Several softwares have been developed for computer-aided design of radio networks. The first constitutive element of such a tool is the propagation modelling algorithm. The second is the planning process providing positions and setup of access points. The last one concerning the frequency channel allocation problem (FAP) is the focus of this paper. Many works devoted to this problem exploit a graph-based modelling that leads to an under-constrained problem as interference between mobile nodes are not taken into account. In this paper a new QoSbased FAP criterion is formulated. This QoS criterion measures the overall throughput taking downlink interference into account. We show on an example that our FAP model outperforms usual graph-based FAP approaches concerning the effective SINR obtained at mobile nodes. Results are based on realistic simulations exploiting a simulator called WILDE (Wireless Lan DEsign) and implementing the MR-FDPF propagation algorithm.