Microwave Fandamental

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Digital Microwave Communication Basics1Page 2Study GoalsAfter studying this course you shouldKnow the concept and features of Digital Microwave CommunicationMaster the function and principle of Digital Microwave devices components Know the common networking and applications of Digital MicrowaveComprehend all the kinds of fading principle and propagation of Digital Microwave communication Master all the kinds of antifading technologiesComprehend the design procedure and key point of microwave network layout2Page 3Chapter 1 Digital microwave communication overview Chapter 2 Induction of digital microwave communication device Chapter 3 Networking and applications of digital microwave communicationChapter 4 Propagation and antifading technologies of digital microwave communication Chapter 5 network layout of digital microwave communication

3Page 4Chapter 1 Digital microwave communication overview

The concept of Digital microwave communicationMicrowave frequency band choice and RF channel arrangements Modulation method of digital microwave Digital Microwave Frame Structure 4Page 5Propagation methods of modern communication networkmicrowave station

fiber communicationmicrowave communicationsatellite communicationmultiplex multiplex

microwave stationPage 6Development Course of Microwave Communicationtransmission capacity ( /ch)155M

34/140M

2/4/6/8M

480 voice channels

SDH digital microwave communication came into fast development

Since 1980sPDH digital microwave communication

1980s middle and little capacity digital microwave communication

1970s Analog microwave communication

1950s Notice: In microwave transmission transmission capacity which is under 10M is called little capacitybetween 10M and 100M is called middle capacity over 100M is called large capacity6Page 7Comparison between microwave communication and fiber communicationCan cross complicated terrains need little areadont limited by land privateNeed to lay fiberoccupy landNeed little investment, short construction time, maintenance is easy Large investment of infrastructure, long construction time Strong ability of resisting disasterrapidly restore the communication Need to maintenance outdoor fiber, easily influence by disasterLimited frequency resourceneed to apply for frequency license

Large transmission capacityRestricted transmission capacityDont limited by frequency, dont need to apply for licenseTransmission quality is stable and reliable, immune to outside influenceInfluence of climate and terrain to transmission quality is greatmicrowave communicationfiber communicationPage 8Definition of MicrowaveMicrowave:Microwave is a kind of electromagnetic wave. Its radio frequency range is from 300MHz to 300GHz, and is a limited frequency band of all electromagnetic wave frequency scope.Based on the features of transmission microwave can be regard as plane wave.In the transmission direction of plane wave, there is no component of electric field and magnetic field, they are vertical of transmission direction of plane wave. So it is called Transverse Electric and Magnetic Field wave (TEM). Sometimes we call this kind of electromagnetic wave as electric wave.811,

Page 9Usually, we call the radio signal whose frequency is from 300MHz to 300GHz as microwave signal.

We call the communication that use microwave as carrier as microwave communication.

Digital microwave communication is mean that baseband transmission signal is digital signal.

Baseband signal is usually processed at intermediate frequency, and then is converted into microwave frequency band by frequency conversion. And it also can be modulated at microwave frequency band, but modulation method is only PSK.

The base theory of microwave communication is electromagnetic theory.Concepts of Digital Microwave Communication91980199070834MB80SDHNx155MB,80

Page 10The use of every frequency bands30 300KHz 3 30 300MHz 3 30 300GHz 3THz 10Km 1 100m 10 1 100mm 10 1 100m VLF(very long freq)LF

(long freq)UHF(ultrahighfreq)VHF(veryhighfreq)MF

(middlefreq)HF

(highfreq)SHF(superhighfreq)EHF(extrahighfreq)OpticalwavefrequencywavelengthnameMain purposenavigationwirelessbroadcastTelevisionSatelliteCommun-icationMicrowave Commun-ication10MFHFVHFUHFUHF

Page 11broadcastingMaximum coverageOne programme per radio channelApplications: Radio (LW, MW, SW, FM); TV etc ...Two base form of radio transmissionMicrowave linksRadio beamOne multiplex per radio channelApplications: Civiliars and military telecommunication networksbroadcastPoint to point eyesight distance11

