300952
Wireless and Mobile Networks
Lecture 4. The Wireless Channel
Lecture objectives
• Understand the different antenna patterns
• Understand how the frequency spectrum is licensed and used
• Understand the different ways that signals propagate in the atmosphere
• Understand the different factors that affect line of sight transmission and how these factors can be mitigated
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ANTENNAS
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Antennas
• Anantennaisanelectricalconductororsystemofconductors – Transmission – radiates electromagnetic energy into space
– Reception – collects electromagnetic energy from space
• In two-way communication, the same antenna can be used for transmission and reception
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Radiation Patterns
• Radiationpattern
– Graphical representation of radiation properties of an antenna – Depicted as two-dimensional cross section
• Beamwidth(orhalf-powerbeamwidth) – Measure of directivity of antenna
• Receptionpattern
– Receiving antenna’s equivalent to radiation pattern
• Sidelobes
– Extra energy in directions outside the mainlobe
• Nulls
– Very low energy in between mainlobe and sidelobes
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Antenna Radiation Patterns
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Types of Antennas
• Isotropicantenna(idealized)
– Radiates power equally in all directions
• Dipole antennas
– Half-wave dipole antenna (or Hertz antenna)
– Quarter-wave vertical antenna (or Marconi antenna)
• ParabolicReflectiveAntenna • DirectionalAntennas
– Arrays of antennas
• Inalineararrayorotherconfiguration
– Signal amplitudes and phases to each antenna are adjusted to create a directional pattern
– Very useful in modern systems Image source: http://www.geofffox.com/MT/archives/tag/omnidirectional-antenna
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Simple Antennas
Source: https://lowpowerlab.com/guide/rf-best-practices/half-wavelength-dipole/ Source: https://www.ead-ltd.com/products/gsm-3g-antennas/pst2100-pentaband-quarter-wave-stub-antenna
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Radiation Pattern in Three Dimensions
yyz
xzx
Side view (xy-plane) Side view (zy-plane) Top view (xz-plane) (a) Simple dipole
yyz
xzx
Side view (xy-plane) Side view (zy-plane) Top view (xz-plane) (b) Directed antenna
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Parabolic Reflective Antennas
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Antenna Gain (1)
• Antennagain
– Power output, in a particular direction, compared to that produced in any
direction by a perfect omnidirectional antenna (isotropic antenna) • Effectivearea
– Related to physical size and shape of antenna
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Antenna Gain (2)
• Relationship between antenna gain and effective area G=4 Ae =4 f2Ae
2 c2
• G=antennagain
• Ae = effective area
• f=carrierfrequency
• c=speedoflight(3108 m/s) • λ=carrierwavelength
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p p
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SPECTRUM
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Spectrum considerations (1)
• Controlledbyregulatorybodies – Carrier frequency
– Signal Power
– Multiple Access Scheme
• Divide into time slots –Time Division Multiple Access (TDMA)
• Divideintofrequencybands–FrequencyDivisionMultipleAccess(FDMA) • Different signal encodings – Code Division Multiple Access (CDMA)
• Australianregulatorybody:AustralianCommunicationsandMedia Authority (ACMA)
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Spectrum considerations (2)
• Industrial,Scientific,andMedical(ISM)bands
– Can be used without a license
– As long as power and spread spectrum regulations are followed
• ISMbandsareusedfor – WLANs
– Wireless Personal Area networks – Internet of Things
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Propagation Modes
• Ground-wavepropagation • Sky-wavepropagation
• Line-of-sightpropagation
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Ground Wave Propagation
• Follows contour of the earth
• Can propagate considerable distances • Frequenciesupto2MHz
• Example
– AM radio
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Sky Wave Propagation
• Signal reflected from ionized layer of atmosphere back down to earth
• Signal can travel a number of hops, back and forth between ionosphere and earth’s surface
• Reflectioneffectcausedbyrefraction
• Example
– Amateur radio
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Line-of-Sight Propagation
• Transmittingandreceivingantennasmustbewithinline of sight
– Satellite communication – signal above 30 MHz not reflected by ionosphere
– Ground communication – antennas within effective line of site due to refraction
• Refraction – bending of microwaves by the atmosphere
– Velocity of electromagnetic wave is a function of the density of
the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums
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LINE OF SIGHT TRANSMISSION
