Lecture 4. Antennas, db, and Introduction to Radio Propagation

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1 Lecture 4 Ateas, db, ad Itroductio to Radio Propagatio

2 Overview 2 Quickly review some cocepts that we make use of repeatedly i this class You may have see these thigs i other classes Primarily a refresher Not iteded to be exhaustive or complete Some cocepts are simplified to just meet the eeds of this class Geeratio, trasmissio ad receptio of sigals Modeled as a liear time ivariat system with sigals as iputs ad outputs Wireless systems ad the radio chael are also ofte modeled as LTI systems

3 Simplified model of a digital 3 commuicatio system Focus of ext few lectures Source Source Ecoder Chael Ecoder Modulator Radio Chael Destiatio Source Decoder Chael Decoder Demod -ulator

4 Compoets of the digital 4 commuicatio system Source Produces a fiite alphabet for trasmissio Examples: Quatized voice samples, ASCII alphabets Source coder Removes the redudacies ad efficietly ecodes the alphabet Example: I Eglish, you may ecode the alphabet e with fewer bits tha you would q Chael ecoder Adds redudat bits to the source bits to recover from ay error that the chael may itroduce Modulator Chael Coverts the ecoded bits ito a sigal suitable for trasmissio over the chael Carries the sigal, but will usually distort it

5 Commuicatio Lik 5 Trasmitter (Tx) Sigal is trasmitted at power P t Receiver (Rx) Sigal is received at power P r Trasmissio medium or chael Tx Chael d Rx x(t) y(t) P t P r time = d/c time

6 Classificatios of Trasmissio Media 6 (the chael) Trasmissio Medium Physical path betwee trasmitter ad receiver Guided Media Waves are guided alog a solid medium Example: Copper twisted pair, copper coaxial cable, optical fiber Uguided Media Provides meas of trasmissio but does ot guide electromagetic sigals Usually referred to as wireless trasmissio Example: Atmosphere, outer space (free space)

7 Uguided Media 7 Trasmissio ad receptio are achieved usually by meas of a atea Ateas Trasducers that allow voltage ad curret waveforms flowig o a wire to be coverted ito electromagetic waves that propagate i free space Capture electromagetic waves propagatig i air ad covert them ito voltage or curret waveforms i a wire Cofiguratios for wireless trasmissio Directioal Omidirectioal

db vs absolute power 8 Power (sigal stregth)

relative quatities) dbm: referece to 1 mw

= -10 dbw 10 log 10 (100 mw / 1 mw) = 20 dbm

dbm value = 30 + dbw value Show i R ad Matlab

8 db vs absolute power 8 Power (sigal stregth) is expressed i db for ease of calculatio (all relative quatities) dbm: referece to 1 mw dbw: referece to 1 W Example: 100 mw = 20 dbm = -10 dbw 10 log 10 (100 mw / 1 mw) = 20 dbm 10 log 10 (100 mw / 1 W) = -10 dbw I geeral dbm value = 30 + dbw value Show i R ad Matlab Other relative values are simply expressed i db

9 Examples of usig Decibels 9 Example 1: Express 2 W i dbm ad dbw dbm: 10 log 10 (2 W / 1 mw) = 10 log 10 (2000) = 33 dbm dbw: 10 log 10 (2 W / 1 W) = 10 log 10 (2) = 3 dbw Example 2: The trasmit power is 2 W, the RSS is 0.12 W. What is the loss i db? Loss = Trasmit power RSS = 33 dbm 20.8 dbm = 12.2 db Or Loss = 3 dbw ( 9.2 dbw) = 12.2 db The loss i Example 2 is usually called the path loss RSS = Received Sigal Stregth

10 Some otes 10 1 bel = 10 decibels Hece the multiplicatio by 10 If voltages are give istead of power values, it is commo to assume a 1 W load resistace The db value is calculated as 20 log 10 (voltage) Path loss Loss i sigal stregth betwee trasmitter ad receiver Primarily due to distace (hece path ), but loss i sigal stregth also due to other reasos

11 Ateas 11 What is a atea? A trasducer for covertig guided sigals i a trasmissio lie or waveguide ito electromagetic radiatio i a ubouded medium or vice versa Coversio should be as efficiet as possible Match the impedace of the trasmissio lie to that of the ubouded medium Prevet uwated reflectios back to the load Focus radiatio i the directio required Needs chage i the velocity of charges carried i the atea for radiatio to occur Atea material, shape ad size impact the radiatio ad impedace The dimesio of a atea is measured i uits of the wavelegth l of the carrier

