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[[TOC]]

General

This is an RF simulator that allows to test OAI without an RF board. It replaces an actual RF board driver. In other words, towards the xNB/UE, it behaves like a "real" RF board, but it forwards samples between both ends instead of sending it over the air. It can simulate simple channels, such as AWGN, hence it simulates RF.

As much as possible, it works like an RF board, but not in real-time: It can run faster than real-time if there is enough CPU, or slower (it is CPU-bound instead of real-time RF sampling-bound).

Architecture

High-level flowchart of the RF Simulator, including the channel simulation feature:

flowchart TD
    subgraph ide1 [RX]
    RU[ru_thread]-->|FH southbound|FH[rx_rf]
    FH-->read
    read -->|apply channel model|rxAddInput
    end
    subgraph ide2 [RFSim initialization]
    A[load_lib] -->|load RFSIM lib| B[device_init]
    B[device_init] -.-> |set trx_read_func|read[rfsimulator_read]
    rxAddInput --> FH 
    B -->cfg[rfsimulator_readconfig]
    cfg -->|read RFSIM CL options|C{rfsimu_params}
    C -->|chanmod|loadchannel[load_channellist]
    C -->|saviq|E[saveIQfile]
    loadchannel-->|new SCM|scm[new_channel_desc_scm]
    end
Loading

Build

From build_oai script

The RF simulator is implemented as an OAI device and always built when you build the OAI eNB or the OAI UE.

Using the -w SIMU option it is possible to just re-build the RF simulator device.

Example:

./build_oai --UE --eNB --gNB --nrUE --ninja -w SIMU

Add the rfsimulator after initial build

After any regular build you can compile the device from the build directory:

cd <path to oai sources>/openairinterface5g/cmake_targets/ran_build/build
ninja rfsimulator

This is equivalent to using -w SIMU when running the build_oai script.

Usage

Overview

To use the RF simulator add the --rfsim option to the command line. By default the RF simulator device will try to connect to host 127.0.0.1, port 4043, which is usually the behavior for the UE. For the eNB/gNB, you either have to pass --rfsimulator.serveraddr server on the command line, or specify the corresponding section in the configuration file.

The RF simulator is using the configuration module, and its parameters are defined in a specific section called "rfsimulator". Add the following options to the command line in order to enable different RFSim features:

CL option usage default
--rfsimulator.serveraddr <addr> IPv4v6 address or DNS name to connect to, or server to behave as a IPv4v6 TCP server 127.0.0.1
--rfsimulator.serverport <port> port number to connect to or to listen on (eNB, which behaves as a tcp server) 4043
--rfsimulator.options list of comma separated run-time options, two are supported: chanmod, saviq all options disabled
--rfsimulator.options saviq store IQs to a file for future replay disabled
--rfsimulator.options chanmod enable the channel model disabled
--rfsimulator.IQfile <file> path to a file to store the IQ samples to (only with saviq) /tmp/rfsimulator.iqs
--rfsimulator.prop_delay simulated receive-path (gNB: UL, UE: DL) propagation delay in ms 0
--rfsimulator.wait_timeout wait timeout when no UE is connected 1

Please refer to this document SIMULATION/TOOLS/DOC/channel_simulation.md for information about using the RFSimulator options to run the simulator with a channel model.

4G case

For the eNB, use a valid configuration file setup for the USRP board tests and start the softmodem with the --rfsim and --rfsimulator.serveraddr server options.

sudo ./lte-softmodem -O <config file> --rfsim --rfsimulator.serveraddr server

Often, configuration files define the corresponding rfsimulator section, in which case you might omit --rfsimulator.serveraddr server. Example:

rfsimulator : {
  serveraddr = "server";
};

For the UE, it should be set to the IP address of the eNB. For instance, if the eNB runs on another host with IP 192.168.2.200, do

sudo ./lte-uesoftmodem -C 2685000000 -r 50 --rfsim --rfsimulator.serveraddr 192.168.2.200

For running on the same host, only --rfsim is necessary.

The UE and the eNB can be used as if the RF is real. The noS1 mode might be used as well with the RF simulator.

