This paper deals with orthogonal Space-Time Block Coded MC-CDMA systems, in outdoor realistic downlink scenarios with up to two transmit and receive antennas. Assuming no channel state information at transmitter, we compare several linear single-user detection and spreading schemes, with or without channel coding, achieving a spectral efficiency from 1 to 2 bits/s/Hz. The different results obtained demonstrate that spatial diversity significantly improves the performance of MC-CDMA systems, and allows different chipmapping without notably decreasing performance. Moreover, the global system exhibits a good trade-off between complexity at mobile stations and performance. Then, Alamouti's STBC MC-CDMA schemes derive full benefit from the frequency and spatial diversities and can be considered as a very realistic and promising candidate for the air interface downlink of the 4 th generation mobile radio systems.I.