Page 12Microwave Frequency Band Choice and RF Channel Arrangements 85432102013040501.52.5region networks long-distance backbone network area and local network, boundary network2834Mbit/s34140155Mbit/s2834140155Mbit/s3.311 GHzGHzFrequency bands frequently used in digital microwave transmission include 7G/8G/11G/13G/15G/18G/23G/26G/32G/38Gdefined by Rec. ITU-R 12(1)PDH(15Km)8GHz25Km11GHz(2)PDH(10Km)11GHz13GHz14GHz15GHz18GHz(3)SDH(15Km)5GHz6GHz7GHz8GHz20Km11GHz 7G,8G,11G,13G,15G,18G23G,4G2G1.5GITUT2G300G1.22.7-3G9G1W1.8G2.4G2.4G.

Page 13 In each frequency band, various frequency ranges, transmitting and receiving (T/R) spacing and channel spacing are defined. Microwave Frequency Band Choice and RF Channel Arrangementsf0(center frequency)Frequency scopeChannel spacingf1f2fnf1f2fnChannel spacingT/R SpacingT/R SpacingLower frequency bandUpper frequency bandProtect spacing13 (1) (2) (3) (4) (5) ITU-R F.746-32.5MHz3.5MHz3.5MHz175MHz1E12E1XSYSZSZlSZ2SDS/fnfn

Page 14Microwave Frequency Band Choice and RF Channel Arrangements

7GHz frequency scope (MHz)F0 (MHz)T/R Space (MHz)Channel SpaceMHzHigh/low station7425--7725757515428Fn=f0-161+28n,Fnf0-7+28n,n1~5757516177110--77507275196287597196287250--755074001613.574001617740016114.f0(7575M)Frequency scope74257725MHz28Mf1=7442f5f1=7596f2f5T/R Space 154Mf2=7470Frequency assignment is very complex

14 (1) (2) (3) (4) (5) ITU-R F.746-32.5MHz3.5MHz3.5MHz175MHz1E12E1XSYSZSZlSZ2SDS/fnfn Page 15Modulation method Digital signal before modulating is called digital baseband signal. For the baseband signal cannot be transmitted in the radio microwave channels, it must be converted into frequency band signal , that is, carrier is modulated digitally by the baseband signals. Digital base signalIntermediate frequency signalBaseband signal velocityChannelbandwidthmodulationService signal for transmittingRFCOH15FM410

Page 16Modulation methods of digital microwave communicationAmplitude Shift Keying (ASK): using digital baseband signal to change carrier A, but Wc AND are not changedFrequency Shift Keying (FSK): using digital baseband signal to change carrier Wc, but A and are not changedPhase Shift Keying (PSK): using digital baseband signal to change carrier , but A and Wc are not changedQuadrature Amplitude Modulation (QAM): using digital baseband signal to change carriers and A, but Wc is not changedA*COSWc*t+Amplitude frequencyphaseFrequently used PSK and QAM modulation method in digital microwave communicationModulation process of digital baseband signal can be simply expressed asPage 17Structure of Microwave FrameRF CoOverHeadRFCOHATPC64Kb/sDMY64Kb/sMLCM11.84Mb/sRSC864Kb/sWS2.24Mb/sXPIC16Kb/sID32Kb/sINI144Kb/sFS288Kb/s15.552Mb/sSOHPayloadSTM-1 155.52Mb/s171.072Mb/sRFCOH: RF CoOverHead MLCM: Multi-Level Coding Modulation DMY: Dummy WS: wayside service XPIC: Cross Polar Interference Cancellation RSC: radio service channel INI: N:1 switch instruction ID: Identity FS: Frame Synchronization ATPCAutomatic Transmit Power Control 17ATPCSDHRFCOHMLCM Page 18Structure of Microwave FrameRFCOH and STM-1 data compose of square bulk Multiframe, and there are six rows in each Multiframe, 3564bit per rows. Each Multiframe is divided into two subframe, and there are 1776 bits in each subframe. The remainder 12 bits are used as FS.C1IIC1IIC1IIC1IIC2IIbIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIC1IIC1IIC1IIC1IIC2IIbIIaIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII12bits first byte12bit the 148th byteSubframe 2 1776bit148bytesFS6bitSubframe 1 1776bit148bytesFS6bitMultiframe 3564bitISTM-1 data bit C1C2two level error correct monitor code FSframe synchronization abother appended overhead6bit18SDHPage 19QuestionsWhat is microwaveWhat is the digital microwave communicationWhich frequency bands are usually used in digital microwave communicationWhich concepts are used in setting microwave frequency bandsWhich modulation methods are usually usedWhich is the most used modulation method at present

