# User-Information-Aware D2D Multicast File Distribution Mechanism
## 1 Introduction  
With the rapid development of Mobile Internet, it is estimated that mobile data traffic will increase by approximately seven times from 2016 to 2021, in which 78% of mobile data traffic is generated by mobile video services [1; 2]. The explosive growth of data traffic will put tremendous pressure on existing mobile communication systems, and D2D communication is recognized as a key technology to solve this problem. D2D communication allows direct data communication between adjacent terminals without the assistance of base stations, which has great potential for alleviating the pressure on base stations, increasing system capacity, enhancing network coverage, and increasing transmission rate [3; 4]. In fact, there is greater redundancy in the transmission of multimedia data such as video and music in the mobile communication system, seriously reducing the efficiency of resource utilization [5]. In order to reduce the redundant data transmission and further improve the system capacity and resources utilization, the use of D2D multicast communication to distribute multimedia content to users has been received widespread attentions and researches in recent years [6; 7; 8]. Cooperative communication is an efficient D2D communication paradigm where devices can serve as relays for each other [9]. The relay-assisted D2D communication occupies a major position in the research of existing D2D multicast communication. The D2D multicast cluster is formed by nearby users with the same service demands, and the cluster head node as relay node selected by base stations assists other nodes in communication. The content requested by the members of theD2D multicast cluster is sent to the cluster head, and then cluster head distributes content to other cluster members by D2D multicast communication. Recently, the relay-assisted D2D communication has been considered in the standardization process of mobile broadband communication systems. For instance, it is proposed in the 3 rd Generation Partnership Project (3 GPP) Long Term Evolution Advanced (LTE-Advanced) to improve the communication quality of cell edge users. Based on the relay-assisted D2D communication and D2D multicast communication, the redundant transmission of multimedia content distribution can be reduced greatly, dramatically improving the efficiency of spectrum and energy, and further mitigating the shortage of spectrum resources. However, the relay node and cluster head node always cause excessive energy consumption during assisting content distribution and its resources also be occupied. The selfishness of the users may cause relay nodes to behave negatively in assisting D2D communication. The user's mobility and social relationships among users can also seriously affect the data transmission performance [10; 11]. What's more, the performance of D2D multicast communication is limited by the worst interference plus noise ratio (SINR). Therefore, formulating a suitable clustering mechanism and selecting the appropriate user as the relay node or cluster head are the key to improve the performance of D2D multicast communication [12; 13]. Currently, relevant research in this field is very active and has achieved fruitful research results. However, by tracking the behavior of users accessing video, [PERSON] et al. [14] found that the amount of video access during the daytime was relatively stable, although the amount of video accessed by users during various hours varied greatly. In other words, videos are heavily accessed by users at any time during the day. For the high popularity videos, the same video in the same area should be transmitted multiple times at different times by using existing D2D multicast communication technology, so there is still much space to improve energy efficiency and spectrum utilization.