wireless course - session 2 031308
TRANSCRIPT
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A Technical Discussion on Wireless Data A Technical Discussion on Wireless Data Transport Systems:Transport Systems:
Wireless Trends, Tools and TipsWireless Trends, Tools and Tips
Session II Session II –– Wireless Link EngineeringWireless Link Engineering
Intelligent Transportation Systems DepartmentIntelligent Transportation Systems DepartmentSouthwest Research InstituteSouthwest Research Institute®®
www.swri.orgwww.swri.org
Copyright Notice: © 2008 by Southwest Research Institute
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2Copyright Notice: © 2008 by Southwest Research Institute
This technical discussion is forThis technical discussion is for……
Any ITS or traffic professional that desires to learn more about wireless solutions, and the components of effective and efficient wireless design.
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Class Format and MethodClass Format and Method
Presentation and discussionTime permitting, questions during the presentation, and at the end of the presentation– During - via the application “question” tool– After – via audio
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Goals for this PresentationGoals for this Presentation
Explain wireless power, application coding and transport protocol.Provide the attendees with usable information concerning wireless power and propagation/attenuation.
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Today'sToday's AgendaAgenda
Session II– Session Focus
– Wireless Link Engineering
– Coding Methods
– Data Communications Protocols
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Session IISession II
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Our Focus is on ISO Layer 1Our Focus is on ISO Layer 1
Layer 1: Physical Layer - Fiber optic cable, wired or wireless properties
Layer 2: Data link Layer – Basic network connection management
Layer 3: Network Layer – Network and individual node management
Layer 4: Transport Layer – Transport and Error correction
Layer 5: Session Layer – Individual Session management, “conversation” management
Layer 6: Presentation Layer – Transformation of data, data conversion and encryption and decryption
Layer 7: Application Layer – Interfaces from and to system applications, not the user
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Wireless Link EngineeringWireless Link Engineering
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Essential Wireless Design ToolsEssential Wireless Design Tools
Radio Link Engineering
– Frequency/Wavelength
– Power
– Propagation/Attenuation
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Frequency / WavelengthFrequency / Wavelength
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An Example (www.onr.mil)An Example (www.onr.mil)
An ocean wave has a peak or crest and a trough or valley; distance between those parts of the wave can be measured
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Frequency/Wavelength Frequency/Wavelength
The length and power of a wave help determine the ability of the wave to pass through obstacles or the immunity of the wave to phenomena
Different length waves have different properties
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Frequency/Wavelength Frequency/Wavelength
Waves travel at the speed of light– A rate of 186,000 miles or 300,000,000 meters
per second– Dividing the rate by the frequency provides the
wavelength (convert to feet, inches or centimeters)
For example:186,000 mps / 1,000,000 (1 MHz) = .186 miles or 982’
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Numbers in Hertz Numbers in Hertz
KHz = 1000 cycles per second
MHz = 1,000,000 cycles per second
GHz = 1,000,000,000 cycles per second
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Frequency/Wavelength Frequency/Wavelength
1 MHz (1000 KHz AM Radio) ~980’
700 MHz (Recent FCC Ruing) ~17”900 MHz (Traffic telemetry) ~15”2.4 GHz (Wireless LANs) ~5”4.9 GHz (Public Safety) ~2.5”5 GHz (WLANs, WiMAX) ~2”10 GHz (Satellite TV) ~1”
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Frequency/Wavelength Frequency/Wavelength
The lower the frequency, the longer the cycle or wave. With power held constant, longer waves are less susceptible to attenuation. A wet leaf may attenuate a 5.7 GHz signal, while having little effect on an AM radio signal.– A conductor (wet leaf) equal to 1/2 of
the wavelength can be a problemHard rain attenuates 10 GHz satellite television.
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PowerPower
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Remember the WaveRemember the Wave……
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Wireless Power is the AmplitudeWireless Power is the Amplitude
The distance above and below the zero reference point (still water). The AC wall receptacle in your office is probably rated at 115 volts at 60 Hz, 15 Amps of power
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Wireless Power MeasurementWireless Power Measurement
Wireless power is in Watts (W), converted to decibel miliwatts (dBm), dBm is decibels from 1 mW
• 1 mW = 0 dBm
• 2 mW = 3 dBm
• 1 W = 30 dBm, 1 Watt, a useful reference point
• 2 W = 33 dBm
• 4 W = 36 dBm
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Power Power -- Very Important!Very Important!
Power in dBm has an interesting numerical relationship.
– 3 dbm more is twice as much power, so
– 33 dBm is twice as much power as 30 dBm
So…how much less power in W or mW is 27 dBm than 30 dBm?
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How much power in W is 27 dBm?How much power in W is 27 dBm?
For a typical power setting in unlicensed commercial and consumer systems
1 W =1000 Miliwatts or 30 dBm
27 dBm is half the power of 30 dBm
1000 miliwatts/2 = 500 mWatts
27 dBm = 500 mWatts or .5 W or ½ W
Expect to see the power given in dBm or dB EIRP (Effective Isotropic Radiated Power), EIRP may be omitted
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Propagation / AttenuationPropagation / Attenuation
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Gains and Losses to PowerGains and Losses to Power
Gains
– The launch power in dBm
– Increases in power due to the antennasLosses– Freespace Loss, loss due to using air as
transport media– Antenna alignment– Obstructions, Line Of Sight and the Fresnel Zone– Diffusion– Diffraction– Cabling Losses - Line and connector insertion
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Power Figures in dBmPower Figures in dBm
Start with radio power (given on cut or data sheet)
– Add gains to power to arrive at total gain figure
Starting with Freespace Loss
– Add additional losses to power to arrive at a total loss figure
Freespace loss is generally the largest loss figure
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Freespace LossFreespace Loss
For distance (d) in miles, frequency (f) in GHz,Freespace Loss in dBm:
96.6 + 10 log(d2) + 10 log(f2)
96.6 dBm is a constant that is used when the distance (d) is in milesFrequency (f) must be in GHz (900 MHz is .9 GHz)
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Freespace LossFreespace Loss
From the equation, note the distance/power relationships
Loss at 16 miles
Loss at 8 miles
Loss at 4 miles
Loss at 2 miles
Loss at 1 mile
118 dBm
110 dBm
100 dBm
112 dBm5.7 GHz
104 dBm2.4 GHz
94 dBm700 MHz
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How to ApproximateHow to Approximate
You can develop an approximation of the in-between values if the loss over the distance is consistent.This does not work if there are obstructions.
