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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.

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Presentation on theme: "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."— Presentation transcript:

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 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 第五讲 射频电波传播 ( Session 5 RF Radio-wave propagation )

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

4 Goal of This Lecture  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 Information 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 6 Transmitter Receiver Earth Sky wave Space wave Ground wave Troposphere ( km) Stratosphere ( km) Mesosphere ( km) Ionosphere ( km) Three types of wave propagation

7 7 Classification BandInitialsFrequency RangeCharacteristics Extremely lowELF< 300 Hz Ground wave Infra lowILF300 Hz - 3 kHz Very lowVLF3 kHz - 30 kHz LowLF30 kHz kHz MediumMF300 kHz - 3 MHzGround/Sky wave HighHF3 MHz - 30 MHzSky wave Very highVHF30 MHz MHz Space wave Ultra highUHF300 MHz - 3 GHz Super highSHF3 GHz - 30 GHz Extremely highEHF30 GHz GHz Tremendously highTHF300 GHz GHz Three types of wave propagation

8 Line of Sight (LOS) Propagation  Free space - wave travels in a straight line  atmospheric absorption (loss)  refraction in the atmosphere  reflections from the ground reflected wave direct (free space) wave refracted wave Tx Rx r Free Space “Spreading” Loss energy intercepted by the red square is proportional to 1/r 2

9 Free Space Path Loss Line of Sight (LOS) Propagation 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). TxRx d G R, D R, η R G T, D T, η T

10 Free Space Path Loss TxRx d G R, D R, η R G T, D T, η T Line of Sight (LOS) Propagation

11 Free Space Path Loss Typical path loss: 145 dB for a 50 km link at 10 GHz (high gain antennas help offset loss ~ 60 dB) 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(4x3.14x5x10 4 /(3x10 -2 ))=146 dB Tr. Loss= =26dB TxRx d G R, D R, η R G T, D T, η T Line of Sight (LOS) Propagation

12 Example: Radio Link (Transmission Loss Calculation) +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 +13 dB antenna gain = −92 dBm into line −3 dB line efficiency = −95 dBm to receiver Receiver Antenna Trans. Line Transmitter Trans. Line Line of Sight (LOS) Propagation

13 Typical Cellular Link Budget Source: Cell TX PO WattsMS TX PO Watts Cell TX PO dBMMS TX PO dBm Cell Combiner Loss dBMS Combiner Loss db Cell Cable Loss dbMS Cable Loss db Cell Antenna Gain dBdMS Antenna Gain dBd EIRP Watts EIRP dBm MS Antenna Gain dBdCell Antenna Gain dBd MS RX Cable Loss dBCell RX Cable Loss MS Diversity Gain dBCell Diversity Gain MS RX Sensitivity dBMCell RX Sensitivity dBM FWD Link Budget, dBREV Link Budget, dB Worst Case Link Budget Input: Calc: Input: Calc: Input: Calc: FWD Path REV Path Imbalance, dB Line of Sight (LOS) Propagation

14 Fresnel Zones Line of Sight (LOS) Propagation 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. Tx Rx 1st Fresnel Zone 2nd Fresnel Zone d=d 1 +d 2 d1d1 d2d2 plane at right angles to path

15 Fresnel Zones  Note that path difference between rays at the edge of the 1st and 2nd Fresnel zones is /2  1st Fresnel Zone is the most important zone. Line of Sight (LOS) Propagation Tx Rx 1st Fresnel Zone 2nd Fresnel Zone d=d 1 +d 2 d1d1 d2d2 plane at right angles to path

16 Fresnel Zones  Huygens principle: different Fresnel zones Z 1,Z 2,Z 3,…  Contribution from Fresnel zones Z 1,Z 2,Z 3,…are B 1,B 2,B 3,… Line of Sight (LOS) Propagation

17 Fresnel Zones Line of Sight (LOS) Propagation B 1 =Contribution from Fresnel zone1 (Z 1 ) B 1 =2B 0 B 0 =received for free-space Contribution from Fresnel zone1 is double of the total field for free space.

18 Fresnel Zones Line of Sight (LOS) Propagation Example: d1=d2=5km F1=17.66m at 2.4 GHz d1d1 d2d2

19 Fresnel Zones Line of Sight (LOS) Propagation 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 Fresnel Zones Line of Sight (LOS) Propagation If the 1 st 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  Absorption by atmospheric gases  Attenuation due to molecular absorption, for example, water vapour and O 2  Important bands at frequencies >10 GHz, especially at 22 GHz (water vapor) and 60GHz (O 2 ) Atmospheric Absorption (clear air)

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

23 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) Atmospheric Attenuation Line of Sight (LOS) Propagation

24 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. n1n1 n2n2 n3n3 n4n4 beam bending Line of Sight (LOS) Propagation θ2θ2 θ1θ1 n1n1 n2n2 y x

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

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

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

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

30 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 R e =K R earth is introduced to “makes “the path straight. Line of Sight (LOS) Propagation

31 Refraction in the Atmosphere Line of Sight (LOS) Propagation 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 R e.