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Five basic propagation mechanisms
1. Free-space propagation
2. Transmission
– Through a medium
– Refraction occurs at boundaries
3. Reflections
– Waves impinge upon surfaces that are large compared to the signal wavelength
4. Diffraction
– Secondary waves behind objects with sharp edges
5. Scattering
– Interactionsbetweensmallobjectsorroughsurfaces
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LOS Wireless Transmission Impairments
• Attenuationandattenuationdistortion • Freespaceloss
• Noise
• Atmosphericabsorption
• Multipath • Refraction
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Attenuation
• Strength of signal falls off with distance over transmission medium • Attenuationfactorsforunguidedmedia:
– Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than noise to be received without error
– Attenuation is greater at higher frequencies, causing distortion
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Free Space Loss
• For any type of wireless communication, the signal disperses with the distance
• Energydispersioncanbeseenasasphereirradiatingfromtheantenna
• Asdistanceincreases,thesphereincrease,resultinginlessenergyper
unit of surface area
• Thus, an antenna with fixed area receives less signal power the further it is from the transmitting antenna
• Free space loss: even in the absence of other sources of attenuation and impairment, the transmitted signal attenuates over distance
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• ThermalNoise
• Intermodulationnoise • Crosstalk
• ImpulseNoise
Noise (1)
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Noise (2)
• Thermalnoise
– Thermal noise due to agitation of electrons
– Present in all electronic devices and transmission media – Cannot be eliminated
– Function of temperature
– Particularly significant for satellite communication
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Noise (3)
• Intermodulation noise: occurs if signals with different frequencies share
the same medium
– Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies
• Crosstalk:unwantedcouplingbetweensignalpaths
• Impulsenoise:irregularpulsesornoisespikes
– Short duration and of relatively high amplitude
– Caused by external electromagnetic disturbances, or faults and flaws in the communications system
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Other Impairments
• Atmospheric absorption: water vapor and oxygen contribute to
attenuation
• Multipath:obstaclesreflectsignalssothatmultiplecopieswithvarying delays are received
• Refraction: bending of radio waves as they propagate through the atmosphere
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FADING
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Fading
• Timevariationofreceivedsignalpowercausedbychangesinthe transmission medium or path(s)
• Fixedenvironments:affectedbychangesintheatmosphericconditions (e.g, rainfall)
• Mobile environments: changes in the relative locations of obstacles as one antenna moves in relation to the other
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The Effects of Multipath Propagation
• Reflection,diffraction,andscattering
• Multiple copies of a signal may arrive at different phases
– If phases add destructively, the signal level relative to noise declines, making detection more difficult
• Intersymbolinterference(ISI)
– One or more delayed copies of a pulse may arrive at the same time as the
primary pulse for a subsequent bit • Rapidsignalfluctuations
– Over a few centimeters
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Examples of Multipath Interference
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Sketch of Three Important Propagation Mechanisms
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Two Pulses in Time-Variant Multipath
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• Large-scalefading
– Signal variations over
large distances – Shadowing
• Small-scalefading – Doppler spread
– Multipath fading
Types of Fading (1)
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• DopplerSpread
– Frequency fluctuations caused by movement
– Coherence time Tc characterizes Doppler shift • Howlongachannelremainsthesame
– Coherence time Tc >> Tb bit time → slow fading • The channel does not change during the bit time
– Otherwise fast fading
Types of fading (2)
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• Multipathfading
Types of fading (3)
– Multiple signals arrive at the receiver
– Coherence bandwidth Bc characterizes multipath
• Bandwidth over which the channel response remains relatively constant
• Related to delay spread, the spread in time of the arrivals of multipath signals
– Signal bandwidth Bs is proportional to the bit rate – If Bc >> Bs, then flat fading
• The signal bandwidth fits well within the channel bandwidth – Otherwise, frequency selective fading
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Flat and Frequency Selective Fading
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CHANNEL CORRECTION MECHANISMS
