8. Pathloss

 

8. Pathloss



17.12.2018

Pathloss – Video: Register for Course


Pathloss – Transcript:

Pathloss

  • The other effect is the signal attenuation. The signal power is reduced along the way when travelling from transmitter to receiver
  • The main two factors impacting the attenuation is the distance and the frequency
  • Shorter waves (high frequencies) are more attenuated than longer

Frequency Loss

  • This shows free space pathloss (attenuation) at 100m in the radio frequency range.
  • In the very high freqs on top of free space loss we have a water and oxygen absorption (for mmWaves)
  • The left lower figure is a selected frequencies that are used in WiFi – 2.4, 5.8 and 60GHz, you can see that 5.8GHz vs 2.4GHz is about 10dB difference and between 5.8 and 60GHz it is around 20dB difference (remember 100x increase in linear scale is 20dB in logarithmic)

Indoor Pathloss

  • The above figures and formulas is free space loss, but in reality we have a slightly more complicated behawior of the radio waves.
  • We have chosen as a baseline a model from ITU for indor, that also has frequency and distance factors, but there are also two other factors – one depending on the environment – and we show a office area and another is depending on the number of floors in buildings – so you can see that depending on the number of floors that the signal has to penetrate, it will be more impacted
  • So taking those into account we can take look on the figure/curves showing the difference between free space loss and indor model application for the propagation – and we can see that the indor propagation is about 6dB „worse” for 50m. This shows that it is important to have a proper method for calculation of the pathloss as it impacts much the answer for the question – where will the signal be still reachable.

https://en.wikipedia.org/wiki/ITU_model_for_indoor_attenuation, seen as a baseline

Received Signal Power

  • We’ve spoken already about the pathloss, but what it impacts is the received signal power
  • So the formula that enables us to get the information on how much power will we „see at the receiver”
  • The received power is increased by the tx & rx antenna gain and decreased by the pathloss

Received Signal Example

  • Here we have a wifi example at 5.8GHz and the distance of 50m
  • The into account the 50m outdoor example and 23dBm tx power and 6dBi of tx antenna gain taken from cisco datasheet, we get a -52,7dBm
  • The pathloss curves can be flipped to the received signal power lelvel taking those parameters into account. So we can see that at at 2.4GHz we get -45dBm, at 5.8GHz à -52.7 and at 60GHz à -72dBm.

 

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