In This article you found Application of Tx diversity modes on UMTS downlink physical channel or in other word transmit diversity mode in UMTS.
Transmitter-antenna diversity can be used to generate multi-path diversity in places where it would not otherwise exist. Multi-path diversity is a useful phenomenon, especially if it can be controlled. It can protect the UMTS UE against fading and shadowing.
UMTS TX diversity is designed for downlink usage. Transmitter diversity needs two antennas, which would be an expensive solution for the UEs.
The UTRA specifications divide the transmitter diversity modes into two categories:
(1) Open-loop mode.
(2) Closed-loop mode.
In the open-loop mode no feedback information from the UE to the UMTS NodeB is available. Thus the UTRAN has to determine by itself the appropriate parameters for the TX diversity. In the closed-loop mode the umts UE sends feedback information up to the UMTS NodeB in order to optimize the transmissions from the diversity antennas.
Thus it is quite natural that the open-loop mode is used for the common channels, as they typically do not provide an uplink return channel for the feedback information.
Even if there was a feedback channel, the UMTS NodeB cannot really optimize its common channel transmissions according to measurements made by one particular umts UE.
Common channels are common for everyone what is good for one umts UE may be bad for another. The closed-loop mode is used for dedicated physical channels, as they have an existing uplink channel for feedback information. Note that UMTS shared channels can also employ closed loop power control, as they are allocated for only one user at a time, and they also have a return channel in the UMTS uplink. There are two specified methods to achieve the transmission diversity in the open-loop mode and two methods in closed-loop mode which explain in other article.