ittc mobile wireless networkingjpgs/courses/mwnets/lecture-physical... · 2011-08-22 · physical...

The University of Kansas EECS 882Physical Layer & MW Environment

James P.G. Sterbenz

Department of Electrical Engineering & Computer ScienceInformation Technology & Telecommunications Research Center

The University of Kansas

jpgs@eecs.ku.edu

http://www.ittc.ku.edu/~jpgs/courses/mwnets

rev. 11.0

22 August 2011 KU EECS 882 – Mobile Wireless Nets – Phy. Layer & Env. MWN-MW-2

Mobile Wireless NetworkingPhysical Layer and Mobile Wireless Environment

PL.1 Physical media and spectrumPL.2 Wireless channels and propagationPL.3 Modulation, coding, and error controlPL.4 Mobile wireless environment

Physical LayerPhysical Layer Communication

networkCPU

end system

R = ∞

• Physical layer communicates digital information– through a communication channel in a medium– digital bits are coded as electronic or photonic signals

• digital or analog coding

– over a link between nodes (layer 2)

Mobile Wireless NetworkingPhysical Layer and Mobile Wireless Environment

• EECS 882 is a networking course– but the operation of the network depends on…– characteristics of the communication channels– physical environment

• Therefore– brief non-mathematical introduction to physical layer

details in EECS 865• review for EE folk• important for CS and IT folk to understand why

– discussion of impact of mobility and wireless on network

Mobile Wireless NetworkingPL.1 Physical Media and Spectrum

Physical MediaGuided

• Guided media– wire

• twisted pair• coaxial cable• power line

– fiber optic cable

role in communication networks?

Physical MediaGuided

traditional Internet & PSTN mostly guided mediaEECS 780 and 881

Physical MediaUnguided

• Unguided media– free space

• radio frequency• optical

Physical MediaUnguided

• Unguided media– wireless free space

• radio frequency• optical

networks with unguided media subject for EECS 882

Wireless Free SpaceSpectrum

• Spectrum– range of frequencies available for communication

• λf = c ; c = 3×105 km/s

– only some spectrum usable for communicationwhich parts?

[http://www.ntia.doc.gov/osmhome/allochrt.pdf]

• λf = c ; c = 3×105 km/s

– RF: radio frequency• frequency determines propagation characteristics

• λf = c ; c = 3×105 km/s

– RF: radio frequency– optical

• infrared 800–900 nm = 333–375 THz 41 THz spectrum

why not higher frequencies?

• λf = c ; c = 3×105 km/s

– RF: radio frequency– optical– higher frequencies: UV, x-ray, γ-ray, …

• health risks of radiation exposure• frequency beyond current transceiver technology• propagation problems

Wireless Free SpaceSpectrum Table

Band Range Propagation Usage Examples

Name Description Frequencies Wavelength Sight

ELF GW

visible visible 400– 900THz 770–330 nm

Attenuation

30– 300 Hz

300–3000 Hz

3– 30kHz

30– 300kHz

300–3000kHz

3– 30MHz

30– 300MHz

300–3000MHz

3– 30GHz

30– 300GHz

300GHz–400THz

ext. low 10– 1Mm home automation

voice 1000–100km voice tel., modem

very low 100– 10km atmos. noise submarine

low 10– 1km daytime maritime

medium 1000–100 m daytime maritime, AM radio

high 100– 10 m daytime transportation

very high 10– 1 m temp, cosmic television, FM radio

ultra high 1000–100mm cosmic noise television, cell tel.

super high 100– 10mm O2, H2O wireless comm.

ext. high 10– 1mm O2, H2O vapor wireless comm.

infrared 1000–770nm optical comm.

Wireless Free SpaceCommunication and Radar Bands

• Communication band designations– UHF, VHF, and SHF bands subdivided– ITU-T B.15 and ITU-R V.431-7– IEEE Std. 251

Wireless Free SpaceCommunication and Radar Bands

Band Range Usage Examples

UHF 300MHz–1GHz 1000–300mm television, p2p radio

L 1–2GHz 300–150mm cordless and mobile phones, satelliteUHF

C 4–8GHz 75– 40mm PSTN relay, satellite, WLAN

X 8–12GHz 40– 25mm satellite links

Ku 12–18GHz 25– 17mm satellite links

K 18–27GHz 17– 10mm satellite, µwave links

Ka 27–40GHz 10– 7.5mm satellite, WMAN

Name Partition Frequencies Wavelength

mm 110–300GHz 27–.77µm

future

10– 1 m television, FM radio30– 300MHz

2–4GHz

40–75GHz

150– 75mm WLAN, WPAN, WMAN, satellite

7500– 40µm emerging WLAN, WMAN

40– 27µm future75–110GHzEHF

Wireless SpectrumAllocation

• Spectrum allocationwhat is it?

