# Network Coding for Efficient Video Multicast in Device-to-Device Communications
## 4 System Evaluation
### Analysis of SDR  
In the experiment, first, we tested how many packets do the destination nodes need to receive before re-encoding to achieve higher efficiency. When performing a re-encoding operation, the nodes need to determine when to re-encode the received packets. The value can be at most \(K\), at least 2. When the number of fragments is large, the linearly mixed data has high availability; when the number of packets is too small, the availability of the combined data is low, and the decoding delay will increase significantly. The experimental results are shown in Figure 4.  
In Figure 4, we observe that the best performance can be achieved when the nodes conduct the re-encoding operation after receiving the complete data because the packets obtained after re-encoding will be more useful. If the nodes conduct re-encoding operation after receiving partial data when other nodes receive the re-encoded packet, they need to check whether the re-encoded packet is linearly independent to their packets, which will increase the decoding delay. Furthermore, if the re-encoding operation is conducted after receiving partial data, it will increase the nodes of re-encoded data slices, which will cause channel jam and significantly reduce the transmission efficiency. According to the diagram, it can be seen that if the re-encoding operation is conducted after receiving complete data, the SDR will decrease. Therefore, we choose to conduct a re-encoding operation after the destination node has received \(K\) packets. Figure 3: Encoding Efficiency in Different Mobile Devices. The relationship between video SDR and the number of destination nodes is shown in Figure 5. Simulation experiments are performed at a 5% loss rate, while real experiments are performed under normal circumstances. In these two experiments, the redundancy value of the proposed scheme is 0, and the redundancy values of the other two schemes are 3. According to the simulation experiments, when the number of mobile devices is large, the SDR of the proposed scheme is almost 1, while the SDR of the RLNC-based scheme is about 0.98, and the replication-based scheme is only about 0.64. Besides, our solution was implemented without redundancy. In real experiments, when there are fewer destination nodes, SDR is lower than 0.7. However, when the mobile phone number is 5, the SDR reaches 0.9. With the increase of mobile phone numbers, the SDR could reach around 0.93. It can be known from the simulation results that when the number of devices is further increased, the decoding efficiency of the proposed scheme will be higher. In the other two schemes, the mobile phone number does not have any influence on the SDR, and the SDR is maintained the same. Also, in the traditional replication-based scheme, high redundancy is required in advance. Otherwise, the SDR will become very low, which in turn incurs the poor decoding quality. In the real scenario, the destination nodes are not always next to the source node, and different distances will lead to different packet loss rates. Therefore, we must consider the effect of distance on wireless data transmission. To this end, we have also conducted corresponding experiments. We performed simulation experiments on the performance of the three schemes under different packet loss rates and conducted real experiments on the effect of distance on the SDR. The results are shown in Figure 6. In the simulation experiment, the number of mobile devices is 25, and the redundancy value of RLNC scheme and the replication-based scheme is set at 3. From the simulation experiment  
Figure 4: Influence of different re-encoding scheme on successful decoding rate (SDR). Figure 5: Influence of equipment quantity on SDR. diagram, we observe that the SDR of the three schemes decreases to varying degrees with the increase of the packet loss rate. The scheme proposed in this paper has a tiny decline, the RLNC-based scheme has a moderate decline, and the replication-based scheme has a significant decline.