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    <td style="border-collapse: collapse; border: none;">
      <a href="http://www.openairinterface.org/">
         <img src="./images/oai_final_logo.png" alt="" border=3 height=50 width=150>
         </img>
      </a>
    </td>
    <td style="border-collapse: collapse; border: none; vertical-align: center;">
      <b><font size = "5">Open Air LTE Emulation</font></b>
    </td>
  </tr>
</table>

**Note: unless you know what you are doing, you likely do not need this!
Rather, you are probably looking for the
[RFsimulator](../radio/rfsimulator/README.md)!**

oaisim has been scraped and replaced by the same programs that are used for the
real-time operation, `lte-softmodem` and `lte-uesoftmodem`. This uses the IF4p5
fronthaul protocol to achieve the communication.

Context: oaisim used to be a simulation mode inside OAI to emulate an eNB and
multiple UEs.

[[_TOC_]]

# Build

Build eNB/UE as normal, as also described in [How to build the eNB and the UE](./BUILD.md):
```bash
./build_oai -c --ninja --eNB --UE
```

# How to run an eNB with the noS1 option

The following paragraph(s) explains how to run the L1 simulator in noS1 mode and using the oai kernel modules.

Modify the configuration file for IF4p5 fronthaul, `/openairinterface5g/ci-scripts/conf_files/rcc.band7.nos1.simulator.conf`, and replace the loopback interface with a physical ethernet interface and the IP addresses to work on your network. Copy your modifications to a new file, let's call YYY.conf the resulting configuration file.

Run lte-softmodem as usual with this configuration. 

```bash
$ source oaienv
$ cd cmake_targets/tools
$ sudo -E ./init_nas_nos1 eNB
$ cd ../ran_build/build
$ sudo -E ./lte-softmodem -O YYY.conf --noS1
```

# How to run a UE with the noS1 option

Similarly modify the example configuration file in `/openairinterface5g/ci-scripts/conf_files/rru.band7.nos1.simulator.conf` and replace loopback interface and IP addresses. Copy your modifications to a new file, let's call XXX.conf the resulting configuration file.

Run it like:

```bash
$ source oaienv
$ cd cmake_targets/tools
$ sudo -E ./init_nas_nos1 UE
$ cd ../ran_build/build
$ sudo ./lte-uesoftmodem -O XXX.conf -r 25 --siml1 --noS1
```

That should give you equivalent functionality to what you had with oaisim including noise and RF channel emulation (path loss / fading, etc.). You should also be able to run multiple UEs. 

# How to ping an eNB from a UE and vice versa (with the noS1 option)

Once your eNB and UE (built with the noS1 option) are running and synchronised, you can ping the eNB from the UE with the following command:

```bash
ping -I oai0 -c 20 $eNB_ip_addr
```
where `$eNB_ip_addr` is the IP address of your eNB.

Similarly, you can ping the UE from the eNB.

The IP adresses of the eNB and the UE are set up by the `init_nas_nos1` program and should have the following values:
* `$eNB_ip_addr` set to 10.0.1.1
* `ue_ip_addr` set to 10.0.1.2
