nRF51822 beacon kit: big notches in RSSI vs distance plot

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

endnode — Wed, 07 Jun 2017 09:47:09 GMT

Btw. I've found one older study (Thesis) showing how doomed RSSI is for indoor location positioning unless you invest years and millions of dollars in effort and SW to it.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

classicman — Fri, 05 May 2017 14:05:12 GMT

I measured the RSSI at different distances, one at a time, then plotted it using Matlab.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

Usman Rashid — Fri, 05 May 2017 06:03:00 GMT

The NRF connect app just give RSSI values so how did you plot RSSI vs distance ?

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

classicman — Tue, 24 Jan 2017 20:47:24 GMT

Thank you all for your insight (and useful links).

The app would not be sold, it's just a University project, so at least that is not an issue.

Yes, I do have more than one beacon available, in fact I was planning to use six for a single room.

May I ask @RogerClark, what problems did you meet when trying to interface with accelorometers? Because that's something my professor mentioned, using accelerometers in aid of the RSSI.

Thanks.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

Roger Clark — Mon, 23 Jan 2017 21:14:52 GMT

Basically RSSI is never going to give you an accurate distance.

If you have lots of beacons and know their exact topology, you could mathematically process the RSSI data, give you a more accurate position.

However I think that the generic maths behind such an algorithm is probably something to ask on another forum.

BTW, Like @endnote, I have also investigated indoor positioning and came to the same conclusions. i.e Using RSSI will not give you an accurate position. I also investigated the use of accelerometers and gyros and magnetometer data to augment the RSSI but it introduces another set of problems.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

endnode — Mon, 23 Jan 2017 13:25:50 GMT

I believe this is domain of Location Service (or RTLS/PLS) solutions which are already explored and offered by several companies for several years. I'm afraid you can spend years of testing and researching in the lab to arrive to pretty much equal conclusions as all these people... also note that there are several patents in this field so whatever smart you want to use you are in danger of IP litigation (if you want to sell it as product or service on the market). Sorry that I'm not helping much but this is conclusion of our 6-12 month side-project in this field (we did it in 2015 already).

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

Sigurd — Mon, 23 Jan 2017 13:19:55 GMT

You can find a lot of resources on this topic online. A common approach seems to be to apply a Kalman filter.

wouterbulten.nl/.../

www.youtube.com/watch

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

classicman — Mon, 23 Jan 2017 12:58:16 GMT

Thank you both for your replies.

While the data is not raw since the nRF Connect app itself plots the average value of the RSSI over different packets received, I do agree that more sophisticated filtering along with additional information might be of great help to calculate the position of the smartphone.

I guess I still need to figure out how to proceed.

But first, since I was asked for a notch-free plot, I'd like to remove those. Do you think the graph could benefit from taking measures in an open field (like, an actual field, completely free of obstacles in every direction) or the reflections of the ground will always be a problem, and I should focus my efforts towards the filtering only?

Thank you all again.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

endnode — Sun, 22 Jan 2017 19:56:57 GMT

Hi Classicman,

As Roger and Daniel pointed out, scalar RSSI is pretty much unusable for distance measurement in real environment unless you will start to do thing such as correlation of measurements at multiple points, filtering, fingerprinting of particular topology (= calibration of your algorithms to known properties of given room/corridor/building) etc. You can find many questions and answers which are pointing to this known property of 2.4GHz radio on this forum.

You can still use RSSI as "rough" indication of relative distance or movement direction but you need to understand that even thing like "I have now mobile phone in back pocket of my jeans" can change the value by 10 or 20dBm and you can only guess if the source suddenly moved 10 meters away or if some shielding/reflection happened.

Cheers Jan

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

Daniel Wang — Sun, 22 Jan 2017 19:35:50 GMT

Is this raw RSSI data? If you plan to do anything useful with the RSSI you need to apply some filtering methods to avoid notches due to reflections, multi-paths, etc

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

classicman — Sun, 22 Jan 2017 19:31:45 GMT

Hi, thanks for your reply.

Both the beacon and smartphone were placed on a stand about 0.80 m high. Unfortunately it didn't come to mind that measuring at different heights would be useful. I guess I could try that.

I know measuring distance using RSSI might be far fetched (in fact, I posted another question regarding the radiation pattern of the beacon to try and rule that out), but that's what I'm asked to do in my project and anyway it's not supposed to be super-accurate, it's more like a demostration.

Thanks again.

RE: nRF51822 beacon kit: big notches in RSSI vs distance plot

Roger Clark — Fri, 20 Jan 2017 20:13:43 GMT

Very interesting data.

I suspect the dips and peaks in the RSSI may be caused by reflections.

When measuring outdoors, what height did you have the beacon?

Did you try varying the height of both the beacon and the receiver ( phone )

In general RSSI is not a reliable indication of distance, as there are loads of different factors that will effect RSSI, not just distance. Including orientation of the antenna in both the Tx and Rx devices, as well as reflections off various types of surfaces at various amplitudes, and absorption of walls and any other material between the Tx and Rx.