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射频工程基础 Fundamentals of RF Engineering

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1 射频工程基础 Fundamentals of RF Engineering
学时:60/20 学分: 3.5 孙利国 中国科技大学信息学院电子工程与信息科学系

2 (Session 5 RF Radio-wave propagation)
第五讲 射频电波传播 (Session 5 RF Radio-wave propagation) 教材:以课堂讲义为主。 主要参考书: [1]“Microwave and RF Design: A System Approach”, Michael Steer, SciTech Publishing, 2010 其它参考书: [2] “射频电路设计-理论与应用”,Reinhold Ludwig等著,王子宇等译,电子工业出版社,2002 [3] “射频微电子学”,拉扎维著,余志平等译,清华大学出版社,2006 [4] “RF and Microwave Circuit Design for Wireless Communications”, Lawrence Larson, Artech House, 1997 [5]”无线网络RF工程:硬件、天线和传播“, Daniel M.Dobkin 著 ,科学出版社 ,2007 [6] “Radio Propagation for Modern Wireless System”, Henry L. Bertoni, Prentice Hall PTR, 2000 [7]”Microwave and millimeter wave propagation”, Xie Yixi, etal. International Academic Publishers, 1995 [8]”Wireless Communications -Principle and Practice”, T.S. Rappaport, Prentice Hall PTR, 2002

3 “Microwave and RF Design: A System Approach”,
Reference “Microwave and RF Design: A System Approach”, Chapter 2 Antenna and RF link Reading: §2.6 to §2.8

4 Goal of This Lecture Outline Propagation Fading
The goal here is to understand propagation in wireless systems. You will be able to calculate loss in a radio link. You will also develop an understanding of fading and the schemes to overcome it. Outline Propagation Fading Information Source Channel Destination

5 Three types of wave propagation
When dealing with radio signals, transmission-reception takes 3 types of wave propagation. Space wave: Line of sight (LOS) Ground wave Sky wave Line of sight (LOS) propagation is very useful in RF frequencies, especially VHF and UHF. Earth Ionosphere Sky-wave Ground-wave Space wave: Line of sight

6 Three types of wave propagation
Ionosphere ( km) Sky wave Mesosphere ( km) Stratosphere ( km) Space wave Ground wave Transmitter Receiver Troposphere ( km) Earth 6

7 Three types of wave propagation
Classification Band Initials Frequency Range Characteristics Extremely low ELF < 300 Hz Ground wave Infra low ILF 300 Hz - 3 kHz Very low VLF 3 kHz - 30 kHz Low LF 30 kHz kHz Medium MF 300 kHz - 3 MHz Ground/Sky wave High HF 3 MHz - 30 MHz Sky wave Very high VHF 30 MHz MHz Space wave Ultra high UHF 300 MHz - 3 GHz Super high SHF 3 GHz - 30 GHz Extremely high EHF 30 GHz GHz Tremendously high THF 300 GHz GHz 7

8 Line of Sight (LOS) Propagation
Rx refracted wave Tx direct (free space) wave reflected wave Free space - wave travels in a straight line atmospheric absorption (loss) refraction in the atmosphere reflections from the ground Free Space “Spreading” Loss energy intercepted by the red square is proportional to 1/r2 r

9 Line of Sight (LOS) Propagation
Free Space Path Loss Tx Rx d GR, DR, ηR GT, DT, ηT The antenna gain G is the factor by which the power density is increased compared with the isotropic case. G is very large for antennas used in microwave point-to-point systems 100 (20 dB) to 10,000,000 (70 dB).

