Handover Interruption time
At handover from a source cell to a target cell, there is a time when the mobile terminal can’t transmit or receive user data. This mobility interruption time can be defined as the shortest time duration supported by a mobile network during handover. In a 4G LTE deployment, the mobility interruption time is around 30-60 milliseconds. To ensure the performance of emerging 5G use cases, the 3GPP decided to make it closer to zero. In 5G networks, short mobility interruption time is possible in few scenarios such as when the mobile terminal moves from one beam to another within the same cell.
The main characteristics of the reduced mobility interruption solution are:
- Continued transmission/reception in the source cell after receiving the handover request.
- Simultaneous reception of user data from source and target cell.
- Uplink transmission of user data switched to the target cell after random access procedure.
Upon receiving the request to perform a handover with reduced interruption time, the mobile terminal continues to send and receive user data in the source cell. At the same time, a new connection to the target cell is established and the mobile terminal performs synchronization and random access in the target cell. The mobile terminal will establish a new user plane protocol stack for the target cell, containing Physical (PHY), Medium Access Control (MAC) and Radio link control (RLC) layers while keeping the source user plane protocol stack active for transmission and reception of user data in the source cell.
The mobile terminal will receive user data simultaneously from both the source cell and the target cell, the PDCP (Packet Data Convergence Protocol) layer is reconfigured to a common PDCP entity for the source and target user plane protocol stacks. In-sequence delivery of user-data and to secure that data, it is much required to have PDCP sequence number continuation which is required to be maintained throughout the handover procedure. Ciphering /deciphering, header compression, and decompression needed to be handled separately.
Due to the beam sweeping used, the handover interruption time can be larger than at lower frequencies. DAPS is introduced in 3GPP Rel-16 which is a make-before-break solution to reduce the interruption time.
The basic way for handling mobility and handover between cells is to use measurement reports from the device, e.g reports on the received power from other neighboring cells.
DAPS Handover Call Flow
DAPS handover is possible over both interface Xn and N2.
- The Source gNB configures the UE measurement procedures and the UE Measurement reports.
- Based on the MR (Measurement Reports) and RRM information, the source gNB decides to hand over the UE.
- The source gNB issues a Handover Request message to the target gNB by passing a transparent RRC container with all the necessary information.
- The target gNB prepares the handover and sends the Handover Request Acknowledge to the source gNB.
- The target gNB also indicates if a DAPS Handover is accepted.
- The source gNB triggers the Uu handover by sending an RRC Reconfiguration message to the UE.
- For DRBs configured with DAPS, the source gNB sends the Early Status Transfer.
- The UE synchronizes to the target cell and completes the RRC handover procedure by sending RRC Reconfiguration Complete message to the target gNB.
- Target gNB sends the handover success message.