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This question is regarding RSSI.
In particular, my understanding is that the RSSI is measured after reception at the antenna and after the ADC (analog to digital converter) and after the Low Noise Amplifier at the receiver.
When calculating RSSI, do we divide by the gain of the amplifier and the gain of the antenna?
lets take an example, if an Access Point sent a packet to a Station, the stations performs RSSI measurement, is it fed back to the AP, or what is used for ?
Can anyone help! Thanks!
The IEEE Std 802.11-2012 says in Table 20-1:
RSSI - The allowed values for the RSSI parameter are in the range from 0 to RSSI maximum. This parameter is a measure by the PHY of the power observed at the antennas used to receive the current PPDU. RSSI shall be measured during the reception of the PLCP preamble. In HT-mixed format, the reported RSSI shall be measured during the reception of the HT-LTFs. RSSI is intended to be used in a relative manner, and it shall be a monotonically increasing function of the received power.
While is is not required to have a precise relationship between physical measurements and RSSI magnitudes, my experience from experiments is that there is often a strong correlation between received power (dBm) and the RSSI. The manufacturers take care of the implementation details, they probably factor out details like amplifier gains. However some receivers just measure in-band power, so good RSSI does not imply a good signal-to-noise ratio.
RSSI is used for example for clear channel assessment (CCA) or the signal quality indicator you see in your OS, as mandated by IEEE 802.11. The manufacturers implement RSSI to be fit for these purposes, it should not be confused with a precise measurement. This indicator was never intended for such a purpose.
RSSI is arbitrarily defined by manufacturer. There is no defined standard for where or how to calculate it.
A simple Google search will provide you with very different ways.
I think the RSSI is the transmit power (dBm) divided by the channel gain (dB), which is calculated in the short OFDM preamble, so it is effectively the average power of the received preamble relative to 1mW, so you'll get some dBm minus some dB, to get RSSI (dBm). The gain involved may be more than just channel gain. When the receiver knows the channel gain, it can normalise i.e. divide the received signal (gs(t) + n(t)) with it, and you get s(t) + n(t)/g. If there are multiple transmit power settings, then it's probably compared to a base level used by the constellation. Similarly it would be the average power of a symbol received, which would be the area under the periodogram divided by the symbol length, or the area under V^2/R divided by the symbol length.
