A wireless communication system is disclosed. A method for constructing a RACH preamble according to a cell radius of a base station (BS) irrespective of a guard time (GT) and a method for allocating the RACH preamble are disclosed. A method for constructing a preamble of a random access channel (RACH) includes acquiring time-length information of a predetermined cyclic prefix (CP) for each cell radius of a base station (BS), acquiring sequence time-length information of a single sequence or a repeated sequence, and constructing the preamble using the predetermined CP time-length information and the sequence time-length information, irrespective of a time length of a guard time (GT).