Closed-form optimality characterization of network-assisted device-to-device communications

International audience—This paper considers the mode selection problem for network-assisted device-to-device (D2D) communications with multiple antennas at the base station. We study transmission in both dedicated and shared frequency bands. Given the type of resources (i.e., dedicated or shared), the user equipment (UE) decides to transmit in the conventional cellular mode or directly to its corresponding receiver in the D2D mode. We formulate this problem under two different objectives. The first problem is to maximize the quality-of-service (QoS) given a transmit power, and the second problem is to minimize the transmit power given a QoS requirement. We derive closed-form results for the optimal decision and show that the two problem formulations behave differently. Taking a geometrical approach, we study the area around the transmitter UE where the receiving UE should be to have D2D mode optimality, and how it is affected by the transmit power, QoS, and the number of base station antennas. I. INTRODUCTION Emerging multimedia services and applications introduce new traffic types and user behaviors [1]. To address the higher demands imposed on wireless networks, more spectrally effi-cient and energy efficient approaches should be developed. Device-to-device (D2D) communication underlaying cellular networks is proposed to improve cell spectral and energy efficiency of the network [2], [3]. In D2D transmission mode, user equipments (UEs) communicate directly to their intended receivers as opposed to the conventional cellular mode where they communicate through the base station (BS). D2D mode can bring proximity gains and reduce the transmission time. Users in the D2D mode can transmit either in a separate frequency band or via spectrum sharing with cellular users. In the former case, D2D communications do not interfere with cellular users. This case is interesting due to its potential applications, such as public safety and multicasting for local multimedia services and robustness to infrastructure failure. On the other hand, spectrum sharing can be employed to efficiently utilize the resources which allows for better area spectral efficiency [4]. The gain from spectrum sharing can be assured if the interference is controlled by proper mode selection and resource management. However, depending on the network topology and channel conditions, it may not always be beneficial to choose the D2D mode for a UE