June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 1

RF Propagation RF Propagation

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RT-RFPRT-RFP

© Copyright 2001 Global Wireless Education Consortium

All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws.

Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners.

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RT-RFPRT-RFP

Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039.

GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.

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Propagation ModelsPropagation Models

Large-scale propagation model Gives power where random environmental effects have been

averaged together

Small-scale propagation model Gives mean power for small distances or short times

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Radio Propagation PathsRadio Propagation Paths

Depending on the frequency, radio signals will follow different paths

Ground Wave Below 2 MHz Sky Waves 2 to 30 MHz LOS – Line of Sight Above 30 MHz

Ground Waves follow the curvature of the earth Sky Waves bounce off the ionosphere LOS must follow a straight line

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Frequency BandsFrequency Bands

VLF 3 – 30 KHz GW LF 30 – 300 KHz GW MF 300 – 3000 KHz GW in Day / SW at Night HF 3 – 30 MHz SW VHF 30 – 300 MHz LOS UHF 300 – 3000 MHz LOS SHF 3 – 30 GHz LOS EHF 30 – 300 GHz LOS

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Impairments to Free- Impairments to Free- Space PropagationSpace Propagation

Reflection Scattering Blocking Absorption Refraction Diffraction Multipath interference

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ReflectionReflection

i r

t

Circuit for voltage applied to input of a receiver

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ScatteringScattering

Incident wave

Reflected waves

Transmitted waves(also refracted)

Wave scattering and transmission

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BlockingBlocking

Blocked and reflected cellular radio waves

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AbsorptionAbsorption

Some substances absorb radio waves Trees and shrubs Clouds, mist and other atmospheric water and dust Metal screen Human head near a hand held

Higher frequency radio waves are absorbed more than lower frequency radio waves

Absorption seldom beneficial in wireless applications

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RefractionRefraction

Refraction is the bending of electromagnetic waves as they pass from medium of one density into medium of another density

Radio waves typically bend due to changes in density of air caused by changes in humidity, temperature or pressure

Dielectric constant describes how the wave will propagate through the material

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Atmospheric RefractionAtmospheric Refraction

troposphere or ionosphere

Earth

reflected radio beam

direct radio beam

Direct and reflected radio beams

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Atmospheric RefractionAtmospheric Refraction

Earth

expected radio beam

radio beam curved by atmosphere

Radio beam curved by atmosphere

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RefractionRefraction

Polarization of waves refracted from the ionosphere are usually different than polarization of a direct waves leaving transmitting antenna

Wave refracted by troposphere maintains same polarization

Index of refraction of earth’s atmosphere changes with altitude Causes a radio beam to be bent downward toward the earth Result of this effect is same as if the radius of the earth has

been flattened

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DiffractionDiffraction

A change in wave pattern caused by interference between waves that have been reflected from a surface or a point

Causes regions of waves strengthening and weakening Results in bending of the wave Can occur in different situations when waves

Pass through a narrow slit Pass the edge of a reflector Reflect off two different surfaces approximately one wavelength

apart

Beneficial effect of radio wave diffraction is the K-factor

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Multipath InterferenceMultipath Interference

Occurs when radio waves reflected off of structures results in multiple copies of signal traveling in reception area Several of these signals are received by the receiver The received signals travel in different paths and take different

amounts of time to arrive Benefit is that radio signals can reach hard-to-reach

areas Disadvantages include:

Delay spread Rayleigh fading Doppler shift

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Small-Scale Fading Small-Scale Fading Propagation ModelPropagation Model

Fading is the rapid fluctuation of the amplitude of a radio signal in a short time over a short distance

Fading is caused by destructive interference between 2 or more versions of the original signal transmitted The multiple signals interfere with each other

Waves can combine constructively or destructively

Signal peaks and valleys are out of phase with each other due to different transmission path lengths

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Causes of FadingCauses of Fading

Primary causes of fading include physical factors Multipath propagation Speed of mobile station Speed of reflecting objects

Doppler shift is involved in fading Multiple paths caused by reflection may have positive or negative effect Positive effect when frequency is shifted higher and negative effect

when frequency is shifted lower If objects reflecting radio waves are moving more rapidly than mobile

station, Doppler shift can be dominant cause of fading If objects reflecting radio waves are moving more slowly than the the

mobile station, Doppler shift can be ignored

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Effects of FadingEffects of Fading

Effects of fading are noticed by the listener Rapid change in volume Random frequency modulation Echoes Distortion Dropped call

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Types of FadingTypes of Fading

Flat fading

Frequency selective fading

Fast fading (Rayleigh fading)

Slow fading

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Industry ContributorsIndustry Contributors

Ericsson (http://www.ericsson.com) Lucent Technologies (http://www.lucent.com) Verizon Wireless (formerly AirTouch Cellular)

(http://www.verizonwireless.com) RF Globalnet (http://www.rfglobalnet.com) Telcordia Technologies, Inc (http://www.telcordia.com) U.S. Navy Verizon (http://www.verizon.com)

The following companies provided materials and resource support for this module:

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Individual ContributorsIndividual ContributorsThe following individuals and their organization or institution provided materials, resources, and development input for this module: Dr. Jamil Ahmed

British Columbia Institute of Technology http://bcit.ca

Mr. John Baldwin South Central Technical College http://means.net

Dr. Derrek Dunn North Carolina A&T State University http://ncat.edu

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Individual Contributors, Individual Contributors, cont.cont.

Dr. Cynthia Furse Utah State University http://www.helios.ece.usu.edu/

Ms. Annette Muga Ericsson http://www.ericsson.com/

Dr. David Voltmer Rose-Hulman Institute of Technology http://www.rose-hulman.edu/

Modified by Dr. Larry Hash State University of NY Institute of Technology

(http://sunyit.edu)