This paper proposes a scheduling algorithm, RTCLA (real-time cross-layer scheduling algorithm for real-time application), at MAC layer for real-time applications traversing heterogeneous networks including wired and wireless links. RTCLA is a cross-layer algorithm, combined with adaptive modulation and coding (AMC) and selective repeat-automatic repeat request (SR-ARQ). It is designed to improve spectrum utilization when satisfying packet error rate (PER) and delay requirements. Simulations are employed to evaluate the performance of RTCLA in three metrics including system packets time-out rate, average system effective throughput and fairness and it is compared with the modified proportional fair (MPF), the earliest deadline first (EDF) and the modified largest weighted delay first (M-LWDF) algorithms. Simulation results show that RTCLA outperforms MPF, EDF and M-LWDF algorithms in terms of the strict delay requirements of real-time applications, scarcity of spectrum and the time-varying channels, especially in the performance of packet time-out rate. Furthermore, the simulation results show that RTCLA performs the same as other three algorithms in stability.