antenna diveersity can be either DL TX, or UL RX, or both. It just means you have two signals transmitted from different antennas, or different branches.
If you have a cell connected to a crosspolar antenna, it is probable that each branch will be used to receive the UL signal. This is UL space diversity. You have one branch with RX-A and one branch with RX-B.
The signal FROM the BTS is transmitted on only one branch. There is only TX-A.
If you want to use DL TX diversity, it means you want to use the second branch to send TX-B.
What do you need to do ? Duplex the RX-B signal and the TX-B signal. Most probably, it means you need :
1. a new type of TRX (with 2 TX output)
2. to re-cable TRX-ANC jumpers
Beware that when using the 2 TX diversity, you’ll need to balance your path loss in uplink as well. You must increase sensitivity of the UL path, either with TMA or with 4 RX diversity.