• Spectrum allocation – how to partition among:– application

• broadcast radio, television, data communication, etc.

– user sector• consumer, business, government, military

– assignees• entity that is allowed to transmit into spectrum• service providers and/or end users

Wireless SpectrumAllocation: Governance

• Governance of spectrum allocation• International

– ITU-R: International Telecommunication –Radio Sector www.itu.int/ITU‐R

– began as International Telegraph Union in 1865– now agency under UN mandate

• incentive for UN members to play nice

Wireless SpectrumAllocation: Governance

• Governance of spectrum allocation• International

– ITU-R: International Telecommunication –Radio Sector www.itu.int/ITU‐R

• National: government agency or appointee– US FCC – Federal Communications Commission

www.fcc.gov

– UK Ofcom – Office of Communicationswww.ofcom.org.uk

– India TRAI – Telecom Regulatory Authority of India www.trai.gov.in

– etc.

Wireless SpectrumRegulation

• Regulations for transmission within allocation– determined and enforced by governing bodies

• Parameters for allowed communication, e.g.– transmission power– field strength– interference parameters

• transmission energy permitted outside allocation

– geographic limits– date and time restrictions

• e.g. AM radio clear channel interference

Wireless SpectrumEnforcement

• Enforcement of transmission regulations– enforcement by national entity– e.g. US FCC EB (enforcement bureau) www.fcc.gov/eb

• Can cause international tension– e.g. former border blasters

• e.g. XETRA-AM (Mighty 690) in Tijuana Mexico• now US-Mexico treaty coordinates AM transmission

Wireless SpectrumLicensing

• Licensed spectrum– most frequency bands require license to transmit– generally issued by national authority (FCC, Ofcom, etc.)

• e.g. broadcast TV, radio, amateur radio, GMRS• e.g. mobile telephony

– some unlicensed bands do not require license to transmit• e.g. US CB (citizen band), FRS (family radio system)• e.g. ISM for cordless telephones and wireless LANS

unlicensed ≠ unregulated!

Licensed SpectrumBroadcast Radio and Television

Allocation Frequency Range

AM long wave 153 – 279 kHz

AM medium wave 520 – 1610 kHz broadcast radio

AM short wave 2.3 – 26.1 MHz transcontinental broadcast radio

VHF TV ch. 2–4ch. 5–6

54 – 72 MHz76 – 88 MHz broadcast television US ch.

65.9 – 74 MHz76 – 90 MHz87.5 – 108 MHz

174 – 216 MHz

512 – 698 MHz698 – 806 MHz806 – 894 MHz

FM radio

VHF TV ch. 7–13

ch. 18–51UHF TV ch. 52–69

ch. 70–83

Typical Use Notes

broadcast radio not US

broadcast radioOIRTJapanInternational

broadcast television US ch.

broadcast television phasing outreclaimed

Licensed SpectrumSelected Telephony Bands

Allocation Frequency Band

AMPS, IS-95, GSM 824 – 894 MHz

GSM 890 – 960 MHz1710 – 1880 MHz

Europe

cdma2000, UMTS 1710 – 1755 MHz

IS-95, GSM 1850 – 1990 MHz Americas PCS

2110 – 2155 MHz

2496 – 2690 MHz

cdma2000, UMTS, W-CDMA

Americas

Wireless Free SpaceUnlicensed Spectrum

• Unlicensed spectrum– regulations for use (FCC Title 47 Part 18 and 15.243–249)

• max transmit power (e.g. 1W)• field strength• spread spectrum parameters

– ISM: industrial, scientific, and medical• … 900 MHz, 2.4 GHz, 5.8 GHz, 24GHz …

– UNII: unlicensed national information infrastructure• 5.8 GHz

– may be use by anyone for any purpose (subject to regulations)

problem?

Wireless Free SpaceUnlicensed Spectrum

• Unlicensed spectrum– regulations for use (FCC 15.243–249)

• e.g. max transmit power• e.g. spread spectrum parameters

– ISM: industrial, scientific, and medical• … 900 MHz, 2.4 GHz, 5.8 GHz, 24GHz …

– UNII: unlicensed national information infrastructure• 5.8 GHz

– may be use by anyone for any purpose (subject to regulations)

– interference a significant problem• e.g. 2.4 GHz FHSS cordless phones against 802.11b• e.g. interference among 802.11 hubs in dense environments

Unlicensed SpectrumISM Bands

Band Center & BW Freq

7 MHz 6.78 ± 0.15 MHz

13 MHz 13.560 ± 0.007 MHz

27 MHz 27.120 ± 0.163 MHz

61 GHz 61.250 ± 0.250 GHz 802.11ad, millimeter-wave links

40 MHz 40.680 ± 0.020 MHz

433.92 ± 0.87 MHz

915.00 ± 13.00 MHz

2.450 ± 0.050 GHz

5.800 ± 0.075 GHz

24.125 ± 0.125 GHz

122.500 ± 0.500 GHz

245.000 ± 1.000 GHz

433 MHz

900 MHz

2.4 GHz

5.8 GHz

24 GHz

122 GHz

245 GHz

Typical Use Notes

Europe

cordless phones, WLANs (historic)

cordless phones, WLANs, WPANs

cordless phones, WLANs (limited use)

Microwave mesh link

future

Wireless SpectrumAllocation Process

• ITU allocates and regulates international spectrumproblem?

