Antennas & Propagation

Antennas & PropagationCS 6710Spring 2010Rajmohan Rajaraman

Introduction An antenna is an electrical conductor orsystem of conductorso Transmission - radiates electromagnetic energyinto spaceo Reception - collects electromagnetic energyfrom space In two-way communication, the sameantenna can be used for transmission andreception

Radiation Patterns Radiation patterno Graphical representation of radiation properties of anantennao Depicted as two-dimensional cross section Beam width (or half-power beam width)o Measure of directivity of antennao Angle within which power radiated is at least half of thatin most preferred direction Reception patterno Receiving antenna’s equivalent to radiation pattern Omnidirectional vs. directional antenna

Types of Antennas Isotropic antenna (idealized)o Radiates power equally in all directions Dipole antennaso Half-wave dipole antenna (or Hertz antenna)o Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antennao Used for terrestrial microwave and satellite applicationso Larger the diameter, the more tightly directional is thebeam

Antenna Gain Antenna gaino Power output, in a particular direction,compared to that produced in any direction bya perfect omnidirectional antenna (isotropicantenna) Expressed in terms of effective areao Related to physical size and shape of antenna

Antenna Gain Relationship between antenna gain and effectivearea4!Ae 4!f 2 AeG 2 2"c G antenna gainAe effective areaf carrier frequencyc speed of light ( 3 x 108 m/s)λ carrier wavelength

Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation

Ground Wave Propagation

Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Exampleo AM radio

Sky Wave Propagation

Sky Wave Propagation Signal reflected from ionized layer of atmosphereback down to earth Signal can travel a number of hops, back andforth between ionosphere and earth’s surface Reflection effect caused by refraction Exampleso Amateur radioo CB radioo International broadcasts

Line-of-Sight Propagation

Line-of-Sight Propagation Above 30 MHz neither ground nor sky wavepropagation operates Transmitting and receiving antennas must bewithin line of sighto Satellite communication – signal above 30 MHz notreflected by ionosphereo Ground communication – antennas within effective lineof site due to refraction Refraction – bending of microwaves by theatmosphereo Velocity of electromagnetic wave is a function of thedensity of the mediumo When wave changes medium, speed changeso Wave bends at the boundary between mediums

Line-of-Sight Equations Optical line of sightd 3.57 h Effective, or radio, line of sightd 3.57 !h d distance between antenna and horizon(km) h antenna height (m) K adjustment factor to account forrefraction, rule of thumb K 4/3

Line-of-Sight Equations Maximum distance between two antennasfor LOS propagation:(3.57 !h1 !h2 h1 height of antenna one h2 height of antenna two)

LOS Wireless TransmissionImpairments Attenuationo Free space loss Distortion Dispersion Noise Other effects:o Atmospheric absorptiono Multipatho Refraction

Attenuation Strength of signal falls off with distance overtransmission medium Attenuation factors for unguided media:o Received signal must have sufficient strength so thatcircuitry in the receiver can interpret the signalo Signal must maintain a level sufficiently higher thannoise to be received without erroro Attenuation is greater at higher frequencies, causingdistortion

Free Space Loss Free space loss, ideal isotropic antenna22Pt (4!d ) (4!fd ) 22Pr"c Pt signal power at transmitting antenna Pr signal power at receiving antenna λ carrier wavelength d propagation distance between antennas c speed of light ( 3 x 108 m/s)where d and λ are in the same units (e.g., meters)

Free Space Loss Free space loss equation can be recast:LdB 10 logPt& 4(d # 20 log !Pr% ' " !20 log(" ) 20 log(d ) 21.98 dB' 4(fd 20 log%" 20 log( f ) 20 log(d )! 147.56 dB& c #

Free Space Loss Free space loss accounting for gain of antennas2222(Pt (4" ) (d ) (!d )cd ) 22PrGr Gt !Ar Atf Ar At Gt gain of transmitting antennaGr gain of receiving antennaAt effective area of transmitting antennaAr effective area of receiving antennao In the above formula, the powers correspond to that ofthe input signal at the transmitter and output at thereceiver, respectively

Free Space Loss Free space loss accounting for gain ofother antennas can be recast asLdB 20 log(" ) 20 log(d )! 10 log(At Ar ) !20 log( f ) 20 log(d )! 10 log(At Ar ) 169.54dB

Path Loss Exponents The free space path loss model is idealizedPt" AdPr Here the exponent α depends on the transmissionenvironmento Urban vs suburban, medium-city vs large-city,obstructed vs unobstructed, indoors vs outdoorso Generally between 2 and 4o Obtained empirically! Two-ray, ten-ray, and general statistical models

