# Transmission Power Adaption for Full-Duplex Relay-Aided Device-to-Device Communication
## 1 Introduction  
Device-to-device (D2D) communication enables users in proximity to exchange information directly, without traversing to the base station (BS) or core network in two hops and has attracted increasing attention from both industrial and academic communities [1; 2; 3; 4; 5; 6; 7]. Due to the potential proximity gain, reuse gain, and hop gain, D2D communications can significantly increase system spectral efficiency and energy efficiency, which has been considered a promising candidate technique for next generation cellular networks [8]. Accordingly, many researches have focused on a D2D communications scheme that underlayings the cellular network infrastructure to increase the cellular capacity, improving the users' throughput, and extending the battery lifetime of user equipment (UE) [9; 10; 11]. However, the existing D2D communication scheme, such as the underlaying cellular spectrum, bring more complex interference environment to cellular users which have higher priority than the D2D users, and lead to deteriorative performance on quality of service for cellular users [12; 13; 14]. In particular, for cellular users in the cell edge who are susceptible to suffered deep fading or shadowing, it would be unadvisable to share an allocated spectrum with D2D peers [15; 16; 17]. To avoid the problem mentioned above, we outline a relay-aided D2D communication scheme to fulfill the transmission gap that affects cellular users which are in the cell edge or strongly-shadowed urban environment. Unlike existing works, a D2D transmitter in our scheme is ordered to aid a cellulardownlink transmission by acting as a relay node between the BS and the cellular UE. In return, the D2D transmitter is allowed to directly communicate with the intended D2D receiver by underlaying the cellular downlink spectral resource [18; 19; 20]. Few works have been carried out so far to explore relaying-based D2D communication schemes. By scanning existing research papers, we can classify this relay-aided D2D communication schemes into two categories in terms of relay mode, namely, half-duplex (HD) and full-duplex (FD). In fact, few works have been carried out so far to explore HD relaying based scheme to transmit forward signals from BS to cellular users in two transmission phases. In this scheme, respective reception and transmission at the relay are allocated to one frame which is divided into double orthogonal time slots which are equivalent to a time division access way for the D2D relay node [21; 22; 23; 24; 25; 26]. Then the superimposed signal, which is a linear weighted combination of cellular user's signal and the D2D signal is generated by the relay. The weight factor represents the power allocation for the cellular user which is a fraction of the total transmitting power of D2D relay. Therefore, the residual power is used for broadcasting the D2D signal in the second time slot. The drawback of HD is lower efficiency in spectrum and energy. Therefore, as a replacer for HD relay, the FD mode is more applicable in practice to transmit and receive signals simultaneously on the same frequency resource. For the sake of communication bi-directional for relay, a two-antenna infrastructure is typically deployed to facilitate spectral efficiency in practice. However, FD relay is more susceptible to loopback interference which is generated from transmitting antenna to receiving antenna of relay nodes. Although few works have been done to eliminate the inter-relay interference, it cannot be ultimately mitigated because even the residual loopback interference is significant when compared to the white Gaussian noise [27; 28; 29]. For this reason, the power split factor should be adapted cautiously in FD mode. Moreover, it must be made clear that due to the FD mode being enable relay to forward two components signals simultaneously on the same spectral band, the cellular user's signal and D2D signal will treat each other as interference. Many works have been carried out to explore relaying-based D2D communication schemes.