An apparatus and method is provided to process a digital signal in an orthogonal frequency division multiple access (OFDMA) wireless communication system. In the apparatus, a memory stores the number of subcarriers allocated to a mobile terminal and stores an inverse discrete Fourier transform (IDFT) coefficient. A controller outputs a mode selection signal indicating a transmission mode or a reception mode. An inverse fast Fourier transform (IFFT) block multiplies a first result obtained by determining a bitwidth for the IDFT coefficient according to the number of subcarriers by a second result obtained by determining a bitwidth for data according to the number of subcarriers.