless channelsSignal distortion – wireless channels From waves to bitsFrom waves to bits Channel modelsChannel models


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Radio spectrum for communication Radio spectrum for communication Which part of the electromagnetic spectrum is used for Which part of the electromagnetic spectrum is used for

communication communication Not all frequencies are equally suitable for all tasks – e.g., Not all frequencies are equally suitable for all tasks – e.g.,

wall penetration, different atmospheric attenuation wall penetration, different atmospheric attenuation (oxygen resonances, …)(oxygen resonances, …)


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Frequency allocation Frequency allocation Some frequencies are allocated to Some frequencies are allocated to

specific usesspecific uses Cellular phones, analog television/radio Cellular phones, analog television/radio

broadcasting, DVB-T, radar, emergency broadcasting, DVB-T, radar, emergency services, radio astronomy, … services, radio astronomy, …

Particularly interesting: Particularly interesting: ISM bands ISM bands (“Industrial, scientific, medicine”) – (“Industrial, scientific, medicine”) – license-free operationlicense-free operation

Some typical ISM bands

Frequency Comment

13,553-13,567 MHz

26,957 – 27,283 MHz

40,66 – 40,70 MHz

433 – 464 MHz Europe

900 – 928 MHz Americas

2,4 – 2,5 GHz WLAN/WPAN

5,725 – 5,875 GHz WLAN

24 – 24,25 GHz


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Example: US frequency allocationExample: US frequency allocation


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Transmitting Data Using Radio WavesTransmitting Data Using Radio Waves Basics: Wireless communication is performed through radio Basics: Wireless communication is performed through radio

waveswaves Transmitter can send a radio waveTransmitter can send a radio wave Receiver can detect the wave and its parametersReceiver can detect the wave and its parameters

Typical radio wave = sine function: Typical radio wave = sine function:

Parameters: amplitude A(t), frequency f(t), phase Parameters: amplitude A(t), frequency f(t), phase (t)(t) Modulation: Manipulate these parametersModulation: Manipulate these parameters

)()(2sin)()( tttftAts


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ModulationModulation Data to be transmitted is used to select transmission Data to be transmitted is used to select transmission

parameters as a function of timeparameters as a function of time These parameters modify a basic sine wave, which serves These parameters modify a basic sine wave, which serves

as a starting point for as a starting point for modulatingmodulating the signal onto itthe signal onto it This basic sine wave has a This basic sine wave has a center frequency fcenter frequency fcc

The resulting The resulting signalsignal requires a certain requires a certain bandwidthbandwidth to be to be transmitted (centered around center frequency)transmitted (centered around center frequency)


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Modulation (Keying) examplesModulation (Keying) examples Use data to modifyUse data to modify

Amplitude - Amplitude - Amplitude Amplitude Shift Keying (ASK)Shift Keying (ASK)

Frequency - Frequency - Frequency Frequency Shift Keying (FSK)Shift Keying (FSK)

Phase - Phase - Phase Shift Phase Shift Keying (PSK) Keying (PSK)


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Receiver: DemodulationReceiver: Demodulation Receiver tries to match the received waveform with the tx’ed Receiver tries to match the received waveform with the tx’ed

data bitdata bit Necessary: one-to-one mapping between data and waveformNecessary: one-to-one mapping between data and waveform

Problems (Wireless Channel Errors)Problems (Wireless Channel Errors) Carrier synchronization: Frequency can vary between sender Carrier synchronization: Frequency can vary between sender

and receiver (drift, temperature changes, aging, …)and receiver (drift, temperature changes, aging, …) Bit synchronization: When does symbol representing a Bit synchronization: When does symbol representing a

certain bit start/end?certain bit start/end? Frame synchronization: When does a packet start/end? Frame synchronization: When does a packet start/end?

Biggest problem: Received signal is Biggest problem: Received signal is notnot the transmitted signal!the transmitted signal!


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Bit Error Rate Mica2 nodes use frequency shift keying (FSK)

RBSNRNoEbep N

NoEbFSKb

/ ;

21 2

/ Bandwidth

Bit rate


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Bit Error Rate CC2420 (MicaZ, Tmote, SunSPOT) use offset quadrature

phase shift keying (O-QPSK) with direct sequence spread spectrum (DSSS)

)1(/34

)/(2/

/

KNoEbN

NoEbNNoEb

NoEbQp

DS

DSOQPSKb

# of chips per bit (16) =2 for MicaZ


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Documents

Wireless Sensor Networks Akyildiz/Vuran 1 PROTOCOL STACK Application Layer Transport Layer Network Layer Link Layer Physical Layer Power Management Plane