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The Wonders of
   Wireless
    Professor Philip Levis
     Stanford eDay 2012
Today You’ll Learn

•   What are wireless signals?
•   How does wireless work?
•   Why does wireless seem so capricious?
•   How can you improve your signal?
Today You’ll Learn

•   What are wireless signals?
•   How does wireless work?
•   Why does wireless seem so capricious?
•   How can you improve your signal?
Electromagnetic
             Radiation
•   Visible light, x-rays, gamma rays, ultraviolet
    light, cosmic rays, and radio waves
•   Intangible (almost!) waves of energy
•   Travel at “speed of light”
    ‣   180,000 miles/second
    ‣   1,000,000,000 ft/second (1 Gft/sec)
    ‣   1 ft/nanosecond
Describing a Wave
                                  Amplitude


                  Phase



                     Wavelength


Frequency: how many waves per second (e.g., 1,000Hz)
Wavelength: speed of wave / wave frequency
Phase: timing of waves within a wavelength
Example: Audio

  •   Waves through a physical medium: air
  •   Audible: 20Hz to 20,000Hz
  •   Speed of sound ≈ 1,100 ft/second
      ‣   Wavelength of piano’s lowest C (33Hz)?
      ‣   Wavelength of middle C (261Hz)?
      ‣   Wavelength of highest flute note (2100Hz)?




Wavelength = speed of wave / wave frequency
Light is Different
•   Not waves through a medium: pass through
    vacuum like empty space
    ‣   Michelson-Morley and the luminiferous aether

•   Waves in water and air are energy in the
    form of motion of matter
•   Electromagnetic radiation is “pure” energy: it
    has no mass
    ‣   Carried by photons
Spectrum
Spectrum
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis
The Wonders of Wireless: Phil Levis

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The Wonders of Wireless: Phil Levis

  • 1. The Wonders of Wireless Professor Philip Levis Stanford eDay 2012
  • 2. Today You’ll Learn • What are wireless signals? • How does wireless work? • Why does wireless seem so capricious? • How can you improve your signal?
  • 3. Today You’ll Learn • What are wireless signals? • How does wireless work? • Why does wireless seem so capricious? • How can you improve your signal?
  • 4. Electromagnetic Radiation • Visible light, x-rays, gamma rays, ultraviolet light, cosmic rays, and radio waves • Intangible (almost!) waves of energy • Travel at “speed of light” ‣ 180,000 miles/second ‣ 1,000,000,000 ft/second (1 Gft/sec) ‣ 1 ft/nanosecond
  • 5.
  • 6. Describing a Wave Amplitude Phase Wavelength Frequency: how many waves per second (e.g., 1,000Hz) Wavelength: speed of wave / wave frequency Phase: timing of waves within a wavelength
  • 7. Example: Audio • Waves through a physical medium: air • Audible: 20Hz to 20,000Hz • Speed of sound ≈ 1,100 ft/second ‣ Wavelength of piano’s lowest C (33Hz)? ‣ Wavelength of middle C (261Hz)? ‣ Wavelength of highest flute note (2100Hz)? Wavelength = speed of wave / wave frequency
  • 8. Light is Different • Not waves through a medium: pass through vacuum like empty space ‣ Michelson-Morley and the luminiferous aether • Waves in water and air are energy in the form of motion of matter • Electromagnetic radiation is “pure” energy: it has no mass ‣ Carried by photons