Multiuser space-time code for OFDM/SDMA systems

The OFDM/SDMA system with single-antenna users (OFDM/SDMA-SAU) employs both orthogonal frequency-division multiplexing (OFDM) modulation and space division multiple access (SDMA) techniques to efficiently suppress multipath interference and significantly enhance the spectral efficiency for Wireless Local Area Networks (WLAN). To further improve the performance of WLAN, an OFDM/SDMA system with double-antenna users (OFDM/SDMA-DAU) has been investigated, where the user terminals are installed with two diversity antennas and the base station with multiple antennas. In the uplink, Alamouti code transmit diversity is preferred at user terminals and linear signal processing algorithms can be adopted for multi-user space-time decoding at the base station. On the other hand, the problem of signal transmission in the downlink of such a system is also very important and still has no satisfactory solutions. For this reason, a multi-user space-time code (MU-STC) with channel state information (CSI) at the transmitter is presented in this paper. The key property of MU-STC is that its decoding scheme of the user terminals requires no CSI and thus is of very low-complexity. In contrast to the OFDM/SDMA-SAU system with the pre-equalization algorithm in the downlink, the OFDM/SDMA-DAU system with MU-STC is able to achieve better BER performance and lower transmitted power fluctuation.