12 What ca be a atea? 12 Ay coductor or dielectric ca serve as the trasducer The properties may make it iefficiet ad thus usuitable for the applicatio Needs careful desig of the structure of the atea Thi Dipole Bicoical Dipole Loop Parabolic Reflector Microstrip Hor Atea Show Prigle s catea

13 Radiatio Sources ad Atea 13 Types Radiatio Sources Currets Aperture fields Curret sources Example: Loops, dipoles Time varyig curret creates a electromagetic field that is radiated Atea Types Passive Ateas Most commo Active (Smart) Ateas More expesive Possibly widespread i the future Aperture sources Example: Hor atea Fields across the aperture serve as the source of the radiatio

14 + The Near ad Far Fields l = wavelegth There are two distict regios of electric ad magetic fields aroud a atea The ear field is called the Fresel regio

plae wave propagatio This is usually the regio of iterest for most applicatios Allows us to simplify the characteristics of the atea The boudary betwee the ear

14 14 + The Near ad Far Fields l = wavelegth There are two distict regios of electric ad magetic fields aroud a atea The ear field is called the Fresel regio Close to the atea (aroud oe l) The far field is called the Frauhofer regio Far away from the atea (several l s away) The radiatio i the far field is similar to plae wave propagatio This is usually the regio of iterest for most applicatios Allows us to simplify the characteristics of the atea The boudary betwee the ear ad far fields is a arbitrary sphere of radius R ff = 2d 2 /l d is the physical dimesio of the atea Diameter of the smallest sphere that completely ecloses the atea

15 Example of Far Field Calculatio 15 What is the far field of a atea for a 1000 MHz carrier if the atea is a half wavelegth dipole? d = l/2 R ff = 2(l/2) 2 / l = l/2 l = c/f = 3 x 10 8 /1000 x 10 6 = 0.3 m R ff = 0.15 m Far Field Near Field d Reactive fields R Radiatig fields

16 Basics of Ateas (I) 16 Radiatio patter G(q,j) Also called atea patter Directioal fuctio of the relative distributio of power or itesity i the far field Three dimesioal plot of the relative stregth as a fuctio of the spherical co-ordiates j ad q The radiatio patter is idepedet of distace It is relative! Typically, it is show as two 2-D plots q-directio (also called elevatio plae) j-directio (also called azimuth plae)

17 Example of Atea Patter 17 polar plot x-y rectagular plot q directio i a g q directio agle i degrees

18 More o Atea Patters 18 The atea patters are usually ormalized to the maximum gai G max The gai is ofte expressed i db i such a case I the previous example The patter is the same for all values of j I may cases, there may be a chage with j i which case the azimuthal variatio also eeds to be show

19 Directivity 19 Directivity Describes the atea patter of a lossless atea Idicates how much gai is there due to the directioality D = maximum radiatio itesity/average radiatio itesity D = 1 4 π 4 π G max G( θ, ϕ) dω Solid Agle siq dq dj

20 Isotropic Atea 20 Radiatio propagates equally i all directios Ideal does ot exist What is the directivity of the isotropic atea? D iso = G max 4π 2π 0 G max π 0 = siθdθdϕ 2π π 0 4π =1 siθdθ

21 Radiatio lobes 21 3 db 3 db Beamwidth Ideal Atea Ideal atea Gai = 1 over a certai agle Gai = 0 over the rest of the directios Mai lobe Side lobe Back lobe Real atea Radiates power i uwated directios Has oe or more mai lobes ad may sidelobes Specified beamwidth

22 Radiatio Lobes (II) 22 Atea Beamwidth The agle of coverage where the radiated eergy is 3 db dow from the peak of the beam (half-power) By arrowig the beamwidth we ca icrease the gai ad create sectors at the same time Frot-to-Back Ratio The ratio of the power i the mai lobe to the power i the lobe created at the back of the atea Ratio should be as large as possible Frot to back ratio of a dipole is 0 db!

