# Network Coding for Efficient Video Multicast in Device-to-Device Communications
## 2 Related Work  
There are some previous researches which apply network coding in the D2D networks for video transmission [30; 31; 32]. To improve the quality of service of real-time scalable video without additional infrastructure, [PERSON] et al. [30] proposed to use network coding to assist cooperative transmission. In their scheme, the cooperative transmission is assisted by scheduling coding, which can transmit the scalable real-time video flow to multiple devices. As a result, it could maximally improve video quality. They adopted opportunistic network coding because it does not have to wait for the reception of all linearly independent packets, which can significantly reduce the latency for decoding packets with deadlines. However, opportunistic network coding does not give full play to the computing power of intermediate nodes. Our paper makes full use of the computing power of intermediate nodes and reduces the computing overhead of source nodes by carrying out the re-encoding operation at intermediate nodes. Besides, they studied the QoS by evaluating the decoding quality of video frames. In this paper, both the decoding quality of frames and transmission delay are analyzed. [PERSON] et al. [31] studied how to use IDNC (Instantly Decodable Network Coding) in the D2D environment to distribute the real-time video sequences to wireless equipment for cooperative transmission. To solve the conflict caused by the synchronous transmission of multiple devices, they built an IDNC-based scheme to represent all feasible coding and non-conflict transmission decisions in a uniform framework. The simulation results of real-time video sequences show that compared with existing IDNC algorithms, their algorithm can improve the quality of the received video. IDNC is essentially a network coding scheme over Galois Field (GF) with size 2, which makes the linear correlation of the received data packets higher, thus increasing the number of useless packets. When RLNC is used, it works over GF(256), the linear correlation of received data packets is far lower than that over GF(2). Moreover, the receivers in their scheme obtain the complete data through the exchange of data between intermediate nodes. In this paper, the source node and destination nodes will automatically calculate the redundancy according to the network conditions and actively send repair packets to obtain the complete data. [PERSON] et al. [32] proposed a network-coding-based multimedia broadcasting retransmission scheme in D2D networks. Based on the built IDNC diagram, they designed an optimal broadcasting strategy for bulk transmission and sliding window transmission, and proposed a heuristic maximum weight selection algorithm based on network coding (MWSA-NC) to reduce the complexity of optimal search algorithm. The simulation results show that compared to the traditional scheme, their algorithm can improve the system throughput and reduce distortion of video flow. IDNC broadcasting strategy and coding selection algorithm improve some performance for network transmission, but this scheme has coding conflict and transmission conflict between terminal nodes. Although they construct a new IDNC graph to solve the conflict, it inevitably increases some computing burden. In our scheme, we gain performance benefits by re-encoding intermediate nodes, and effectively reduce the computational overhead of source nodes. [PERSON] et al. [33] combined the RLNC technology with video real-time transmission technology to implement a network-coding-based video conferencing system (NCVCS). The system can not only enable multiple people to have an online video conference but also guarantee video quality. In NCVCS, they introduced a coding server with high performance to perform the re-encoding operation. In this paper, the scheme is designed for mobile devices in D2D communications. Currently, most mobile devices have powerful computational abilities. Therefore, in this multimedia multicast scheme, the re-encoding operation is performed on mobile devices.