5G Handover Types: UE Initiated vs. Network Initiated

This page describes the two main types of 5G handover: handover initiated by the User Equipment (UE, or your phone) and handover initiated by the network (the 5G base station, also called Node B or NB). This information is based on specifications from Verizon 5G TF (Technical Forum) document TS V5G.300.

Network Initiated 5G Handover

Figure 1: Network Initiated 5G Handover

The figure above illustrates the network-initiated handover process. Here’s a breakdown of the messages exchanged between the UE, the serving Node B (NB1), and the target Node B (NB2):

• Step 1: The UE receives handover information from the serving NB1. This is essentially the initial setup for a potential handover.
• Step 2: The UE begins performing quality measurements on Beam Reference Signals (BRSs) transmitted from neighboring cells. Think of this as the UE “listening” to see how strong the signals are from other base stations.
• Step 3: Once a defined measurement event is triggered (e.g., a neighboring cell’s signal becomes stronger than the serving cell’s signal by a certain threshold), the UE sends a measurement report back to NB1.
• Step 4: NB1 decides to initiate a handover and responds with a handover command. This command contains RRC (Radio Resource Control) connection re-configuration information specifically for the target cell, NB2.
• Step 5: The UE completes the handover process by transmitting an “RRC connection reconfiguration complete” message to the target cell, NB2, confirming the successful switch.

UE Initiated 5G Handover

Figure 2: UE Initiated 5G Handover

The figure above depicts the UE-initiated handover process. The message exchange between the UE, serving NB1, and target NB2 is as follows:

• Step 1: The UE receives measurement configuration information from its current serving cell, NB#1.
• Step 2: Following the instructions from NB#1, the UE performs quality measurements on Beam Reference Signals (BRSs) from neighboring cells.
• Step 3: Based on these measurements and predefined triggering events, the UE sends a measurement report to its serving cell, NB#1.
• Step 4: NB#1 responds to the UE with a list of potential neighboring cells that are suitable candidates for handover. This information is delivered in an “RRC connection reconfiguration” message.
• Step 5: NB#1 also provides measurement configuration settings that will help the UE evaluate the target NBs more precisely.
• Step 6: From the list of candidate cells, the UE selects the best cell for handover using an “RRC Connection Reestablishment procedure.” Essentially, the UE independently chooses the best cell to connect to.

Reference: TS V5G.300: “Network and Signaling Working Group; Verizon 5th Generation Radio Access; Overall Description”