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. Orthogonal Walsh-Hadamard codes are used for spreading and in order for the codewords to stay orthogonal at the receiver, the channel response 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, named Time-Frequency Localized CDMA (TFLCDMA), places the codeword-chips in rectangular 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 (MC-DS), in terms of bit error rate. The MCand MC-DS-CDMA systems are special cases of the proposed scheme.