Time switching transmit diversity TSTD in UMTS is used only on UMTS SCH channel
Time-switched transmit diversity (TSTD) can be applied to the UMTS SCH. Just like Space time block coding based transmit antenna diversity STTD, the support of Time switching transmit diversity is optional in the UMTS UTRAN, but mandatory in the UE. The principle of Time switching transmit diversity TSTD is to transmit the synchronization channels via the two base station antennas in turn. In even-numbered time slots the UMTS SCHs are transmitted via antenna 1, and in odd-numbered slots via antenna 2. This is depicted in above Figure. Note that UMTS SCH channels only use the first 256 chips of each time slot (i.e., one-tenth of each slot).