A BLE disconnection issue occurred randomly after the device had been functioning without any problems for many hours

[Tai]

Hi everyone,

We have devices using the BLE module (nRF52) deployed in clinics for testing. They have been functioning without issues until recently when we noticed occasional BLE disconnections. The following image shows data from one of the affected devices. Its RSSI value dropped significantly to -90 dBm, indicating weak BLE signal strength that could explain the disconnect. However, the device was positioned close to a tablet running the application connected to it, where the BLE signal strength should have remained strong. This inconsistency is what we find puzzling.

In another case, the RSSI value was within acceptable range (-68dBm), which couldn't lead to the BLE disconnection. 

We're thinking of if any issue with code optimization. Any suggestions to further investigate this issue would be greatly appreciated. Thanks!

[hmolesworth]

We had a similar issue with a disconnect due to RSSI when the sensor was supposed to be next to a tablet/gateway and it turned out to be a large body of water placed in close proximity to the tablet ie someone picked the tablet up and turned away from the sensor to better read the data and the human body blocked the 2.4GHz signal pretty well. 2nd case looks odd though unless a local microwave got switched on .. SDK or nRFConnect?

[Tai]

Thanks for the reply, hmolesworth ! We're using the latest SDK 17.1.0.

Since deploying it in the clinics, people being around the device might be inevitable. Moreover, some other equipment that have interference might be introduced in the room as well. In this situation, it'd be great if you can suggest any solutions to protect the device from being disrupted by human body or other sources of interference! Thanks!

[hmolesworth]

Maybe post the settings you have; these can be tweaked to reduce disconnection likelihood by delaying as long as possible a disconnect due to loss of signal. Other options are coded PHY or increased Tx power, but the issue may also be with the tablet. Changing orientation or raising height of either device or tablet or both can help, a lot depends on multipath signals so room size and wall coatings.

[Tai]

The following is the setting I have. A patient on the bed which is around 2.5 feet from the floor. Our device is worn in the patient's foot. The tablet location is a subject of change, meaning it can be held by someone or put on a table. Some people walked between the device and the tablet. Some other equipment was also in the room as well.

[hmolesworth]

With that setup good quality antennas should work well, but for example a low-cost nRF52840 Dongle would not work well as that printed circuit antenna is not effective with personnel in close proximity. I prefer chip antennas. Here are a couple of discussions on maintaining connection stability; a lot depends on whether the streaming has to be live with no lag or you can accept some delay in updating (say) a scrolling live display. Also some tablets are better than others, and there are differences in what parameter ranges are allowed.

effect-of-connection-interval-on-connection-stability

throughput-and-long-range-demo

ble-connection-stability

There are lots on the devzone, these just happen to be a couple of links I have to hand

Edit: Orientation of the foot antenna matters; you might consider specifying a solid angle of best performance for the position of the tablet, or change the foot antenna alignment if that position happens to be inconvenient.