Wireless Radio Propagation

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Wireless Radio Propagation Antennas
Fundamentals

• Dr. R. K. Rao

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Propagation Modes

• Ground Wave
• Ground wave propagation more or less follows the
  contour of the earth
• Sky Wave
• Signal from an earth based antenna is reflected
  from the ionized layer of the upper atmosphere
  back down to earth
• Line of Sight wave
• Communication is by line of sight

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Wireless Propagation

• Wireless propagation is the total of everything
  that happens to a wireless signal as the signal
  travels from Point A to Point B.
• The study of how EM waves travel and interact
  with matter can become extremely complex.

• There are several important simplifications which
  can be made.
• In a vacuum, 2.4 GHz microwaves travel at the
  speed of light.
• Once started, these microwaves will continue in
  the direction they were emitted forever, unless
  they interact with some form of matter.
• In the atmosphere, the microwaves are traveling
  in air, not in a vacuum.
• This does not significantly change their speed.
• Similar to light, when RF travels through
  transparent matter, some of the waves are
  altered.
• 2.4 5 GHz microwaves also change, as they
  travel through matter.
• Amount of alteration depends heavily on the
  frequency of the waves and the matter.

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Wireless Propagation

• Mental picture
• Wave is not a spot or a line, but a moving wave.
• Like dropping a rock into a pond.
• Wireless waves spread out from the antenna.
• Wireless waves pass through air, space, people,
  objects,

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Transmission Impairments

• Attenuation
• Free Space Loss
• Noise
• Atmospheric Absorption
• Multi-path
• Reflection
• Refraction

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Attenuation
Same wavelength (frequency), less amplitude.

• Attenuation is the loss in amplitude that occurs
  whenever a signal travels through wire, free
  space, or an obstruction.
• At times, after colliding with an object the
  signal strength remaining is too small to make a
  reliable wireless link.

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Attenuation and Obstructions

• Shorter the wavelength (higher frequency) of the
  wireless signal, the more the signal it is
  attenuated.

Same wavelength (frequency), less amplitude.

• Longer the wavelength (lower frequency) of the
  wireless signal, the less the signal is
  attenuated.

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Attenuation and Obstructions

• The wavelength for the AM (810 kHz) channel is
  1,214 feet
• The larger the wavelength of the signal relative
  to the size of the obstruction, the less the
  signal is attenuated.
• The shorter the wavelength of the signal relative
  to the size of the obstruction, the more the
  signal is attenuated.

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Free-Space Waves

• Free-space wave is a signal that propagates from
  Point A to Point B without encountering or coming
  near an obstruction.
• The only amplitude reduction is due to free
  space loss .
• This is the ideal wireless scenario.

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Noise

• The received signal will consist of the
  transmitted signal, modified by various
  distortions imposed by the transmission system,
  plus additional unwanted signal inserted between
  transmission and reception
• Thermal Noise Crosstalk Impulse noise
• Measure is Signal-to-Noise Ratio, Eb/No

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Reflected Waves

• When a wireless signal encounters an obstruction,
  normally two things happen
• Attenuation The shorter the wavelength of the
  signal relative to the size of the obstruction,
  the more the signal is attenuated.
• Reflection The shorter the wavelength of the
  signal relative to the size of the obstruction,
  the more likely it is that some of the signal
  will be reflected off the obstruction.

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Reflected Waves
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Microwave Reflections

• Microwave signals
• Frequencies between 1 GHz 30 GHz (this can vary
  among experts).
• Wavelength between 12 inches down to less than 1
  inch.
• Microwave signals reflect off objects that are
  larger than their wavelength, such as buildings,
  cars, flat stretches of ground, and bodes of
  water.
• Each time the signal is reflected, the amplitude
  is reduced.

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Reflection

• Reflection is the light bouncing back in the
  general direction from which it came.
• Consider a smooth metallic surface as an
  interface.
• As waves hit this surface, much of their energy
  will be bounced or reflected.
• Think of common experiences, such as looking at a
  mirror or watching sunlight reflect off a
  metallic surface or water.
• When waves travel from one medium to another, a
  certain percentage of the light is reflected.
• This is called a Fresnel reflection (Fresnel
  coming later).

