Method and apparatus for performing handover

A user equipment (UE) configured to perform a handover in a wireless communication system, the UE comprising:

• a transceiver;
• at least one processor;
• and
• at least one computer memory operably connectable to the at least one processor and storing instructions that, when executed, cause the at least one processor to perform operations comprising:
• receiving, from a base station (BS), mobility assistance information informing the UE that conditional mobility is allowed, wherein the mobility assistance information includes information on candidate target cells for the conditional mobility;
• receiving a timer value including a first time period, where a handover to the first target cell may be not succeeded on the first target cell until the timer expires;
• measuring channel quality of the candidate target cells during the first time period;
• receiving, from the BS, a first threshold for a first target cell among the candidate target cells, a second threshold for a serving cell, a third threshold for the first target cell, and a fourth threshold for a second target cell among the candidate target cells;
• initiating a first random access procedure to a first target cell among the candidate target cells by transmitting a random access channel (RACH) preamble to the first target cell;
• initiating a handover to the first target cell as part of the first random access procedure, based on that a channel quality of the first target cell measured during the first time period is equal to or above the first threshold for the first target cell and a channel quality of the serving cell measured during the first time period is equal to or below the second threshold for the serving cell;
• during the first random access procedure after transmitting the RACH preamble to the first target cell:
• measuring channel quality of the candidate target cells during the first time period, based on the mobility assistance information;
• detecting that a channel quality of the second target cell measured during the first time period is greater than a channel quality of the first target cell measured during the first time period;
• and
• stopping the handover to the first target cell before detecting a failure of the handover to the first target cell, based on that the channel quality of the first target cell measured at the first time period is lower than the third threshold for the first target cell and the channel quality of the second target cell measured at the first time period is higher than the fourth threshold for the second target cell;
• and
• performing a second random access procedure to the second target cell by transmitting another RACH preamble to the second target cell based on contention free random access resources.