A comparison of the open loop transmit diversity schemes for third generation wireless systems

Multiple transmit antennas giving rise to diversity (transmit diversity) have been shown to increase the downlink (base station to the mobile) capacity in cellular systems. The third generation partnership project (3Gpp) for WCDMA FDD has chosen space time transmit diversity (STTD) as the open loop transmit diversity technique for two transmit antennas. On the other hand, the CDMA2000 has chosen space time spreading (STS) and orthogonal transmit diversity (OTD) as the open loop transmit diversity techniques. In this paper, we compare the open loop transmit diversity techniques of 3Gpp and CDMA2000. We show that STS involves a QPSK to non-QPSK constellation transformation followed by an STTD encoding of the resulting symbols. Thus, the performance of STS is the same as STTD, while the peak to average ratio (PAR) and complexity of STS is higher than STTD. We also show theoretically that the performance of STTD is 0.3-0.5 dB better than OTD. Further, STTD has another distinct advantage over OTD and STS in its application to the broadcast channels. During the power on and possibly during soft handoff, the mobile does not know the presence/absence of the diversity antenna on the primary/soft handoff base station. In this case, a blind detection of the diversity antenna is possible for STTD with a very small degradation in performance. This is not possible for OTD and STS. We thus show that STTD is a better open loop transmit diversity technique than OTD and STS.