Consistent Rssi values

[mani]

Hi team,

  We are doing indoor navigation project using BLE  nordic beacons and we are using  ble_app_beacon as a  transmitter.

We are calculating the position of person using RSSI.

I stick the beacon to one wall and i monitoring the rssi values on mobile . Let us take 3 positions ->1 meter, ->5 meter, and ->10 meter.

We often found the same repeated RSSI values on 5 meters and 10 meters.it leads to pointing the navigator in mobile app is very tough. Is there any method to send or receive consistent Rssi values???

[Jørn]

Hello mani

Measuring distance using the RSSI values is not easy. First of all the nRF51822 (the chip mounted on the beacon kit) has a valid RSSI range of -50 to -80. If you're outside this the RSSI value can no longer be trusted.

Secondly the RSSI value is the measured received power at the receiver. How much power it receives is highly dependent on the quality of the electronics design and your environment, as the electromagnetic waves will be reflected and absorbed by obstacles and walls. The attenuation of your signal depends on distance between transmitter and receiver, obstacles between them, antenna positioning and environment, position of walls, material of walls, distance traveled by reflected signals (multipath propagation and fading), whether or not there are air ducts exposed, presence of fluorescent lights, presence of people, other wireless sources in the same frequency band etc.

For example, sticking the beacon to a drywall will give different results from sticking it to a concrete wall as they affect the antenna in different ways, and reflect different amounts of the electromagnetic wave. So to properly judge distance based on RSSI, you need to have a highly controlled environment.

Best regards

Jørn Frøysa

[Matthew K]

Hi, Jørn.

Jørn said:

nRF51822 (the chip mounted on the beacon kit) has a valid RSSI range of -50 to -80. If you're outside this the RSSI value can no longer be trusted.

1. Is there a reason for this? It does make sense for not trusting value under -80,

but what about values higher than -50?

I hope to know why I shouldn't trust values which are not in that range.

2. Also, what about the case of pairable peripherals?

For instance, if I flash the NUS (Nordic UART Service) peripheral example on a nRF51-DK and notice that the RSSI value of the advertising packet is out of that range,

does this mean that the board has an antenna issue

or the scanning central is in a noisy condition (too many Wi-Fi/BLE signals nearby)?

3. Lastly, does this apply to other BLE devices (nRF52 series, other vendors like Cypress/Dialog Semiconductor) as well?

[Jørn]

Hello Matthew K

 

I apologise for the late answer, for some reason I didn't get a notification of your comment until today.

 

1.

For the nRF51 series, values above -50 dBm should not be trusted. The loss of accuracy is due to non-linearities.

 

2.

Low RSSI could be caused by any number of things, RSSI is simply measured received power at receiver. The cause of reduced received power cannot be read from RSSI directly. It could be mismatch in RF front-end (matching network/antenna), fading due to multi-path, obstacles. I don't think a noisy environment would necessarily generate a low RSSI (but it would generate a low SNR), as it is related to received power, and not the quality of the signal.

If you experience any problems with a custom board, testing the same setup with a reference device, such as a DK, can give an indication on whether it is a hardware issue.

 

3.

The RSSI range is defined as -50 to -80 dBm for the nRF51 series, and -90 to -20 dBm for the nRF52 series. I cannot speak for the accuracy of RSSI measurements in products from other manufacturers. It depends on their implementation of the measurement, I therefore recommend you always check the product specification for the specific product.

 

Best regards

 

Jørn Frøysa

[Matthew K]

I deeply appreciate your detailed answer, Jørn.

One last thing;

Jørn said:

The loss of accuracy is due to non-linearities.

May I ask about this non-linearity? I tried to find this from the nRF product specification, but I couldn't find it.

Or can I get a link about this?

-Thanks!

[Jørn]

Happy to help!

The non-linearity is not described in the documentation. It is related to the method used to measure the RSSI. Unfortunately, I am not at liberty to disclose more details surrounding it.

Best regards

Jørn Frøysa