RSSI accuracy

RE: RSSI accuracy

Stanislav Silnitskiy — Wed, 10 Feb 2016 20:40:25 GMT

As a sidenote, I have implemented the feature with NRF_RADIO->MODE = RADIO_MODE_MODE_Nrf_2Mbit << RADIO_MODE_MODE_Pos and got the following observation: For the packet with payload 120 bytes long I got 39 RSSI samples. If my calculations correct, every RSSI sampling iteration consumes 12+ us, that corresponds to ref. manual of nrf51 (8.8 us as minimum for RSSI sampling). This result allowed to calculate mean variance of received packet's RSSI and I was pleased to see live float calculation of a such for EVERY packet (it must be pretty tough for float point emulation, I suppose). Experiment was done with four identical beacons, emitting 120 byte packets every 10 ms. No idea, either it could be described as a high load, I noticed something about 5% packets with CRC errors. For completeness: 2500 MHz frequency was used.

RE: RSSI accuracy

Stanislav Silnitskiy — Tue, 09 Feb 2016 11:37:41 GMT

Thank you, Kenneth! I'll try to implement the feature.

RE: RSSI accuracy

Kenneth — Tue, 09 Feb 2016 11:35:57 GMT

I guess it depends on the packet, but presuming an empty packet in BLE, that will be 40bit (40us) including 3byte CRC after the address match. So maybe you can do 2-3 sampling in best case during that time.

RE: RSSI accuracy

Stanislav Silnitskiy — Tue, 09 Feb 2016 10:52:07 GMT

So, theoretically, how many samples it is possible to grab while receiving standard BLE packet?

RE: RSSI accuracy

Kenneth — Tue, 09 Feb 2016 10:27:46 GMT

The length of time doesn't vary depending on packet lenght no, the radio simply take one single sample of the receive signal strength each time the RSSISTART task is triggered. You are free to trigger the task as frequently as you like, though you would like to wait for the RSSIEND event to read out the actual RSSISAMPLE before next RSSISTART task.

RE: RSSI accuracy

Stanislav Silnitskiy — Tue, 09 Feb 2016 09:28:01 GMT

Hi Kenneth! Yes, I saw it in docs and I like the concept of SHORTS. I wonder, is the length of time, when RSSI collected can vary depending on the packet length and is this appropriate field to play with, improving RSSI accuracy (overall, of course)? My vision is to receive more RSSI samples per second and filter them somehow. It is crucial in building positioning system, so every percent of improvement is important.

RE: RSSI accuracy

Kenneth — Tue, 09 Feb 2016 08:08:59 GMT

The RSSI measurement is done one time for each packet received, there is a radio short 'ADDRESS_RSSISTART' that can be enabled to do one RSSI sample after each radio address match.