In UMTS channelization codes are othogonal but scrambling codes which distinguish each transmitter is not completely orthogonal. So in UL each UE has seperate scrambling code(not completely orthogonal) and using same frequency. so with increasing user ul interference increase and hence capacity reduced.
In LTE each subcarrier is completely orthogonal, i.e. the frequency is different for different user.
So interference is less.
So such limitations in interference.
In Ericsson there are RBS parameter which can increase maximum sustainable interference in a NodeB (maxrotcove, and few simililar in same mo i dont able to recall fullname) + you have some features like interference suppression, fast congestion control and many more.
Please check if all are activated and you can tune this parameters as well.
I guess basic admission parameter you already maximized and suppressed r99 rabs already.
All parameter, feature I mentioned are available for ericsson vendor,I guess similar will be there for other vendor as well.