I have a device in which a user connects their mobile device to an arduino uno via a HC-05 bluetooth transponder. I want to be able to calculate the approximate distance between the user and the transponder by hopefully using the RSSI methodology:
d = 10 ^ ((TxPower - RSSI) / 20)
My goal is for the user to send a connection request to the HC-05, the uno to then calculate the distance, and if greater than x meters away, reject the request. I realize RSSI based distance is an estimate at best but that is fine for my purposes.
With that in mind, I have two distinct but related questions:
1.) Is an HC-05 able to calculate the RSSI strength of a mobile device? (Iphone)
2.) Can it do so WITHOUT connecting first
Thank you!
The approximate path loss for a radio signal in free air is
43 + 20*log(number of wavelengths between rx and rx) in dB
433MHz has a wavelength of about 0,7 metres. 2.4GHz about 0.13 m.
In general wavelength is about 300/F(MHz) metres where F is the frequency.
Note you must factor in antenna gain at each end . This value is for 0dBi antennas at each end.
This is only true for 'far field' ie greater than 10 wavelengths.
so : convert your known tx level and path loss to rx signal level
Your rx will have a signal level to RSSI specification and a sensitivity specification in microvolts or dBm. The conversion factor is that 1uV into 50 ohms is -107dBm.
Professionals use dBm.
The signal will have to be considerably above the minimum sensitivity level for reliable performance - typically 20dB or 10 times the voltage.
Then starting with an RSSI you can work backwards from here to get ( very approximately ) the range.
in practice the measurement could be enormously affected by reflections and absorbtion , both depending on wavelength, and, particularly, within buildings. Variations of 20dB or more are not uncommon. that's 10 times in range.
regards
Allan.
That's why it's called a Received Signal Strength INDICATOR - it isn't enormously useful except for setting the gain of amplifiers in a receiver so as to get a constantish level to drive an a/d in digital receivers such as mobile phones and datalinks. Such PGA's usually have many 2 or 3dB gain steps.