Fall 2015Fall 2015

Afshin HemmatyarAfshin Hemmatyar

Sharif University of Technology

Wireless CommunicationWireless Communication

IntroductionIntroduction

ReferencesReferences

A. Goldsmith, Wireless Communications, Cambridge University Press, 2005. D. Tse and D. Vaswanth, Fundamentals of Wireless Communications, Cambridge University Press, 2005. T. Rappaport, Wireless Communications, Principles and Practice, 2nd Edition, Prentice Hall. J. Fayyaz, Radio Design of Cellular Networks, Naghoos Press, 2011.

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ContentsContents

Background and PreviewBackground and Preview Wireless Propagation ChannelWireless Propagation Channel Multiple Access MethodsMultiple Access Methods Cellular SystemsCellular Systems Diversity IssuesDiversity Issues Information Transmission CapacityInformation Transmission Capacity Multiple Antenna TechnologiesMultiple Antenna Technologies Cooperative Communications Cooperative Communications

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Wired Vs. Wireless CommunicationsWired Vs. Wireless Communications

WiredWirelessEach cable is a different channel One media (cable) shared by all

Signal attenuation is low High signal attenuation

No interference High interference

noise; co-channel interference; adjacent channel interference

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AdvantagesAdvantages• Sometimes it is impractical to lay cablesSometimes it is impractical to lay cables• User mobilityUser mobility• Cost Cost

LimitationsLimitations• BandwidthBandwidth• FidelityFidelity• PowerPower• SecuritySecurity

Why Wireless?Why Wireless?

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Wireless ≈ WavesWireless ≈ Waves

Electromagnetic radiationElectromagnetic radiation Emitted by sinusoidal current running Emitted by sinusoidal current running

through a wire (transmitting antenna)through a wire (transmitting antenna) Creates propagating sinusoidal magnetic Creates propagating sinusoidal magnetic

and electric fields according to Maxwell’s and electric fields according to Maxwell’s equations:equations:

Fields induce current in receiving antenna. Fields induce current in receiving antenna. 66

Propagation PrinciplePropagation Principle

electricfield

magneticfield

propagation direction

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Propagation MechanismsPropagation Mechanisms

Non Line-of-Sight

Reflection

λ << D

Diffraction

λ D

Scattering

λ >> D

S D

Line-of-Sight

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Propagation in the “Real World”Propagation in the “Real World”

a wave can be absorbed

reflectreflect

penetratepenetrate

bend

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The Cluttered World of Radio WavesThe Cluttered World of Radio Waves

walls

hallwayswindows

trees

vehicles

rain

hills

girders

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Electromagnetic SpectrumElectromagnetic 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 ra

dio

S/W ra

dio

FM ra

dio

TV TV cellu

lar

902 – 928 Mhz

2.4 – 2.4835 Ghz

5.725 – 5.785 Ghz

ISM band

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

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

3GHz

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Evaluating FrequenciesEvaluating Frequencies

50 MHz-250 MHz50 MHz-250 MHz: Good for outdoor range, large : Good for outdoor range, large antenna size, bending and penetrating. No foliage antenna size, bending and penetrating. No foliage problems. “Sees” metallic building structures, problems. “Sees” metallic building structures, doesn’t pass through windows or down corridors. doesn’t pass through windows or down corridors.

450 MHz to 2 GHz450 MHz to 2 GHz: - Good compromise for cellular-: - Good compromise for cellular-type systems. Small antenna, but big enough for type systems. Small antenna, but big enough for outdoor range. Minor foliage effects. OK for windows outdoor range. Minor foliage effects. OK for windows walls and corridors.walls and corridors.

5-20 GHz5-20 GHz: Antenna too small for outdoor range. : Antenna too small for outdoor range. Foliage and rain effects. Indoor microcells, Point-to-Foliage and rain effects. Indoor microcells, Point-to-point, and Satellites to ground stations.point, and Satellites to ground stations.

