Get advertising rssi from target channel(37 or 38 or 39)

[LeDucTai]

Hi,

I have some problem when scan advertising packet using nrf52840dk with ncs v2.5.0.

1. when I scan during 30s in the same position, I got a list rssi value same that:

   timestamp	rssi
   0.3	-40
   0.4	-40
   0.6	-40
   0.8	-40
   1.2	-57
   1.4	-57
   1.5	-56
   1.7	-57
   1.8	-57

   I think [0->0.8] is the advertising packet from same channel and [1.2->1.8] is the other one. I think in scan mode have a  mechanism to switch between channels. It is right?
2. when i scan until there is an advertising package then turn off scan and turn on advertising and turn on scan again alternately in the same position, I got a list rssi value same that:

   timestamp	rssi
   0.3	-40
   0.4	-50
   0.6	-47
   0.8	-40
   1.2	-50
   1.4	-57
   1.5	-46
   1.7	-40
   1.8	-51

   I think there is an alternating reception of advertising between channels on the antenna. Does it mean that every time the scan is turned off, the next scan listening packet will be received from a different channel in order, for example 37->38->39?

i have not found any specific technical documentation about this part, but from my experiment i found that there is a cyclic repetition of rssi. thanks for listening to my question and i would be very grateful if you can give me an exact answer to my problem.

[Edvin]

Hello,

I think in scan mode have a  mechanism to switch between channels. It is right?

That is correct. The scanner will scan for one scan window on one channel, then wait until one scan interval has passed, and then start scanning on the next channel for the next scan window. And as you say, the channels are 37->38->39.

i have not found any specific technical documentation about this part,

This is not decided by HW, but it is implemented in the SoftDevice controller (BLE stack). Exactly how it is implemented I am not sure. I assume it will continue where it left off. 

That being said, there can be many reasons why you are seeing difference in RSSI. It can be radio noise from other surrounding devices. That is the idea of the channel hopping in BLE in general. In case one channel is completely blocked by noise, you still have others to use.

Best regards,

Edvin