If you reach 'RA not active' on UE, be careful to generate a valid SIM.

$OPENAIR_DIR/cmake_targets/ran_build/build/conf2uedata -c $OPENAIR_DIR/openair3/NAS/TOOLS/ue_eurecom_test_sfr.conf -o .

5G case

Similarly as for 4G, first launch the gNB, here in an example for the phytest:

run gNB:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-LTE-EPC/CONF/gnb.band78.tm1.106PRB.usrpn300.conf --gNBs.[0].min_rxtxtime 6 --phy-test --rfsim --rfsimulator.serveraddr server

where --gNBs.[0].min_rxtxtime 6 is due to the UE not being able to handle shorter RX/TX times. As in the 4G case above, you can define an rfsimulator section in the config file.

and run UE:

sudo ./nr-uesoftmodem --rfsim --phy-test --rfsimulator.serveraddr <TARGET_GNB_IP_ADDRESS>

To run OAI RFSimulator for SA mode, gNB:

sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb.sa.band78.fr1.106PRB.usrpb210.conf --rfsim --rfsimulator.serveraddr server

and UE:

sudo ./nr-uesoftmodem --rfsim --rfsimulator.serveraddr <TARGET_GNB_IP_ADDRESS>

In the commands for the UE, TARGET_GNB_IP_ADDRESS can be 127.0.0.1 if both UE and gNB run on the same machine.

If necessary the user can provide a custom UICC configuration file to the UE with command line option -O ue.conf. In case of a multi-UE scenario, the user shall provide a different IMSI to each UE with the command line option --uicc0.imsi followed by the IMSI, e.g. --uicc0.imsi 001010000000001.

Notes:

  1. This starts the gNB and UE in the phy-test UP-only mode where the gNB is started as if a UE had already connected. See RUNMODEM.md for more details.
  2. <TARGET_GNB_IP_ADDRESS> should be the IP interface address of the remote host running the gNB executable, if the gNB and nrUE run on separate hosts, or be omitted if they are on the same host.
  3. To enable the noS1 mode, --noS1 option should be added to the command line, see again RUNMODEM.md.
  4. Information on operating the gNB/UE with a 5GC can be found here. here.

Store and replay

You can store emitted I/Q samples. If you set the option saviq, the simulator will write all the I/Q samples into this file. Then, you can replay with the executable replay_node.

First compile it like other binaries:

make replay_node

You can use this binary as I/Q data source to feed whatever UE or gNB with recorded I/Q samples.

The file format is successive blocks of a header followed by the I/Q array. If you have existing stored I/Q, you can adapt the tool replay_node to convert your format to the rfsimulator format.

The format intends to be compatible with the OAI store/replay feature on USRP.

How to use OAI RFSIM with a channel model

Please refer to this document channel_simulation.md to get familiar with channel simulation in RFSIM and to see the list of commands for real-time usage with telnet.

How to simulate a simple GEO satellite channel model

A simple channel model for satellites on a geostationary orbit (GEO) simulates simply one line-of-sight propagation channel.

The most basic version is to simply simulate a constant propagation delay, without any other effects.

In case of a transparent GEO satellite, the minumum one-way propagation delay (DL: gNB -> satellite -> UE, or UL: UE -> satellite -> gNB) is 238.74 ms.

So, additionally to other parameters, this parameter should be given when executing the gNB and the UE executables:

--rfsimulator.prop_delay 238.74

Note:

To successfully establish a connection with such a GEO satellite channel, both gNB and UE need to have the NTN support configured.

Caveats

There are issues in power control: txgain/rxgain setting is not supported.

How to improve performance

Most importantly, note that the RFsimulator is not designed to be as performant as possible, nor is it designed to run close to real-time. It might run faster or slower than realtime, depending on CPU, and by design, as this allows to stop the entire system for inspection, e.g., using a debugger.

In order to improve performance, you can modify the radio parameters of the gNB to reduce the amount of transported samples:

  • Use option -E for three-quarter sampling (also to be done on the UE-side!)
  • Prefer smaller cell bandwidths

A possible, unimplemented optimization would be to compress samples.

You can further tune your machine