Thanx for your answers my friends.
I forgot about power reducing. Thanks.
You mean it`s better to use 2 different cells istead one, don`t you?
(higher cell for coverage the last one for capacity)
We already use GSM1800 everywhere for capacity.
And traffic control algorithms seem very good.
But problem with GSM900 is increase with building of new sites. It`s due the lack of available ARFCN.
So the idea to add GSM900 microcell layer to reduce traffic of high cells and put as much traffic as possible to underlayer.