Distortion Signals at higher frequencies attenuatemore than that at lower frequencies Shape of a signal comprising ofcomponents in a frequency band isdistorted To recover the original signal shape,attenuation is equalized by amplifyinghigher frequencies more than lower ones

Dispersion Electromagnetic energy spreads in spaceas it propagates Consequently, bursts sent in rapidsuccession tend to merge as theypropagate For guided media such as optical fiber,fundamentally limits the product RxL,where R is the rate and L is the usablelength of the fiber Term generally refers to how a signalspreads over space and time

Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices andtransmission media Cannot be eliminated Function of temperature Particularly significant for satellitecommunication

Thermal Noise Amount of thermal noise to be found in abandwidth of 1Hz in any device orconductor is:N 0 kT (W/Hz ) N0 noise power density in watts per 1 Hz ofbandwidth k Boltzmann's constant 1.3803 x 10-23 J/K T temperature, in kelvins (absolute temperature)

Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz(in watts):N kTBor, in decibel-wattsN 10 log k 10 log T 10 log B !228.6 dBW 10 log T 10 log B

Other Kinds of Noise Intermodulation noise – occurs if signals withdifferent frequencies share the same mediumo Interference caused by a signal produced at afrequency that is the sum or difference of originalfrequencies Crosstalk – unwanted coupling between signalpaths Impulse noise – irregular pulses or noisespikeso Short duration and of relatively high amplitudeo Caused by external electromagnetic disturbances, orfaults and flaws in the communications systemo Primary source of error for digital data transmission

Expression Eb/N0 Ratio of signal energy per bit to noise powerdensity per HertzEb S / RS N0N0kTR The bit error rate for digital data is a function ofEb/N0o Given a value for Eb/N0 to achieve a desired error rate,parameters of this formula can be selectedo As bit rate R increases, transmitted signal power mustincrease to maintain required Eb/N0

Other Impairments Atmospheric absorption – water vapor andoxygen contribute to attenuation Multipath – obstacles reflect signals so thatmultiple copies with varying delays arereceived Refraction – bending of radio waves asthey propagate through the atmosphere

Fading Variation over time or distance of receivedsignal power caused by changes in thetransmission medium or path(s) In a fixed environment:o Changes in atmospheric conditions In a mobile environment:o Multipath propagation

Multipath Propagation Reflection - occurs when signal encounters asurface that is large relative to the wavelength ofthe signal Diffraction - occurs at the edge of animpenetrable body that is large compared towavelength of radio wave Scattering – occurs when incoming signal hits anobject whose size is in the order of thewavelength of the signal or less

Effects of Multipath Propagation Multiple copies of a signal may arrive atdifferent phaseso If phases add destructively, the signal levelrelative to noise declines, making detectionmore difficult Intersymbol interference (ISI)o One or more delayed copies of a pulse mayarrive at the same time as the primary pulsefor a subsequent bit

Types of Fading Fast fadingo Changes in signal strength in a short time period Slow fadingo Changes in signal strength in a short time period Flat fadingo Fluctuations proportionally equal over all frequencycomponents Selective fadingo Different fluctuations for different frequencies Rayleigh fadingo Multiple indirect paths, but no dominant path such as LOS patho Worst-case scenario Rician fadingo Multiple paths, but LOS path dominanto Parametrized by K, ratio of power on dominant path to that onother paths

Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

Forward Error Correction Transmitter adds error-correcting code to datablocko Code is a function of the data bits Receiver calculates error-correcting code fromincoming data bitso If calculated code matches incoming code, no erroroccurredo If error-correcting codes don’t match, receiver attemptsto determine bits in error and correct

Adaptive Equalization Can be applied to transmissions that carry analogor digital informationo Analog voice or videoo Digital data, digitized voice or video Used to combat intersymbol interference Involves gathering dispersed symbol energy backinto its original time interval Techniqueso Lumped analog circuitso Sophisticated digital signal processing algorithms

Diversity Techniques Space diversity:o Use multiple nearby antennas and combine receivedsignals to obtain the desired signalo Use collocated multiple directional antennas Frequency diversity:o Spreading out signal over a larger frequency bandwidtho Spread spectrum Time diversity:o Noise often occurs in burstso Spreading the data out over time spreads the errors andhence allows FEC techniques to work wello TDMo Interleaving

Ground Wave Propagation Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example oAM radio. Sky Wave Propagation. Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and