23 Example: Beamwidth ad Directivity 23 Compute the 3 db beamwidth ad directivity of a atea that has the patter defied by the followig equatio: G( θ, ϕ) = 2 # cos θ, if 0 θ π/2 "! 0, elsewhere Note that this atea patter is idepedet of the azimuth Set G(q,j) = 0.5 to fid the 3 db beamwidth (solve for q) You ca fid the directivity by itegratio The aswer is D = 6

24 Atea Gai 24 The gai of a atea i a give directio is the ratio of the power desity produced by it i that directio divided by the power desity that would be produced by a referece atea i the same directio Two types of referece ateas are geerally used Isotropic atea: gai is give i dbi Half-wave dipole atea: gai is give i dbd Maufacturers ofte use dbi i their marketig To show a slightly higher gai J dbi = dbd db 0 dbi 0 dbd Dipole Isotropic Other 5 dbd = 7.15 dbi

25 Basics of Ateas (II) 25 Reciprocity A atea ca be used both for trasmissio ad receptio It performs equally well for both tasks The radiatio patter is idetical for trasmissio ad receptio Impedace Exceptios: Solid state ateas It is importat to match the impedace of the atea to that of the trasmissio lie feedig it

26 Omidirectioal Ateas 26 Omidirectioal atea Radiatio patter is costat i the azimuth plae Half-wave dipoles ad quarter-wave moopoles with a groud plae are good approximatios Typically made from some type of colliear array of half-wave dipoles Radiatio patter is i the shape of a dout At l/2, impedace matchig occurs with the trasmissio lie 2

27 Effective Area 27 Characterizes the ability of a atea to Capture eergy from a icidet wave ad covert it ito a itercepted power Also called effective aperture ad receivig cross-sectio It is ot depedet o the physical area of the atea although that could affect it You ca show that the effective area is give by A e = l 2 D/4p for ay atea (D = directivity) Assumes matched impedace What is it for a isotropic atea? (remember free space loss)

28 Importace of ateas 28 Capacity of the system ca be icreased Co-chael iterferece ca be reduced with directioal ateas Multipath effects ca be reduced If a highly directioal atea is used for both trasmissio ad receptio, the umber ad spread of multipath compoets are reduced Diversity gais are possible Usig atea elemets that are spaced apart, spatial diversity gais are achieved MIMO Multiple Iput Multiple Output ad smart ateas Rx 2 Tx Rx 1 Three sector atea for a cellular system with two orders of receive diversity There are two receivig elemets per sector ad oe trasmittig elemet

29 Atea Examples 29 Moopole Omidirectioal Pael Array of dipoles for sectored cell Grid Reflector Atea

30 Atea Locatio 30 Tred is to co-locate cells from multiple compaies due to cost of cell site lad/tower America Tower Crowcastle

31 The Radio Chael 31 The radio chael is differet Extremely harsh eviromet compared to wired or guided media Chael is time variat Movemet of people Switchig off ad o of iterferece Movemet of mobile termials Sesitivity to a variety of other factors Fadig ad Multipath Need a framework that characterizes the radio chael Commo to approximate it as a LTI system

32 What is Radio Propagatio? 32 How is a radio sigal trasformed from the time it leaves a trasmitter to the time it reaches the receiver What is the radio chael? Importat for the desig, operatio ad aalysis of wireless etworks Where should base statios be placed? What trasmit powers should be used? What radio chaels eed be assiged to a cell? How are hadoff decisio algorithms affected?

33 Propagatio Mechaisms (1) 33 EM radiatio propagates as various waves depedig o wavelegth ad distace Groud (surface) wave travels close to groud level Domiat for low frequecies (30 khz - 3 MHz) Scatters off terrai ad buildigs Tropospheric waves propagate i lower atmosphere ad refract back to groud level Amout of refractio icreases with frequecy, causes sigificat aoyace above 30 MHz Ioospheric waves ca be reflected betwee upper atmosphere ad groud to propagate thousads of miles Effected by suspot activity, cause sigal distortio

34 Propagatio Mechaisms (2) 34 For a high frequecy sigal (> 500 MHz) A electromagetic wave ca be modeled as a ray Basic mechaisms Trasmissio (propagatio through a medium) Scatterig (small objects less tha wavelegth) Reflectio (objects much larger tha wavelegth) Waves may be reflected by statioary or movig objects Diffractio at the edges

35 Reflectio ad Trasmissio 35 Electromagetic ray impiges o object larger tha the wavelegth l It bouces off the object Examples: Walls, buildigs, groud Sigal is atteuated by a reflectio factor Atteuatio depeds o Nature of material Frequecy of the carrier Agle of icidece Nature of the surface Usually trasmissio through a object leads to larger losses (absorptio) tha reflectio Multiple reflectios ca result i a weak sigal

