Multiple access regardless of time-and frequency-selective fading

Relying on block spreading and despreading, where the spreading and despreading matrices corresponding to the same user only depend on this user's signature code, we propose a novel multi-user separation in the presence of unknown time- and frequency-selective fading. More important, Maximum Likelihood (ML) optimality is preserved in this multi-user separation step in the presence of additive white Gaussian noise. In addition to MUI-resilient reception, channel-irrespective symbol detectability is also guaranteed. The maximum achievable spectral efficiency is analyzed, and an implementation based on conventional symbol spreading and despreading combined with an (I)FFT operation and interleaving is presented. Simulation results show improved performance of the proposed transceiver over a multi-user time-frequency RAKE receiver for Direct-Sequence (DS) Code Division Multiple Access (CDMA).