• ITU allocates and regulates international spectrum– competing national interests– agreement can be difficult– compromises are frequently poor solutions

• ITU allocates and regulates international spectrum• Government agencies allocate and regulate spectrum

problem?

• ITU allocates and regulates international spectrum• Government agencies allocate and regulate spectrum

– competing business, consumer, and government interests– agreement can be difficult– compromises are frequently poor solutions

• Spectrum allocated in fixed frequency ranges– exclusive use : spectrum within defined area

• government, military, public safety, public interest

• Spectrum allocated in fixed frequency ranges– exclusive use

– command-and-control : license assignment• comparative bidding

– process: broadcasters and service providers submit proposalsproblem?

– command-and-control• comparative bidding

– process: broadcasters and service providers submit proposals– problem: fairness (real and perception) and appeals

– command-and-control• comparative bidding• lottery

– process: assignees picked by lotteryproblem?

– command-and-control• comparative bidding• lottery

– process: assignees picked by lottery– problem: companies bid with intent to resell spectrum

– command-and-control• comparative bidding• lottery• auction

– process: competitive auction for spectrumproblem?

– process: competitive auction for spectrum– problem: complex process

free market but antitrust issues

– commons : unlicensed (including ISM)• process: anyone can use subject to regulation

problem?

– commons• process: anyone can use subject to regulation• problem: interference

– among applications (e.g. WLANs, cordless phones, µwave ovens– among providers and users of given application (e.g. WLANs)

• Spectrum allocated in fixed frequency rangesproblem?

• Spectrum allocated in fixed frequency ranges– problem: very inefficient use of spectrum

• spectrum reserved for future use• difficult to reclaim unused spectrum

– example: UHF TV reclaimed for GSM in 850MHz

• inability to load balance among different allocations

Wireless SpectrumFCC/NTIA Spectrum Allocation

• FCC allocates and licenses spectrum in US– static allocations lead to significant inefficiency in use

alternative?

– alternative: dynamic spectrum allocation• significant technical, political, and policy challenges• fear of making things worse• currently hot topic for research

– technical and policy

Wireless SpectrumDynamic Allocation

• FCC Spectrum Policy Task Force recommendationwww.fcc.gov/sptf– command-and-control should only be used when needed:

• to accomplish important public interest objectives• conform to treaty obligations

– significant expansion of commons allocation

• Dynamic spectrum management– SDR (software defined radios) a key technology– new algorithms and protocols– significant policy change

Mobile Wireless NetworkingPL.2 Wireless Channels and Propagation

PL.1 Physical media and spectrumPL.2 Wireless channels and propagation

PL.2.1 Digital and analog signalsPL.2.2 Wireless propagationPL.2.3 Channel characteristics and challenges

PL.3 Modulation, coding, and error controlPL.4 Mobile wireless environment

Mobile Wireless NetworkingPL.2.1 Digital and Analog Signals

CommunicationSignal Types

• Transmission of a signal through a medium

• Analog signal: time-varying levels– electromagnetic wave amplitude– electrical: voltage levels– photonic: light intensity

• Analog signal: time-varying levels– electrical: voltage levels– photonic: light intensity

• Digital signal: sequence of bits represented as levels– electrical: voltage pulses– photonic: light pulses– two levels for binary digital signal

– more levels in some coding schemes more later

CommunicationDigital vs. Analog

• Digital bits are reconstructed at the receiver

[Tannenbaum]

• Digital bits are reconstructed at the receiver– all transmission is actually analog!– frequency response determines

• pulse rate that can be transmitted• shape of pulse → ability for receiver to recognise pulse

[Tannenbaum]

harmonic

• Digital bits are reconstructed at the receiver– all transmission is actually analog!– frequency response determines

• pulse rate that can be transmitted• shape of pulse → ability for receiver to recognise pulse

– high-frequency attenuation reduces quality of pulse

attenuated frequencies

adapted from [Tanenbaum 2003]

CommunicationDigital vs. Analog in Free Space

[Tannenbaum]

• Digital transmission is baseband– frequency spectrum begins at 0Hz– only practical for dedicated (guided media)

• wire and fiber optic cable

• Free space transmission generally broadband– digital information modulated over a range of frequencies

