Multi User Detection for Cdma- Ofdm/oqam System Combined with Space Time Coding

In this paper, we propose the combination of Multi Carrier (MC) OFDM/OQAM modulation and CDMAcalled MC-CDMA-OQAM system with Space Time (ST) coding in a multi user context. This combination takes advantages from multicarrier modulation, spread spectrum and spatial time diversity. Indeed, the use of OFDM has proved its ability to fight against frequency selective channels but the insertion of guard interval yields spectral efficiency loss and sensitivity to frequency dispersion due to the use of rectangular pulse shape. Thus, cyclic prefix OFDM is replaced by an advanced filterbank-based multicarrier system OFDM/OQAM that operates without guard interval. However, OFDM/OQAM provides orthogonality only on the real domain, so transmitted symbols must be real valued. In the other hand, the CDMA component has two advantages: multiple access interference cancellation and providing orthogonality in the complex domain. From the orthogonality property provided, the Alamouti ST code can be combined with MC-CDMA-OQAM system. The resulting MIMO-MC-CDMA-OQAM system improves the spectral efficiency of wireless system, combat channel fading and reduce narrowband interference. Numerical results show the utility of this new wireless communication system; a significant BER versus Signal to Noise Ratio (SNR) was achieved thus Multiple access interference (MAI) is suppressed and so supporting a large number of users.