Wireless Media

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Chapter 4 Transmission Media Wireless Media
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Wireless Transmission Frequencies

• 1GHz to 40GHz
• Microwave frequency range
• highly directional beams are possible
• Point-to-point transmission
• Used also in satellite communications
• 30MHz to 1GHz
• Radio frequency range
• Omnidirectional (all directions)
• broadcast radio and TV
• 3 x 1011 to 2 x 1014 Hz
• Infrared portion of the spectrum
• Local point-to-point and multipoint
  applications(e.g., infrared remote control,
  wireless LAN)

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Antennas

• Electrical conductor used to radiate or collect
  electromagnetic energy
• Transmission antenna
• radio frequency energy from transmitter converted
  to electromagnetic energy by antenna
• radiated into surrounding environment
• Reception antenna
• electromagnetic energy impinging on antenna
  converted to radio frequency electrical energy
• fed to receiver
• Same antenna is often used for both purposes

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Antenna Radiation Pattern

• Power radiated in all directions
• Not same performance in all directions
• as seen in a radiation pattern diagram
• An isotropic antenna is a (theoretical) point in
  space
• reference antenna
• radiates in all directions equally
• with a spherical radiation pattern

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Parabolic Reflective Antenna
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Antenna Gain

• Measure of directionality of antenna
• Defined as the output power in particular
  direction compared to that produced by an
  isotropic antenna
• The antenna gain is related to the effective area
  of an antenna
• The isotropic antenna has a gain of 10dB with
• Example what is the gain of a parabolic antenna
  with effective area
  , radius r1m and f12GHz?

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Terrestrial Microwave

• Used as an alternative to coaxial and fiber
  cables
• Requires fewer repeaters but line-of-sight
• Used also for short point-to-point links between
  buildings
• Use a parabolic dish to focus a narrow beam onto
  a receiver antenna
• 1-40GHz frequencies
• Higher frequencies give higher data rates
• Main source of loss is attenuation the loss

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Satellite Microwave

• A satellite is a microwave relay station
• Receives on one frequency band (uplink),
  amplifies or repeats signal and transmits on
  another frequency band (downlink)
• eg. uplink 5.925-6.425 GHz downlink 3.7-4.2 GHz
• A satellite operate on a number of frequency
  bands called transponders
• The satellite requires geo-stationary orbit
• height of 35,784km
• spaced at least 3-4 apart (to minimize
  interference from other satellites)
• typical uses
• Television distribution
• long distance telephone transmission
• private business networks
• Global Positioning System (GPS)

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Satellite Point-to-Point Link
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Satellite Broadcast Link
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Broadcast Radio

• Use broadcast radio, 30MHz - 1GHz, for
• FM radio
• UHF and VHF television
• Is omnidirectional
• Still need line-of-sight
• Suffers from multipath interference
• reflections from land, water, other objects

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Infrared

• modulate infrared light
• are blocked by walls
• no licenses required
• typical uses
• TV remote control
• Infrared WLAN

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Wireless Propagation Ground Wave

• Ground wave is found in frequencies up to 2MHz
• The best-known example of ground wave
  communication is AM radio

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Wireless Propagation Sky Wave

• Sky wave propagation is found in frequencies from
  2MHz to 30MHz
• A sky wave signal bounces back and forth between
  the ionosphere and the earth surface

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Wireless Propagation Line-Of-Sight (LOS)

• LOS propagation is found in frequencies above
  30MHz
• the transmitting and receiving antennas must be
  within a line of sight of each other

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Refraction

• velocity of electromagnetic wave is function of
  material density
• 3 x 108 m/s in vacuum, less in anything else
• speed changes as move between media
• Index of refraction (refractive index) is
• have gradual bending if medium density varies
• density of atmosphere decreases with height
• results in bending of radio waves towards earth
• hence optical LOS horizon and radio LOS horizon
  are not the same

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Optical and Radio LOS

• The optical LOS to the horizon can be expressed
  as
• The radio LOS to the horizon can be expressed as
• The max. distance between two antennas for LOS
  propagation

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EXAMPLE

• The max. distance between two antennas for LOS
  transmission if one antenna is 100m high and the
  other at ground level is
• Now suppose that the receiving antenna is 10m
  high. To achieve the same distance, how high must
  the transmitting antenna be? The result is

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Free Space Loss

• The free space loss for ideal isotropic antenna
  is
• For other antennas, we must take into account
  antenna gain

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Free Space Path Loss

• The free space path loss is a function of both
  the distance and the carrier frequency

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Example

• Determine the free space loss at 4GHz for the
  shortest path to a satellite from earth
  (35.863Km).
• Now consider the antenna gain of both the
  satellite and earth station antennas. Typical
  values are 44dB and 48dB, respectively. The free
  space path loss
• What is the received power at the satellite
  antenna if the transmitted power from the earth
  station antenna is 250W?

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Impairments in wireless LOS transmission

• Free space loss
• loss of signal with distance
• Atmospheric Absorption
• from water vapour and oxygen absorption
• Multipath
• multiple interfering signals from reflections
• Refraction
• bending signal away from receiver

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Examples of Multipath Interference
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