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RF Considerations for

Wireless Systems Design

Frank Jimenez

Manager, Technical Support & Service

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The Presentation Objective

� We will cover..

�The available wireless spectrum

�802.11 technology and the wireless time domain

�Fresnel zones and their impact

�Application wireless transport requirements

�The importance of the Site and RF Survey

�System design recommendations

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Radio Channels in the Unlicensed Frequency Domain

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Radio Channels in the 2.4 GHz Frequency Domain

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Radio Channels in the 4.9 GHz Frequency Domain

4.94 GHz 4.99 GHz

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The 802.11 Standard

�Originally Developed for indoor WLANs

�Is RF Half-Duplex technology, but bi-directional

�Bi-directional via Time Division Duplexing

�Is a CSMA/CA vs. CSMA/CD Technology

�Collision Avoidance achieved through transmit deferral

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A Near Ideal Wireless Time Domain

�T = 0 �T = 1

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A Highly Congested Wireless Time Domain

�T = 0 �T = 1

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Fresnel zones

� Named after Augustin Fresnel (1788-1827), who contributed significantly to wave optic theory. (1)

� RF signal reflected off of First Fresnel zone boundary travels exactly ½ wavelength farther than direct path signal.

� Each subsequent Fresnel zone adds another ½ wavelength in distance to length of reflected path.

(1) From Wikipedia, the free encyclopedia

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Why are Fresnel zones important?

� As Fresnel zone numbers increase, separation between Fresnel zones decreases

� Fresnel zones are important with respect to reflected signals depending on their number

� If an RF path’s reflection point is blocked from view of the antennas, then Fresnel zone relationships are not a factor

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Fresnel zones and antenna height

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Fresnel zones and antenna height

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Fresnel zones and antenna height

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Where do I start with my system design?

�Start by confirming the application to be supported, and

it’s wireless transport requirements.

�What protocols does it use?

�What is the transport bandwidth required?

�What is the allowable latency and jitter?

�Where are the traffic origins and destinations?

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Where do I start with my system design?

�The Site and RF Survey

� Identify location of all application traffic interface points to

the proposed system

�Determine all node and antenna locations

�Confirm line-of-sight (LOS) exists between nodes

� If LOS does not exist, find alternate wireless path(s)

�Perform RF spectrum survey of deployment environment

� Identify any special equipment needed (lifts, cranes, etc.)

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What about non-line-of sight?

� Products in the marketplace are being advertised as non-

line-of-sight (NLOS) technology. Is NLOS possible?

� NLOS Feasibility depends on the specific path, not all NLOS

paths are possible

� NLOS systems are inherently signal distorted with variable

bandwidth, latency and packet jitter performance

� NLOS systems may be a good fit for low data rate TCP

applications, ie. Smart Grid and data acquisition systems

� NLOS systems are not recommended for video applications

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Back to the system design..

� Based on site survey, design a wireless topology that

supports the application traffic routing, bandwidth,

and latency requirements.

� Systems for streaming protocol applications should

always be based on clear line-of-sight radio paths.

� NLOS signal distortion relegates these systems to

acknowledgement protocol applications that do not

require high bandwidth, low latency, and low packet

jitter.

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Antenna Selection

� Use narrowest beam width antenna available that will

provide signal coverage to the intended nodes

� Usable beam width of an antenna is considerably broader

than specified beam width

� Beam width is defined by the points on either side of the

main signal beam where gain is reduced by 3 dB

� Refer to an antenna’s gain pattern to determine its effective

beam width, so that you can properly utilize it in your

system design

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AS-050-N Sector Azimuth Gain Pattern

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AS-050-N Sector Elevation Gain Pattern

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And Finally…

� ALWAYS specify a dual-radio nodes to avoid

� Bandwidth constriction

� Latency

� Packet jitter

� When reusing frequencies, isolate the links involved to

avoid time domain congestion

� Accomplished by physical line-of-sight blockage, antenna

cross-polarization, and directional antennas.

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A high-performance design example

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A robust linear loop mesh design example

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In Summary

� A successful project outcome requires:

�Understanding the transport requirements of the application

to be supported

�Understanding and managing the wireless frequency and

time domains

�Performance of a proper site and RF survey

�Proper wireless topology design and implementation

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Questions?