19Page 20Brief SummaryIn this chapter, the content which we mainly studied is as follows: The concept of digital microwave communication The features of digital microwave communicationThe usually used frequency bands in digital microwave communicationThe usually used modulation method in digital communicationThe frame structure of digital microwave

20Page 21Chapter 1 Digital microwave communication overview Chapter 2 Induction of digital microwave communication device Chapter 3 Networking and applications of digital microwave communicationChapter 4 Propagation and antifading technologies of digital microwave communication Chapter 5 network layout of digital microwave communication

21Page 22Chap 1 Induction of Digital Microwave Communication Device

Classes of microwave deviceIntroduction of microwave device components functionThe signal flow of microwave device systemPage 23Classes of microwave deviceModeDigital microwavePDHSDHSplit microwaveAll indoor microwaveTrunk MWAll outdoor microwave216E134MSTM-0STM-12 x STM-1CapacityStructureFall into disuseAnalog microwaveMultiple methodPage 24All Indoor Microwave DeviceTrunk MicrowaveHigh cost, high transmission capacity, high device stability, is suit for long distance and backbone transmission.RFIFsignal processmultiplex units and some other units are all indooronly antenna and feeder are outdoor.SDH microwave device

Structure of subrack

(7+1)*155MBRU: Branch of RF unitMSTU: Main signal transceiver unit (transceiver, modem, SDH electric interface, hitless module)SCSU: surveilcontrolswitch unitBBIU: baseband interface unit(optional: STM-1 optical interface, C4 PDH interface)

PM1M224Page 25All outdoor microwave deviceAll outdoor Microwave device

All units are outdoorEasy to installSave equipment roomIF and baseband signal processing unitIF cable

RF signal processing unitService and power cable25Page 26Split Microwave DeviceODU is outdoor, IFsignal processingmultiplex and some other units are indoor. There is a IF cable between ODU and IDU.It can directly mount ODU to antenna or with a soft waveguide, avoiding attenuationTransmission capacity of the split microwave is contrastly small, but it is easy to install and maintenance, convenient to build a network, and is the most popular microwave device at present.

Antenna Unit

IF Cable

ODU (Outdoor Unit)IDU (Indoor Unit)Split microwave device26Page 27Split microwave device

AntennaODUIF cableAntenna: focus RF signal sent by ODU, enlarge signal gainODU: RF signal processingconversion between IF signal and RF signal.IF cable: Transmission for IF service signal and IDU/ODU communication signal, and supply power for ODUIDU: service access and distribute, multiple, modem and so on

IDUPage 28Installation of split microwave deviceIDU

Standard antennaODU

Standard antennaODUIF cable

IF interfaceSplit mountDirect mountSoft waveguide28ODUPage 29

Installation of digital microwave deviceMicrowave project is very different from optical transmissions, since the proportion of outdoor upper air in a project is very large. Mount workload is different according to work environment and device type.Building spend of microwave project is a big part of contract amount, and also is the main profit source of a project.29Page 30The alignment of antenna

side elevationplanformMain lobeSide lobeMain lobeThe beam widthSide lobe30Page 31In the procedure of vertical and horizontal alignmentit will get a voltage waveform as the lower-right picture. If you meet with this situation, the position which the biggest voltage corresponds is the position of main lobe. It doesnt need big adjustment at this direction, just need adjusting voltage to the biggest position by inching.The vertical adjusting method is the same with the horizontals. If the antennas dont align exactly, it maybe only get a small voltage in any direction. So the both sites need coarse adjusting cooperatingly, and align the antennas approximately.The position of side lobeThe position of main lobeRSSI: Detect point of received voltageVRSSIazimuth