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

902 Mhz

928 Mhz

26 Mhz 83.5 Mhz 125 Mhz

2.4 Ghz

2.4835 Ghz5.725 Ghz

5.850 Ghz

cordless phonesbaby monitorsWaveLan

802.11bBluetoothMicrowave oven

802.11a

33cm 12cm 5cm

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Free-space Path-loss Free-space Path-loss

Power of wireless transmission reduces with Power of wireless transmission reduces with square of distance (due to surface area square of distance (due to surface area increase of sphere).increase of sphere).

Reduction also depends on wavelength:Reduction also depends on wavelength:• Long wave length (low frequency) has less lossLong wave length (low frequency) has less loss• Short wave length (high frequency) has more lossShort wave length (high frequency) has more loss

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..

d

LP

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Path-loss ModelsPath-loss Models

Path-Loss Exponent Depends on environment:Path-Loss Exponent Depends on environment:

L(d) = L(dL(d) = L(d00)(d/d)(d/d00))nn

Free spaceFree space n = 2n = 2

Urban area cellularUrban area cellular n = 2.7 to 3.5n = 2.7 to 3.5

Shadowed urban cellShadowed urban cell n = 3 to 5n = 3 to 5

In building LOSIn building LOS n = 1.6 to 1.8n = 1.6 to 1.8

Obstructed in buildingObstructed in building n = 4 to 6n = 4 to 6

Obstructed in factoriesObstructed in factories n = 2 to 3n = 2 to 3

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Multi-path PropagationMulti-path Propagation

Electromagnetic waves bounce off of Electromagnetic waves bounce off of conductive (metal) objects.conductive (metal) objects.

Reflected waves received along with Reflected waves received along with direct wave.direct wave.

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Multi-Path EffectMulti-Path Effect

Multi-path components are delayed Multi-path components are delayed depending on path length, causes depending on path length, causes delay spreaddelay spread..

Phase shift Phase shift causes frequency causes frequency dependent constructive / destructive dependent constructive / destructive interference.interference.

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Multi-transmitter InterferenceMulti-transmitter Interference

Similar to multi-pathSimilar to multi-path

Two transmitting stations will Two transmitting stations will constructively/destructively interfere constructively/destructively interfere with each other at the receiver.with each other at the receiver.

Receiver will “hear” the sum of the Receiver will “hear” the sum of the two signals, which usually means two signals, which usually means garbage.garbage.

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ModulationModulation Modulation allows the wave to carry Modulation allows the wave to carry

information by adjusting its properties information by adjusting its properties ((AmplitudeAmplitude, , FrequencyFrequency, , and Phaseand Phase) in a ) in a time varying way.time varying way.

Digital modulation using discrete “steps” Digital modulation using discrete “steps” so that information can be recovered well so that information can be recovered well despite noise/interference.despite noise/interference.• 8VSB - US HDTV8VSB - US HDTV• BFSK - Mote Sensor NetworksBFSK - Mote Sensor Networks• QPSK - 2 Mbps 802.11 & CMDA(IS-95)QPSK - 2 Mbps 802.11 & CMDA(IS-95)

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Symbol Rate & BandwidthSymbol Rate & Bandwidth Modulation allows transmission of one of Modulation allows transmission of one of

several possible symbols (two or more).several possible symbols (two or more).

Data stream is encoded by transmitting Data stream is encoded by transmitting several symbols in succession.several symbols in succession.

Symbol rate ≈ BandwidthSymbol rate ≈ Bandwidth• Symbol Rate or Baud Rate (symbols/sec)Symbol Rate or Baud Rate (symbols/sec)• Bit Rate or Throughput (bits/sec)Bit Rate or Throughput (bits/sec)• Spectrum Usage or Bandwidth (Hz)Spectrum Usage or Bandwidth (Hz)

Inter-symbol interference (ISI) occurs unless Inter-symbol interference (ISI) occurs unless delay spread << symbol time.delay spread << symbol time.

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Thermal NoiseThermal Noise

Ever-present thermal noise in wireless Ever-present thermal noise in wireless medium.medium.

Sums with any wireless transmission.Sums with any wireless transmission.

Potentially causes errors in reception Potentially causes errors in reception (digital) or degradation of quality (analog).(digital) or degradation of quality (analog).

