Device-to-Device (D2D) communications is considered as one of the key technologies for 5G wireless communications system, since it offers several advantages compared with traditional cellular network, i.e. low end-to-end latency and high spectral efficiency. By allowing underlaying direct D2D communications, a D2D link which reuses the spectral resources of a cellular user causes increased interference to this cellular link. Further, the D2D link suffers from interference caused by the cellular user. In this paper, we propose a network controlled algorithm with low computation complexity to efficiently maximize the reuse of cellular network spectrum with the help of using channel information at base station. This algorithm can be used to select the transmission mode of potential D2D pairs. Besides, we also propose to use an auction algorithm for maximizing system capacity. The low complexity of these two algorithms reduces the setup delay of D2D links. Simulation results demonstrate improved system performance in terms of overall system capacity and number of established D2D links.