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Reflection

• Radio waves can bounce off of different layers of
  the atmosphere.
• The reflecting properties of the area where the
  WLAN is to be installed are extremely important
  and can determine whether a WLAN works or fails.
• Furthermore, the connectors at both ends of the
  transmission line going to the antenna should be
  properly designed and installed, so that no
  reflection of radio waves takes place.

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Microwave Reflections
Multipath Reflection

• Advantage Can use reflection to go around
  obstruction.
• Disadvantage Multipath reflection occurs when
  reflections cause more than one copy of the same
  transmission to arrive at the receiver at
  slightly different times.

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Multipath Reflection

• Reflected signals 1 and 2 take slightly longer
  paths than direct signal, arriving slightly
  later.
• These reflected signals sometimes cause problems
  at the receiver by partially canceling the direct
  signal, effectively reducing the amplitude.
• The link throughput slows down because the
  receiver needs more time to either separate the
  real signal from the reflected echoes or to wait
  for missed frames to be retransmitted.
• Solution discussed later.

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Multi-path Interference
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Two Pulses in Multi-path
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Propagation Mechanisms
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Modeling Multi-path environment

• Channel is often modeled as
• Rayleigh Fading Channel When there are multiple
  indirect paths between transmitter and receiver
  and no distinct dominant path such as LOS-
  applicable to outdoor environment
• Rician Fading Channel When there is a direct
  LOS path in addition to number of indirect
  multi-path signals applicable to indoor
  environment

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Diffraction
Diffracted Signal

• Diffraction of a wireless signal occurs when the
  signal is partially blocked or obstructed by a
  large object in the signals path.
• A diffracted signal is usually attenuated so much
  it is too weak to provide a reliable microwave
  connection.
• Do not plan to use a diffracted signal, and
  always try to obtain an unobstructed path between
  microwave antennas.

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Weather - Precipitation

• Precipitation Rain, snow, hail, fog, and sleet.
• Rain, Snow and Hail
• Wavelength of 2.4 GHz 802.11b/g signal is 4.8
  inches
• Wavelength of 5.7 GHz 802.11a signal is 2 inches
• Much larger than rain drops and snow, thus do not
  significantly attenuate these signals.
• At frequencies 10 GHz and above, partially melted
  snow and hail do start to cause significant
  attenuation.

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Weather - Precipitation

• Rain can have other effects
• Get inside tiny holes in antenna systems,
  degrading the performance.
• Cause surfaces (roads, buildings, leaves) to
  become more reflective, increasing multi-path
  fading.
• Tip Use unobstructed paths between antennas, and
  do not try to blast through trees, or will have
  problems.

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Weather - Ice
Collapsed tower

• Ice buildup on antenna systems can
• Reduce system performance
• Physically damage the antenna system

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Weather - Wind

• The effect of wind
• Antenna on the the mast or tower can turn,
  decreasing the aim of the antenna.
• The mast or tower can sway or twist, changing the
  aim.
• The antenna, mast or tower could fall potentially
  injuring someone or something.

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Refraction
Sub-Refraction
Refraction (straight line)
Normal Refraction
Earth

• Refraction (or bending) of signals is due to
  temperature, pressure, and water vapor content in
  the atmosphere.
• Amount of refractivity depends on the height
  above ground.
• Refractivity is usually largest at low
  elevations.
• The refractivity gradient (k-factor) usually
  causes microwave signals to curve slightly
  downward toward the earth, making the radio
  horizon father away than the visual horizon.
• This can increase the microwave path by about 15,

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Refraction

• Radio waves also bend when entering different
  materials.
• This can be very important when analyzing
  propagation in the atmosphere.
• It is not very significant in WLANs, but it is
  included here, as part of a general background
  for the behavior of electromagnetic waves.