Effectively limits transmission range whenEffectively limits transmission range when

transmitting signal strength falls below a transmitting signal strength falls below a threshold.threshold.

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Thermal Noise CalculationThermal Noise Calculation

Noise power depends on channel bandwidth:Noise power depends on channel bandwidth:

Noise Power = -174dBm/Hz + 10log(BW in Hz)Noise Power = -174dBm/Hz + 10log(BW in Hz)

So for 802.11So for 802.11• BW = 25 MHz for 802.11b or 802.11a channelBW = 25 MHz for 802.11b or 802.11a channel

• Thus noise power is about -100 dBmThus noise power is about -100 dBm

• -100 dBm = 10-100 dBm = 10-10-10 mW mW

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Physical Channel Properties ReviewPhysical Channel Properties Review Received signal power Received signal power depends on:depends on:

• Transmit powerTransmit power• Loss over distance (falls off by dLoss over distance (falls off by dnn))• Shadowing (e.g. absorption by walls)Shadowing (e.g. absorption by walls)• Multi-path (e.g. bouncing off of metal objects)Multi-path (e.g. bouncing off of metal objects)

Noise power Noise power depends on:depends on:• Thermal noise Thermal noise • Environmental noise (e.g. microwave ovens)Environmental noise (e.g. microwave ovens)

Channel QualityChannel Quality• Related to Related to Signal to Noise RatioSignal to Noise Ratio

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Current Wireless TechnologiesCurrent Wireless Technologies

Cellular Telephony (GSM, CDMA2000)

Fixed Wireless Access (WiMax) Wireless Local Area Networks

Local Networks (Bluetooth, UWB)

Satellite Communications

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Cellular Telephony (1)Cellular Telephony (1)

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Cellular Telephony (2)Cellular Telephony (2)

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Cellular Telephony (3)Cellular Telephony (3)

Data is bursty, whereas voice is continuous.

3G widens the data pipeo 384Kbps to few Mbpso Standard based of WCDMAo Packet-based switching for both voice and data

New generationso HSDPA, HSPA+, LTEo WiMAx added to 3G

4G systems start to come upo Mostly based on OFDM

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Fixed Wireless Networks (WiMax)Fixed Wireless Networks (WiMax)

Provide broadband wireless access to homes/offices in a few Km range.

A potential replacement of ADSL

Provide high speed Internet and VOD

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Wireless Local Area Networks (1)Wireless Local Area Networks (1)

WLANs connect local computers (100m to a few Km range)

Breaks data into packets

Channel is shared (random access)

Backbone internet provides best-effort service

Poor performance in some applications (e.g. video)2929

802.11b (oldest)o Standard for 2.4GHz ISM band (80MHz BW)o Frequency hopped spread spectrumo 1.6-10Mbps, few hundred meter range

802.11a (old)o Standard for 5.7GHz NII band (300MHz BW)o OFDM with time division o 20-70Mbps, variable range

802.11g (newer)o Standard for 2.4GHz and 5.7GHz bandso OFDMo Up to 54Mbps, few hundred meter range

802.11n (current)o Up to 140Mbpso Uses smart antenna technology

Wireless Local Area Networks (2)Wireless Local Area Networks (2)

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Ultra Wide-BandUltra Wide-Band

Also known as “Impulse Radio”.

Use very high bandwidth to decrease power level.

Hard synchronization

No license problem, but seems to interfere with GPS

Now mainly targeted to small distance applications. (home networks to replace Bluetooth)

Smaller delay and longer range than Bluetooth

Emerging as competitor for 802.11

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Satellite CommunicationsSatellite Communications

Cover very large areas.