32 Refraction in the Atmosphere Line of Sight (LOS) Propagation An equivalent Earth radius K R earth “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

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

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

36 36 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 Line of Sight (LOS) Propagation

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

38  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) d h r d h t h MULTIPATH h t d h r h d h t h r h KNIFE-EDGE DIFFRACTION h t h r h TREE SCATTERING Line of Sight (LOS) Propagation Other Propagation Mechanisms

39 Ground Reflections - Path Clearances and Antenna Heights  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 Tx Rx direct Line of Sight (LOS) Propagation

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

41 Ground Reflections - Path Clearances and Antenna Heights  Smoth surface- specular reflection Line of Sight (LOS) Propagation  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  Rough surface -diffuse reflection Line of Sight (LOS) Propagation

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

44 Knife-Edge Diffraction  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 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. Tx Rx d1d1 d2d2 H Atten. (dB) Line of Sight (LOS) Propagation

45 Knife Edge Diffraction Example 1/3 What is the total path loss at 1 GHz? Tx Rx d1d1 d2d2 H 5 m 100 m 150 m A B Loss = (Path Loss, no diffraction) + (Diffraction Loss) Path Loss (dB) (no diffraction) = 20 log 10 (4  d/ ) = 20 log 10 (4  250/   dB Diffraction Loss Atten. (dB) Use = -5 sqrt[2/0.3 * (1/ /150)] = Loss = = 97.9 dBAdd atmospheric absorption loss to this. Line of Sight (LOS) Propagation Diff. Loss = 17.5 dB

46 Atten. (dB) dB Diffraction Loss = 17.5 dB Knife Edge Diffraction Example 2/2 Line of Sight (LOS) Propagation

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

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

50 50  The received signal power: where G r is the receiver antenna gain, L is the propagation loss in the channel, i.e., L = L P L S L F Fast fading Slow fading Path loss Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

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

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 55  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 Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

56 17 March 1999 Radio Propagation56  Value of  characterizes different environments Rappaport, Table 3.2, pp. 104 Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

57 Reflection with Partial Cancellation 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 TX EIRP DBM HT FT D MILES Comparison of Free-Space and Reflection Propagation Modes Assumptions: Flat earth, TX EIRP = MHz. Base Ht = 200 ft, Mobile Ht = 5 ft. FS Link Loss Free- Space DBM FS Link Loss using Reflection DBM Distance MILES Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

58 Pass Loss in a Mobile System Power of a signal drops off as 1/D n n ranges from 2 to 4 n = 4 dense urban area n = 3 suburban area n = 2 rural area (dBm) Strength Signal Received Distance from cell site (km) Signal Measured 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. F = 1900 MHz Example: Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

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

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

67 67 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) Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

68 68 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 Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

69 69 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., 10log 10 m, M is the area average signal level, i.e., the mean of M,  is the standard deviation in decibels Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

71 71 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 r m of envelope signal within sample range to be satisfied by  We have r m =  Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

72 72 The pdf of the envelope variation r P(r)  =1  =2  =3 Rayleigh Distribution Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

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

75 75 Rayleigh Distribution Line of Sight (LOS) Propagation PDF of E is given by The magnitude ( envelop) of the field is defined as The probability of E

76 76 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, I 0 (x) is the zero-order Bessel function of the first kind,  is the amplitude of the direct signal Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

78 78 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. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

79 79  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 f C is the frequency of source carrier, f D is the Doppler frequency.  Doppler Shift in frequency: where v is the moving speed, is the wavelength of carrier. MS Signal Moving speed v Doppler Shift Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

80 80 Moving Speed Effect Time V1V1 V2V2 V3V3 V4V4 Signal strength Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

81 81 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”. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

83 83 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. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

85 85 Coherence Bandwidth  Coherence bandwidth B c :  Represents correlation between 2 fading signal envelopes at frequencies f 1 and f 2.  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. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

87 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. 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 /2 apart. Amplitude Time dB  Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

88 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). Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

89 Space Diversity Method for Combating Rayleigh Fading  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 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. Signal received by Antenna 1 Signal received by Antenna 2 Combined Signal D Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

90 Coding Technologies that Overcome Multipath Fades 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. 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. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

92 Radio Link: Reciprocity in Cellular? 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 MHz MHz MHz Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

94 Statistical Propagation Model Power of a signal drops off as 1/D n n ranges from 2 to 4 n = 4 dense urban area n = 3 suburban area n = 2 rural area Signal Measured (dBm) Strength Signal Received  dB V/m Strength Field Distance from cell site (km) Okumura-Hata Propagation Model 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 Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

95 Statistical Propagation Models : Okumura-Hata Model A (dB) = log (F) – log(H) + (44. 9 – 6.55 log(H) )*log (D) + C Where: AFDHCCAFDHCC ============ 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 Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

96 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. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

97 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 RSSI, dBm Distance 10% of locations exceed this RSSI 50% 90% Percentage of Locations where Observed RSSI exceeds Predicted RSSI Median Signal Strength , dB Occurrences RSSI Normal Distribution Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

98 Statistical Techniques Example of Application of Distribution Statistics  Suppose you want to design a cell site to deliver at least - 95 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  ( 1.29 x 10 ) = -82 dbm  Design for an average signal strength of -82 dbm! Cumulative Normal Distribution Standard Deviations from Median (Average) Signal Strength 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

99 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 + Frequency Planning © 2009 by Remcom, Inc. Used with permission. Line of Sight (LOS) Propagation Radio Propagation in Mobil Communication

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

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

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

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

104 Free Space Communications  3 – 30 kHz VLFLong distance, low fading, military & Navigation. Earth-ionosphere waveguide (sky wave).  30 – 300 kHzLFLW band, small fading high atmospheric noise. Ground Wave, some sky wave.  300 kHz – 3 MHzMFMW band, more fading, less a atmospheric noise. Ground wave & sky wave.  3 – 30 MHzHFFading severe, less noise, 3-6 MHz continental, 6-30 intercontinental mobile land comms. Sky wave  30–300 MHzVHFLine-of-sight VHF links, fading problems. Space Wave.  300 MHz–3 GHzUHFLine-of-sight, fading a problem. Space wave + scatter wave.  3–30 GHzSHFLine-of-sight, satellite links, radio relay, atmospheric absorption at higher frequencies. Space wave.  30–300 GHzEHFLine-of-sight. Satellite links, local distribution networks. Atmospheric absorption is a problem.

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

106 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 to diffraction? 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? Exercises


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