# Presentation: 090505

##  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

**Copyright Notice:  ****© 2008 by Southwest Research Institute**

## 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.**

## 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**

## Goals for this Presentation

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

## Today's Agenda

**Session II**
**Session Focus**
**Wireless Link Engineering**
**Coding Methods**
**Data Communications Protocols**

## Session II

## Our Focus is on ISO Layer 1

## Wireless Link Engineering

## Essential Wireless Design Tools

**Radio Link Engineering**
**Frequency/Wavelength**
**Power**
**Propagation/Attenuation**

## Frequency / Wavelength

## 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 **

## 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   **

## 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’**

## Numbers in Hertz 

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

## 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”**

## 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.**

## Power

## Remember the Wave...

## 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**

## 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**

## 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?**

## 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**

## Propagation / Attenuation

## 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**

## 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***_

## Freespace Loss

**For distance (d) in miles, frequency (f) in GHz,**
**	****Freespace Loss in dBm:**
**96.6 + 10 log(d****2****) + 10 log(f****2****)**
**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)**

## Freespace Loss

**From the equation, note the distance/power relationships**

|  | Loss at 1 mile | Loss at 2 miles | Loss at 4 miles | Loss at 8 miles | Loss at 16 miles |
| --- | --- | --- | --- | --- | --- |
| 700 MHz | 94 dBm | 100 dBm |  |  |  |
| 2.4 GHz | 104 dBm | 110 dBm |  |  |  |
| 5.7 GHz | 112 dBm | 118 dBm |  |  |  |

## 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 |  |  |

## The Fresnel Zone

## 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**

## 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**

## nLOS 

**Near Line of Sight**
**Less  than 60% Fresnel zone available**

## NLOS 

**Non Line of Sight**

## 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**

## Rain, Snow Attenuation (NASA)

## 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**

## 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**

## 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.**

## Wireless Data Coding 

**Three Most Used Methods**
**Frequency Hopping Spread Spectrum (FHSS)**
**Direct Sequence Spread Spectrum (DSSS)**
**Orthogonal Frequency Division Multiplexing (OFDM)**

## 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

## 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

## 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

## Data Communications 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.**

## Summary: Power, Coding and Protocol

**Radio Power Management**
**And Coding**

## Questions and Answers

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

## 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 **