103 dBm
Loss at 3 miles
107 dBm100 dBm94 dBm700 MHz
Loss at 5 miles
118 dBm
110 dBm
Loss at 2 miles
112 dBm5.7 GHz104 dBm2.4 GHz
Loss at 1 mile
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The Fresnel ZoneThe Fresnel Zone
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Line of Sight (LOS)Line of Sight (LOS)
Human line of sight: what you see
Radio line of sight: what the radio sees
Near-Line of Sight (nLOS): A human can see the receiver, but there is an obstruction in the Fresnel Zone
Non-Line of Sight (NLOS): A human cannot see the receiver
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Fresnel Fresnel -- LOS, nLOS, NLOS LOS, nLOS, NLOS
Radio Line of Sight (Distance “d” in miles, f in GHz)
– Formula for radius, r = 72.2 * sqrt( d/4f )
– 60% of Fresnel zone of r needed, more is better
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nLOS nLOS
Near Line of Sight
– Less than 60% Fresnel zone available
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NLOS NLOS
Non Line of Sight
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Reflection, Scattering or Diffusion Reflection, Scattering or Diffusion and Diffractionand Diffraction
Reflection: Shorter wavelengths relative to the size of an obstacle will tend to “bounce off”obstructions and reflectors.
– For example, buildings and calm water
– Signals can arrive late, but intact
Scattering: Called multi-path fading
– Signals through trees
Diffraction: Standing in the shadow of an obstruction
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Rain, Snow Attenuation (NASA)Rain, Snow Attenuation (NASA)
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Summary of Effects and ActionsSummary of Effects and Actions
Digital signals can arrive at the receiver out of sequence and out of time
Rules of Thumb for maintaining signal
– Allow .5 dB for antenna aiming error
– Allow .5 dB per meter of tree canopy if you have to go through that tree. Expect lower bit-rates.
– Allow .02 dB per Km for rain fade below 10 GHz, concentrate on the stability of the mount
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Insertion and Cabling LossesInsertion and Cabling Losses
Read the manufacturer’s data sheet for the standard values
– Evaluate the cable and the connectors
Ethernet radios with integrated amplifiers and antenna will not have insertion or cabling losses
Grounding is important
– Noise control
– Lightning protection
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Wireless Data CodingWireless Data Coding
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Wireless Data Coding Wireless Data Coding
Note:
Coding of data for specific frequencies is subject to change. If you have questions call the FHWA Division Office.
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Wireless Data Coding Wireless Data Coding
Three Most Used Methods– Frequency Hopping Spread Spectrum
(FHSS)– Direct Sequence Spread Spectrum
(DSSS)– Orthogonal Frequency Division
Multiplexing (OFDM)
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Most Used Methods Most Used Methods
FHSS - Transmission is switched from one frequency to another. The frequencies that are utilized are within the allocated bandDSSS - Transmission is spread out over the allocated band. There is no hopping. There is a variant named High Rate Direct Sequence Spread Spectrum (HR/DSSS). HR offers improvements in processing speed
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Most Used Methods cont.Most Used Methods cont.
OFDM – Orthogonal Frequency Division Multiplexing. Each portion of the signal is encoded on a sub-channel within the frequency range. The signal is encoded on multiple sub-channels and transmitted at the same time.
This is a widely utilized method
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Coding Applications Coding Applications
FHSS: Noisy environments that have more channels available DSSS: Noisy Environments, where power is minimizedOFDM: NLOS or nLOS in which the receiver may not be seen or seen well. A popular method for backhauls
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Data Communications ProtocolsData Communications Protocols
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IEEE Protocols IEEE Protocols
802.11 a, b, g or n– Mostly indoors, some outdoor use.
Proprietary Wireless Ethernet (802.3)– Mostly outdoors. Popular with Wireless
Internet Service Providers. 802.16e: – Outdoors. An emerging standard in the
US. Promises high data rates.
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Summary: Power, Coding and ProtocolSummary: Power, Coding and Protocol
Layer 1: Physical Layer - Fiber optic cable, wired or wireless properties
Layer 2: Data link Layer – Basic network connection management
Layer 3: Network Layer – Network and individual node management
Layer 4: Transport Layer – Transport and Error correction
Layer 5: Session Layer – Individual Session management, “conversation” management
Layer 6: Presentation Layer – Transformation of data, data conversion and encryption and decryption
Layer 7: Application Layer – Interfaces from and to system applications, not the user
Radio Power ManagementAnd Coding
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Questions and AnswersQuestions and Answers
Questions or comments?
Patrick (Pat) Clair PMP
210-522-3019
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ReferencesReferences
Wave Graphic from Office of Naval Research (ONR):– http://www.onr.navy.mil/Focus/ocean/motion/waves1.htm
Power loss graph from the National Aeronautics and Space Administration (NASA)
– http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19830021220_1983021220.pdf