Presentation on theme: "Cambium College Radio Wave Propagation and Antennas/Reflectors"— Presentation transcript:
1 Cambium College Radio Wave Propagation and Antennas/Reflectors David HensleySenior Engineer Network & Systems
2 Before we get started… This presentation has some math. In fact, this presentation has some algebra.Don’t worry. This presentation ends well.It ends with addition and subtraction.It’s just like life.Sometimes we have to schedule our pain.Sometimes we have to suffer a little before life gets easier.If you’ve always wondered how you’d ever use what you’d learned in your freshman algebra class, this is it!
3 Radio Wave Propagation and Antennas/Reflectors Parts of a RadioConversationWave PropertiesRadio WavesAntennasDecibelsLink BudgetFor further study
4 Parts of a Radio formatter demodulator receiver antenna channel transmittermodulatortransmitreceiveinputinformationoutputStart with an example like an iPhoneInput information can be voice or dataFormatter converts the input information into a binary stream—a stream of ones and zeroes.The modulator converts the stream of ones and zeros:Adds additional binary digits (redundant information) for error detection and error correctionChooses the right method of varying the carrier wave that’s appropriate for the channel (the correct modulation mode)Produces an information stream composed of symbolsThe transmitter produces a wave that carries the information from the transmitter to the receiver—this is the carrier waveThere can be one or more carrier wavesThe transmitter antenna focuses the carrier wave so that it can be sent from the transmitter to the receiver with intentThe channel carries the carrier wave through thin air!There may be obstructions, objects, etc. in the channelThe channel may have varying characteristics like temperature, pressure, density, etc.The receiver antenna captures and focuses the carrier wave with intent (extreme prejudice)Focuses the wanted signalRejects the unwanted signalThe receiver removes the carrier wave and provides the demodulator with the information stream, in symbol formatThe demodulator converts the information stream in symbol format to an information stream of binary digitsThe demodulator can detect errors that were sentThe demodulator can often correct errors that were sent: this is called Forward Error CorrectionThe demodulator strips off any additional binary digits that were used to “protect” the information streamThe formatter converts the binary stream into the output information format—voice or data
5 It’s like a conversation! (Radio waves are like sound waves) The speaker or transmitter (mouth) must be intelligible:LanguageVolumePitchTone of voiceDirectionOxford commaThe channel must be clear:Extraneous noises, music, conversations, etc.Objects in the wayThe listener or receiver (ear) must be available:SensitivityDistraction“Eats, shoots and leaves!”It’s mostly all about delivering enough power to the receiver
6 Wave Properties Drop a rock into a pond. What happens? Waves travel or radiate or propagate outwards in a big circleTake a point source, radiating radio energy in all directions.Capture or receive the signal. What do you have?point sourcecapture area
7 Properties of WavesWhat happens to radio waves as they travel in free space?They get smallerTheir amplitude gets smallerThis is called attenuation or lossAmplitudeHow tall is the wave?Frequency or WavelengthHow often does the wave repeat?PhaseHow far did the wavestart from a referencestarting point?
8 Radio Waves As the signal propagates, it gets spread out over the area of a sphere.What’s the surface area of a sphere? (4πR2)The amount of power we can capture is related to the part of the area capturedPt (λ / 4πR)2Pt is Power transmitted, Wattsλ is the wavelength of the signal, metersR is the distance between the transmitter and receiver, metersAssuming that our transmitter and receiver have no focus or gainpoint sourcecapture area
9 Let’s be more intentional Then the amount of power we can capture isPr = Gt Gr Pt (λ / 4πR)2Pr is Power received, WattsGt is transmitter antenna Gain (output/input ratio)Gr is receiver antenna Gain (output/input ratio)Pt is Power transmitted, Wattsλ is the wavelength of the signal, metersR is the distance between the transmitter and receiver, metersThe Free Space Path Loss is the power lost between transmitter and receiverFSPL = (4πR / λ)2 = (4πRf / c)2f is the signal’s frequency, Hertzc is the speed of light in a vacuum, 300 x 106 meters per secondpoint sourcecapture area
10 How do we make antennas with large gain, Gt or Gr? Make them bigger!This is useful, but it becomes impractical (after a while)There is a point of diminishing returnsLarge antennas on tall towers are similar to sails on sail boatsFocus the energy by reflecting captured signal to a single pointUse a parabolic reflectorA parabola has a focusCollect the signal and reflects it to a single pointthe signal can be two, three, ten, or even thousands of times stronger
11 So what’s all of this look like at home? Here’s an example using your home Wi-Fi Access PointNotice:the farther we get away from the Access Point, the weaker the received powerThese power levels are small!10-6 Watts or microwatts or μW or millionths of a Watt10-9 Watts or nano Watts or nW or billionths of a WattGo watch “Powers of Ten” (1977) on YouTube right now!There’s a way to make these power levels easier to understand: the decibel or dB!Power transmitted, Pt, mWTransmitter antenna gain, Gtdistance, R, mReceiver antenna gain, GrPower received, Pr, W203.16510162.6E-615.7E-6307.0E-6502.5E-6100626.3E-9200156.6E-9
12 But first, we have to agree on the following… Multiplication and Division are difficult!Addition and Subtraction are easy!Dealing with thousandths, millionths, billionths, picos, and femtos is hard!Wait. Where does multiplication and division show up in the radio?Antennas amplify (or increase) the signal—these are multipliersAntennas can magnify (or multiply) the signal by two or three or ten or a thousand timesThe channel attenuates (or reduces) the signal—this is a dividerChannels can decrease (or divide) the signal byA thousandA millionA billionOr more!
