A method for minimizing interference is applicable to a primary network whether or not spectrum resources are assigned to its users using a block-wise subcarrier assignment scheme or a randomized allocation scheme. The identified unused spectrum resources that are to be assigned to the users of the opportunistic network exclude un-used subcarriers adjacent to subcarriers used by the users of the primary network to avoid interference. The opportunistic network may assign the identified unused spectrum resources using a scheme that selects a block size for an adaptive modulation and coding scheme or for avoidance of waste of spectrum resources.