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Channel Correction Mechanisms
• Forwarderrorcorrection
• Adaptiveequalization
• Adaptivemodulationandcoding • DiversitytechniquesandMIMO • OFDM
• Spreadspectrum
• Bandwidthexpansion
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Forward Error Correction
• Transmitter adds error-correcting code to data block – Code is a function of the data bits
• Receivercalculateserror-correctingcodefromincomingdatabits
– If calculated code matches incoming code, no error occurred
– If error-correcting codes don’t match, receiver attempts to determine bits in error and correct
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Adaptive Equalization
• Canbeappliedtotransmissionsthatcarryanalogordigitalinformation – Analog voice or video
– Digital data, digitized voice or video
• Usedtocombatintersymbolinterference
• Involves gathering dispersed symbol energy back into its original time
interval
• Techniques
– Lumped analog circuits
– Sophisticated digital signal processing algorithms
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Adaptive modulation and coding (AMC)
• The modulation process formats the signal to best transmit bits – To overcome noise
– To transmit as many bits as possible
• Coding detects and corrects errors
• AMCadaptstochannelconditions – 100’s of times per second
– Measures channel conditions
– Sends messages between transmitter and receiver to coordinate changes
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Diversity Techniques
• Diversityisbasedonthefactthatindividualchannelsexperience independent fading events
• Space diversity – techniques involving physical transmission path, spacing antennas
• Frequencydiversity–techniqueswherethesignalisspreadoutovera larger frequency bandwidth or carried on multiple frequency carriers
• Time diversity – techniques aimed at spreading the data out over time
• Useofdiversity
– Selection diversity – select the best signal – Combining diversity – combine the signals
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MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) ANTENNAS
• Useantennaarraysfor
– Diversity – different signals from different antennas
– Multiple streams – parallel data streams
– Beamforming – directional antennas
– Multi-user MIMO – directional beams to multiple simultaneous users
• Modernsystems
– 4 × 4 (4 transmitter and 4 receiver antennas)
–8×8
– Two dimensional arrays of 64 antennas
– Future: Massive MIMO with many more antennas
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Four Uses of MIMO
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OFDM and Spread Spectrum
• Orthogonal Frequency Division Multiplexing (OFDM)
– Splits signal into many lower bit rate streams called subcarriers
– Overcomes frequency selectivity from multipath
– Spaces subcarriers apart in overlapping yet orthogonal carrier frequencies
• Spreadspectrum
– Expand a signal to 100 times its bandwidth
– An alternative method to overcome frequency selectivity
– Users can share the channel by using different spreading codes • Code Division Multiple Access (CDMA)
• Topics of Lecture 6
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Bandwidth expansion
• A signal can only provide a limited bps/Hz – Morebandwidthisneeded
• Carrier aggregation
– Combinemultiplechannels
• Example: Fourth-generation LTE combines third-generation carriers
• Frequency reuse
– Limit propagation range to an area
– Use the same frequencies again when sufficiently far away
– Use large coverage areas (macro cells) and smaller coverage areas (outdoor picocells or relays and indoor femtocells)
• Millimeter wave (mmWave)
– Higher carrier frequencies have more bandwidth available
– 30 to 300 GHz bands with millimeter wavelengths
– Yet these are expensive to use and have problems with obstructions
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Summary
• Electromagnetic signals are transmitted wirelessly with the use of antennas
– The same antenna can be used to bidirectional traffic
– Directional antennas transmit to a direction with a gain compared to a idealized isotropic antenna
• Spectrum for wireless communications are assigned and regulated by national authorities
– Spectrum in the ISM band can be used without license
• There are three basic propagation modules, with Line of Sight (LOS) being of special
interest in this unit
• LOS transmissions are affected by a number of impairments
• Fading causes time variation in the receive power of a signal
• Impairments and fading cannot be prevented, so a number of error correction mechanisms were developed to compensate for their effect on communications
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Sources for this lecture
Cory Beard, William Stallings. Wireless Communication Networks and Systems, 1st edition. Pearson Higher Education, 2016
(Chapter 6)
All material copyright 2016
Cory Beard and William Stallings, All rights reserved
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