## 2 Related Works  
To improve spectral efficiency and network throughput, many recent studies have investigated resource allocation in D2D communications underlying cellular network. Based on a social-aware D2D system, [PERSON] et al. in [18] proposed a joint optimization algorithm to allocate the communication resources to improve the system performance. The authors in [19] considered an LTE network where the D2D users reuse the downlink subchannel, and investigated the resource allocation to improve network throughput. However, system performance improvement is limited due to the limited licensed spectrum resources. Therefore, there have been numerous works investigating D2D communications underlaying LTE-U networks to improve system throughput and spectrum efficiency. The main problem in LTE-U networks is how to make the users accessing the unlicensed spectrum coexist with the Wi-Fi users in an appropriate manner. Thus, several mechanisms of coexistence have been proposed in the previous works, i.e., LBT method [20; 21] and duty-cycle method [13]. The LBT method uses carrier sensing and backoff rules which is similar to CSMA/CA [22]. Two schemes based on LBT named Frame-based-equipment (FBE) and Load-based-equipment (LBE) have been proposed in [23], where the Clear Channel Assessment (CCA) is executed in a uniform subframe in per period or before per data transmission. Some studies use the duty-cycle method where the LTE-U and Wi-Fi users are controlled by the BS, and the LTE-U users transmit data in on-time, but in off-time they vacate the spectrum for the Wi-Fi users to guarantee the orthogonality [24]. More studies on LTE-U technology based on these two mechanisms can be carried out. In [25], [PERSON] et al. proposed an algorithm based on Q-Learning, which can maximize the use rate of the white spaces for the LTE-U user transmissions, while decreasing the latency of data transmissions for the Wi-Fi network by adjusting the LTE-U duty-cycle to Wi-Fi activity. [PERSON] et al. in [16] proposed a matching-based algorithm to maximize the utility of all the CUs while guaranteeing the throughput requirements of the Wi-Fi users, and optimized the resource allocation of the licensed and the unlicensed spectrum. Facilitated by the LTE-U technology, the subchannel allocation and mode selection in D2D-U networks are investigated in recent years. A joint mode selection and resource allocation algorithm for D2D-U network has been proposed in [26] to maximize the total throughput of all the CUs and Wi-Fi users. Moreover, [PERSON] et al. in [26] proved that the duty cycle can achieve better performance than the LBT method. However, one subchannel in licensed spectrum only can be reused at most by one D2D pair in [26], which caused a waste of spectrum resources. The authors in [27] adopted particle swarm optimization to solve a joint mode selection, subchannel allocation and power control optimization problem to improve the total throughput of CUs and D2D users in licensed spectrum. Few works on spectrum access and resource allocation of D2D underlaying both licensed and unlicensed since the interference among LTE/D2D, LTE-U/D2D-U, and Wi-Fi users is difficult to manage. In [28], a subchannel allocation problem of system in which D2D communications operate as an underlay in LTE network over licensed and unlicensed spectrum, they propose a user-subchannel swap matching algorithm. However, they do not consider the required minimum rate of the Wi-Fi users. In [29], the authors leveraged stochastic geometry in a unified network including LTE, D2D and Wi-Fi users in licensed and unlicensed spectrum to investigate resource allocation problem. They modeled the deployment of users as PPPs, and then proposed an SQP-based algorithm to obtain a sub-optimal solution. In [30], the authors investigated the content placement problem to maximize the cache hit ratio in D2D communications overlaying CNs. Simulation results show that interference can easily be solved in overlaying D2D communications of CN.