Introduction:
In telecommunications, a received signal strength indicator (RSSI) is a measure of the power present in a received radio signal. RSSI of -74dBm (or 66 ASU) displayed on a smartphone. Also shown are signal bars for 2 cellular networks and signal bars for one Wi-Fi network.
RSSI is usually invisible to the user of a receiving device. However, because signal strength can vary widely and affect functionality in wireless networks, IEEE 802.11 devices often make measurement available to users. In zero-IF systems, it is tapped into the baseband signal chain, before the baseband amplifier. The RSSI output is often an analogue DC level. It can also be sampled by an internal analogue-to-digital converter (ADC) and the resulting codes are available directly or via an internal or peripheral processor bus.
Figure 1: Transmission of RSSI signal
Background:
A Received Signal Strength Indicator (RSSI) is defined on the antenna input connector, but is not fully specified, because there are no unit definitions and no performance requirements, such as accuracy or testability.
An RSSI might have limited use for evaluating access point (AP) options within a station, such as a wireless local area network (WLAN) station, and within a given PHY, but it is not useful for evaluations between Phys. An RSSI rescales between direct sequence spread spectrum (hereinafter DSSS) physical layers and orthogonal frequency division multiplexing (OFDM) physical layers.
For the load balancing or the transition of the transmission, RSSI is not usable by the network management. An RSSI from one station is not related to an RSSI from any other station. In high interference environments, an RSSI is not a suitable indicator of expected signal quality.
Functions and Features:
First, you have to understand the difference between a digital and an analogue RSSI. The RSSI is normally an analogue value (a voltage level). But this is not always the case, and could also exist as a PWM (Pulse Width Modulation) signal with a variable ratio depending on the strength of the signal. Therefore, there could be an incompatibility in the system when reading the RSSI (which is why the RSSI in the OSD of many people always marks 0.5 or 100). To solve this, we need to convert the PWM signal to a voltage value using a "Digital to Analog Converter", or simply a LowPass filter (Resistor-Capacitor filter).
Some OSD modules and flight controllers accept a direct connection to the RSSI PWM output because they have built-in LowPass filters.
Figure 2: Circuit for input and output
Calculation Criteria
The transmission power of the wireless router is generally 100 mW (or even higher). When the RSSI at the receiving end is -50dBm, the power at the receiving end is 0.01μW, and the difference in transmitting and receiving power is 10 million times; but in real situations, the RSSI only If it is greater than -50dBm, the signal is very good; once the RSSI is less than -75dBm, it may cause unstable transmission. The most ideal situation is that the RSSI is 0dBm, and the received power at this time is 1mW. However, in real situations, the interference received during transmission is relatively large, so the RSSI value is generally less than 0. We calculate it as;
dB: "It is a value that represents the relative value."
The voltage gain is expressed as: 
The current gain is expressed as: 
The power gain is expressed as: 
dBm: "It is a value that represents the absolute value of power, a ratio based on 1 mWatt of power."
The calculation formula is: 
dBw: "It is a value that represents the absolute value of power, a ratio based on 1 Watt of power."
The calculation formula is: 
For example:
Assuming that the power of (a) is twice that of b, then 10lg2 = 3dB, which means that the power of (a) is 3 dB higher than b; if the power of (a) is 46dBm and the power of b is 40dBm, then the power of (a) is greater than b by 6 dB.