Mobile Wireless NetworkingPL.2.1 Wireless Propagation

Wireless Free SpaceMedium Sharing

• Dedicated– single transmitter attached to medium– signals may be multiplexed by a single transmitter

• link multiplexing

Wireless Free SpaceMedium Sharing

• Dedicated– single transmitter attached to medium– signals may be multiplexed by a single transmitter

• link multiplexing

• Shared: multiple access– multiple transmitters transmit into a the same medium

Lecture ML

Wireless Free SpacePropagation Mechanisms

• Direct signal• Reflection• Diffraction• Scattering

Wireless Free SpacePropagation Mechanisms: Direct

• Direct signal– direct transmission from transmitter to receiver

• Reflection• Diffraction• Scattering

Wireless Free SpacePropagation Mechanisms: Reflection

• Direct signal• Reflection

– reflected off object large relative to wavelength

• Diffraction• Scattering

Wireless Free SpacePropagation Mechanisms: Diffraction

• Direct signal• Reflection• Diffraction

– bending by object comparable to wavelength

• Scattering

Wireless Free SpacePropagation Mechanisms: Scattering

• Direct signal• Reflection• Diffraction• Scattering

– by many objects smaller than wavelength– multiple weaker signals

Wireless Free SpacePropagation Mechanisms: Multipath

• Multipath– multiple signals using different propagation mechanisms

problem?

Wireless Free SpacePropagation Mechanisms: Multipath

• Multipath interference or distortion – multiple signals using different propagation mechanisms– time-shifted versions of signal interfere with one another

Wireless Free SpaceAntennæ and Transmission Pattern

• Omnidirectional antennæ– RF radiated in all directions

advantages and disadvantages?

• Omnidirectional antennæ– RF radiated in all directions– advantage: simple cheap design

• Omnidirectional antennæ– RF radiated in all directions– advantage:

simple cheap design– disadvantage:

no spatial reuse

• Omnidirectional antennæ• Directional antennæ

– focused beam of radiationadvantages and disadvantages?

– focused beam of radiation– advantage

• reduces contention with spatial reuse

– focused beam of radiation– advantage

• reduces contention with spatial reuse– disadvantages

• more complex antenna design• significantly complicates

network design: beam steering MACLecture ML

Wireless Free SpacePropagation Modes

• Ground-wave propagation < 2 MHz• Sky wave propagation 2 – 30 MHz• Line-of-sight propagation > 30 MHz

ionosphere

Wireless Free SpacePropagation Modes: Ground Wave

• Ground-wave propagation < 2 MHz– signals follow curvature of earth– scattered in upper atmosphere

• Sky wave propagation 2 – 30 MHz• Line-of-sight propagation > 30 MHz

ionosphere

Wireless Free SpacePropagation Modes: Sky Wave

• Ground-wave propagation < 2 MHz• Sky wave propagation 2 – 30 MHz

– signals refracted off ionosphere– communication possible over thousands of kilometers

• Line-of-sight propagation > 30 MHz

ionosphere

Wireless Free SpacePropagation Modes: Line of Sight

• Ground-wave propagation < 2 MHz• Sky wave propagation 2 – 30 MHz• Line-of-sight propagation > 30 MHz

– antennæ must be in view of one-another– terrain and earth curvature block signature

ionosphere

Wireless Free SpaceSpectrum Table

ELF GW

Attenuation

30– 300 Hz

300–3000 Hz

3– 30kHz

30– 300kHz

300–3000kHz

3– 30MHz

30– 300MHz

300–3000MHz

3– 30GHz

30– 300GHz

300GHz–400THz

Wireless Free SpaceVelocity

• Velocity v = c /n [m/s]– speed of light c = 3×105 km/s– index of refraction n

Consequences?

Wireless Free SpaceVelocity and Delay

• Velocity v = c /n [m/s]– speed of light c = 3×105 km/s– index of refraction n

• this is why velocity slower than c in fiber and wire

• Delay d = 1/v [s/m]– generally we will express delay in [s] given a path length

Mobile Wireless NetworkingPL.2.3 Channel Characteristics and Challenges

Wireless ChannelCharacteristics and Challenges

• Goal for communications link– receiver reconstructs signal transmitter sent– propagation (PL.2.2)

• Challenges to meeting this goal– path loss and attenuation– fading– noise and interference– Doppler Shift– transmission rate constraints

Wireless Channel ChallengesPath Loss and Attenuation

• Path loss and attenuation• Fading• Noise and interference• Doppler Shift• Transmission rate constraints

Path Loss and AttenuationTransmission Length

• Link Length– distance over which signals propagate– constrained by physical properties of medium

Consequences?

• Attenuation: decrease in signal intensity– over distance expressed as [dB/m]– at a particular signal frequency

how to compute?

