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USRP driver: describe patch, --continuous-tx, -E options

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Robert Schmidt
2026-03-04 09:18:49 +01:00
parent 732f970442
commit 76b4899ecb
2 changed files with 55 additions and 2 deletions
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doc/NR_SA_Tutorial_COTS_UE.md
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@@ -201,7 +201,7 @@ With devices like the USRP N300 and especially the X300, there is noise in the D
There are two possible solution that can be enabled in OAI:
* `--tune-offset`: it consists in shifting away the operational bandwidth to avoid the center frequency
* `ul_prbblacklist`: can be used to define specific PRBs that should not be used for uplink scheduling
* `gNBs.[0].ul_prbblacklist`: can be used to define specific PRBs that should not be used for uplink scheduling
A spectrum clean from the noisy PRBs will eventually result in an enhanced UL throughput.

55
radio/USRP/README.md
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@@ -15,6 +15,20 @@ 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).
## Build
The OAI USRP driver in leverages the [USRP Hardware
Driver](https://github.com/EttusResearch/uhd) 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`](./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
@@ -95,6 +109,45 @@ RUs = (
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
Furthur information about synchronization on the USRP N3xx devices can be found
[in the knowledge base article "Using Ethernet-Based
Synchronization"](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.
## 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](./NR_SA_Tutorial_COTS_UE.md).