A method and apparatus of a UE in a wireless communication system supporting a shared spectrum channel access is provided. The method comprises: receiving, from a BS, a set of higher layer parameters; identifying, from the received set of higher layer parameters, a set of SS/PBCH blocks assumed to be transmitted by the BS; identifying, from the received set of higher layer parameters, a PRACH slot including at least one random access RO; determining the at least one RO as at least one valid RO; and transmitting, to the BS, a PRACH preamble in the at least one valid RO.