Method and apparatus for preamble transmission based on the first PDCCH order and the second PDCCH order in mobile wireless communication system

A method performed by a user device, the method comprising:

• transmitting, to a base station, capability information associated with the user device;
• receiving, from the base station, a first radio resource control (RRC) message, wherein the first RRC message comprises:
• at least one configuration parameter associated with a serving cell;
• and
• at least one configuration parameter associated with layer-1/layer-2 triggered mobility (LTM);
• receiving, from the base station, downlink control information (DCI) associated with a physical downlink control channel (PDCCH) order, wherein the DCI comprises:
• a first field comprising a value associated with the PDCCH order;
• and
• a second field configured to comprise a first value associated with the serving cell or a second value associated with a LTM candidate cell;
• based on the second value being comprised in the second field, transmitting, to the LTM candidate cell, a random access preamble;
• and
• after transmitting the random access preamble, receiving a second message comprising a timing advance value associated with the LTM candidate cell.