The problem of demodulation of multiuser orthogonal frequency division multiplex-code division multiple access (OFDM-CDMA) in the presence of both multiple access interference (MAI) and intersymbol interference (ISI) is considered. Three types of OFDM-CDMA are presented: multicarrier code division multiple access (MC-CDMA), multicarrier direct sequence code division multiple access (MC-DS-CDMA) and multi-tone code division multiple access (MT-CDMA). The dispersive OFDM-CDMA channel is first cast into a multiple input multiple output (MIMO) signal framework. Then, using a subspace method, it is shown that each user's channel impulse response can be estimated using the knowledge of the user's signature waveform. It is further shown that, using the user's channel impulse response and the identified signal subspace, a blind linear minimum mean square error (MMSE) detector and a blind zero-forcing decorrelator detector can be constructed in closed form. The proposed detectors are attractive for their higher spectral efficiency, which is achieved by the elimination of cyclic prefix and training sequences. Further a comparison of MC-CDMA/MC-DS-CDMA/MT-CDMA with direct sequence code division multiple access (DS-CDMA) is then presented. The paper's major contribution is the development of the sub-space blind techniques.
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