Method of random access procedure in wireless communication system and apparatus therefor

A user equipment (UE) for performing a random access (RA) procedure in a wireless communication system, the UE comprising:

• a transceiver;
• at least one processor;
• and
• at least one computer memory operably connected to the at least one processor and storing instructions that, when executed, cause the at least one processor to perform operations comprising:
• selecting an uplink carrier for the RA procedure between a first uplink carrier for a serving cell and a second uplink carrier for the serving cell,
• wherein selecting the uplink carrier comprises:
• selecting the first uplink carrier when a downlink signal quality of the serving cell is less than a first threshold, and
• selecting one of the first and second uplink carriers randomly when the downlink signal quality of the serving cell is equal to or above the first threshold;
• performing a first random access preamble (RAP) transmission for the RA procedure through the selected uplink carrier;
• reselecting an uplink carrier for the RA procedure when a random access response (RAR) reception for the first RAP transmission or contention resolution for the first RAP transmission fails and when the downlink signal quality of the serving cell changes more than X;
• and
• performing a second RAP transmission through the reselected uplink carrier.