The alignment of antenna31Page 32The alignment of antennaIn the procedure of antenna alignment, it usually meet with the wrong situation as the right picture, that is, antenna is aligned to the side lobe, so the received signal level cant reach the designed level.

rightwrongwrong

32Page 33Antenna Unit of Microwave

Paraboloid antennaKasai Green antennaThe function of antenna is to send the power of electric wave from transmitter in one direction, and receive the power of electric wave. The usually used antenna classes are Paraboloid antenna and Kasai Green antenna.The diameter of antenna made in China is usually as follows: 0.3 0.61.21.62.02.53.2m etc.The diameter of antenna imported from other country is usually as follows: 0.3 0.6 1.21.82.43.0m etc.Backfeeder2 reflectorsFrontfeeder1 reflector33Page 34 Several channels can use one antenna in one frequency bandTxRxTxRxChannelChannel11'nn'11'nn'Antenna unit of microwavePage 35FeederElliptical waveguideThe attenuation of Elliptical waveguide per length is smalland it is the most used waveguide for all indoor microwave device at present.Flexible Twist WaveguideIt is used to connect ODU and antenna. It is easy to mount, can get high precision connectivity, and can be twisted, but its attenuation is large.IF CableFor split microwave device, it is divided into two parts, namely IDU and ODU. It uses IF cable to connect ODU and IDU.

35Page 36Split microwaveantennaThe gain of antennaIt is defined that when you want to get the same electric field intension at somewhere using direction antenna or omni-antenna, the ratio of input power Pio(when using ) and Pi(when using ) The gain of microwave antenna can be calculated as follows

The antenna gain usually means the gain in the biggest radiate direction (main lobe), and use dB as unit.Beam widthFrom the main lobe departure to the both sides, we call the point where the power only is the half of the maximum power as half power point(-3dB point). The angle between the two half power points is beam width. Approximate calculation formula is as follows

beam width36 Page 37Cross polarization interference cancellation (XPIC) It stands for the suppressive intensity for the other polarized wave. Usually, it should be over 30db XdB10lgPo/Px Pothe received power of the main polarized wave. Pxthe received power of the wave converted from the main polarized wave.Suppressive intensity It stands for the attenuation degree in some direction according to the main lobe direction. The suppressive intensity of 180 degree is called the front and back directions suppressive intensity . Split microwaveantennaThis should be X polarization discriminationPage 38Split microwave deviceODUCable interface Tx IFRx IFSignal of monitor and controlDCDC/DC convertTransmission oscillationSideband filterPower amplifierRF attenuation Tx RF Rx RFPower detectorAPCBandpass filterRf LoopNoiseless amplifiermixingfilterIF amplifierReceive oscillationAlarm and controlconversion from IF signal to RF signalconversion from RF signal to IF signalFrom/to IDU350MHz140MHz-48VmixingIF amplifierAPC: Automatic Power ControlPage 39The main specification of transmitterWork frequency band: The backbone microwave usually use the frequency bands of 6,7 and 8 GHz. 11, 13 GHz and some above frequency bands usually used in access layer (eg, basestation access)Output power: Output power means that the power at the output port of transmitter. The range of power is from 15 to 30 dBm.Stability of the oscillation frequency If the work frequency of transmitter dont stable, the amplitude of demodulated effective signal will descend, and bit error rate will increase. The oscillation frequency stability of microwave device is usually from 3 to 10 ppm. Transmitting signal mask Transmitting signal mask must comply with some regulation, lest occupy too large bandwidth and bring too serious interference to adjacent channels. The restricted frequency scope is frequency mask.Split microwave deviceODUPage 40Split microwave deviceODUThe main specification of receiverWork frequency band: Receiver must work with transmitter corporately. The receiving frequency of local station is the same with the remote station.Stability of the oscillation frequency The requirement is basically the same with the transmitter. It is usually range from 3 to 10ppm.Noise Figure The noise figure of digital microwave receiver is usually range from 2.5 to 5dB.Transmission bands We should select suitable transmission bands and fine amplitude-frequency characteristic for effectively suppressing interference and getting optimal signal transmission. The transmission bands of receiver is depend on the IF filter. Selectivity It means the suppressing ability against interference beyond transmission bands, especially suppressing interference from adjacent channel, mirror frequency and interference between transmission signal and received signal, etc. Voltage range which the AGC can control The output level from receiver will change when transmission encounter fading, and it may lead demodulator cant work well. So, the gain of receiver must be controlled to basically keep the level of IF signal when the input RF signal changes among a limited range.Page 41Split microwaveODUSpecification of ODU is related with frequency. Because one ODU cant cover a frequency band, one band may be divided into several subbands, and ODUs of different subbands are different.ODUs with different T/R spacing are different.ODUs of high/low station are different.

