Bluetooth mesh oscillating RSSI

[richardjallen]

Hi,

I have been working with Bluetooth mesh. The test I have been performing repeatedly polls a single node and measures the RSSI (as measured from p_message->meta_data.rssi) and number of hops (as measured from p_message->meta_data.ttl) of the reply. I've noticed when a node is at a certain distance away from a receiver, recording the data, the RSSI output oscillates. This has been tested using the nRF52840 pdk as the receiving node, and the nRF52840 & nRF52832 as the polling node. All experimenting was done using nRF5 SDK for Mesh v0.10.0-alpha.

Attached are two graphs demonstrating the behaviour.

polling every 500 ms, it must be noted that the timestamp on this graph reset to 0 during the test, the oscillating behaviour is still clear

polling every 200 ms, at 200 ms the oscillating behaviour begins to look more like a sine wave

note: the green line displays the RSSI (y-axis on the left), the blue line displays the number of hops (y-axis on right), number of hops is not relevant to this question.

Is there an explanation for why the RSSI is behaving like this?

[Matt]

Signal interference, bouncing, multipath channel fading, ... etc. You are not expecting a clean, constant rssi value right?

[Sigurd]

Hi,

In addition to the things Matt mentioned, you will also have different RSSI value on different BLE channels(frequency) . You can find more information about this topic in this post here.

Quote:

The RSSI on each channel can also vary depending on your surroundings. A signal on a given frequency might e.g. travel more easily through a wall than a different frequency.

[richardjallen]

Thanks a lot, this is what I was looking for.

Richard