36 Oxyge absorptio at 60 GHz 36 Sigals are atteuated (fade) over distace depedig o frequecy ad weather coditios f = 60 GHz I oxyge with rai I oxyge Loss i db I vacuum log (distace) For illustratio oly, ot to scale

37 Diffractio 37 The radio sigal is icidet upo the edge of a sharp object Example: Wall, roof edge, door Each such object becomes a secodary source Losses are much larger tha with reflectio or trasmissio Importat i micro-cells for o-lie of sight trasmissio Propagatio ito shadowed regios Not sigificat i idoor areas because of large losses

38 Scatterig 38 Caused by irregular objects comparable i size to the wavelegth These objects scatter rays i all directios Each scatterer acts as a source Sigal propagates i all directios Large losses i sigal stregth Isigificat except whe the trasceiver is i very cluttered eviromets Examples of scatterers Foliage, furiture, lampposts, vehicles

39 Multipath Propagatio 39 Multipath Receiver gets combied radio waves from differet directios with differet path delays Received sigal is very depedet o locatio - differet phase relatioships ca cause sigal fadig ad delay spread Causes time variatio ad iter-symbol iterferece i digital systems Causes burst errors Limits maximum symbol rate

40 Time Variatio of Sigals 40 A movig receiver ca experiece a positive or egative Doppler shift i received sigal, depedig o directio of movemet Results i wideig frequecy spectrum Rapid fluctuatios of sigal evelope Sigal evelope i db 1 2 time Fadig

41 Time Dispersio ad ISI 41 Suppose we trasmit a sigle arrow pulse Assume there are three paths What do we receive? What happes if we sed two arrow pulses? Iitial Tx pulse Received sigal

42 First possibility 42 Write Maxwell s equatios Solve Maxwell s equatios Difficult if ot impossible Details? Approximatios may help FDTD Ray tracig

43 Secod possibility 43 Simplify! Measuremets Macroscopic characterizatio Empirical models how sigals are affected vis-à-vis some parameters

44 Summary 44 Scatterig Trasmissio Diffractio Reflectio RX TX Several paths from Tx to Rx Differet delays, phases ad amplitudes Add motio makes it very complicated Very difficult to look at all of the effects i a composite way Use empirical models Use statistical models Breakdow pheomea ito differet categories

45 Radio chael characterizatio 45 Radio propagatio is modeled as a radom pheomeo Measuremets followed by statistical modelig Sigal stregth measuremets RMS delay spread measuremets Use spread spectrum or liear FM Measuremets to fie tue simulatios ad simulatios followed by statistical modelig Ray tracig: Approximate the radio propagatio by meas of geometrical optics

46 Classified based o site/applicatio 46 specificity Propagatio Coditios Idoor Commercial Office Residetial Tuel Outdoor to Idoor Outdoor Urba Rural Suburba Forest/Jugle Moutaious Ope areas/free space Over Water Frequecy depedece 700, 900 MHz : Cellular 1.8, 1.9 GHz : PCS 2.4 GHz : WLANs, BT, Cordless 5 GHz : WLANs, RF tags, MMDS 10 GHz : MMDS 30 GHz : LMDS LMDS: Local multipoit distributio service MMDS: Multichael multipoit distributio system

47 Commuicatios Issues i Radio 47 Propagatio Coverage How far does the sigal propagate over a give terrai at a particular frequecy? Power or received sigal stregth (RSS) Performace Bit error rate Statistics of fadig amplitudes ad duratios Data rate (capacity) Multipath structure MIMO Some issues are predomiat for certai applicatios

48 Coverage 48 How far does the sigal propagate over a give terrai at a give frequecy? Same as lik budget (i a sese) Determies Trasmit power required to provide service i a give area Iterferece from other trasmitters Number of base statios or access poits that are required Parameters of importace Path loss Shadow fadig

49 Sigal propagatio rages 49 Trasmissio rage Commuicatio possible Low error rate Detectio rage Detectio of the sigal possible No reliable commuicatio possible Iterferece rage Sigal may ot be detected Sigal adds to the backgroud oise seder trasmissio detectio iterferece distace