f0(7575M)Frequency band74257725MHzSubband A7442T/R Space 154M7498Subband BSubband CSubband ASubband BSubband CLow stationHigh stationClasses of ODUNO of bandsNO of T/R spacingNO of subbands2ODUs of some factory is different from transmission capavityThe classes of ODUs are manybut quantity per batch is smallthey are produced by little company, and assembled by big company.Page 42Cable interface From/to ODU Tx IFRx IFmodulationdemodulationMultiplex of microwave frameDemultiplex of microwave frameCross connectionTributary unitLine unitIF unitService channelService channelDC/DC convertMonitor and control unitInterface of OMSplit microwaveIDU42IDUIDUIDUIDUSDESD1SLESL1PH1PO1PXCSCCIDU Page 43Signal flow of microwave transmission

Radio spacePage 44QuestionsWhich classes of microwave device can be divided into?Which components does split microwave compose of? And their functionHow to align antennasWhich are the main specifications of antennaWhich are the main specifications of transceiver of ODUCould you please describe the signal of microwave transmission system

Page 45Brief summary Give a brief list of classes of microwave deviceDetailedly introduce the components of split microwave device and their functionInstallation of antenna and their main specificationsThe components of ODU and its main specificationsThe components of IDUThe signal flow of microwave transmission

Page 46Chapter 1 Digital microwave communication overview Chapter 2 Induction of digital microwave communication device Chapter 3 Networking and applications of digital microwave communicationChapter 4 Propagation and antifading technologies of digital microwave communication Chapter 5 network layout of digital microwave communication

46Page 47Chapter 3 Networking and applications of digital microwave communication

The usually used networking of digital microwaveThe station type of digital microwaveThe applications of digital microwavePage 48The usually used networking of digital microwaveTree shapeRing shapeStar shapeChain shapeBasic unit1 hop point to point48Page 49The station types of digital microwaveHinge stationHinge station: it is located at backbone, need to complete communication in multi-directionTerminal stationTerminal station: it is located at the both ends of a line or the end of a branch.Branch stationBranch station: It is located at the middle of a line, can add/drop tributary, or connect communication of backbone stations at two direction. Relay stationRelay station: it is located at the middle of a line , without adding or dropping service.

Terminal stationTerminal station

49Page 50RelayThe classes of relat station back-to-back antennas reflective panelActivePassive REG relay IF relay RF relay50Page 51Direct RF amplify station:It is a kind of active, bidirectional and without frequency conversion relay system. because it directly amplify RF signal, it is called as Direct RF amplify station.Regeneration relay station:It is a kind of high performance and high frequency repeater. It can be used to breach distance limitation of microwave communication system, or change the transmission direction to round eyesight obstacle, without signal deterioration. The received signal undergo thorough regeneration and amplify, and then forwarded.

