Information Transmission Chapter 4, Channels OVE EDFORS ELECTRICAL - - PowerPoint PPT Presentation

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Information Transmission Chapter 4, Channels OVE EDFORS ELECTRICAL - - PowerPoint PPT Presentation

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Information Transmission Chapter 4, Channels OVE EDFORS ELECTRICAL AND INFORMATION TECHNOLOGY Learning outcomes After this lecture the student should understand the basic properties of wired channels, such as cables and optical

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Information Transmission Chapter 4, Channels

OVE EDFORS ELECTRICAL AND INFORMATION TECHNOLOGY

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Learning outcomes

  • After this lecture the student should

–

understand the basic properties of wired channels, such as cables and optical fibers,

–

know the basic properties of wireless channels, including propagation loss in free space and antenna gains,

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understand how noise enters the system and how it is characterized,

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understand the basic principles of how movements and multiple wireless propagation paths create Doppler effects and fading (variations in signal strength), and

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be familiar with the principle of the magnetic recording channel (for storing data).

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Wires, cables and fibers

  • Twisted pair
  • Standard telephone line

» Coaxial cable » Used for high frequency transmission » Shielded and controlled properties

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Model of a transmission line (wire)

Model of short (unit length) section of line:

  • resistive loss
  • inductance from wires
  • “short circuit” resistance
  • capacitance between wires

... Model of entire wireline unit length sections in series

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Wires, cables and fibers

  • Wires and cables have quite high attenuation
  • Where the propagation constant is given by
  • Sinus in – sinus out, but with an attenuation and phase

shift

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Attenuation of a wire pair (telephone line)

  • For longer wire

lengths the attenuation is huge at higher frequencies.

  • They are already in

place, so let’s use them…

Frequency [Hz] Received power [dB]

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Propagation in a fiber

Fibers have low attenuation (< 0.5 dB/km). Reflections inside the fiber lead to dispersion – the light pulse will Smear out in time.

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Radio Channels – Free space

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Free-space loss

d If we assume RX antenna to be isotropic: Attenuation between two isotropic antennas in free space is (free-space loss):

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Antenna gain

  • An antenna will collect its power from an effective area A.

The larger antenna the more power it will collect

  • Similarly, it will focus its transmit power in a certain

direction where the power density then will be higher

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Free-space loss, Friis’ law

Received power, with antenna gains GTX and GRX:

In free space, the received power decays with distance at a rate = 20 dB/decade

If we write the expression in dB ...

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Noise sources

The noise situation in a receiver depends on several noise sources

Analog circuits Detector Noise picked up by the antenna Thermal noise Output signal with requirement

  • n quality

Wanted signal

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Receiver noise: Noise sources (1)

The power spectral density of a noise source is usually given in one of the following ways: 1) Directly [W/Hz] 2) Noise temperature [Kelvin] The power N of the noise is also determined by the bandwidth Here k is Boltzmann’s constant (1.38x10-23 W/Hz) and TK is the is the temperature of the noise source in Kelvin.

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Distribution of the noise

  • The noise is most often assumed to have a Gaussian

distribution

  • With this distribution it is possible to calculate the

probability that a noise sample exceeds a certain level.

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Receiver noise: Noise sources (2)

Antenna example Noise temperature

  • f antenna 1600 K

Noise free antenna Na Model Multiply with bandwidth to get noise power Power spectral density of antenna noise is

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Multi-path propagation, Two waves

Wave 1 + Wave 2 Wave 2 Wave 1 At least in this case, we can see that the interference pattern changes on the wavelength scale.

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2013-01-21 Fredrik Tufvesson - ETIN10 17

Small-scale fading

Illustration of interference pattern from above Transmitter Reflector

Movement

Position

A B

A B Received power [log scale]

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Small-scale fading - Rayleigh fading

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Doppler shifts

Frequency of received signal: where the Doppler shift is

Receiving antenna moves with speed vr at an angle θ relative to the propagation direction

  • f the incoming wave, which

has frequency f0.

c

r

v  f f   

 

cos

r

v f c    

The maximal Doppler shift is

max

v f c  

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More than one incoming wave

Spectrum of received signal when a f0 Hz signal is transmitted. RX RX movement

Incoming waves from several directions (relative to movement or RX)

All waves of equal strength in this example, for simplicity.

1 1 2 2 3 3 4 4

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Magnetic recording

  • Store magnetic field with different orientation
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SUMMARY

  • Wires, cables and fibers

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Wirels and cables are LTI systems

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Bandwidth of wires and cables depend on length

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Coaxial cables can carry higher bandwidths than wires

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Fibers have low attenuation

  • Radio channels

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Free-space propagation

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Antenna gains

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Friis' law

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Noise properties and calculation

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Multi-path propagation: Fading and Doppler shifts

  • Magnetic recording

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Storing messages by changing magnetization of tape (Transmitting to another time)

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