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# USRP device documentation

## General

OAI works with most common USRP models, like B200, B200mini/B205mini, B210,
X310, N300, N310, N320, X410. This is achieved by using the Ettus Universal
Hardware Driver (UHD) (https://github.com/EttusResearch/uhd). The file
[`usrp_lib.cpp`](./usrp_lib.cpp) provides an abstraction layer of UHD to OAI.

Example files can be found in the `ci-scripts/conf_files/` directory with a
`usrp` in the name, for instance
[`gnb.sa.band78.106prb.usrpn310.ddsuu-2x2.conf`](../../ci-scripts/conf_files/gnb.sa.band78.106prb.usrpn310.ddsuu-2x2.conf).

## Configuration

The USRP can be configured in the RU section of the config file. The field
`sdr_addrs` uses the [USRP device identification
string](https://files.ettus.com/manual/page_identification.html). Here are a
few examples

```bash
sdr_addrs = "addr=192.168.10.2" # uses a single 10Gb Ethernet interface on an N3x0 or X3x0 or X4x0
sdr_addrs = "addr=192.168.10.2,second_addr=192.168.20.2" # uses 2 10Gb Ethernet interfaces on a N3x0 or X3x0 or X4x0 (requires that you flashed the FPGA wth the XG image)
sdr_addrs = "serial=ABC1234" # globally unique identifier of this device
```

you can also use the multi USRP feature and specify multiple USRPs, in which case you will get the aggregated number of channels on all the devices

```bash
sdr_addrs = "addr0=192.168.10.2,addr1=192.168.30.2"
```

You can specify to use external or interal clock or time source either by
adding the parameters in the `sdr_addrs` field or by using the fields
`clock_src` or `time_src`

```bash
sdr_addrs = "addr=192.168.10.2,clock_source=external,time_source=external"
```

is equivalent to

```bash
sdr_addrs = "addr=192.168.10.2"
clock_src = "external"
time_src  = "external"
```

Valid choices for clock and time source are `internal`, `external`, and `gpsdo`.

Note 1: the USRP remembers the choice of the clock source. If you want to make
sure it uses always the same, always specify the `clock_source` and
`time_source`.

Note 2: when using multiple USRPs they always have to be synchronized using
`external` or `gpsdo`

Last but not least you may specify that only a specfic subdevice of the USRP is
used. See also the section [Specifying the
Subdevice](https://files.ettus.com/manual/page_configuration.html#config_subdev)
in the UHD manual.

For example, on a USRP N310, the following fields will specify that you use
channel 0 of subdevice A. You can enter this information after providing the ip
addresses of the USRPs in `sdr_addrs`.

```bash
tx_subdev = "A:0"
rx_subdev = "A:0"
```
Here is an example of the RUs configuration for a setup with 2 USRP N310. 

```bash
RUs = (
{
  local_rf       = "yes"
  nb_tx          = 2
  nb_rx          = 2
  att_tx         = 0
  att_rx         = 0;
  bands          = [78];
  max_pdschReferenceSignalPower = -27;
  max_rxgain                    = 75;
  eNB_instances  = [0];
  sdr_addrs = "addr0=192.168.10.2,addr1=192.168.20.2,clock_source=external,time_source=external"
  tx_subdev ="A:0 B:0"
  rx_subdev ="A:1 B:1"
}
);
```

Hint: You cannot see TX/RX spread over multiple USRPs, if you use `internal` as a refrence of the clock and time source.

Furthur information about synchronization on the USRP N3xx devices can be found here: https://kb.ettus.com/Using_Ethernet-Based_Synchronization_on_the_USRP%E2%84%A2_N3xx_Devices

When combining this with the multi USRP feature you can create a distributed antenna array with only 1 channel used at each USRP.