10 Line of Sight (LOS) Propagation
Free Space Path Loss Tx Rx d GR, DR, ηR GT, DT, ηT

11 Line of Sight (LOS) Propagation
Free Space Path Loss Tx Rx d GR, DR, ηR GT, DT, ηT Example: Calculate the path loss and Tr. loss for antennas 50 km apart, for a 10 GHz, system with antenna gains of 60 dB Path loss=20log(4x3.14x5x104/(3x10-2))=146 dB Tr. Loss= =26dB Typical path loss: dB for a 50 km link at 10 GHz (high gain antennas help offset loss ~ 60 dB)

12 Example: Radio Link (Transmission Loss Calculation)
Line of Sight (LOS) Propagation Example: Radio Link (Transmission Loss Calculation) Transmitter +43 dBm TX output −3 dB line efficiency = +40 dBm to antenna +13 dB antenna gain = +53 dBm EIRP −158 dB path attenuation =−105 dBm if intercepted by dipole antenna = −92 dBm into line = −95 dBm to receiver Trans. Line Antenna Antenna Trans. Line Receiver

13 Typical Cellular Link Budget
Line of Sight (LOS) Propagation Typical Cellular Link Budget Source: Cell TX PO Watts MS TX PO Watts Cell TX PO dBM MS TX PO dBm Cell Combiner Loss dB MS Combiner Loss db Cell Cable Loss db MS Cable Loss db Cell Antenna Gain dBd MS Antenna Gain dBd EIRP Watts EIRP dBm MS RX Cable Loss dB Cell RX Cable Loss MS Diversity Gain dB Cell Diversity Gain MS RX Sensitivity dBM Cell RX Sensitivity dBM FWD Link Budget, dB REV Link Budget, dB Worst Case Link Budget Input: Calc: FWD Path 45.00 46.53 -3.00 +10.00 113.03 50.53 0.00 155.53 -2.00 REV Path 3.00 34.77 +5.00 5.99 37.77 10.00 +4.00 155.77 -0.24 Imbalance, dB

14 Line of Sight (LOS) Propagation
Fresnel Zones Tx Rx 1st Fresnel Zone 2nd Fresnel Zone d=d1+d2 d1 d2 plane at right angles to path As radio waves propagate they spread out. The spreading out is understood from Huygens principle that every point of a propagating EM wave reradiates in every direction. One of the consequences of this is that an obstruction that is not in the LOS path can still interfere with signal propagation. The appropriate clearance is determined from the Fresnel zones.

15 Line of Sight (LOS) Propagation
Fresnel Zones Tx Rx 1st Fresnel Zone 2nd Fresnel Zone d=d1+d2 d1 d2 plane at right angles to path Note that path difference between rays at the edge of the 1st and 2nd Fresnel zones is l/2 1st Fresnel Zone is the most important zone.

16 Line of Sight (LOS) Propagation
Fresnel Zones Huygens principle: different Fresnel zones Z1,Z2,Z3,… Contribution from Fresnel zones Z1,Z2,Z3,…are B1,B2,B3,…

17 Line of Sight (LOS) Propagation
Fresnel Zones B1=Contribution from Fresnel zone1 (Z1) B1=2B0 B0=received for free-space Contribution from Fresnel zone1 is double of the total field for free space.

18 Line of Sight (LOS) Propagation
Fresnel Zones d1 d2 Example: d1=d2=5km F1=17.66m at 2.4 GHz

19 Line of Sight (LOS) Propagation
Fresnel Zones Fresnel zones are a number of concentric ellipsoids with TX and RX in focus. Odd-numbered Fresnel zones have relatively intense field strengths, whereas even numbered Fresnel zones are nulls.

20 Line of Sight (LOS) Propagation
Fresnel Zones If the 1st Fresnel zone is clear, it can be thought as free-space propagation (LOS). If the 1st Fresnel zone is not clear, then free-space loss (LOS) does not apply and an adjustment term must be included. To avoid this : Use an antenna with a narrower lobe pattern, usually a higher gain antenna will achieve this. Raise the antenna mounting point on TX and/or RX.

