In a transmitter of an orthogonal frequency division multiple access (OFDMA) system, a subchannelization module generates an OFDMA symbol with data on multiple subcarriers, from received incoming data packets. An input controller applies a first formula to determine a first index of each received data packet, and stores each received data packet at an address in memory according to its first index. An output controller applies a second formula to determine the nature of the data to be carried by each subcarrier in the OFDMA symbol and, if said second formula indicates that a data subcarrier should be output, reads the data from said memory, wherein said data packets are stored in said memory at addresses such that the data can be read out at least piecewise sequentially when generating the OFDMA symbol.