• Attenuation– signal strength decreases as 1/r 2 in perfect medium

• Attenuation– signal strength decreases as 1/r 2 in perfect medium– signal may decrease as 1/r x with multipath interference

• rural environments: x > 2• urban environments: x → 4

more later

Path Loss and AttenuationFrequency Response

• Attenuation: decrease in signal intensity– over distance expressed as [dB/m]– at a particular signal frequency

• Frequency response of media– wire: generally falls off above a certain fmax

– fiber optic cable & free space transparent to certain rangesanalogy:UV blocking sunglasses (high attenuation )

vs.standard glass (moderate attenuation )

vs.UV transparent black light glass (low attenuation )

Path Loss and AttenuationSpectrum Table: Frequency Response

ELF GW

Attenuation

30– 300 Hz

300–3000 Hz

3– 30kHz

30– 300kHz

300–3000kHz

3– 30MHz

30– 300MHz

300–3000MHz

3– 30GHz

30– 300GHz

300GHz–400THz

Path Loss and AttenuationFrequency Response

• Atmospheric transparency bands– RF: 10MHz – 10GHz

• VHF meter band, UHF millimeter band– Infrared: N-band

Wavelength [m] | Frequency [Hz]

10µm0.1nm 10nm 10m 100m 1km1m10cm1cm1mm100µm1µm1nm 100nm

adapted from[coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irwindows.html]

10THz1EHz 10Pz 10MHz 1MHz 100KHz100MHz1GHz10GHz100GHz1THz100THz100PHz 1PHz

UHF VHF

microwave shortwave

Wireless Channel ChallengesFading

Wireless Channel FadingDefinition

• Channel fading– changes of signal intensity over time

Wireless Channel FadingTypes and Cause

• Fast fading– rapid fluctuations in intensity– mobility on the order of 1/2 wavelength

• Slow fading– long-term fluctuations in intensity (seconds to minutes)

causes?

• Slow fading– long-term fluctuations in intensity (seconds to minutes)– causes

• obstructions such as rain (rain fade )• micro-mobility among buildings in urban areas• macro-mobility between base stations

• Flat fading (nonselective)– uniform fade across frequency range

• Selective fading– different frequency components suffer different attenuation

Wireless Channel ChallengesInterference

Noise and InterferenceDefinition

• Superposition interaction of waves is interferenceCauses?

Noise and InterferenceCauses

• Superposition interaction of waves is interference• Causes

– noise– co-channel interference– adjacent channel interference

NoiseCauses

• Noise interferes with signals• Thermal noise [W/Hz]

– caused by agitation of electrons: function of temperature t– independent of frequency: white noise

N = kTB ; k = 1.38×10-23 [J/K] (Boltzmann constant)B = bandwidth [Hz]

NoiseCause and Effect: SNR

• Noise interferes with signals• Thermal noise [W/Hz] = [(J/s)/s–1] = [J]

– caused by agitation of electrons: function of temperature t– independent of frequency: white noise

N = kTB ; k = 1.38×10-23 [J/K] (Boltzmann constant)B = bandwidth [Hz]

• Background noise No– thermal noise + other sources (e.g. cosmic radiation)– No interferes with the signal bit energy Eb

– SNR: signal to noise ratio = 10 log10 (Eb /No ) [db] (decibels)

InterferenceCo-Channel Interference

• Co-channel interference within given frequency bandCauses and solutions?

• Co-channel interference within given frequency band• Multiple users sharing channel

– motivates medium access control (MAC) Lecture ML

• Malicious attack: jamming of channel– faraday cage to repel – spread spectrum techniques Lecture ML– resilience techniques EECS 983

• Malicious attack: jamming of channel– Faraday cage to isolate when possible – spread spectrum techniques Lecture ML– resilience techniques EECS 983

• Natural phenomena– e.g. sunspots

InterferenceAdjacent Channel Interference

Solutions?

• Adjacent channel interference between freq. bands

Guard bands– reserved bandwidth between frequency bands

InterferenceAdjacent Channel Interference

• Spatial partitioning– adjacent channels not used in same geographic area

• e.g. TV broadcast channels (2,4,5,7,9,11,13 vs. 3,6,8,10,12)• e.g. FM broadcast stations • e.g. cellular frequency plan for mobile telephony

InterferenceAdjacent Channel Partitioning: Broadcast TV

• Spatial partitioning example: broadcast television– large cities get channels 2,4,5,7,9,11,13

recall guard band between 4/5 and 6/7– small towns in-between get channels 3,6,8,10,12

• Spatial partitioning example: broadcast FM radio– adjacent frequencies not used in same city

METROPOLISCh. 2,4,5,11GOTHAM

Ch. 2,4,5,7,9,11,13

SmallvilleCh. 10, 12

Elk’s BreathCh. 6

MayberryCh. 3,8

InterferenceAdjacent Channel Partitioning: Cellular

• Spatial partitioning example: cellular telephony– most efficient circular packing is hexagonal– frequency channels mapped to hexagonal tiling– adjacent cells assigned different frequencies Lecture MT

Wireless Channel ChallengesDoppler Shift

Doppler ShiftDefinition

• Doppler shift– explained by Austrian scientist Christian Doppler in 1800s

what is it?