21 Line of Sight (LOS) Propagation
Atmospheric Absorption (clear air) Absorption by atmospheric gases Attenuation due to molecular absorption, for example, water vapour and O2 Important bands at frequencies >10 GHz, especially at 22 GHz (water vapor) and 60GHz (O2)

22 Line of Sight (LOS) Propagation
Atmospheric Attenuation (not clear air) Attenuation due to precipitations Rain: usually at frequency >10GHz Snow, cloud, fog, sand

23 Line of Sight (LOS) Propagation Atmospheric Attenuation
Atmospheric Absorption - rainfall (snow/fog) - water vapour (molecular res. at 22 GHz) - oxygen (molecular res. at 60 GHz) Typical path loss: 140 dB for a 50 km link at 10 GHz (high gain antennas help offset loss ~ 60 dB)

24 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere Refraction effect- ray bending Refractive index n of atmosphere changes with height and temperature This leads to beam bending . Normally n decreases with increasing height. n4 n3 n2 n1 beam bending θ2 θ1 n1 n2 y x

25 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere Radio waves get “bent” downwards. Be able to propagate beyond the geometric horizon, which extends range.

26 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere

27 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere negative refraction zero refraction positive refraction

28 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere standard refraction critical refraction super refraction

29 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere

30 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere Earth curvature Radio wave go behind the geometrical horizon due to refraction:the air refractivity changes with height, temperature, water vapor contents, etc. In standard conditions the radio wave travels along arc bent slightly downward。 An equivalent Earth radius Re=K Rearth is introduced to “makes “the path straight.

31 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere Except zero refraction, the ray is curve. We can “make” it a straight line by pretending the Earth has a larger radius which we call the equivalent Earth radius Re.

32 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere An equivalent Earth radius K Rearth “makes “the path straight. K-factor is a scaling factor of the ray path curvature. K=1 means a straight line. For standard atomsphere K=4/3.

33 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere

34 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere

35 Refraction in the Atmosphere
Line of Sight (LOS) Propagation Refraction in the Atmosphere

36 Other Propagation Mechanisms
Line of Sight (LOS) Propagation Other Propagation Mechanisms Reflection Propagation wave impinges on an object which is large as compared to wavelength - e.g., the surface of the Earth, buildings, walls, etc. Diffraction Radio path between transmitter and receiver obstructed by surface with sharp irregular edges Waves bend around the obstacle, even when LOS (line of sight) does not exist Scattering Objects smaller than the wavelength of the propagation wave - e.g. foliage, street signs, lamp posts 36

37 Other Propagation Mechanisms
Line of Sight (LOS) Propagation Other Propagation Mechanisms Building Direct Signal Reflected Signal hb Diffracted Signal hm d Transmitter Receiver 37

38 Other Propagation Mechanisms
Line of Sight (LOS) Propagation Other Propagation Mechanisms r h t r TREE SCATTERING h t MULTIPATH h d d h t r KNIFE-EDGE DIFFRACTION h t d r MULTIPATH d Any object which is half a wavelength ( /2) long can interfere with signal.  /2 = 7.0 in., cellular radio (860 MHz)  /2 = 3.2 in., PCS (1.9 GHz)

39 Ground Reflections - Path Clearances and Antenna Heights
Line of Sight (LOS) Propagation Ground Reflections - Path Clearances and Antenna Heights direct Rx Tx If the ground is in the 1st Fresnel zone, the ground reflection needs to be considered. There are two kinds of reflections Smoth surface- specular reflection Rough surface -diffuse reflection

40 Ground Reflections - Path Clearances and Antenna Heights
Line of Sight (LOS) Propagation Ground Reflections - Path Clearances and Antenna Heights Smoth surface- specular reflection

41 Ground Reflections - Path Clearances and Antenna Heights
Line of Sight (LOS) Propagation Ground Reflections - Path Clearances and Antenna Heights Smoth surface- specular reflection According to image theory, the ground reflection can be thought as field radiated from image point A’. The 1st Fresnel zone due to image point A’ is shown in the figure. It is called efficient reflection zone.

42 Ground Reflections - Path Clearances and Antenna Heights
Line of Sight (LOS) Propagation Ground Reflections - Path Clearances and Antenna Heights Rough surface -diffuse reflection

43 Direct propagation –curvature of the Earth
Line of Sight (LOS) Propagation Direct propagation –curvature of the Earth Propagation: curvature of the Earth is considered.

