In the future 5G networks, deploying ultra-dense smallcells for enhancing the spectrum utilization and network throughput is expected. Moreover, the advanced techniques proposed in LTE-A, the carrier aggregation (CA) and the coordinated multi-point (CoMP), can further improve the network throughput. For maximizing the network throughput, we introduce the software-defined network (SDN) architecture that it is a centralized management method. The controller of the SDN collects the global information of the smallcells and the UEs, and then finds out a sub-optimal solution for the resource allocation. In this paper, we propose a downlink resource allocation algorithm for SDN-based multicell orthogonal frequency division multiple access (OFDMA) systems in small scale ultra-dense smallcell networks. The simulation results show that the proposed scheme can improve the throughput and fairness significantly.