RSSI

Take a look on the internet for calculators for the "Friis" or "link" equasion. It is valid in the far field, which with lower antenna gain is as close as one wavelength to the antenna.

You will need all of the following variables except one, to calculate the one you are looking for: antenna gains, transmit power, distance, and receiver input signal strength, usually in dBm referred to 50 ohms.

Due to thermally induced noise, idealized receiver noise floor at 1 Hz bandwidth is -174 dBm at room temperature. It is -114 dBm at 1 MHz, or -144 dBm at 1 KHz. Noise floor varies with bandwidth in first order fashion. Assuming you know the bandwidth of the receiver, you will then need to know its noise figure, which is to be subtracted from the idealized noise floor. Therefore, if you are working with 1 MHz bandwith and have a 10 dB receiver noise floor, the received signal will approach extinction at -104 dBm.

The simplest way to correlate RSSI data to receiver signal strength is by measurement, using a signal generator at the receiver antenna port. RSSI is dependent upon receiver design. The only psuedo-standard which comes to mind is the "S" meter used in ham radio equipment. You can make a table of RSSI voltage level vs. signal generator level. That coupled with Friis calculations will be an approximate answer to your question. At higher frequencies, obstacles tend to reduce range as compared with open field measurements.

Bernie Katz

berniek, thanks for your reply. Can you tell me if there are any papers already available describing such algorithms. I want to get to use more reference points to find the distance vs RSSI calculation.

Also how can I find a more accurate RSSI value as with each measurement it will be varying?