range to tx power conversion

[pramithkv]

Hi

I need to configure my nrf51 tx power so that it is discoverable only at a particular range/distance.

for example: i need my device to be discoverable only within 3m of device, so what should be the value of tx power i may use.

And also i would like to know whether Nordic provides any standard rssi values at 1m distance for various tx power

Thanks Pramith

[Asbjørn]

This is a very difficult and complicated question to answer as there's not one simple answer to it. However, the way to do this is to test range, output power and RSSI readings with your application and in scenarios where your application might be used. That way you would at least have some ballpark estimates on what range leads to which RSSI values when you are using a certain output power.

So there is no standard RSSI value for 1 meter. This will vary from application to application and also based on the environment that the application is in. Normally it is better to average RSSI over a few samples, this will give you a better estimate on what it actually is than just one reading.

For your question though, without knowing the details of your PCB and application, I would say that you could drop the output power down to -12 dBm and still get 3 meters of range. You can probably lower it even more than that, but this requires testing for optimizing.

It all comes down to testing with your specific application and implementation.

[Bouke]

Is the RSSI value calibrated? That is, if I use one sender, would two receivers (with identical hardware) both give the same RSSI value if they were placed in the same position?

[Asbjørn]

There's a specified variance in the nRF51 chip, but ideally, yes, they should give the same value. It's better if you average over a few samples, this makes it more accurate.