In this paper, a novel analytical model for resource allocation is proposed for a device-to-device (D2D) assisted cellular network. The proposed model can be applied to underlay and overlay D2D systems for sharing licensed bands and offloading cellular traffic. The developed model also takes into account the problem of unlicensed band sharing with Wi-Fi systems. In the proposed model, a global system state reflects the interaction among D2D, conventional cellular, and Wi-Fi packets. Under the standard traffic model assumptions, a threshold-based flow control is proposed for guaranteeing the quality-of-service (QoS) of Wi-Fi. The packet blockage probability is then derived. Simulation results show the proposed scheme sacrifices conventional cellular performance slightly to improve overlay D2D performance significantly while maintaining the performance for Wi-Fi users. Meanwhile, the proposed scheme has more flexible adjustments between D2D and Wi-Fi than the underlay scheme.