Active RepeatersActive relayPage 52

Double Parabolic reflectorsIt composes of two parabolic reflectors which are connected back to back with a section of waveguide.Plane reflectorsIt is a metal panel with a smooth surface and effective acreage. And the distance and azimuth of the panel relating to the two communication points.Passive repeaters52Page 53In this situation, large diameter antennas are usually used. It has to use apparatus to adjust antennas and usually takes long time. Near-end distance should be under 5KMPassive double Parabolic reflectors532Page 54Passive repeater with plane reflector

Passive plane reflectors54Page 55The free space whole distance attenuation:

A stands for the effective acreage of reflector(m2)

kmkmAcreage A

Passive plane reflectorsPage 56Passive repeatersactual picture

Plane reflectorsDouble Parabolic reflectorsPage 57The applications of digital microwave

Supplement for Optical networkthe first mile access Backhaul transmission for mobile base station

Redundant backup for important linkAccess for big client Emergency communicationlarge-scale conference, activity, crisisSpecial transmission situationriver, lake, islandMicrowave Application57Page 58Core NetworkTDMAIS-136GSMCDMAIS-95Radio Network (Base Station)Air Interface4-16 T1/E1 LinksBTSBTSBTS 3 T1/E1 Links1 T1/E1 Link2 T1/E1 LinkBSCBackhaul Transmission for Mobile Base stationPage 59

Supplement for Optical networkPage 60

Redundant backup of Significant LinePage 61

Enterprise Private NetworkPage 62Access for very important client

Page 63QuestionsThe common networking of digital microwave? The station types of digital microwave?The types of relay station?The main applications of digital microwave?

Page 64Brief summary Briefly listed the networkings of digital microwaveIntroduced the types of digital microwaveDetailedly introduced several types of relay stationListed the main applications of microwave

Page 65Chapter 1 Digital microwave communication overview Chapter 2 Induction of digital microwave communication device Chapter 3 Networking and applications of digital microwave communicationChapter 4 Propagation and antifading technologies of digital microwave communication Chapter 5 network layout of digital microwave communication

65Page 66Chapter 4 Propagation and anti-fading technologies of digital microwave communication

Several important parameters of microwave propagationThe factors may effect the propagation of electric waveAll kinds of microwave propagation fadingThe anti-fading technologies of digital microwavePage 67The Fresnel Zone the Fresnel radiusWhen the distance sum from point P to point T and point R satisfies the following equation: TP+PR=TR + n /2n =1,2,3,), the track of the point P is The Fresnel Zone.Several important parameters of microwave propagation

The vertical distance PO from point P to the line between point T and point R is called as the Fresnel radius. F1(n=1) stands for the first Fresnel radius.67Page 68The formula for calculating the first Fresnel radius F1 Several important parameters of microwave propagationThe first Fresnel Zone is the most centralized zone of microwave propagation power, and we should reduce the effect of obstacle as much as possible in this zone. Along with the sequence number augmenting, the field intensity of receiving point will descend as a arithmetical progression

68Page 69Several important parameters of microwave propagationClearance In the actual path of microwave propagation, it may be blocked by buildings, trees or mountains, etc. if the obstacle comes into the first Fresnel zone, it will bring in additive attenuation, reduce the received level, and influence propagation quality. To avoid this situation, we import the concept that Clearance. The vertical distance from the obstacle point to the line AB is called as this obstacles clearance. For calculation convenience, we usually use the line hc in the vertical direction of ground to approximatively denote the clearance. And hc/F1 stands for the relative clearance(F1 is the first Fresnel radius of this point).

69Page 70The factors may effect the propagation of electric wave terrainThe main influence is that the received level is influenced by the reflected wave from the ground. Part of the signal power from the transmitter antenna may be reflected by the smooth ground or the water and interfere the main signal which undergo the direct propagation direction. The vector sum of reflected wave and main wave lead the sum wave augment or reduce, so the propagation is not stable. Therefore when we design the propagation path of microwave, we should reduce the reflected wave, and if there is some reflected wave, we should use the fluctuation of terrain to block the reflected wave.