44 Knife-Edge Diffraction
Line of Sight (LOS) Propagation Knife-Edge Diffraction d1 d2 Tx Rx H Sometimes a single well-defined obstruction blocks the path. This case is fairly easy to analyze and can be used as a manual tool to estimate the effects of individual obstructions. First calculate the parameter n from the geometry of the path Next consult the table to obtain the obstruction loss in db Add this loss to the otherwise-determined path loss to obtain the total path loss. Other losses such as reflection cancellation still apply, but computed independently for the path sections before and after the obstruction. n Atten. (dB) +5 +10 +15 +20 +25 -4 -3 -2 -1 1 2 3 -5

45 Knife Edge Diffraction Example 1/3
Line of Sight (LOS) Propagation Knife Edge Diffraction Example 1/3 Tx Rx d1 d2 H 5 m 100 m 150 m A B What is the total path loss at 1 GHz? Diffraction Loss Use Loss = (Path Loss, no diffraction) + (Diffraction Loss) Path Loss (dB) (no diffraction) = 20 log10 (4pd/l) = 20 log10 (4p250/0.3) = 80.4 dB n Atten. (dB) +5 +10 +15 +20 +25 -4 -3 -2 -1 1 2 3 -5 = -5 sqrt[2/0.3 * (1/ /150)] = Diff. Loss = dB Loss = = 97.9 dB Add atmospheric absorption loss to this.

46 Line of Sight (LOS) Propagation
Knife Edge Diffraction Example 2/2 n Atten. (dB) +5 +10 +15 +20 +25 -4 -3 -2 -1 1 2 3 -5 17.5 dB -1.68 Diffraction Loss = 17.5 dB

47 Radio Propagation in Mobil Communication
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Effect of mobility Channel varies with user location and time Radio propagation is very complex Multipath scattering from nearby objects Shadowing from dominant objects Attenuations effects Results in rapid fluctuations of received power

48 Radio Propagation in Mobil Communication
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Large scale model Path lossn Slow fading(long term fade) Small scale model fast fading(short term fade)

49 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Fast Fading (Short-term fading) Slow Fading (Long-term fading) Signal Strength (dB) Path Loss Distance 49

50 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication The received signal power: where Gr is the receiver antenna gain, L is the propagation loss in the channel, i.e., L = LP LS LF Fast fading Slow fading Path loss 50

51 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Definition of path loss LP Path Loss in Free-space: 51

52 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Path Loss in Free-space: 52

53 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication

54 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Most radio propagation models are derived using a combination of analytical and empirical methods. Both theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance, whether in outdoor or indoor radio channel Long distance path loss model is as following

55 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication In general a simplest Formula is given: where d : distance between transmitter and receiver α and β : propagation constants which are determined experimentally for different environments β : value of 2 for free space value of 3 ~ 4 in typical urban area 55

56 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Value of  characterizes different environments Rappaport, Table 3.2, pp. 104 17 March 1999 56 Radio Propagation

57 Reflection with Partial Cancellation
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Reflection with Partial Cancellation TX EIRPDBM Analysis: physics of the reflection cancellation predicts signal decay approx. 40 db per decade of distance twice as rapid as in free-space! observed values in real systems range from 30 to 40 db/decade HTFT HTFT DMILES Comparison of Free-Space and Reflection Propagation Modes Assumptions: Flat earth, TX EIRP = MHz. Base Ht = 200 ft, Mobile Ht = 5 ft. DistanceMILES 1 2 4 6 8 10 15 20 FS Link Loss Free-SpaceDBM -45.3 -51.4 -55.3 -57.4 -63.4 -65.4 -68.9 -71.4 FS Link Loss using ReflectionDBM -69.0 -79.2 -89.5 -95.4 -99.7 -103.0 -109.0 -113.2

58 Pass Loss in a Mobile System
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication (dBm) Strength Signal Received 4 16 20 24 28 32 12 8 -50 -60 -70 -80 -90 -100 -110 -120 Distance from cell site (km) Measured Pass Loss in a Mobile System Power of a signal drops off as 1/Dn n ranges from 2 to 4 n = 4 dense urban area n = 3 suburban area n = 2 rural area Free-Space 20 dB per decade of distance Reflection Cancellation 40 dB per decade of distance Real-life cellular propagation decay rates are typically somewhere between 30 and 40 dB per decade of distance. Example: F = 1900 MHz

59 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication

60 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Outdoor propagation models There are lots of out-door models to predict signal strength at a particular receiving point or in a specific local area. The methods vary widely in their approach, complexity, and accuracy. Okumura’s model is one of the most widely used models for signal prediction in urban areas. The more common form is a curve fitting of Okumura’s original results, which is called Okumura-Hata model.