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Antenna Fundamentals
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Antenna Directivity

• Antennas radiate wireless power
• Accept wireless signal energy from the
  transmission line connected to a transmitter
• Launch that wireless energy into free-space

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Antenna Directivity

• Antennas focus wireless energy like a flashlight
  reflector (focusing element) focuses light from a
  flashlight bulb.
• Without the focusing element, the bulb radiates
  light energy in all direction.
• No direction receives more light than any other
  direction.

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Antenna Directivity
Theoretical Isotropic Antenna

• Light energy from an unfocused flashlight bulb is
  similar to the wireless energy radiated from a
  theoretical isotropic antenna.
• Like a light bulb, an isotropic antenna radiates
  wireless energy equally in all directions and
  does not focus the energy in any single direction.

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Antenna Directivity

• A flashlight focuses the light into a beam that
  comes out the front of the flashlight.
• The flashlight (reflector) does not amplify the
  power or total amount of light from the bulb.
• The flashlight simply focuses the light so all of
  it travels in the same direction.

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Antenna Directivity

• By focusing the light, the flashlight provides
  more directivity (beam focusing power).
• An antenna provides directivity for the wireless
  energy that it focuses.
• Depending upon the design of the antenna,
  antennas focus and radiate their energy more
  strongly in on favored direction.
• When receiving, antennas focus and gather energy
  from their favored direction and ignore most of
  the energy arriving from all other directions.

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Antenna Radiated Patterns
Top View
Main Lobe
Front
Null
Side Lobes
Back

• Antennas exhibit directivity by radiating most of
  their power in one direction.
• Major or Main Lobe Main direction of the power
  from the antenna
• Minor or Side Lobes Small amount of power in
  other directions
• Nulls Where no power is radiated

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Antenna Radiated Patterns
Top View
Main Lobe
Front
Null
Side Lobes
Back

• Antennas provide the same directivity for
  transmitting and receiving.
• Antennas radiate transmitter power in the favored
  direction(s) when transmitting.
• Antennas gather signals coming in from the
  favored directions(s) when receiving.

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Antenna Radiated Patterns
Patch Antenna (Directional Antenna)

• When selecting antennas, remember
• When receiving, antenna directivity not only
  gathers incoming signals from the favored
  direction, but also reduces noise, interference,
  and unwanted signals coming in from other
  directions.

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Antenna Radiated Patterns
Top View (H)
Side View (V)
Dipole Antenna (Omnidirectional Antenna)

• An omnidirectional antenna radiates equally well
  in all horizontal directions around the main
  lobe, surrounding the antenna like a donut.

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Antenna Gain
Like a flashlight, there is always a tradeoff
between gain, which is comparable to brightness
in a particular direction, and beamwidth, which
is comparable to the narrowness of the beam.
(coming)

• Antenna gain Measurement of the power in the
  main lobe of an antenna and comparing that power
  to the power in the main lobe of a reference
  antenna.
• Gain - This refers to the amount of increase in
  energy that an antenna appears to add to an RF
  signal.
• Measure in dBi or dBd
• dBd d is the gain measured relative to the
  gain of a dipole reference antenna.
• dBi i is the gain measure relative to the
  gain of a theoretical isotropic antenna.

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Antenna Gain
21 dBi or about 100 times the signal strength
when comparing it to an isotropic antenna
Top View

• The dBi is a unit measuring how much better the
  antenna is compared to an isotropic radiator.
• An isotropic radiator is an antenna which sends
  signals equally in all directions (including up
  and down).
• An antenna which does this has an 0dBi gain.
• The higher the decibel figure the higher the
  gain.
• For instance, a 6dBi gain antenna will receive a
  signal better than a 3dBi antenna.

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Antenna Gain
Dipole antenna

• A dBd unit is a measurement of how much better an
  antenna performs against a dipole antenna.
• As a result a dipole antenna has a 0dBd gain.
• Note Wireless power never stops exactly on a
  sharp line like the lobe drawings show, but
  tapers off.
• More later

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Antenna Beamwidth

• Beamwidth The width of the main beam (main
  lobe) of an antenna.
• Measures the directivity of an antenna
• The smaller the beamwidth in degrees, the more
  the antenna focuses power into its main lobe.
• The more power of the main lobe, the further the
  antenna can communicate.

