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NR-DC tutorial

[[TOC]]

This tutorial explains how to test NR-DC with OAI gNB.

F1 NR-DC

It is possible to test F1-based NR-DC with OAI gNB.

In F1-based NR-DC, the UE is connected to a unique CU (that handles upper layer protocols, PDCP and above and is connected to the core network). Its signalling radio bearers are using the FR1 DU for lower layers (RLC, MAC and PHY). Its data radio bearer (DRB) (in this scenario, we suppose that the UE requests only one bearer) is also using the FR1 DU for lower layers when it is established but its lower layers (RLC and below) are immediately transferred to the FR2 DU.

Setup

The setup is as follows:

  • one CU
  • one FR1 DU connected to the CU
  • one FR2 DU connected to the same CU

Both DUs need to be tightly synchronized. This can be achieved in different ways depending on the hardware used.

For O-RAN 7.2 RUs, ensure that you use the same PTP clock for both units.

For USRP-based setups (for example, the FR1 DU is using a B210 and the FR2 DU is using an X410 connected to an Interdigital MHU) you need to set the following in the RUs section for both units:

         clock_src = "external";
         time_src = "external";

And you need to connect both units to the same clock and time sources (PPS and 10 MHz), for example, a USRP OctoClock.

To set up the CU and the DUs, refer to the documents F1AP/F1-design.md and handover-tutorial.md that explain how to run one CU with two different DUs connected to it.

UEs

NR-DC was successfully tested with the following UEs:

  • Samsung S25 (FR1 band n77 40MHz and FR2 band n261 100MHz)
  • Quectel RG530 (FR1 band n77 40MHz and FR2 band n261 100MHz)

Compilation

Refer to the standard ways, depending on your hardware setup.

Configuration

Add the following in the CU configuration file (at the end, or anywhere but outside of other blocks).

nrdc = {
  combinations = (
    { mcg = 1, scg = 2 },
    { mcg = 3, scg = 4 }
  )
}

This will enable two different possible band combinations. The first combination is for an MCG with nr_cellid==1 and an SCG with nr_cellid==2. The second combination is for an MCG with nr_cellid==3 and an SCG with nr_cellid==3.

If you run one DU in band n77 (this DU being the MCG with nr_cellid==1) and another one in band n261 (this DU being the SCG with nr_cellid==2), then a UE that supports this NR-DC combination will be put in NR-DC mode. The same applies if you run one DU in band n78 (with nr_cellid==3) and another one in band n257 (with nr_cellid==4).

The configuration file uses nr_cellid, not band, to select an MCG/SCG combination.

Testing

Once you have your three proper configuration files for the CU and the two DUs and the system is up and running, connect a UE to the FR1 band. And if the UE supports the configured band combination, its first PDU session will be transferred to the FR2 DU.

For example, when testing a Quectel RM530 with bands n77 and n261, run the following AT command so that it will connect only to band n77 and not attempt to connect to band n261, which it is otherwise capable of doing.

  AT+QNWPREFCFG="nr5g_band",77

For the Samsung S25, you need to enable all bands and band combinations. Refer to this external link.

Limitations

Only one data radio bearer and one PDU session are supported at the moment.

Disable IMS if possible.

What actually happens is that only the first PDU session that is established will be transferred to the FR2 DU.

A multi-PDU session scenario was not tested.

Traces

Some NR-DC traces may be found in the wiki.

Message flow

sequenceDiagram
  participant ue as UE
  participant dufr1 as DU FR1
  participant dufr2 as DU FR2
  participant cu as CU

  note over cu,ue: initial connection, PDU session establishment

  note over cu: CU decides to<br/>initiate NR-DC

  cu ->> dufr1: DL RRC Message Transfer<br/>(UE Capability Enquiry)
  dufr1 ->> ue: 1. UE Capability Enquiry
  ue ->> dufr1: 2. UE Capability Information
  dufr1 ->> cu: UL RRC Message Transfer<br/>(UE Capability Information)

  note over cu: CU checks the UE capabilities,<br/>finds a valid NR-DC combination

  cu ->> dufr1: DL RRC Message Transfer<br/>(RRC Reconfiguration)
  dufr1 ->> ue: 3. RRC Reconfiguration<br/>(measConfig for FR2 DU)
  ue ->> dufr1: RRC Reconfiguration Complete
  dufr1 ->> cu: UL RRC Message Transfer<br/>(RRC Reconfiguration Complete)
  cu ->> dufr1: UE Context Modification Request<br/>(RRC Reconfiguration Complete Indicator)
  dufr1 ->> cu: UE Context Modification Response
  ue ->> dufr1: 4. Measurement Report
  dufr1 ->> cu: UL RRC Message Transfer<br/>(Measurement Report)
  cu ->> dufr2: 5. UE Context Setup Request
  cu ->> dufr1: 6. UE Context Modification Request
  dufr1 ->> cu: UE Context Modification Response
  dufr2 ->> cu: 7. UE Context Setup Response
  cu ->> dufr1: DL RRC Message Transfer<br/>(RRC Reconfiguration)
  dufr1 ->> ue: 8. RRC Reconfiguration<br/>(activate NR-DC)
  ue ->> dufr1: RRC Reconfiguration Complete
  dufr1 ->> cu: UL RRC Message Transfer<br/>(RRC Reconfiguration Complete)
  cu ->> dufr1: UE Context Modification Request<br/>(RRC Reconfiguration Complete Indicator)
  dufr1 ->> cu: UE Context Modification Response
  ue ->> dufr2: 9. Random Access<br/>(using provided CFRA configuration)
  dufr2 ->> ue: 10. RAR

  note over cu,ue: the PDU session is now using FR2 DU
Loading
  1. The UE Capability Enquiry contains the includeNR-DC field and a CellGrouping to specify the MCG and SCG bands
  2. The UE Capability Information contains the supported Band Combination List that is checked by the CU to activate NR-DC
  3. The RRC Reconfiguration contains a measurement configuration for the FR2 DU.
  4. The Measurement Report contains a measurement of the FR2 DU cell
  5. The UE Context Setup Request contains the UE Capabilities and the DRB to set up
  6. The UE Context Modification Request contains the DRB to remove (the same that is to be set up at step 5)
  7. The UE Context Setup Response contains the Cell Group Config to be sent to the UE. This Cell Group Config contains the RLC bearer to set up and all the parameters of the cell, plus the CFRA configuration that the UE has to use to connect to the FR2 DU
  8. The RRC Reconfiguration contains several elements:
    • recoverPDCP for the DRB to transfer from FR1 to FR2
    • removal of the measConfig for the FR2 DU (not necessary for completion of NR-DC; added for cleaner processing)
    • removal of RLC bearer from the Master Cell Group (FR1)
    • mrdc-SecondaryCellGroupConfig that contains an RRCReconfiguration with the Cell Group Config sent by the FR2 DU, configuring the RLC bearer on the FR2 Cell and all the parameters of the FR2 DU cell, plus the CFRA configuration the UE has to use to connect to the FR2 DU
  9. The Random Access is done according to the configuration sent by the FR2 DU
  10. After the transmission of the RAR, the connection to the FR2 DU is established and data traffic for the DRB is now using the FR2 cell