INTER-gNB Xn handover procedure in 5G NR
January 26, 2021

The core of mobility management in 5G NR is very similar to the one in 4G LTE. As specified by 3GPP Rel. 15, mobility in mobile networks is divided into RRC_IDLE mobility, RRC_INACTIVE mobility, and RRC_CONNECTED mobility.

RRC_IDLE and RRC_INACTIVE state in which UE does not have any established RRC connection. RRC_CONNECTED means that UE has an active RRC connection to the network and, therefore, is ready to receive or transmit user-plane data.
In case UE changes its location (or triggers
other event leading to mobility management action) in the RRC_CONNECTED state, the network changes means of communication with the UE.

Cell-level mobility or Handover refers to transferring the ongoing data transmission from one network component (gNodeB (gNB), Remote Radio Head (RRH), etc.) to another. The handover procedure consumes resources, causes connection interruption, and differs depending on the handover type (INTER/INTRA-gNB, with or without 5G Core Access and Mobility Management Function (AMF) change, etc.), the network requires a varying level of internal communication and signaling, where the least complex one. 

INTER-gNB Xn handover procedure

The INTER-gNB Xn handover procedure includes four components:

  • Based on the received measurements, Source gNB initiates the handover through Xn interface.
  • Target gNB provides a new RRC configuration and performs admission control.
  • Source gNB forwards the Handover Request Acknowledge message to the UE, along with cell ID, access information, and beam-specific information.
  • UE moves to RRC connected state with Target gNB and sends the RRC Reconfiguration Complete message.

Xn handover does not include AMF and User Plane Function (UPF) in the handover procedure. In case they are included (e.g., in Next-Generation (NG) handover), the procedure consists of additional steps, further increasing the complexity and the delay required by the process.
The frequency of inter-cell handovers is one of the critical parameters that influence the efficiency of mobility management, along with the high signal to interference and noise ratio (SINR) communication.

Source: Cell-Level and Beam-Level Mobility Management System

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