Role of Central's Tx Power in connection with Peripheral

RE: Role of Central's Tx Power in connection with Peripheral

endnode — Wed, 16 Aug 2017 11:01:38 GMT

And better stop thinking about receivers in the same scale as Tx Power: they are having only "sensitivity" limit which is lower threshold where they stop being able to recognize signal from noise (typical -80 to -100dBm for BLE receivers). So as long as the receiver recognize the packet it will also measure some relative signal strength and that's combination of all the topology around (= how strongly transmitter shouts, what was around when waves traveled, how antenna and other stages before receiving circuit in SoC look etc.) Thinking about receivers as 0dBm or other fixed value doesn't help to understanding this picture, at least not me...

RE: Role of Central's Tx Power in connection with Peripheral

endnode — Wed, 16 Aug 2017 10:57:28 GMT

Yes indeed, if you put more power to transmitting then your fixed (not only Android but ANY device) will see higher RSSI. If you think that RSSI means distance then your device will be (by mistake) thinking the device is close but it only radiates stronger.

RE: Role of Central's Tx Power in connection with Peripheral

Abe — Wed, 16 Aug 2017 10:48:29 GMT

@endnode thank you for the explanation. However can you shed more light on the following. An Android device(the Central) would have a fixed sensitivity for the receiver(say 0dB), so would it make a difference if we have the peripheral's Tx power set at 4dB instead of 0dB, since the central cannot exceed 0dB ?

RE: Role of Central's Tx Power in connection with Peripheral

endnode — Wed, 16 Aug 2017 10:15:50 GMT

Note that most of low power radio chips (including all nRF5x) are having pretty much "fixed" sensitivity of receiver. So there is nothing like "Tx Power" or "RSSI threshold" on scanner's receiving HW stage. They simply either register and decode the packet through tuned antenna or not. On the other side transmitting stage have typically some wider or narrow possibility to tune the HW stage and emit more or less. So there you can do trade-offs how much power you want to invest into your "broadcasting" vs. what theoretical range of sight you need. But note that it's always combination of transmitter and receiver so you will have observer/central devices which will register some packets and other devices which will miss it when being side by side.

Also note that when establishing connection you have two transmitter/receiver pairs and they can behave differently so in extreme cases (when your radio topology makes your signal close to low thresholds of receivers) you can see that packets from one side to another are OK while the other way are being "lost". So then you start to care about Tx Power on Central side (because that's used during transmitting part of connection interval from Master's point of view).