T3 Webinar:

A Technical Discussion on Wireless Data Transport Systems: Wireless Trends, Tools and Tips Session II - Wireless Link Engineering

April 8, 2008

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"A Technical Discussion on Wireless Data Transport Systems:
Wireless Trends, Tools and Tips Session II - Wireless Link Engineering"

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Slide 1: A Technical Discussion on Wireless Data Transport Systems
Wireless Trends, Tools and Tips

Session II – Wireless Link Engineering
Intelligent Transportation Systems Department
Southwest Research Institute®
www.swri.org

Slide 2: This 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.

Slide 3: Class Format and Method

Presentation and discussion
Time permitting, questions during the presentation, and at the end of the presentation
- During - via the application "question" tool
- After – via audio

Slide 4: Goals for this Presentation

Explain wireless power, application coding and transport protocol.
Provide the attendees with usable information concerning wireless power and propagation/attenuation.

Slide 5: Today's Agenda

Session II

Session Focus
Wireless Link Engineering
Coding Methods
Data Communications Protocols

Slide 6: Session II

Slide 7: Our Focus is on ISO Layer 1

Layer 7: Application Layer - Interfaces from and and to system applications, not the user
Layer 6: Presentation Layer - Transformation of data, data conversion and encryption and decryption
Layer 5: Session Layer - Individual Session management, "conversation" management
Layer 4: Transport Layer - Transport and Error Correction
Layer 3: Network Layer - Network and individual node management
Layer 2: Data link Layer - Basic network connection management
Layer 1: Physical Layer - Fiber optic cable, wired or wireless properties

Slide 8: Wireless Link Engineering

Slide 9: Essential Wireless Design Tools

Radio Link Engineering

Frequency/Wavelength
Power
Propagation/Attenuation

Slide 10: Frequency / Wavelength

Slide 11: 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

[Image of two ocean waves, both peaking with a trough formed between the two waves. The image shows the crest as the highest points on the waves; the wavelength as the horizontal distance between the two crests, and wave height vertical distance between the trough and the crest. ]

Slide 12: 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

Slide 13: 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´

Slide 14: Numbers in Hertz

KHz = 1000 cycles per second
MHz = 1,000,000 cycles per second
GHz = 1,000,000,000 cycles per second

Slide 15: 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"

Slide 16: 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 problem
- Hard rain attenuates 10 GHz satellite television.

Slide 17: Power

Slide 18: Remember the Wave…

Slide 19: Wireless 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

[Image shows a single cycle or wavelength ]

Slide 20: Wireless 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

Slide 21: Power - 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?

Slide 22: 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 1/2 W
Expect to see the power given in dBm or dB EIRP (Effective Isotropic Radiated Power), EIRP may be omitted

Slide 23: Propagation / Attenuation

Slide 24: Gains and Losses to Power

Gains

The launch power in dBm
Increases in power due to the antennas

Losses

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

Slide 25: Power 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

Slide 26: Freespace 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 miles
Frequency (f) must be in GHz (900 MHz is .9 GHz)

Slide 27: Freespace Loss

	Loss at 1 mile	Loss at 4 miles	Loss at 6 miles	Loss at 8 miles	Loss at 14 miles
700 MHz	94 dBm	100 dBm
2.4 GHz	104 dBm	110 dBm
5.7 GHz	112 dBm	118 dBm

Slide 28: How 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.

	Loss at 1 mile	Loss at 2 miles	Loss at 3 miles	Loss at 5 miles
700 MHz	94 dBm	100 dBm	103 dBm	107 dBm
2.4 GHz	104 dBm	110 dBm
5.7 GHz	112 dBm	118 dBm

Slide 29: The Fresnel Zone

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

Slide 30: 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

Slide 31: Fresnel - 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

[Images of three buildings that shows an example of how line of sight (LOS) is measured. ]

Slide 32: nLOS

Near Line of Sight

Less than 60% Fresnel zone available

[Images of three buildings that shows an example of how near line of sight (nLOS) is measured. ]

Slide 33: NLOS

Non Line of Sight

[Images of three buildings that shows an example of non Line of Sigh (NLOS). The taller building is between the two shorter buildings, blocking the line of sight between the two shorter buildings.]

Slide 34: Reflection, Scattering or Diffusion and 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

Slide 35: Rain, Snow Attenuation (NASA)

[Layer 7: Application Layer – Interfaces from and to system applications, not the user
Layer 6: Presentation Layer – Transformation of data, data conversion and encryption and decryption
Layer 5: Session Layer – Individual Session management, "conversation" management
Layer 4: Transport Layer – Transport and Error correction
Layer 3: Network Layer – Network and individual node management
Layer 2: Data link Layer – Basic network connection management
Layer 1: Physical Layer - Fiber optic cable, wired or wireless properties]

Slide 36: Summary 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

Slide 37: Insertion 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

Slide 38: Wireless Data Coding

Slide 39: Wireless Data Coding

Note:
Coding of data for specific frequencies is subject to change. If you have questions call the FHWA Division Office.

Slide 40: Wireless Data Coding

Three Most Used Methods

Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS)
Orthogonal Frequency Division Multiplexing (OFDM)

Slide 41: Most Used Methods

FHSS - Transmission is switched from one frequency to another. The frequencies that are utilized are within the allocated band
DSSS - 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

Slide 42: 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

Slide 43: Coding Applications

FHSS: Noisy environments that have more channels available
DSSS: Noisy Environments, where power is minimized
OFDM: NLOS or nLOS in which the receiver may not be seen or seen well. A popular method for backhauls

Slide 44: Data Communications Protocols

Slide 45: 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.

Slide 46: Summary: Power, Coding and Protocol

Slide 47: Questions and Answers

Questions or comments?
Patrick (Pat) Clair PMP
210-522-3019
pclair@swri.org

Slide 48: References

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