50 Rate of Chael Fluctuatios 50 What are the chages i the chael? How fast are these chages? How do they ifluece performace? Determies Performace of the commuicatio system Outage, probability of error Receiver desig Codig, diversity etc. Power requiremets Parameters of importace Fluctuatio characteristics Fade rate, fade duratio ad Doppler spectrum

51 Data Rate Support 51 What is the maximum data rate that ca be supported by the chael? What limits it? Determies Capacity of the system Complexity of the receiver Applicatio support Parameters of importace Multipath delay spread ad coherece badwidth Fadig characteristics of the multipath compoets

52 Radio Propagatio 52 Characterizatio Fadig Chaels Large Scale Fadig Small Scale Fadig Path Loss Shadow Fadig Time Variatio Time Dispersio Coverage Amplitude fluctuatios Distributio of amplitudes Rate of chage of amplitude Doppler Spectrum Multipath Delay Spread Coherece Badwidth Itersymbol Iterferece Receiver Desig (codig) Performace (BER) Receiver Desig, Performace Maximum Data Rates

53 Summary 53 Large Scale Fadig Histogram of Deviatios is Shadow Fadig Power i db Liear Fit of RSS i db to log(distace) Slope is the distace-power gradiet Small Scale Fadig Histogram of Deviatios is Multipath Fadig Fourier Trasform of Deviatios is Doppler Spectrum Distace from Base Statio i Logarithmic Scale

54 The Free Space Loss 54 Assumptio Trasmitter ad receiver are i free space No obstructig objects i betwee The earth is at a ifiite distace! The trasmitted power is P t, ad the received power is P r The path loss is L p = P t (db) P r (db) Isotropic ateas Ateas radiate ad receive equally i all directios with uit gai d

55 The Free Space Model 55 The relatioship betwee P t ad P r is give by P r = P t l 2 /(4pd) 2 The wavelegth of the carrier is l = c/f I db P r (dbm)= P t (dbm) log 10 (l) 20 log 10 (d) L p (d) = P t P r = log 10 (l) + 20 log 10 (d) = L log 10 (d) L 0 is called the path loss at the first meter (put d = 1) We say there is a 20 db per decade loss i sigal stregth

56 A simple explaatio of free space 56 loss Isotropic trasmit atea: Radiates sigal equally i all directios Assume a poit source At a distace d from the trasmitter, the area of the sphere eclosig the Tx is: A = 4pd 2 The power desity o this sphere is: P t / 4pd 2 Isotropic receive atea: Captures power equal to the desity times the area of the atea Ideal area of atea is A at = l 2 /4p The received power is: P r = P t / 4pd 2 l 2 /4p = P t l 2 /(4pd) 2

57 Isotropic ad Real Ateas 57 Isotropic ateas are ideal ad caot be achieved i practice Useful as a theoretical bechmark Real ateas have gais i differet directios Suppose the gai of the trasmit atea i the directio of iterest is G t ad that of the receive atea is G r The free space relatio is: P r = P t G t G r l 2 /(4pd) 2 The quatity P t G t is called the effective isotropic radiated power (EIRP) This is the trasmit power that a trasmitter should use were it havig a isotropic atea

where d is i meters P r = Pl 2 t 2 2 ( 4p ) d I db, we have: P r (dbm)= P t (dbm) - 21.

58 Summary: Free space loss 58 Trasmit power P t ad received power P r Wavelegth of the RF carrier l = c/f Over a distace d the relatioship betwee P t ad P r is give by: where d is i meters P r = Pl 2 t 2 2 ( 4p ) d I db, we have: P r (dbm)= P t (dbm) log 10 (l) 20 log 10 (d) Path Loss = L p = P t P r = log 10 (l) + 20log 10 (d)

59 Example 59 The trasmit power of a wireless commuicatio system is 2 W. If the propagatio is similar to free space, what is the received power at a frequecy of 1 GHz at a distace of 1 km? Assume isotropic trasmit ad receive ateas. What do we kow?

60 Next Week 60 Impact of frequecy Impact of distace Other path-loss models

Introduction to Wireless Communication Systems ECE 476/ECE 501C/CS 513 Winter 2003

Introduction to Wireless Communication Systems ECE 476/ECE 501C/CS 513 Winter 2003 troductio to Wireless Commuicatio ystems ECE 476/ECE 501C/C 513 Witer 2003 eview for Exam #1 March 4, 2003 Exam Details Must follow seatig chart - Posted 30 miutes before exam. Cheatig will be treated