61 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Okumura-Hata Model Urban area: where Suburban area: Open area: 61

62 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Okumura-Hata Model Path loss in decreasing order: Urban area (large city) Urban area (medium and small city) Suburban area Open area

63 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication indoor propagation models Indoor models are less generalized Environment comparatively more dynamic Significant features are physically smaller Shorter distances are closer to near-field More clutter, scattering, less LOS

64 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Indoor propagation models Physical effects: Signal decays much faster Coverage contained by walls, etc. Walls, floors, furniture attenuate/scatter radio signals People moving around: Path loss formula:

65 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Fading The result of variation (with time) of the amplitude or relative phase, or both, or one or more of the frequency components of the signal. Cause: changes in the characteristics of the propagation path with time. There are slow fading and fast fading

66 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Fast Fading (Short-term fading) Slow Fading (Long-term fading) Signal Strength (dB) Path Loss Distance 66

67 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Area-mean power (pass loss) is determined by path loss is an average over 100 m - 5 km Local-mean power (slow fading) is caused by local 'shadowing' effects has slow variations is an average over 40 λ (few meters) Instantaneous power (fast fading) fluctuations are caused by multipath reception depends on location and frequency depends on time if antenna is in motion has fast variations (fades occur about every half a wave length) 67

68 Slow Fading Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Slow Fading The long-term variation in the mean level is known as slow fading(shadowing or log-normal fading). This fading caused by shadowing. Example of Shadow: Local obstacles cause random shadow attenuation 68

69 Slow Fading Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Slow Fading The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). Received power in dB follows normal distribution. Log-normal distribution: - The pdf of the received signal level is given in decibels by where M is the true received signal level m in decibels, i.e., 10log10m, M is the area average signal level, i.e., the mean of M,  is the standard deviation in decibels 69

70 Log-normal Distribution
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Log-normal Distribution M 2 p(M) The pdf of the received signal level 70

71 Fast Fading Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Fast Fading The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles. When MS far from BS, the envelope distribution of received signal is Rayleigh distribution. The pdf is where  is the standard deviation. Middle value rm of envelope signal within sample range to be satisfied by We have rm = 1.777 71

72 Rayleigh Distribution
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication r 2 4 6 8 10 P(r) 0.2 0.4 0.6 0.8 1.0 =1 =2 =3 Rayleigh Distribution The pdf of the envelope variation 72

73 Proof of Rayleigh Distribution
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Proof of Rayleigh Distribution There are many objects in the environment that scatter the radio signal. If the electric field is divided into Ex and Ey, the total fields as superposition of scattering field are given by If there are sufficiently much scatters, the Ex and Ey obey Gauss distribution respectively according to the central limit theorem. 73

74 Rayleigh Distribution
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Rayleigh Distribution PDF of Ex and Ey are given by Because Ex and Ey are independent with each other, the joint PDF is 74

75 Rayleigh Distribution
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Rayleigh Distribution The magnitude ( envelop) of the field is defined as x y R φ The probability of E<R is PDF of E is given by It is Rayleigh distribution 75

76 Fast Fading (Continued)
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Fast Fading (Continued) When there is a dominant stationary(no fading) signal component (direct signal), the envelope distribution of received signal is Ricean distribution. The pdf is where  is the standard deviation, I0(x) is the zero-order Bessel function of the first kind,  is the amplitude of the direct signal 76

77 Ricean Distribution Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Ricean Distribution r Pdf p(r)  = 2  = 1 = 0 (Rayleigh)  = 1  = 3 The pdf of the envelope variation 77

