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An orthogonal frequency-division multiplexing (OFDM)-code-division multiple-access (CDMA) system employing various diversity schemes is considered as a possible candidate for broadband wireless access network applications. With an emphasis on a preamble design for multichannel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in the space-time block-coded (STBC) OFDM-CDMA systems, respectively. Also, for a simple construction of the OFDM-CDMA system with the low peak-to-average power ratio (PAR) and reduced transmitter complexity, we investigate a simple PAR reduction scheme and its combination with both decimated selective mapping (SLM) and partial transmit sequence (PTS) methods. Taking into account the false side information introduced by the PAR reduction scheme, the effects of diversity techniques and PAR on the performance of the STBC OFDM-CDMA system are investigated and the maximum achievable diversity gain for a two-path Rayleigh-fading environment is evaluated. From the presented results, we find that the OFDM-CDMA signal with the low PAR is easily constructed by a simple phase transform and its combination version with the SLM and PTS approaches can be implemented to be less complex by decimating the OFDM samples. At the same time, the OFDM-CDMA system applying a space-time-frequency diversity with two and four transmit antennas can give the diversity of D=4 and D=8 for the multiuser case with half user capacity, respectively, even if encountered with the false side information.