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Antenna Beamwidth
15 dBi
-3 dBi
12 dBi
15 dBi

• Beamwidth is a measurement used to describe
  directional antennas.
• Beamwidth is sometimes called half-power
  beamwidth.
• Half-power beamwidth is the total width in
  degrees of the main radiation lobe, at the angle
  where the radiated power has fallen below that on
  the centerline of the lobe, by -3 dB (half-power).

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• Remember, wireless power does not stop and start
  exactly along a straight line, but declines
  gradually with distance.
• The smooth outlines of the main lobes show the
  approximate intensity of the wireless power at
  various distances away from the antenna.
• The dotted lines pass through the half-power
  points the points on each side of the center of
  the main lobe where the wireless power is
  one-half as strong as it is at the center of the
  lobe.

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Line-of-Sight (LOS)
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Line of Sight
Attenuated Signal
Diffracted Signal

• When a wireless signal encounters an obstruction,
  the signal is always attenuated and often
  reflected or diffracted.
• It is important to try and obtain a wireless
  line-of-sight whenever possible, especially in a
  wireless WAN environment (outdoor connections
  between building or different parts of a campus).
• A wireless LOS typically requires visual LOS plus
  additional path clearance to account for the
  spreading of the wireless signal (Fresnel Zone
  coming).

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Visual LOS
I see you!
And, I see you!
1 Mile
1 Mile

• There is a difference between visual LOS and
  wireless LOS.
• This is because of the difference in wavelengths.
• The wavelength of visual light is very small.
• For example, the wavelength of a green light is
  only about 1/50,000th of an inch
• Remember, the wavelength of a 2.4 GHz WLAN signal
  is about 4.8 inches.

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LOS
1 Mile
1 Mile

• A lightwave and a wireless wave are similar.
• Both are forms of electromagnetic radiation.
• Both must obey the same laws of physics as they
  propagate.
• Wireless signals are like lightwaves that you
  cannot see.

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LOS
1 Mile
1 Mile

• The shorter the wavelength of an electromagnetic
  wave, the less clearance it needs form objects
  that it passes as it travels between two points.
• The less clearance it needs, the closer it can
  pass to an obstruction without experience
  additional loss of signal strength.
• The clearance distance is known as the Fresnel
  Zone.

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LOS
1 Mile
1 Mile

• The green light has a shorter wavelength so only
  needs a fraction of an inch to avoid additional
  attenuation.
• A 2.4 GHz (802.11b/g) wireless signal has a
  larger Fresnel zone and needs to clear the
  building by quite a few feet (about 10 feet in
  this example).

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Fresnel Zone

• Fresnel zone (pronounced frA-nel the s is
  silent).
• Provides a method for calculating the amount of
  clearance that a wireless wave (or light wave)
  needs from an obstacle to avoid additional
  attenuation of the signal.

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Fresnel Zone

• Fresnel Zone 72.1 SqrRoot (dist1Mi dist2Mi
  / FreqGHz DistanceMi)
• At least 60 of the calculated Fresnel Zone must
  clear to avoid significant signal attenuation.

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19.7 feet
1 Mile
1 Mile

• Example
• Diameter 72.1 SquareRoot (D1 D2) /
  FreqGhZ (D1 D2)
• 72.1 SquareRoot (1 1) /
  2.4 (1 1)
• 72.1 SquareRoot 1 / 2.4
  (2)
• 72.1 SquareRoot 1 / 4.8
• 72.1 SquareRoot .208
• 72.1 .456
• 32.9 feet
• 60 of FZ 0.6 (32.9) ft. 19.7 feet

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9.85 feet

• 60 of FZ 0.6 (32.9) ft. 19.7 feet
• So the wireless wave must clear the building by
  one-half of the 19.7 ft. diameter or or 9.85 feet

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Fresnel Zone Calculators

• http//www.wisp-router.com/calculators/fresnel.php
  
• http//www.tuanistechnology.com/education/calculat
  ors/fzc.htm
• http//www.firstmilewireless.com/calc_fresnel.html