Device-to-Device Communications in Unlicensed Spectrum: Mode Selection and Resource Allocation

In this paper, a novel technology, namely, device-to-device communications in the unlicensed spectrum (D2D-U) is proposed, which can allow D2D users to transmit on the unlicensed spectrum and coexist with the incumbent Wi-Fi networks. In D2D-U networks, D2D users can share the licensed spectrum with the existing cellular users or share the unlicensed spectrum with legacy Wi-Fi networks. Therefore, mutual interference across different networks and different users should be properly coordinated to optimize the system performance. In this paper, within the framework of D2D-U, we propose a joint mode selection and resource allocation algorithm to minimize the overall interference that cellular and Wi-Fi users suffer from the D2D communications while guaranteeing the signal-to-noise-and-interference ratio requirements of all users, including those of cellular, D2D, and Wi-Fi. Through theoretical analysis and numerical simulation, we show that using unlicensed spectrum can significantly mitigate the interference to both cellular and Wi-Fi users. Moreover, the duty cycle-based unlicensed spectrum access method achieves better system throughput than the listen-before-talk-based access method in most of the cases.