Directly propagatereflectionbadShould avoid

Directly propagatereflectiongood70Page 71The reflection of different terrain is different, so their influence for electric wave propagation is different too. We can classify the terrain into four classes, they are as follows: class Amountainous region (or the region of dense buildings) class Bfoothill (the fluctuation of ground is gently) class Ccampagna class Dbig acreage of waterThe reflectance of mountainous region is the least, and it is the most suitable terrain for microwave propagation. And the terrain of foothill is the second. We should avoid water or other smooth surface when we design the microwave propagation path.The factors may effect the propagation of electric wave terrain71Page 72The troposphere is the low atmosphere which is from the ground. Troposphere is low-altitude atmosphere ranging from the ground to 10 kilometers higher upward . Because the height of microwave antenna cant exceed the troposphere, we just need to study the electric wave propagates in the troposphere which can substitute the study of in the atmosphere. The influence of troposphere on the electric wave has following types: Atmosphere absorption loss because of aerosphere molecule syntony. This kind absorption mainly influence the microwave transmission whose frequency is over 12 GHz. Scattering and absorption loss caused by rain, fog and snow. This kind loss mainly influence the microwave transmission whose frequency is over 10 GHz. Because of the inhomogeneity of atmosphere, the transmission of electric wave in the troposphere may lead refraction, reflection, dispersion, etc. And the influence against microwave propagation of aerosphere refraction is the maximum among all the phenomenon.The factors may effect the propagation of electric wave atmospherePage 73The fading characteristics of electric wave propagationFADING means the received level fluctuates randomly. The variation is irregular, and the reason is various.According to different principle, we can classify the fading as follows:mechanismAbsorption lossFading of rain and fogBlinking fadingK type fadingDuct fadingSustained time lengthReceived levelDifference of influenceFast FadingSlow FadingUpward Fading Downward fading Flat fadingFrequency selective fading Fading in free space73VdB

Page 74Free Space Loss A = 92.4 + 20 log d + 20 log f

Where d = distance in km f = frequency in GHz (refer to isotropic antennas)0dfIf the D or f become twice, the attenuation will increase by 6 dB Free Space Basic Transmission Loss74Page 75Free Space LossPower Level P = TX PowerPTXDistanceGTXGRX PRX G = Antenna GainA0A0 = Free Space LossMReceiver ThresholdM = Fading MarginGPG75 Pr(dBm)=Pt+Gt+Gr-Lf-Lt-Lr-Lb Pt GtGr LfLtLrLb

Page 76Attenuation due to atmosphereMolecule of any material is constituted by charged particles. Those particles have intrinsic electro-magnetic resonance frequency. When the microwave frequency is close to their resonance frequency, those materials generate resonance absorption to microwave power.Statistically, when the frequency is under 12 GHz, Attenuation due to atmosphere is under 0.1 dB. By contrast with free space loss, it can be ignored.

Page 77Generally, for frequency band is less than 10 GHz, fading caused by rain and fog is not serious. Normally the fading between two stations is only several dB.For the frequency band is over 10 GHz, relay distance is limited by loss caused by rains. Take the frequency band above 13 GHZ for example, the rainfall of 100mm/hour may bring into attenuation of 5dB/kmso in the frequency bands of 13GHz and 15GHzthe common relay distance is about 10km.In the frequency bands above 20GHZbecause of the influence of rainfall, the relay distance is only about several kilometers.Higher frequency band, larger attenuation due to rainHigh frequency bands can be used as user class transmissionAttenuation due to Rain and Fog77

78G100107141Page 78Because of the refraction of atmosphere, the transmission path of electric wave is curving. The last effect of atmosphere refraction can be equivalent as the electromagnetic wave transmits in the space above a earth with a equivalent radius . The equivalent radius can be calculated as follows: =KR , R is the actual earth radius.The actual measurement average value of K is about 4/3. but the real value of K is depend on the local meteorological phenomena, and can changes among a large range. So it will influent eyesight transmission.

Refraction in the atmosphere:The K type attenuation

Wothe path of microwave is curvingRRe7863704wa ae (Hc)Page 79Microwave Propagation

k>1,positive refractionk=1, no refractionk