The radio signal power depends on the transmitter power or the power supplied to the antenna. Keep in mind that the transmitter and the radiating element are connected through a feeder, which loses the power, so the total radiated power is slightly less than the input.
Equivalent isotropic-radiated power (EIRP)
In section 3. Antennas an isotropic antenna model and the antenna gain definition are described. The EIRP allows to estimate the power radiated in a particular direction, is equal to the product of the input power and the antenna gain. In case if values in dB are used, the product is replaced by the sum:
Note that radio signals affect the operation of other wireless systems, as well as human health, therefore the permissible radiated power is regulated by the state policy.
Signal-to-noise ratio (SNR)
On the receiving side, the antenna, in addition to the useful signal, receives noises. The greatest contribution is made by thermal noise, the radiation of which is related to heat exchange in the medium near the receiving antenna. The ratio of the signal powers to the total noise power at the receiving side is calculated by the formula:
Signal Interference + Noise Ratio (SINR)
In addition to environmental noise, the receiver can hear signals from other wireless communication systems, or other sources of signals, which will interfere with the useful signal. The SINR is used more often then SNR, is calculated by the formula:
The difference between SNR and SINR is shown in Figure 1:
Figure 1 Definition of SNR and SINR
Received signal strength indicator (RSSI)
RSSI is the total power of the received signal, takes into account the antenna gain, and is estimated after the frequency conversion, i.e. after transferring the signal to the low-frequency spectrum, but before amplification.
In case the link power is at a low level, errors in the transmission appears, some can be corrected by using corrective codes, as shown in 6. Analog and digital signals. Modulation. Reasons for the radio parameters deterioration may be weather conditions, the appearance of a source of interference etc. There are several diagnostic parameters such as the errors level, the repetitions level, etc., but the absolute value of the errors is not always indicative, so more often the errors rate is used: