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