Skip to content

README.md

Latest commit

 

History

History
153 lines (116 loc) · 5.76 KB

README.md

File metadata and controls

153 lines (116 loc) · 5.76 KB

[[TOC]]

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 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.

Build

The OAI USRP driver in leverages the USRP Hardware Driver to interface with a USRP. build_oai has support for installing UHD from package manager (Ubuntu) or from source (other distributions or on request). See BUILD.md for more information.

Note that OAI comes with a patch for UHD as found in cmake_targets/tools/. This patch is automatically applied when building UHD from source with build_oai, and improves the TX/RX switching times of UHD. If you cannot apply this patch, the --continuous-tx option can be an alternative as described further below.

Configuration

The USRP can be configured in the RU section of the config file. The field sdr_addrs uses the USRP device identification string. Here are a few examples

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

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

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

is equivalent to

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 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.

tx_subdev = "A:0"
rx_subdev = "A:0"

Here is an example of the RUs configuration for a setup with 2 USRP N310.

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 in the knowledge base article "Using Ethernet-Based Synchronization".

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

Further considerations

TX/RX switching times

When using TDD (e.g., in 5G/NR band n78), the USRP has to frequently switch between TX and RX directions. Certain USRPs, e.g., B210, might be too slow for this, which can result in degraded radio link performance, especially in 5G/NR operation. In order to get a good performance, there are two workarounds:

  1. At runtime: you can use the --continuous-tx options at gNB/nrUE. This will continuously send TX samples, also when the current link direction is to receive, and we found this to improve the radio link.
  2. At compile/build time: apply the UHD patch described further above, which obliveates the need for --continuous-tx.

Three-quarter sampling

Not all sampling rates can be used with (a) given master sampling rate(s) of a USRP. Depending on the bandwidth, you might employ (or not) three-quarter sampling using option -E. As a basic rule of thump, 40MHz on B210 needs -E, whereas most other bandwidths on USRPs do not (bu there are exceptions).

Noise on DC carrier

Some USRPs have noise on the DC carrier, which can degrade the radio signal. There are two workarounds:

  1. Use the --tune-offset parameter, which shifts the operating frequency of the USRP to avoid the use of the DC carrier.
  2. --tune-offset has a maximum shift frequency. If half the bandwidth is larger than --tune-offset, the DC carrier will still lie within the carrier. In this case, you can also mask these RBs by using gNBs.[0].ul_prbblacklist in the configuration file.

You can also find more information on this in the 5G/NR gNB with COTS UE tutorial.