How dose RSSI and TXPOWER affect device work

[Rain_July]

As we known,RSSI means Received Signal Strength Indicator,TXPOWER decide the power of output port.How do the two factors combine affect the device work?

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That's extreme simplification of real world;) There are many more factors:

• Tx Power says what energy goes to chip's radio transmitting stage (circuit).
• Then it goes through characteristic of that circuit (stop thinking that setting in some SW registers of nRF5x chip certain TXPOWER value means you can measure exactly that signal strength on antenna PINs!), tuning of the antenna and antenna itself. Alternatively you can have some active components like Power Amplifier etc. This already makes the radiation physics complex and radio waves hardly left the thing!;)
• Then the biggest surprise: all around the antenna influences how 2.4GHz waves radiate! This starts with PCB, other components of the device, packaging, if there are any close objects like human body or walls and goes on with more distant objects significant to GHz radio like all metal... Already here you can solve only basic equations how the wave radiates from source in ideal topology so what signal strength (measured as RSSI) is at what distance. Some more complex topologies can be simulated (so computed indirectly) but in real life it's simply a mess!
• Finally you want to measure the signal? Then again all the properties of antenna, tuning and chip's HW receiving circuits make the game interesting. One receiver will tell you something and another something else, without precise lab calibration you can just guess what is the offset from "reality" at the antenna.

There are many scientific publications about sub-GHz or 2.4GHz radiation and influence of environment and antenna/chip so you can comfortably study it for the rest of your life. If you don't want to spend so much time then popular intro to the subject would be: from BLE RSSI you can guess how far (depending on circumstances you will be +-5 up to +-30m out of reality) the transmitter is or if it goes closer/further. For simple applications like "is my beacon 5 or 20 meters away" it is in 80% of cases OK. If you want anything better and more reliable then you are in front of several moth (or rather year) project.