This method cannot improve spectrum utilization since the D2D pairs use separate resource with the cellular users (CUs). In the overlay case, D2D transmitters can only access the channels which are not used by nearby cellular users. In addition, the access to the spectrum in D2D communication can be done in two ways: Overlay spectrum sharing or underlay spectrum sharing. D2D communication is defined as direct communication between two mobile users without traversing the base station, which can potentially increase the cellular capacity, improve the user throughput, and extend the battery lifetime of users. In order to alleviate this pressure, device-to-device (D2D) communication is introduced to improve the spectrum utilization of cellular networks. With the rapid popularity of intelligent devices, the cellular systems are suffering an unprecedented pressure imposed by the explosive growth of data traffic and a massive increase of interconnected devices. Numerical results reveal that the performance of the proposed scheme of jointly uplink and downlink is better than that of the schemes for independent allocation. The proposed scheme can improve the system capacity performance and increase the spectrum efficiency. The Hungarian algorithm is developed to achieve joint uplink and downlink channel assignment. It is proved that the objective function of power control is a convex function, in which the optimal transmission power can be obtained. To achieve the reasonable resource allocation, the optimization problem is divided into two sub-problems including power allocation and channel assignment. The optimization problem is formulated as a mixed integer nonlinear problem that is usually NP hard. In this paper, we propose the joint uplink and downlink resource allocation scheme which maximizes the system capacity and guarantees the signal-to-noise-and-interference ratio of both cellular users and device-to-device pairs. In cellular networks, device-to-device communications can increase the spectrum efficiency, but some conventional schemes only consider uplink or downlink resource allocation.
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