Different orbit heightso GEO (36000 Km), MEO and LEO (<2000 Km)

Optimized for one-way transmissiono Radio (DAB) and TV (DVB-S) broadcast

Two-way systemso Expensive alternative to terrestrial systems

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Evolution of Current TechnologiesEvolution of Current Technologies

Link: Modulation, Coding, Adaptivity, Smart Antennas

Network: Dynamic resource allocation, Mobility support

Hardware: Better batteries, Circuits/Processors

Applications: Soft and adaptive QoS (main issue in real-time multi-media services)

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Moving toward Real MultimediaMoving toward Real Multimedia

VoiceDataVideo

Delay< 100 mSec-< 100 mSec

Packet loss< 1%0<1%

BER10-310-610-6

Data Rate8-32Kbps1-100 Mbps1-20 Mbps

TrafficContinuousBurstyContinuous

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Design ChallengesDesign Challenges

Wireless channels are capacity-limited broadcast communication medium.

o Two main problems in wireless media: Fading Interference

Traffic patterns, user locations, and network conditions are constantly changing.

Energy and delay constraints change design principles across all layers of the protocol stack.

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Emerging SystemsEmerging Systems

Ad-hoc Wireless Networks

Wireless Sensor Networks

Distributed Control Networks

Cooperative Networks

Cognitive Radio Networks

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Ad-hoc Wireless Networks (1)Ad-hoc Wireless Networks (1)

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Ad-hoc Wireless Networks (2)Ad-hoc Wireless Networks (2)

Peer-to-peer communications No backbone infrastructure Multi-hop routing Dynamic topology Fully connected with different link SNRs

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Ad-hoc Wireless Networks (3)Ad-hoc Wireless Networks (3)

Ad-hoc Wireless Networks provide a flexible network infrastructure for any emerging application.

The capacity of such a network is generally unknown.

Transmission, access, and routing strategies are generally ad-hoc

Energy constraints impose interesting design tradeoffs for communication and networking.

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Wireless Sensor Networks (1)Wireless Sensor Networks (1)

40404040

Wireless Sensor Networks (2)Wireless Sensor Networks (2)

Energy is the driving constraint. Nodes powered by non-rechargeable batteries. Data flows to central location. Low per-node rates but up to 100000 nodes. Data highly correlated in transmission. Nodes can cooperate in transmission, reception, compression, and signal processing. 4141

Wireless Sensor Networks (3)Wireless Sensor Networks (3)

(Energy-constrained nodes) Short-range networks must consider transmit, circuit, and processing energy.

o Sleep modes save energy but complicate networking.

Changes everything about the network design:

o Optimization of bit allocation across all protocolso Delay vs. throughput vs. node/network lifetime tradeoffso Optimization of nodes cooperationo Efficient MAC layer communication and Scheduling

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Distributed Control (1)Distributed Control (1)

Packet loss and/or delay impacts controller performance. Controller design should be robust to network faults. Joint application and communication network delay. Interesting ideas in packet-based communication and file transfer.

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There is no methodology to incorporate random delays or packet losses into control system design. The best rate/delay tradeoff for a communication system in distributed control can not be determined. Current autonomous vehicle platoon controllers are not string stable with any communication delay. What are the best routing technologies?

Distributed Control (2)Distributed Control (2)

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Cooperative Networks (1)Cooperative Networks (1)

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Increase coverage area Reduce number of “blind spots” Reduce transmit power per node

Increase in transceiver complexity

More complex synchronization problems More interference to be handled properly Higher end-to-end delays Additional delay to be handled in real-time applications

Cooperative Networks (2)Cooperative Networks (2)

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Cognitive Radio (1)Cognitive Radio (1)

Available spectrum looks scarce.

Measurements show the allocated spectrum is vastly underutilized.

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Cognitive Radio (2)Cognitive Radio (2)

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Cognitive Radio (3)Cognitive Radio (3) Sense, learn, and exploit the environment One “simple” option:

o Use “un-used” spectrumo Agile Radioso Give priority to ”primary” users

But not receiving a signal in wireless environment does not mean that no signal is actually transmitted at that frequency! Even in simplest form is very challenging. Many ideas such as:

o Game Theoryo Near-Noise Level Signal detectiono BW transactionso Trust theories (how to identify users with “bad” intensions?)

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SummarySummary

The wireless vision encompasses many exciting systems and applications.

Technical challenges go across all layers of the system design.

Wireless systems today still have limited performance and interoperability.

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