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Direct sequence (DS) and multicarrier (MC) are code-division multiple-access (CDMA) schemes based on single-carrier and MC modulations, respectively. While in DS CDMA, spreading is performed by increasing the transmission rate, or in the time domain, in MC CDMA, spreading is performed using several subcarriers, or in the frequency domain. In this paper, it is shown that MC and DS CDMA are time-frequency dual systems. Their analysis in time- and frequency-dispersive multiple-input multiple-output channels revealed that they can be described equally in terms of dual-channel functions, therefore exhibiting the same performance on dual channels. Furthermore, the duality property is used to derive matched filters for highly time-variant and wide-band channels. Finally, other issues involved in the comparison of DS and MC, namely spreading-sequence design and multiuser detection, are addressed.