78 Characteristics of Instantaneous Amplitude
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Characteristics of Instantaneous Amplitude Level Crossing Rate: Average number of times per second that the signal envelope crosses the level (some threshold) in positive going direction. Fading Rate: Number of times signal envelope crosses middle value in positive going direction per unit time. Depth of Fading: Ratio of mean square value and minimum value of fading signal. Fading Duration: Time for which signal is below given threshold. 78

79 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Doppler Shift Doppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source. When they are moving toward each other, the frequency of the received signal is higher than the source. When they are opposing each other, the frequency decreases. Thus, the frequency of the received signal is where fC is the frequency of source carrier, fD is the Doppler frequency. Doppler Shift in frequency: where v is the moving speed,  is the wavelength of carrier. Moving speed v MS Signal 79

80 Moving Speed Effect Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Moving Speed Effect Time V1 V2 V3 V4 Signal strength 80

81 Delay Spread Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Delay Spread When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections. This forces signal to follow different paths. Each path has different path length, so the time of arrival for each path is different. This effect which spreads out the signal is called “Delay Spread”. 81

82 Delay Spread Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Delay Spread Delay Signal Strength The signals from close by reflectors The signals from intermediate reflectors The signals from far away reflectors 82

83 Intersymbol Interference (ISI)
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Intersymbol Interference (ISI) Caused by time delayed multipath signals Has impact on burst error rate of channel Second multipath is delayed and is received during next symbol For low bit-error-rate (BER) R (digital transmission rate) limited by delay spread d. 83

84 Intersymbol Interference (ISI)
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Intersymbol Interference (ISI) Transmission signal 1 1 Time Received signal (short delay) Time Propagation time Delayed signals Received signal (long delay) Time 84

85 Coherence Bandwidth Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Coherence Bandwidth Coherence bandwidth Bc: Represents correlation between 2 fading signal envelopes at frequencies f1 and f2. Is a function of delay spread. Two frequencies that are larger than coherence bandwidth fade independently. Concept useful in diversity reception Multiple copies of same message are sent using different frequencies. 85

86 Cochannel Interference
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Cochannel Interference Cells having the same frequency interfere with each other. rd is the desired signal ru is the interfering undesired signal  is the protection ratio for which rd  ru (so that the signals interfere the least) If P(rd  ru ) is the probability that rd  ru , Cochannel probability Pco = P(rd  ru ) 86

87 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Rayleigh Fading Named after the statistical model that describes it. Due to cancellation resulting from individual paths drifting in and out of phase. Varies over distances of roughly l/2 apart. Amplitude Time 10-15 dB l/2 Fixed Wireless System: Due to rapidly changing atmosphere with small regions of different values of refractive index. Varies over a few seconds. Mobile Wireless System: Principally due to movement of mobile terminal unit or moving reflect objects. Slow variations come from blockage and shadowing by large objects such as hills and buildings as for fixed wireless systems. Time interval between fades depends on speed.

88 Diversity Techniques Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Diversity Techniques Multipath fading is minimised by using Diversity Techniques Many different methods for diversity. Major diversity methods: Frequency signal transmitted at 2 or more frequencies. Space 2 spaced Rx antennas used. Polarization two antenna polarizations (not very effective because the paths are still similar).

89 Space Diversity Method for Combating Rayleigh Fading
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Space Diversity Method for Combating Rayleigh Fading D Signal received by Antenna 1 Signal received by Antenna 2 Combined Signal Fortunately, Rayleigh fades are very short and last a small % of the time Two antennas separated by several wavelengths generally will not fade concurrently. Switch instant-by-instant to whichever is best. Required separation D for good decorrelation is 10-20l MHz, MHz. Space Diversity can be applied only on the receiving end of a link. On the downlink the only way to overcome fading (in TDMA and narrow band CDMA) is to boost the transmit power by 10–15 dB.

