Inter-gNB-DU Mobility in 5G NR

We discuss Inter-gNB-DU-Intra-gNB-CU Mobility in 5G NR as per TS 38.401.

Discloser: Following is an excerpt (with some additional details from Section 8.2.1.1, TS 38.401)

Scenario

Let’s assume a gNB-CU is connected to two DUs. gNB-DU1 is the source DU and for some reason (e.g. radio link failure or handover-trigger condition met), UE switches to gNB-DU2. This scenario is depicted in Fig. 1 below.

System model for UE connectivity with gNb

The message flow for the inter-gNB-DU-Intra-CU Handover is as follows (Fig. 8.2.1.1–1, TS 38.401).

Inter-gNB-DU-Intra-CU Handover means that HO is between two DUs that are tied to a single CU i.e. no HO between the CU.

1. The UE sends a MeasurementReport message to the source gNB-DU.
2. The source gNB-DU sends a UL RRC MESSAGE TRANSFER message to the gNB-CU to convey the received MeasurementReport message.

2(a). The gNB-CU may send a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU to query the latest configuration.