Time-frequency localized CDMA for downlink multi-carrier systems

An OFDM-CDMA system that utilizes the channel-correlation in both time and frequency to reduce multiple access interference is proposed. The mutually orthogonal Walsh-Hadamard codes are used for spreading, and in order for the codewords to stay orthogonal at the receiver the channel must be flat in both time and frequency. A frequency-selective fading channel will, however, distort the codewords and thus introduce correlation. The proposed system, time-frequency localized CDMA (TFL-CDMA), places the codewords in blocks on the time-frequency-grid in a such a way that the difference in scaling of the codeword chips is reduced. The proposed system is found to outperform competing schemes, such as multi-carrier (MC) and multi-carrier direct-sequence CDMA (MCDS), in terms of bit error rate. The MC- and MCDS-CDMA systems are shown to be special cases of the proposed scheme.