Complex spreading for STBC/DSTBC under frequency selective fading

Abstract

Space time block codes (STBC) have become a popular transmit-diversity technique due to their simplicity and high gain under a frequency-fiat-fading channel and so have been adopted for the 3G wideband code division multiple access system (W-CDMA). However the performance of a STBC is degraded under a multi-user and frequency-selective fading environment. The goal of this paper is to present an improvement in the STBC under such environments by spreading the information symbols with a complex pseudo-noise (PN) sequence, called a Song-Park (SP) sequence. The SP sequence has a good even and odd cross-correlation property so that the inter-symbol interference (lSI) and multi-user access interference (MAl) can be almost negligible. Therefore, a significant result in this paper is that the performance of the proposed scheme under a synchronous or asynchronous multi user environment is close to that of a single user. In addition, the complexity of the proposed system is much simpler than that of the known multi-user detection (MUD) schemes such as minimum mean square error (MM SE) and zero-forcing (ZF), because the proposed system simply spreads and de-spreads the information symbols with the SP sequence consisting of complex numbers on the unit circle. Furthermore, this paper also extends the proposed idea to a differential STBC (D-STBC) system because a D-STBC does not require any channel estimation. Again, the proposed D-STBC spread with a SP sequence shows superior performance over the ordinary D STBC with no spreading or a D-STBC spread with another sequence such as a Walsh orthogonal sequence.