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It is a big challenge to design an effective scheduling algorithm for multimedia transmissions over a wireless channel that employs adaptive modulation and coding (AMC). On one hand, the scheduling algorithm needs to schedule user transmissions by taking advantages of the channel diversity so that the overall system performance can be improved. On the other hand, the algorithm also needs to maintain fairness among users and to provide QoS guarantee based on user requirements. In this paper, we propose a novel scheduling algorithm called QoS-based cross-layer scheduling algorithm (QoS-CLS) to address the aforementioned issues. To maximize the system throughput, the algorithm takes into consideration both the physical layer and data link layer information to schedule user transmissions. Using the cross-layer information, the scheduling problem is formulated into a Markov decision process (MDP) and the optimum decision policy (based on the channel status, traffic state and buffer status of each traffic flow) is precalculated by linear programming and stored in the system for scheduling in real-time. Results show that QoS-CLS can greatly enhance the channel throughput compared to the opportunistic scheduling algorithm because of its cross-layer QoS consideration and the optimization method. Moreover, it can provide QoS guarantee and facilitate resource sharing among different traffic flows.