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contiki-ng / cooja

This is a fork of the Cooja network simulator from contiki-os/contiki
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LogisticLoss: increasing path loss exponent improves radio performance #54

Closed jeppenodgaard closed 3 years ago

jeppenodgaard commented 3 years ago

I'm using the LogisticLoss radio medium and it looks like the radio performance improves when I increase the path_loss_exponent in the .csc file. I would expect the radio performance to decrease based on https://en.wikipedia.org/wiki/Path_loss#Loss_exponent. Am I missing something?

To reproduce: When I use the default value, 3, and click mote 1 the radio environment value on mote 2 varies a lot e.g. 18.5% or 90.2%. When I use the value 5, and click mote 1 the radio environment value on mote 2 is almost always 99.x%. I used this simulation (used with contiki-ng in path tests/14-rpl-lite) 11-rpl-logisticloss.zip

atiselsts commented 3 years ago

Hello @jeppenodgaard, I'm the author of the LogisticLoss model.

While I'm not an expert in the radio models, I believe the log-distance path loss is modeled correctly in the code. See the formula in: https://en.wikipedia.org/wiki/Log-distance_path_loss_model#The_model

When d < d_0, the logarithm becomes negative. Therefore, the bigger the exponent, the smaller the path loss.

It may be best to think of the exponent as a parameter that controls the "sharpness" of the signal drop near the edge of the reception distance. A sharper drop (larger exponent) means that all positions inside the reception distance receive a stronger signal.

jeppenodgaard commented 3 years ago

Hi @atiselsts, Thank you for clarifying. The text "where 2 is for propagation in free space, 4 is for relatively lossy environments" in the link I gave confuses me. Just to be sure I understand the exponent correctly you mean the "sharpness" of the 2nd graph?

atiselsts commented 3 years ago

No, the path loss modeling is relevant to the RSSI graph. not to the packet reception rate directly, Perhaps "sharpness" is not a good word to use here, as in all cases the approach to the Rx sensitivity is quite gradual. It's simply that this approach will be slightly less gradual with an increased path loss exponent, which will look like the average RSSI value in the first graph being higher. (The value at the maximal tx distance is -100 dBm, i.e. the receiver sensitivity, in all cases, irrespective of path loss exponent).

jeppenodgaard commented 3 years ago

Alright. I'll go ahead and close this issue.

Thank you for taking the time to explain this!