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Signal propagation in Wireless Transmission

Dec, 17 2020, 02:30 pm [IST]
Signal propagation in Wireless Transmission

Signal propagation

Like wired networks, wireless connection networks also possess senders and receivers of signals. In wireless networks, this signal has no wire to define the direction of propagation, whereas sickness in wired networks only goes along the wire. As long as where is not disrupted or damaged, it typically shows the same features at each point.

  • Transmission range: Within a certain radius around the center transmission is possible, i.e., a receiver receives the signals with an error rate low enough to be able to communicate and it can also act as sender.
  • Detection range: Within a second radius around the sender transmission is possible, i.e., the transmitted power is big enough to vary from background noise. However, the error rate is too expensive to set communication.
  • Interference range: Within a third radius which is even larger, this sender may interfere with another transmission by adding to the background noise. A receiver will not be able to identify the signals, but the signals may disrupt other signals.

Path loss of radio signals

In free space radio signals originate as light it does, i.e., the following a straight line. If a straight line lives between a sender and a receiver it is named the line of sight (LOS). If nothing exists between the sender and the receiver, this signal still feels the free space loss. Received power Pr is proportional to 1/d2 with d beginning the distance between sender and receiver.

Additional signal propagation effects

Signal propagation in free space always follows a straight line like light does. But in real life,  we rarely have a line of sight between sender and receiver of radio signals. Mobile phones are typically used in big cities with skyscrapers, on mountains, inside buildings, while driving through an alley, etc.
 
An ultimate form of attenuation is blocking or shadowing of radio signals due to large Obstacles. The larger the frequency of a signal, the more it acts like right. Thus, even smaller obstacles like a simple wall, a truck on the street, or trees in an alley may block signals. If an object is big compared to the wavelength of the signal, this signal is reflected. The reflected signal is not as effective as the original, as objects can receive some of the signal power.

While shadowing and Reflection are caused by objects much larger than the wavelength of the signals, the following two effects exhibit the wave character of radio signals. If the size of an obstacle is in the order of the wavelength or less, then the wave can be scattered. An incoming signal is scattered into several weaker outgoing signals. Radio waves will be deflected at an edge and propagate in a different direction. The result of scattering and diffraction patterns with varying signal strength depending on the location of the receiver.

Multipath propagation

Together with the direct transmission from a sender to a receiver, called multipath propagation. Due to the measurable speed of light, signals going along various ways with various lengths reached at the receiver at various times. This effect is called a delay spread.

Effects of delay spread

  • The first effect is that a short impulse will be smeared out into a broader impulse, or rather into several weaker impulses. The impulse at the sender will result in three smaller impulses at the receiver.
  • On the sender side, both impulses are separated. At the receiver, both impulses interfere, i.e., the overlap in time. Now consider that each impulse should represent a symbol and that one or several symbols could represent a bit. The energy intended for one symbol now spills over to the adjacent symbol, an effect which is called intersymbol interference (ISI). ISI limits the bandwidth of a radio channel with multipath propagation. Due to this interference, the signals of different symbols can cancel Each Other leading to misinterpretations at their receiver and thus causing transmission errors.
  • One more effort is the long-term fading of the received signals. Typically, senders can compensate for long term fading by increasing or decreasing sending power so that the received signal always stays within certain limits.

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