feat(rf_emulator): documentation

radio/emulator/README.md

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+# RF emulator library
+
+The RF emulator is a simple network stack terminator on the RF side.  
+It is meant to run the network stack without real radio and without RF simulation.
+It simply drops TX signal and generate noise as an RX signal.
+It can be used in `phy-test` mode to simply benchmark the network stack in a determined scenario.  
+It synchronizes the network stack on the real time clock of the host to guarantee a realistic behavior of the network stack.
+It detects and report late samples reads and writes to assess the real time behavior of the network stack.
+
+## Building
+
+`librf_emulator.so` is built by default by the `build_oai.sh` script.  
+It is then located in the root build directory (usually `cmake_targets/ran_build/build`).
+
+## Usage
+
+### Enabling `rf_emulator`
+
+Use the option `--device.name rf_emulator` of the softmodem to use the RF emulator.
+
+### Options
+
+Two options are available to configure the RF emulator:
+* `--rf_emulator.enable_noise <enable>` Enables (enable = 1) or disables (enable = 0) noise generation on RX. By default noise generation is enabled.
+* `--rf_emulator.noise_level_dBFS <noise_level>` Sets the level of noise generated in dB. If omitted no noise is generated.