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Space-time transmit diversity, as an effective scheme to mitigate multipath fading, has been proposed for IMT-2000 systems. Space-time transmit diversity (STTD) has been adopted as an open loop diversity scheme for WCDMA. Orthogonal transmit diversity (OTD) and space-time spreading (STS) are two optional schemes for cdma2000. This paper focuses on their differences in principle and performance. Theoretical analysis presents that the schemes of OTD, STS and STTD are similar in terms of spread structure. Simulation results show that STS scheme have 3 dB advantage over STTD. STS and STTD have better performance than OTD and NTD (none transmit diversity) scheme.