5G NR Handover Types: Inter gNB and Intra-RAN

This page describes 5G NR Handover Types, specifically focusing on 5G Inter gNB handover and 5G intra RAN handover (C-Plane Handling). This information aligns with specifications from 3GPP, as documented in 3GPP TS 38.300.

Introduction

There are two main types of 5G NR handovers:

• Network controlled
• UE controlled

Network controlled handover applies to 5G NR UEs in RRC_CONNECTED mode. This type is further broken down into two subtypes: cell level and beam level.

• Cell Level: Explicit RRC signaling is required to trigger handover at the cell level. Messages are exchanged between gNBs to facilitate this inter-gNB handover type.
• Beam Level: This type of mobility/handover doesn’t require explicit RRC signaling. It is handled by the lower layers. The RRC doesn’t need to know which beam is being used at any given time.

5G NR Inter gNB Handover

Figure 1: 5G Inter gNB Handover

Figure 1 illustrates the 5G Inter gNB handover process. Here’s a breakdown of the steps:

1. The source gNB initiates the handover and sends a “Handover Request” to the target gNB via the Xn interface.
2. The target gNB performs admission control and responds with a “Handover Acknowledgement,” which includes the RRC configuration.
3. The source gNB then provides the RRC configuration to the UE in a “Handover Command.” This command at a minimum includes the cell ID and the information needed for the UE to access the target cell without having to read system information. It can also contain information for contention-based and contention-free random access, as well as beam-specific information if applicable.
4. The UE moves its RRC connection to the target gNB and confirms the handover with a “Handover Complete” message.

5G NR Intra-RAN Handover (C-Plane Handling)

Figure 2: 5G NR intra RAN handover

Figure 2 illustrates the 5G intra-NR RAN handover process (C-Plane Handling). This handover type doesn’t involve the 5GC (5G Core Network), meaning neither the AMF nor the UPF changes. U-Plane handling handover is still under development.

In this scenario, messages are exchanged directly between the gNBs. The target gNB triggers the release of resources at the source gNB during the handover completion phase.

Reference: 3GPP TS 38.300 section 9 for 5G NR handover types.