Thanx for your answers my friends.
To Pix:
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)
To aso:
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.