# Network Coding for Efficient Video Multicast in Device-to-Device Communications
## 1 Introduction  
Device-to-Device (D2D) communication is one of the critical supporting technologies for the fifth-generation mobile communication system [1; 2; 3]. When the mobile devices are close to each other, they are allowed to conduct direct communication without the support of the base station [4; 5], which could significantly increase the bandwidth efficiency. Nowadays, people's demand for mobile video services is growing rapidly, which brings a great burden to base stations, and the characteristics of D2D communication technology bring hope to reduce the burden of base stations. Therefore, in recent years, there have been more and more studies combining D2D communication technology with video services [6; 7; 8]. However, the mobile devices in D2D communications may suffer from interference from the base station and other devices working in the same frequency, which will reduce the transmission reliability. The unreliable transmission will reduce the user experience when transmitting video. During the video transmission process, various problems, such as loss, reversal, and repetition of video frames, may occur. These problems cause abnormal situations at receiving devices, which results in blurriness, crash, green screen, and other problems during video play. The technologies of multicast and broadcast are always used in practical networks because they are faster and more convenient for one-to-many transmission and can significantly improve the throughput of overall networks. However, in such networks, the problems will be more serious [9; 10]. The traditional retransmission mechanism can improve transmission reliability, but it will increase transmission overhead and increase the transmission delay. In the communication environment with interference, it is necessary to improve the transmission efficiency [11]. The technology of network coding had provided a new idea to improve this problem [12]. The network coding theory was proposed by [PERSON] et al. [13], and they showed that the network throughput could be effectively improved through data re-encoding operation at the intermediate nodes of the networks. Network coding has changed the opinion that the intermediate nodes in the network can only forward data, and it is unfeasible to conduct any additional operation. Network coding has attracted attention from many researchers [14; 15]. [PERSON] et al. [16] proposed linear network coding and proved that linear network coding could achieve maximum multicast capacity. [PERSON] et al. [17] extended network coding to an arbitrary network. In order to solve the coding problem in a distributed network environment, [PERSON] et al. [18] proposed Random Linear Network Coding (RLNC), which is considered to be feasible in practical applications. Network coding has been widely studied in video distribution [19; 20], P2P content distribution [21; 22], network security [23; 24] and other fields. Although the theoretical research on network coding has become mature, few studies are focusing on the application of network coding. The main reason is that the intermediate nodes of traditional networks, such as switch and router, cannot perform re-encoding operations, which has generated significant difficulty in its practical applications. The network coding technology has outstanding performance on the aspects of improving transmission reliability and increasing the network throughput [25; 26; 27; 28; 29]. In a D2D network, the devices have powerful computation ability, so it could be regarded as an intermediate node to perform the re-encoding operation. Therefore, the difficulty of implementing re-encoding operation in traditional networks no longer exists in D2D networks, which implies that the merge of these two technologies has good prospects. This paper proposes an RLNC-based video transmission scheme for the D2D communication environment. In this scheme, the re-encoding operation is allocated to other nodes during the transmission.