Doppler ShiftDefinition

• Doppler shift– explained by Austrian scientist Christian Doppler in 1800s– wavelength changes with relative velocity– recall decreasing pitch of horn as train passes

• Doppler effectfd = v/λ

Consequences?

Doppler ShiftConsequences

• Doppler shift– explained by Austrian scientist Christian Doppler in 1800s– wavelength changes with relative velocity– recall decreasing pitch of horn as train passes

• Doppler effectfd = v/λ

• Consequences– frequency perceived by receiver different from expected– concern networks with high node mobility

Wireless Channel ChallengesTransmission Rate Constraints

• Path loss and attenuation• Fading• Propagation modes and interference• Doppler Shift• Transmission rate constraints

Transmission Rate ConstraintsOverview

• Transmission rate constraints– transceiver switching frequency– Nyquist rate– Shannon rate

Transmission Rate ConstraintsSwitching Frequency

• Rate constrained by switching frequency– transmitter and receiver [b/s]– dictated by electronic circuits

• switching time of transistors• propagation delay through circuits

Transmission Rate ConstraintsNyquist Rate

• Channel capacity C [b/s]– Harry Nyquist: Swedish physicist at Bell Labs in early 1900s– determined by bandwidth

(max CS bandwidth determined by EE bandwidth)– constrained by number of quantization levels per bit

C = 2B log2 LB = channel bandwidth [Hz] = [1/s]L = number of quantization levels

Transmission Rate ConstraintsShannon Theorem

• Maximum data rate for noisy channel– Claude Shannon: American at Bell Labs in mid-1900s

• “father of information theory” and pioneer in digital logic

– noise reduces data rate

C = B log2 (1+ S/N ))C = channel capacity [b/s]B = channel bandwidth [Hz] = [1/s]S = signal power [dB]N = noise power [dB]

Mobile Wireless NetworkingPL.3 Modulation, Coding, and Error Control

PL.1 Physical media and spectrumPL.2 Wireless channels and propagationPL.3 Modulation, coding, and error control

PL.3.1 Modulation and codingPL.3.2 Error control

PL.4 Mobile wireless environment

Mobile Wireless NetworkingPL.3.1 Modulation and Coding

Modulation and CodingDigital Communication

• Digital Communication– we consider only digital communication for networking

• transmission of binary data (bits) through a channel

– recall: in free space digital signal is analog modulated• broadband, not baseband

Modulation and CodingLine Coding

• Line coding– way in which bits are encoded for transmission– digital codes (binary, trinary, …)– analog modulation

Modulation and CodingLine Coding and Symbol Rate

• Line coding– way in which bits are encoded for transmission– digital codes (binary, trinary, …) EECS 780– analog modulation

• Symbol rate– baud rate [symbols/s]

• baud = b/s only if one symbol/bit

– clever encodings (e.g. QAM) allow high baud rates

Analog Line CodingAnalog Modulation

• Analog line coding– modulate an analog carrier

with a digital signal

0 0 1 0 0 1 0 1 1 1

Analog Line CodingAmplitude Modulation

• Analog line coding– modulate an analog carrier

• Amplitude modulation– each symbol a different level of carrier

• one may be zero voltage

– compare to AM radio• modulate an analog carrier with

an analog signal

0 0 1 0 0 1 0 1 1 1

Analog Line CodingFrequency Modulation

• Analog line coding– modulate a carrier

• Amplitude modulation– each symbol a different level

• Frequency modulation– each symbol a different frequency– FSK (frequency shift keying)– compare to FM radio

• modulate an analog carrier withan analog signal

0 0 1 0 0 1 0 1 1 1

Analog Line CodingPhase Modulation

• Analog line coding– modulate a carrier

• Frequency modulation– each symbol a different frequency

• Phase modulation– each symbol a different phase– e.g. 0°, 180°

0 0 1 0 0 1 0 1 1 1

Analog Line CodingCombination Codes

• Analog line coding: – modulate a carrier

• Phase modulation– each symbol a different phase

• Combinations possiblewhy?

