I would like to ask how small scale fading can affect a transmission with time. The particular system is an antenna transmitting an ASK modulated signal at 915MHz and with bandwidth between 200KHz to 500KHz.

The receiver is static (does not move) and is placed at a distance not greater than 12 meters.

The question is, if the communication consists of slots of time of milliseconds, does the fading change for each time-slot or only will change when the receiver moves?

Thank you for the help



1 Answer 1


Small scale fading results from a superposition of a large number of propagation paths between TX and RX, which can be constructive or desctructive depending on their phase offset. As the phase offset varies rapidly with the propagation distance, the phase offsets between different paths are likely to vary rapidly too, leading to a pseudo-random behaviour of the received amplitude. We refer to this as small scale fading.

Technically, this behavior requires some form of change to the propagation conditions. So again, technically, if you did an experiment in a closed chamber with noone inside and no movements, there would not be any fading but the amplitude would remain constant.

However, in reality, there is always some movement. Most of the propagation paths are due to reflections and reflecting objects can move on their own (think of wind, people, cars, etc.). So in practice, even if TX and RX are stationary, you should expect small-scale fading to be present. A way to reduce it is using directive antennas that put stronger emphasis on the line of sight component, if that is an option.

  • $\begingroup$ Thank you. The answer is clarifying. So in a real-world scenario, like a logistic portal (e.g., 4m width) where the receiver is somewhere in the middle of two transmitters at both sides of the portal, I should take into account a different random component of fading almost every time (or every x milliseconds), isn't it?. $\endgroup$ Nov 22, 2019 at 12:00
  • $\begingroup$ Yes, I'd highly advise you to do so. Logistic applications typically have a lot to do with moving things around. ;-) $\endgroup$
    – Florian
    Nov 22, 2019 at 13:27

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