We consider a cross-layer scheme for the downlink multiuser Orthogonal Frequency Division Multiple Access (OFDMA) for mobile WiMAX (Worldwide Interoperability for Microwave Access) based on IEEE802.16e. We propose an intelligent Medium Access Control (MAC) that adapts with physical layer in response to the different QoS requirements of the service flows in the system. Our scheme satisfies the maximum delay requirement for real-time service flows as well as minimum reserved data rate for non real-time service flows sessions while maintaining a high capacity of the whole system. We provide extensive simulation results showing the performance aspects of the proposed cross-layer scheme.