0 0 1 0 0 1 0 1 1 1

Analog Line CodingCombination Codes

• Analog line coding: – modulate a carrier

• Phase modulation– each symbol a different phase

• Combinations possible– e.g. amplitude and phase

0 0 1 0 0 1 0 1 1 1

00 10 01 01 11 00 10 01 01 11

Analog Line CodingQPSK and QAM

• Combination of amplitude- and phase-modulation– allows more bits per symbol

• QAM: quadrature amplitude modulation– quadrature = 4 phases carried on two sine waves– PAM is case for only one phase– QPSK is case for only one amplitude

Name Amplitudes Phases Bits/Symbol

QPSK 1 4 2

QAM-16 2 4 4

QAM-64 3 4 6

QAM-256 4 4 8

Analog Line CodingQPSK and QAM

• QAM: amplitude- and phase modulation• Represented by constellation diagram

– amplitude is distance from origin– phase is angle

0°180°

QAM-64QAM-16QPSK

Mobile Wireless NetworkingPL.3.2 Error Control

Error ControlIntroduction and Motivation

Motivation for error control?

Error ControlIntroduction and Motivation

• Motivation for error control– channels are imperfect

• cause: noise and interference• result: bit errors

– components can fail– packets my be dropped due to congestion EECS 780

• Therefore need error controlwhere to perform?

physical layer?link layer?transport layer?

Error ControlHop-by-Hop vs. End-to-End

• Per-hop error control for frame transfersWhy?

Error ControlHop-by-Hop vs. End-to-End

• Per-hop error control for frame transfers• Recall end-to-end arguments:

– if error checking and correction needed E2E …… it must be done end-to-end by transport (or application)

• Hop-by-hop control to improve overall performance– physical layer for bit errors in noisy channel– link layer at frame granularity Lecture ML

Error ControlDetection Techniques

• Byte, word, or (small) block: done at physical layer– parity– 2-dimensional parity– Hamming codes

• Frame: done at link layer Lecture ML– checksum– CRC

Block Error DetectionParity

• Parity: detect single errors– no ability to correct errors– only an odd number of bit errors detected

• only useful if bit error probability very low

• Parity bit covers n bit block• Even parity: even number of 1s (including parity)

– example: 0111 0001 1010 1011 ?

• Odd parity odd number of 1s (including parity)– example: 0111 0001 1010 1011 ?

• Parity bit covers n bit block• Even parity: even number of 1s (including parity)

– example: 0111 0001 1010 1011 1

• Odd parity odd number of 1s (including parity)– example: 0111 0001 1010 1011 0

Block Error Detection & Correction2-Dimensional Parity

• 2-dimensional parity: correct single bit errors– detects which bit flipped and can therefore correct

• n + m parity bits covers n × m bit block– n row parity bits cover m data bits each– m column parity bits cover n data bits each

• Example (odd parity)0111 00001 01010 11011 01000

• Example (odd parity)0111 0 0111 00001 0 0001 01010 1 1110 1 ← detects and can correct flip1011 0 1011 01000 1000

Block Error Detection & CorrectionHamming Codes

• Hamming codes: correct single bit errors– detects which bit flipped and can therefore correct– k parity bits per block cover different sets of n data bits

• SECDED: single error correct double error detect

• SECDED: single error correct double error detect• Example 4 data bits covered by 3 parity bits

– parity bits p2 p1 p0 interleaved with data bits d3 d2 d1 d0

1011010 d3 d2 d1 p2 d0 p1 p0

1‐1‐0‐1 d3 d1 d0 covered by p0

10‐‐00‐ d3 d2 d0 covered by p1

1011‐‐‐ d3 d2 d1 covered by p2

• SECDED: single error correct double error detect• Example 4 data bits covered by 3 parity bits

– parity bits p2 p1 p0 interleaved with data bits d3 d2 d1 d0

1011010 d3 d2 d1 p2 d0 p1 p0 11110101‐1‐0‐1 d3 d1 d0 covered by p0 1‐1‐0‐110‐‐00‐ d3 d2 d0 covered by p1 11‐‐00‐ p1 detects error1011‐‐‐ d3 d2 d1 covered by p2 1111‐‐‐ p2 detects error

Error Detection & CorrectionForward Error Correction

• Forward error correction (FEC)– redundant information added to data– allows detection of errors– also allows correction of errors

Error Detection & CorrectionForward Error Correction

• Forward error correction (FEC)– redundant information added to data– allows detection of errors– also allows correction of errors

• Types of FEC codes– block codes: FEC header over link layer frame Lecture ML– convolutional code: redundant bits interspersed in stream– turbo codes: iterative convolutional coder

• performs closer to theoretical Shannon limit

EECS 869

Forward Error CorrectionConvolutional Coder

• Convolutional code– redundant bits interspersed in stream

Mobile Wireless NetworkingPL.4 Mobile Wireless Environment

PL.4.1 Network impact of wireless channelPL.4.2 Network impact of mobility

Mobile Wireless EnvironmentImpact on the Network

• Recap: brief introduction to physical layerWhy does this matter to the network?

