June 2001Copyright 2001 Global Wireless Education ConsortiumRT-RF Propagation 1 RF Propagation.
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Transcript of June 2001Copyright 2001 Global Wireless Education ConsortiumRT-RF Propagation 1 RF Propagation.
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 1
RF Propagation RF Propagation
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 2
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.
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 3
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.
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 4
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 5
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 6
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 7
Impairments to Free- Impairments to Free- Space PropagationSpace Propagation
Reflection Scattering Blocking Absorption Refraction Diffraction Multipath interference
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 8
ReflectionReflection
i r
t
Circuit for voltage applied to input of a receiver
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 9
ScatteringScattering
Incident wave
Reflected waves
Transmitted waves(also refracted)
Wave scattering and transmission
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 10
BlockingBlocking
Blocked and reflected cellular radio waves
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 11
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 12
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 13
Atmospheric RefractionAtmospheric Refraction
troposphere or ionosphere
Earth
reflected radio beam
direct radio beam
Direct and reflected radio beams
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 14
Atmospheric RefractionAtmospheric Refraction
Earth
expected radio beam
radio beam curved by atmosphere
Radio beam curved by atmosphere
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 15
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 16
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 17
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 18
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 19
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 20
Effects of FadingEffects of Fading
Effects of fading are noticed by the listener Rapid change in volume Random frequency modulation Echoes Distortion Dropped call
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 21
Types of FadingTypes of Fading
Flat fading
Frequency selective fading
Fast fading (Rayleigh fading)
Slow fading
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 22
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:
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 23
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
June 2001 Copyright 2001 Global Wireless Education Consortium RT-RF Propagation 24
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)