90 Coding Technologies that Overcome Multipath Fades
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Coding Technologies that Overcome Multipath Fades Wideband CDMA (WCDMA) Fades tend to have an instantaneous bandwidth of ½ MHz independent of the carrier frequency. In CDMA the information is spread out of the operating bandwidth. The bandwidth of WCDMA is around 5 MHz and with error correcting codes the bit errors due to the 10% loss due to fading can be recovered. OFDM (Orthogonal Frequency Division Multiple Access) In OFDM a channel is subdivided in frequency and independent bit streams transmitted in each subchannel. Rapid fading tends to affect only a portion of the channel and so only a fraction of the symbols are affected by multipath. In commercial wireless OFDM investigations the total spectrum utilized is a small fraction of the operating frequency.

91 Space-Time Coding (for combating fading)
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Space-Time Coding (for combating fading) Space-time coding techniques exploit the presence of multiple transmit antennas to improve performance on multipath radio channels. Commercial space-time modulation methods use accurate channel estimates at the receiver. The information is transmitted by two or more antennas using different codes but at the same frequency. The terminal unit has one but preferably two or more receive antennas. The antenna sets (Tx and Rx) can be as little as λ/2 apart and yet we can overcome fading. D

92 Radio Link: Reciprocity in Cellular?
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Radio Link: Reciprocity in Cellular? db @ MHz db @ MHz db @ MHz Between two antennas, on the same exact frequency, path loss is the same in both directions But things aren’t exactly the same in cellular -- Different transmit and receive frequencies. antenna: gain/frequency slope different Rayleigh fades up/downlink often, different TX & RX antennas RX diversity Notice also the noise/interference environment may be substantially different at the two ends.

93 Propagation Analysis What is needed, and when?
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Propagation Analysis What is needed, and when? Initial System Planning how many cells required? statistical models for average propagation, along with traffic and link budget requirements actual measurement data for example sites to fine-tune models number of cells is whatever needed for coverage & capacity Cell Planning, Frequency Planning, Interference Planning where should the site(s) be placed? propagation software tool: coverage, interference analysis possible test measurements for specific problem sites Ongoing Growth Planning, Interference Control example: what can I do to solve the interference along the highway? test measurements propagation software tool iterative interactive changes to frequency plan, cell configuration

94 Statistical Propagation Model
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Statistical Propagation Model Signal Measured (dBm) Strength Received m dB V/m Field 4 16 20 24 28 32 12 8 -50 -60 -70 -80 -90 -100 -110 -120 90 80 70 60 50 40 30 Distance from cell site (km) Okumura-Hata Propagation Model Power of a signal drops off as 1/Dn n ranges from 2 to 4 n = 4 dense urban area n = 3 suburban area n = 2 rural area Deterministic Propagation Prediction Models: Use of terrain data for construction of path profile Path analysis (ray tracing) for obstruction, reflection analysis. Commonly-required Inputs: Frequency,Station Height,Distance,Obstacle Height,Geometry,Separation, Radius of first Fresnel zone ,Forest/Roof Height, Loss allowances based on: land use; building/vehicle penetration

95 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Statistical Propagation Models:Okumura-Hata Model A (dB) = log (F) – log(H) + (44. 9 – 6.55 log(H) )*log (D) + C Where: A F D H C = Path loss Frequency in MHz Distance between base station and terminal in km Effective height of base station antenna in m Environment correction factor 0 dB - 5 dB - 10 dB - 17 dB = Dense Urban Urban Suburban Rural

96 Statistical Propagation Models Walfisch-Ikegami Model
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Statistical Propagation Models Walfisch-Ikegami Model Useful in dense urban environments, but often superior to other methods in this environment. Based on “urban canyon” assumption a “carpet” of buildings divided into blocks by street canyons Uses diffraction and reflection mechanics and statistics for prediction Input variables relate mainly to the geometry of the buildings and streets Useful for two distinct situations: macro-cell - antennas above building rooftops micro-cell - antennas lower than most buildings © 2009 by Remcom, Inc. Used with permission.