• Recap: brief introduction to physical layer• Network consists of nodes interconnected by links

– characteristics of links and nodes impact network

• Recap: brief introduction to physical layer• Network consists of nodes interconnected by links

– characteristics of links and nodes impact network

• Traditional PSTN and Internet– static (non-mobile) nodes– reliable wired links– many design decisions based on these assumptions

What is different and why does it matter?

Mobile Wireless EnvironmentPL.4.1 Network Impact of Wireless Channel

Impact of Wireless ChannelChannel Connectivity

Impact of wireless channel on connectivity?

Wireless ChannelChannel Connectivity

• Weak time-varying connectivity– limited bandwidth of shared medium– time-varying channel capacity

• noise• interference• fading

• Weak time-varying connectivity• Intermittent and episodic connectivity

– long fades (e.g. rain fades)– interference and jamming

• Weak time-varying connectivity• Intermittent and episodic connectivity• Asymmetric connectivity due to heterogeneous nodes

– unequal transmitter power• design• available power over battery life

– different up/downlink characteristics• mobile phones• satellite links

Wireless ChannelImpact of Weak Connectivity

• Traditional networks– strong symmetric connectivity assumed– weak connectivity treated as failure

Impact of weak connectivity?

• Impact of weak connectivity– bit errors → packet loss → performance impact– link failures → routing reconvergence– loss discrimination important Lecture WI

• Mobile wireless networks– weak, asymmetric, intermittent, episodic connectivity routine– network architecture and protocol design for this

Example Weak Connectivity ScenarioWDTN Millimeter-Wave Mesh Network

• Millimeter-wave links– 60–90 GHz, 1–10 Gb/s– severe rain attenuation

• Mesh architecture– high degree of connectivity– alternate diverse paths

• WDTN solution– reroute before failures occur– avoid high error links– P-WARP, XL-OSPF

[Jabbar Rohrer Oberthaler Çetinkaya Frost Sterbenz 2009]

802.163–4G

CO/POP

Wireless ChannelOpen Channel

• Open channelproblems?

Wireless ChannelOpen Channel

• Open channel subject to attack– eavesdropping

• network and traffic analysis

– interference• jamming and denial of service

– injection of bogus signalling and control messages

Wireless ChannelImpact of Open Channel

• Open channel subject to attack– eavesdropping

• network and traffic analysis

– interference• jamming and denial of service

– injection of bogus signalling and control messages

• Security and resilience more importantLecture RSECS 983

Mobile Wireless EnvironmentPL.4.2 Network Impact of Mobility

Impact of MobilityOverview

Impact of mobility?

Impact of MobilityOverview

• Impact of mobility– connectivity– dynamic topologies– QoS

Impact of MobilityConnectivity

• Mobility impacts connectivity– nodes move in and out of range of one another– long fades– episodic and intermittent connectivity

• Design for weak connectivity– as for wireless channel impacts

Impact of MobilityDynamic Topologies

• Mobility means nodes and subnets move– result: dynamic topology– changing links, clustering, and federation topology– difficult to achieve routing convergence– mobility may exceed ability of control loops to react

Impact of MobilityDynamic Topologies

• Mobility means nodes and subnets move– result: dynamic topology– changing links, clustering, and federation topology– difficult to achieve routing convergence– mobility may exceed ability of control loops to react

• Design for mobility– addressing mechanisms must not assume static location– routing algorithms must assume dynamic topologies

• predictive• reactive

Impact of MobilityQuality of Service

• Mobility impacts QoS (quality of service)

• Mobility impacts QoS (quality of service)– changes in inter-node distance

• requires power adaptation• changes node density and impacts degree of connectivity

– latency issues (routing optimisations temporary)

• Mobility impacts QoS (quality of service)– changes in inter-node distance

• requires power adaptation• changes node density and impacts degree of connectivity

– latency issues (routing optimisations temporary)

Example High-Mobility ScenarioAirborne Ad Hoc Networking

• Very high relative velocity– Mach 7 ≈ 10 s contact– dynamic topology

• Communication channel– limited spectrum– asymmetric links

• data down omni• C&C up directional

• Multihop– among TAs, through RNs

[Rohrer Jabbar Perrins Sterbenz 2008]

Internet

TA – test articleRN – relay node

GS – ground stationGW – gateway

Mobile Wireless EnvironmentImpact on Network

• Now we are ready for the rest of the course…

Physical LayerFurther Reading

• William Stallings,Data and Computer Communications, 8th ed.Pearson Prentice Hall, Upper Saddle River NJ, 2007.

Physical LayerAcknowledgements

Some material in these foils is based on the textbook• Murthy and Manoj,

Ad Hoc Wireless Networks:Architectures and Protocols

Significant material in these foils enhanced from EECS 780 foils

ittc mobile wireless networkingjpgs/courses/mwnets/lecture-physical... · 2011-08-22 · physical...

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