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RSSI variation with Bluetooth LE Advertisement
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RSSI variation with Bluetooth LE Advertisement

by kevinjwalters on Wed May 20, 2020 4:58 pm

If I run this code on a Circuit Playground Bluefruit

Code: Select all | TOGGLE FULL SIZE
Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit Circuit Playground Bluefruit with nRF52840
>>>
>>> from adafruit_ble import BLERadio
>>> from adafruit_ble.advertising.adafruit import AdafruitColor
>>> ca = AdafruitColor()
>>> ca.color = 0xbadb1e
>>> ble = BLERadio ()
>>> ble.start_advertising(ca, interval=0.08001)


And running a scanning app on an Android Tablet 60cm away I see this:

cpb-advertising-androidtabletrssi-fivesecondvariation-1.jpg
RSSI from nearby CPB sending Advertisement packets
cpb-advertising-androidtabletrssi-fivesecondvariation-1.jpg (149.21 KiB) Viewed 70 times


There are three clear values (-67.5, -82.5, -70.5) which change every 5 seconds. I had assumed these related to the three channels that are for advertising, 37, 38 and 39 but I've also read that devices advertise in a very rapid burst across all three channels for each Advertisement sent? The graph suggests the nRF52840 using CircuitPython is not doing this. Does anyone know more about this?

This might be the same discussion as Silcon Labs: Bluetooth Wireless Forum: Scanning vs Advertising with BLE

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Re: RSSI variation with Bluetooth LE Advertisement

by DarTheMadScientist on Wed May 20, 2020 5:40 pm

Yes, those are channels. I'll find a few links.

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Re: RSSI variation with Bluetooth LE Advertisement

by DarTheMadScientist on Wed May 20, 2020 6:12 pm

The first reference is behind an IEEE pay wall, but I found a link to the author's university. This has graphs similar to yours.
Assessing the Impact of Multi-Channel BLE Beacons on Fingerprint-based Positioning
Powar, Jovan & Gao, Chao & Harle, Robert (2017)

An Analysis of the Accuracy of Bluetooth Low Energy for Indoor Positioning Applications
Faragher, Ramsey & Harle, Robert

Ramsey Faragher is on the Covid Watch team. He recommends taking the maximum over a short duration.

My thinking is more general, taking an aggregation such as the mean (the dB of the mean of the un-dB). However, those are not far apart and those would be even closer if the aggregation is, say, the mean of power of the top 10%.

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Re: RSSI variation with Bluetooth LE Advertisement

by kevinjwalters on Wed Jun 24, 2020 4:05 pm

I made a video of an Android tablet BLE scanner which shows RSSI for 2 Adafruit CLUEs and 2 CPB on my desk. This was mainly to show how they rather strangely synchronise their channel switching for advertising but it also demonstrates how much they can differ by both from each other and across the three channels despite being same distance from the tablet which was about 70cm but with an obstacle to take out the direct line of sight: RSSI monitoring of 4 nRF52840 Adafruit boards starting to transmit BLE advertisements (YouTube). Variations at 1:39 are just me moving a tiny bit nearby, no straight lines if I fidget!

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Re: RSSI variation with Bluetooth LE Advertisement

by kevinjwalters on Tue Jul 14, 2020 1:12 pm

This is now less mysterious, the appearance of synchronisation is actually the receiver (tablet) changing channel rather than the nRF52840 as these advertise in short sequential blasts across all three channels.

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Please be positive and constructive with your questions and comments.