97 Practical Application of Distribution Statistics
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Practical Application of Distribution Statistics Technique: use a model to predict RSSI (Received Signal Strength Indication) compare measurements with model obtain median signal strength obtain standard deviation now apply correction factor to obtain field strength required for desired probability of service Applications: Given a desired signal level the standard deviation of signal strength measurements a desired percentage of locations which must receive that signal level We can compute a “cushion” in dB which will give us that % coverage Percentage of Locations where Observed RSSI exceeds Predicted RSSI RSSI, dBm Distance 10% of locations exceed this RSSI 50% 90% Normal Distribution Occurrences Median Signal Strength RSSI s, dB

98 Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Statistical Techniques Example of Application of Distribution Statistics Cumulative Normal Distribution Standard Deviations from Median (Average) Signal Strength 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% -3 -2.5 -2 -1.5 -1 -0.5 0.5 1 1.5 2 2.5 3 Suppose you want to design a cell site to deliver at least dBm to at least 90% of the locations in an area Measurements you’ve made have a 10 db. standard deviation above and below the average signal strength On the chart: to serve 90% of possible locations, we must deliver an average signal strength 1.29 standard deviations stronger than -95 dbm -95 + ( 1.29 x 10 ) = -82 dbm Design for an average signal strength of -82 dbm!

99 Deterministic Propagation Prediction Models
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Deterministic Propagation Prediction Models Based on deterministic methods use of terrain data for construction of path profile path analysis (ray tracing) for obstruction, reflection analysis appropriate algorithms applied for best compliance with radio physics. Commonly-required Inputs: Frequency, Distance from transmitter to receiver, Effective Base Station Height Obstacle Height & Geometry, Radius of the first Fresnel zone, Forest Height / Roof Height , Distance between buildings, Arbitrary loss allowances based on land use (forest, water, etc.), Arbitrary loss allowance for penetration of buildings/vehicles © 2009 by Remcom, Inc. Used with permission. + Frequency Planning

100 Propagation in a Rural Area
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Propagation in a Rural Area © 2009 by Remcom, Inc. Used with permission. Calculated using Wireless Insight®.

101 Propagation in an Urban Area
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Propagation in an Urban Area

102 Propagation: Indoor Line of Sight (LOS) Propagation
Radio Propagation in Mobil Communication Propagation: Indoor

103 Propagation: Diffraction
Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication Propagation: Diffraction

104 Free Space Communications
3–30 kHz VLF Long distance, low fading, military & Navigation. Earth-ionosphere waveguide (sky wave). 30–300 kHz LF LW band, small fading high atmospheric noise Ground Wave, some sky wave. 300 kHz–3 MHz MF MW band, more fading , less a atmospheric noise. Ground wave & sky wave. 3–30 MHz HF Fading severe, less noise, 3-6 MHz continental, 6-30 intercontinental mobile land comms. Sky wave 30–300 MHz VHF Line-of-sight VHF links, fading problems. Space Wave. 300 MHz–3 GHz UHF Line-of-sight, fading a problem Space wave + scatter wave. 3–30 GHz SHF Line-of-sight, satellite links, radio relay, atmospheric absorption at higher frequencies Space wave. 30–300 GHz EHF Line-of-sight. Satellite links, local distribution networks. Atmospheric absorption is a problem.

105 Summary RF Link and Calculations Propagation
Fading, Diffraction, Refraction, Scattering Diversity Techniques for overcoming fading.

106 Exercises 1. A transmitter and receiver operating at 2 GHz are at the same level, but the direct path between them is blocked by a building and the signal must diffract over the building for a communication link to be established. This is a classic knife-edge diffraction situation. The transmit and receive antennas are separated from the building by 4 km and the building is 20 m higher than the antennas (which are at the same height). Consider that the building is very thin. It has been found that the path loss can be determined by considering loss due to free-space propagation and loss due to diffraction over the knife edge. (a) What is the additional attenuation (in decibels) due to diffraction? (b) If the operating frequency is 100 MHz, what is the attenuation (in decibels) due to diffraction? (c) If the operating frequency is 10 GHz, what is the attenuation (in decibels) due 2. A transmit antenna and a receive antenna are separated by 10 km. (a) What is the radius of the first Fresnel zone? (b) What is the radius of the second Fresnel zone? (c) To ensure LOS propagation, what should the clearance be from the direct line between the antennas and obstructions such as hills and vegetation?


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