a preamble sequence generating apparatus and method in a bwa communication system using ofdm . an arm code generator generates at least one arm code of a different length . a sub - carrier selection and frequency mask unit maps a second number of arm code components to a second number of sub - carriers among a third number of ofdm sub - carriers distributed equally in a frequency band and maps null components to sub - carriers excluding from the second number of sub - carriers . here , the second number being the number of minus . then , an ifft generates a preamble sequence by inverse - fast - fourier - transforming the sub - carriers received from the sub - carrier and frequency mask unit .