In this paper, antenna diversity (AD) schemes are presented for frequency-domain (FD) multi-user detection (MUD) in the up-link (UL) of cellular systems which apply CP (Cyclic Prefix) assisted DS-CDMA (direct sequence code division multiple access). Alamouti-based space-time-code (STC) was applied to exploit transmitter (TX) diversity. The MUD and the diversity combining in the receiver (RX) are MMSE (minimum mean square error) based. Simulation results show that with moderate computational complexity, the SIMO and MIMO schemes have achieved better performance than the iterative block decision feedback equalization (IB-DFE) based scheme in. It was also shown that SIMO is superior to MISO in terms of diversity exploitation and achieves the best tradeoff between complexity and performance.