This letter proposes a novel geographic channel assignment (GCA) framework for dynamic channel allocation (DCA) in broadband wireless access networks (BWANs). The proposed GCA scheme is based on the characteristics of radio propagation, which focuses on the relationship between transmission distances and communication parameters, e.g., signal-to-noise ratio (SNR) etc., and uses a signal-aware distance estimation scheme to determinate an appropriate channel for communication. This method significantly increases the capacity of the BWA system. Simulation results show that the GCA framework can yield approximate two times throughput of the IEEE 802.16 standard specifications as well as obtain significantly lower call blocking probability compared with classical channel assignment methods.