for DAS, you’re are looking for an entire training course or asking a question ?
as for 5th floor interference, you will experience bad quality on the GROUND if you have cells with equal RxLev! you don’t have to go any higher. Whole concept of cells and freq re-use relies on the fact that in each cells coverage area, only that cell is dominant … when you go to higher floors in the building, then you are exposed to a greater noise floor.. thus the need for IBS and DAS apart from capacity considerations