1. A method of a UE (User Equipment) for performing random access procedure, comprising:the UE receives a first back-off information from a first base station in a dedicated message, wherein the first back-off information includes a granularity information;
the UE transmits a first preamble to a second base station;
the UE determines a back-off interval based on the first back-off information and a second back-off information, wherein the second back-off information is received when the UE monitors a response for the first preamble; and
the UE transmits a second preamble to the second base station after a back-off time, wherein the back-off time is randomly selected within the back-off interval.