# Energy Efficiency of D2D Multi-User Cooperation
## 4 Simulation Results  
In order to achieve D2D multi-user cooperation with a high energy efficiency in different communication environments, the relationships among the number of cooperative users \(K\), the length of shared data \(L\), the number of date packets \(n\), and energy efficiency are studied in this section. We evaluate the system performance in terms of link outage probability and system energy efficiency. Note that the bandwidth consumed in the D2D communication is 5 MHz and single side band noise power spectral density \(N_{0}\) is \(-174\) dB\(/\)Hz. We suppose that all channel gains followed independent exponential distribution (IED) where variance is 1. The packet error rate can be calculated according to the SNR. We adopt binary quadrature amplitude modulation (QAM) in the Additive Gaussian White Noise (AWGN) channel. Other simulation parameters are listed in Table 1.  
Figure 3 shows the relationships between the average SNR, average delay threshold of D2D link and outage probability. We suppose that the total length of data is \(L=512\) bits, the number of D2D users within cluster is \(K=100\), and the number of data packets was \(n=6\). Figure 3 shows that the outage probability decreased as average SNR increased since the higher SNR indicates better channel quality, which is conducive to successful transmission. The higher delay constraint led to higher outage probability since the link is interrupted when the delay is beyond the maximum delay the D2D user willing to tolerate. Figure 4 shows the relationships between the total length of data \(L\), the number of data packets \(n\) and the energy efficiency. The number of D2D users within the cluster is \(K=100\), the average SNR of D2D link is 10 dB, and the average delay threshold of D2D link is \(\tau_{\rm th}=2\) us. Figure 4 shows that when the total length of data \(L\) is less than a certain value, the energy efficiency of one data packet is higher than the others. This behavior can be attributed to excessive data packets leading to more extraordinary relaying energy consumption on overhead, which wastes energy rather than improving energy efficiency. Meanwhile, the energy efficiency of a six data packet is higher than that of the others when the total length of data \(L\) is larger than a certain value. This is due to the shorter data packet, which reduces the outage probability and ensures a reliable transmission for a long data stream. Figure 5 shows the relationships between the number of D2D users within cluster \(K\), the total length of data \(L\) and energy efficiency. The number of data packets is \(n=6\), the average SNR is 10 dB and the average delay threshold is \(\tau_{\rm th}=2\) us. Figure 5 shows that the energy efficiency of the proposed D2D multi-user cooperation scheme for the case of \(L=512\) bits outperforms other cases in the AWGN environment. This occurs when the number of data packets \(n\) is constant as the excessive long data sequence leads to long data packets creating a high outage probability, which results in decreasing \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Distances between D2D users (m) & 10–25 \\ Transmission power (dBm) & 15–30 \\ Average SNR of D2D link (dB) & 10 \\ Total length of data L (bits) & 256, 512, 1024 \\ Average delay threshold \(\tau\)th (\(\mu\)s) & 1, 2, 3 \\ The number of D2D users within cluster K & 10–300 \\ The number of data packets n \(\tau_{\rm min}^{\mu}\) & 1, 3, 6 \\ Transmission power of cluster head S (dBm) & 24 \\ Constant overhead of each data packet (bits) & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters. Figure 3: Relationships between the average SNR, average delay threshold \(\tau\)th of D2D link and outage probability, \(L=512\) bits, \(K=100\), \(n=6\). energy efficiency. Meanwhile, when the data sequence is short, excessive data packets leads to wasted energy for relaying overhead.