In the 3G architecture, most operators are allotted 2 frequencies for use. So, interference could
also be reduced by allowing femtocells to transmit on the unused second frequency. However,
this still does not resolve interference that may arise between two femtocells that operate on, say
two floors of a building. This scenario may also affect the quality of service and capacity of each
individual femtocell. Another alternative suggested, was for mobile operators to implement a
two-tier femtocell network [2], by sharing spectrum rather than splitting spectrum between the two
tiers.