Device-to-device (D2D) communications enable the proximity communications among users with or without the use of the network infrastructure. D2D communications can increase the overall spectrum efficiency and reduce the power consumption thus a massive utilization of D2D is expected in the future 5G networks. The paper proposes a D2D resource allocation method based on the reuse of primary uplink resources. The method guarantees both a fixed quality of service (QoS) for D2D communication as well as the assigned QoS for cellular communications. It relies on the estimation of distances of all the actors related to D2D position: D2D pairs, base station and the primary devices. These distances are used to evaluate the minimum D2D transmission power which guarantees the D2D reception and jointly produces a controlled and acceptable level of interference on the primary system. The performance of primary system achieved by the proposed method is compared with the one of an unaware interference approach. The results obtained for the scenario with a D2D pair show how the method can limit the interference on primary system. For the scenario with different D2D pairs, the results highlight a better scalability with respect to the other method.