# Transmission Power Adaption for Full-Duplex Relay-Aided Device-to-Device Communication
## 4 Simulation Results  
In this section, we show that the simulation results demonstrate high achievement transmit rate and power efficiency of the proposed FD relay-based D2D communication scheme and guarantee the rate requirement for the cellular user in the primary. We choose the HD relaying mode as a benchmark. It should be made clear that the spectrum band consumed in a traditional cellular system is double that of our underlaying D2D communication due to one orthogonal subchannel that is needed for the cellular user and D2D users respectively. Therefore, the D2D communication we proposed achieves an improvement of spectral efficiency for cellular system. In this section, we study relaying and transmit power adaptation in the case of static channels and, thus, exploiting instantaneous channel state information. Alternatively, the following analysis explains the performance during one instantaneous snapshot within channel coherence time in a slow-fading environment. The parameter of system as rate requirement for cellular is set as \(R^{\pi}=20\) kb/s, which represents a typical voice signal characteristic and the bandwidth \(W=0.1\) MHz which corresponds to the width of the subchannel allocated to one cellular user. In Figure 2, we show the impact of loopback interference on the optimal transmission power of D2D relay by setting \(\gamma_{BR}=\gamma_{RC}=10\) dB and \(\gamma_{BC}=\gamma_{BD}=-10\) dB which means that channels from BS to cellular user and D2D receiver suffer from deep fading or shadowing. In the simulation, we change the channel gain of the loopback link at the D2D relay \(\gamma_{L}\) from 5 dB to 30 dB. It can be seen that the optimal transmit power is decreasing as the channel gain of loopback link is stronger. We can also observe that for a particular loopback gain, enlargement of the transmission power of relay is even harmful for the achievable rate for the cellular user which demonstrates our analysis we mentioned before. Therefore, in view of this problem, it is desirable to determine the optimal allocation power for the relay. In Figure 3, we observe the impact of power split factor on the system performance when we set \(\gamma_{L}=10\) dB. As we can observe from Figure 3, in the range of \(\alpha\geq 0.24\), the scheme we proposed could satisfy the rate requirement for the cellular while at the same time the achievable rate for D2D communication is appropriate. Furthermore, this scheme provides significant performance improvement in terms of the achievable sum rate of the system. It also can be seen that the choice of power split factor has a great significance on system performance. The transmit rate of the cellular user will increase with a large power split factor. However, enlarging power split factor will also lead to the deterioration of D2D communication performance. Therefore, the selection of power split factor for the relay node is important. Figure 2: Achievable rate for cellular user with different transmission power usage. In Figures 4 and 5, we make a comparison to the HD D2D relay mode in terms of optimal transmit power \(p_{R}^{*}\) and power splitting factor \(\alpha\). Due to transmitting signals in two orthogonal time slots, D2D relay in HD mode uses the maximum power as the optimal transmit power. In contrast, the scheme we proposed uses less power and guarantees the rate requirement for the cellular user which makes it much better in extending the battery life. It also can be observed that the optimal power is decreasing as the channel gain between D2D relay and cellular is stronger in our proposed scheme. Figure 5 shows that the power splitting factor gets smaller as the self-interference becomes weaker in FD mode, which means the D2D relay could share more of its power to support its own communication demand. In Figure 6, with the optimal transmit power and power split factor as we illustrated above, it is obvious that the D2D link can achieve significant rate gain in our proposed FD relaying-based D2D communication scheme.