Transmit diversity is default MIMO mode and used to improve the signal-to-noise ratio and makes transmission more robust. It sends the same information via various antennas, whereby each antenna stream uses different coding and different frequency resources. By this way it improves the signal-to-noise ratio and makes transmission more robust.
In LTE, transmit diversity is used as a fallback option for some transmission modes, such as when spatial multiplexing (SM) cannot be used. Control channels, such as PBCH and PDCCH, are also transmitted using transmit diversity. For two antennas, a frequency-based version of the Alamouti codes (space frequency block code, SFBC) is used, while for four antennas, a combination of SFBC and frequency switched transmit diversity (FSTD) is used.
Ref: http://www.rohde-schwarz.cz/file_17063/1ma186
