adding a new BTS is very expensive !!
these 3×2 1800 TRX will be put into existing 900 BTS, if they can support it. I think they can, can’t they ??
Then, it’s up to you:
1/ you create 3 new cells “1800”, which will be considered as neighbors to the collocated 900 cells. It’s quite complex to manage.
2/ you include these TRX in the existing 900 cells, meaning that you’re going to create a site with 3 “multiband” cells. This is the cleanest solution, if you master it properly and know its limitations (depending on vendor).
The TRX1800 are located in the inner zones of each cell, while the TRX900 will be located in the outer zones.