Space–time block coding based transmit diversity
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STTD stands for Space Time Block Coding Based Transmit Diversity in UMTS systems. It is a method of transmit diversity. STTD is optional in the UTRAN but mandatory for user equipment (UE). STTD utilizes Space-time Block Code in order to exploit redundancy in multiply transmitted versions of a signal.
A generic STTD encoder for QPSK creates two sets of STTD encoded symbols which will each be transmitted on two different antennas. If the transmitted symbols consist of b0, b1, b2 and b3, the two tranmitted sets of symbols will consist of b0, b1, b2 and b3 on antenna 1 and !b2, b3, b0 and !b1 on antenna 2.