Impact of Mode Selection on the Performance of D2D-Unlicensed Communications

Device-to-Device (D2D) communication, which enables direct connection between the nearby user equipments (UEs), is one of the key technologies in 5G network. In this paper, D2D communication on the unlicensed spectrum, namely, D2D-U is discussed. D2D system performance can be efficiently enhanced by utilizing the unlicensed band. However, it has a huge impact on the performance of other unlicensed networks. Therefore, the fairness of the coexistence scheme is a key problem for D2D-U. To solve the coexistence problem, two access schemes called Listen Before Talk (LBT) and Duty Cycle Mechanism (DCM) have been discussed extensively. D2D users choose the transmission mode according to transmission environments. In this paper, we discuss the performance of these two modes. Furthermore, the problem of how to access the unlicensed band with combination of these two modes is also discussed.