13 Decibel or dB! 10 log( P / P0 ) dB A decibel is a ratio of two things: Compare one thing to anotherCompare a measured quantity to a reference quantityIn our case,We are comparing power levelsWe are comparing our measured quantity to 1 (one) milliwatt1 milliwatt or a thousandth of a Watt or 10-3 WattsThe ratio is received power divided by one milliwatt (or referenced to a mW)Pr / 1 mWIt’s called a dBm1 mW is the reference PowerTo make the math easier, we take the logarithm of Pr / 1 mW and multiply it by ten10 log( Pr / 1 mW )10 log( P / P0 ) dB
14 Example Power Levels in milliwatts, Watts, and dBm Observations (rules of thumb)1000 mW is 1 WattDouble the power (in Watts) add 3 dBHalve the power (in Watts) subtract 3 dBMultiply the power by ten (in Watts) add 10 dBDividing the power by ten (in Watts) subtract 10 dBAntenna gains and channel losses become easierAn antenna with 1000 times gain has 30 dB of gain (add 30 dB)A channel that divides the signal by one million has 60 dB of loss (subtract 60 dB)A channel that divides the signal by one billion has 90 dB of loss (subtract 90 dB)Free Space Path Loss becomes easier(4πR / λ)2 = (4πRf / c)2 becomes 20 log( R ) + 20 log( f ) dBmWWdBm0.10.0001-10.010.0010.020.0023.050.0057.0100.0110.0200.0213.010020.02000.223.0100030.0200033.0As an exercise, you can create your own chart just using these four rules for power levels3 dB and half/double are approximationsRules of them are often wrong!
15 Let’s go back to the parts of a radio and create a Link Budget Link Budget, 2.4 GHz, varying distance between AP and clientdistanceR, distance, m101005001000frequencyf, frequency, GHz2.4E+9AP transmit PowerPt, transmit power, dBm13AP antenna GainGt, Tx antenna gain, dBi5 Free Space Path LossFSPL, dB-60-80-94-100client antenna GainGr, Rx antenna gain, dBiclient received PowerPr, received Power, dBm-42-62-76-82Pr = Pt + Gt + FSPL + Gr
16 In the end, it’s all about… Getting sufficient power to the receiverShorter distancesChoosing a lower frequencyBigger antennasAntennas that focusMore sensitive receiversOne transmitter and multiple receivers (receiver diversity)Multiple transmitters and multiple receivers (Multiple Input Multiple Output or MIMO)
17 For Further StudyDigital Communications, Fundamentals and Applications, Bernard SklarNetworks, The Definitive Guide, Matthew GastBuilding Wireless Community Networks, Rob FlickengerWikipedia!Parabolic reflectorFriis transmission equationDecibeldBm (not dbm)Link budgetPower of 10ModulationTrellis modulationRadio propagationSphereFrequencyWavelengthSpeed of lightAntenna (radio)ParabolaLogarithmHalf-power pointRadio waveWaveLongitudinal waveDirectional antennaSerial commaRadio receiverMIMOIsotropic radiatorWi-FiIEEE