Abstract: Overlay and underlay carriers are used to transmit traffic over a cellular network for a call associated with a mobile terminal. A method involves determining which one of the underlay carrier and the overlay carrier is to carry the traffic as a function of a resource parameter associated with a resource for transmitting the traffic and a mobility parameter of the mobile terminal. In some implementations, the method is applied at for example a base station to a plurality of calls associated with various mobile terminals within a cell and the calls are assigned to the underlay carrier and the overlay carrier using an optimization routine that optimizes a function associated with the capacity while imposing a constraint such as for example a constraint on a hand-off rate between the carriers.