RF Link Budget Calculation Or RF Receive Power calculation

RF Link Budget Calculation is the calculation of the receive RF power at the Receiver based on the transmitter RF power, RF cable losses, Tx Antenna Gain, Free Space Loss(FSL) Rx Cable Loss, Rx Antenna Gain

So the equation is

Receive Power (dBm)  = Tx Power (dBm)  - RF Cable Loss(dB) + Tx Antenna Gain(dB)  - Free Space Loss(dB) - Rx Cable Loss(dB) + Rx Antenna Gain (dB)

The calculation of the Free Space Loss

With Kilo Meter as the unit for distance
Free Space Loss(FSL) dB = 32.44 + ( 20 * Log(Frequency in MHz) ) + ( 20 * Log(Distance_In_KM) )

With Miles as the unit for distance
Free Space Loss(FSL) dB = 36.6 + ( 20 * Log(Frequency_In_MHz) ) + ( 20 * Log(Distance_In_Miles) )

Error Vector Magnitude (EVM) db to percentage calculation

For converting  Error Vector Magnitude (EVM) from db to %(percentage), we can use the below formula

EVM in % = 100 * (10^(EVM in dB/20))


Example:

EVM = -18 dB

EVM % = [100 * 10^(-18/20)] %
EVM % = [100 * 0.125892] %
EVM % = 12.589 %

Difference between RSSI and RSS or RSS vs RSSI

Lot of people mix the term RSSI and RSS in the RF industry, the  Field engineers, developers, testers.

RSSI - Received Signal Strength Indicator
RSS  - Received Signal Strength

RSSI is an indicator and RSS is the real value. Ok, now what do you mean by indicator, indicator mean it can be a relative value and RSSI is  always a positive value and there is no unit for the RSSI.

We can say RSSI is for common man to understand. RF values are always told in dBm and the values are negative values most of the time. To make it easy for the people to understand these negative values are converted to positive values through scaling.

Say for example, if the maximum signal strength is 0dBm and minimum is -100dBm. we can scale it like as explained. We can put 0 dBm and more (RSS) as 100 RSSI(ie maximum RSSI) and -100  dBm(or less) as 0 RSSI(minimum RSS).

RSS is the actual signal strength received at the receiver and the unit of measurement can be in dBm, dB, milli Watt, Watt. So RSS will always have a unit.