1. The introduction

In recent years, the continuous emergence of new multimedia businesses represented by Augmented Reality (AR,Augmented Reality) and Virtual Reality (VR,Virtual Reality) has brought new challenges to the 5th Generation of mobile communication networks (5G,5th Generation). At present, the content of XR (especially VR) is mainly local content broadcast or wired content transmission, which limits VR application scenarios to a certain extent. Currently XR video transmission only can satisfy the user the basic experience of XR, from high quality XR experience anytime and anywhere there is still a huge gap between the industry application of demand, resolving the real-time wireless transmission of XR is the key to meet XR mobility, especially with the popularization and application of 5 g, how to use the 5 g network support XR development is a hot spot in current industry attention.

Wireless, mixed reality of virtual reality, augmented reality as a very special application, at the same time for large and super reliable bandwidth and low latency have high requirements: under the constraint condition of low latency (VR delay MTP asked 20 milliseconds, AR latency requirements in 5 milliseconds), thousands of megabit per second (Gigabits) data is sent to a user terminal. Therefore, to achieve wireless XR(VR/AR) interconnection, intelligent network design is required to meet reliable, low latency and seamless support for different network scenarios.

2. XR content transmission based on D2D

The demand of high bandwidth in XR undoubtedly puts great pressure on the limited spectrum resources. As one of the key candidate technologies for 5G, it is an important issue worth exploring and studying to utilize D2D's own technical advantages to facilitate XR content transmission.

D2D is device-to-device, also known as terminal pass-through. D2D communication technology refers to a communication mode in which two peer user nodes communicate directly. In a distributed network composed of D2D communication users, each user node can send and receive signals and has the function of automatic routing (forwarding messages). Network participants share some of the hardware resources they own, including information processing, storage, and network connectivity. These shared resources provide services and resources to the network and can be accessed directly by other users without going through intermediate entities. In D2D communication network, user nodes play the roles of server and client at the same time. Users can be aware of each other's existence and self-organize to form a virtual or real group.

D2D allows direct data transmission between users, avoids cellular wireless communication, and does not occupy bandwidth resources, thus greatly improving the utilization rate of spectrum. Resource sharing between adjacent users can provide better user experience. Using D2D communication to improve the efficiency of XR video transmission and ensure the user's QoE may be a feasible solution in future XR application scenarios.

Aiming at the characteristics of HIGH bandwidth, low latency and high reliability of XR, we proposed a D2D based XR content transmission mechanism in 5G scenario, as shown in Figure 1 below:

Content server and MEC server 1) mainly used for storage and transcoding XR content, including MEC server can through intelligent autonomous caching, XR users could be interested in the contents of the cache in advance to the edge of the computing server and advance transcoding, makes the appropriate content closer to users, reduce the content of XR preparation (mainly refers to the transcoding) latency overhead.

2)XR user device (smart glasses or terminal) transmits XR content data through 5G module and 5G network; At the same time, with the help of the D2D module, XR users can transfer XR content to nearby XR users or receive XR content transferred by nearby XR users.

1.png

Figure 1 XR content transmission based on D2D in 5G scenario

Obviously, the XR content transmission based on D2D in the 5G scenario needs to consider the application characteristics of XR and the complexity of the transmission mechanism, which requires in-depth research and exploration. This article mainly from the applicable scenario and the main challenges two main aspects of the brief.

Applicable Scenario:

Through D2D communication function, data transmission can be carried out between adjacent users such as XR content sharing, interactive games, etc. XR users can find interested XR users in adjacent areas through D2D discovery function.

With the increasing growth of multimedia services with high traffic characteristics such as XR, it brings great challenges to the core layer and spectrum resources of the network. Using the local characteristics of D2D communication to carry out local multimedia services can greatly save the network core layer and spectrum resources, and the cellular communication between close XR users can be switched to THE D2D communication mode to realize the unloading of cellular network traffic. Of course, you can multipath XR content by combining D2D with cellular networks.

Major challenges:

The application of D2D technology in 5G communication network can effectively solve the problem of insufficient spectrum resources, and greatly improve the utilization rate of spectrum resources. However, the sharing of spectrum resources may cause interference to users' communication, thus affecting users' communication experience

XR users can receive other users' XR content data through one-hop or multi-hop D2D communication. However, XR is sensitive to delay, so how to select appropriate relay users while ensuring delay is a comprehensive optimization problem.

Close XR users find that it is the key for XR to carry out content transmission based on D2D, especially in XR scenarios with strong interactivity, how to choose users with the same interests or hobbies is the key to meet the delay requirement of XR content to carry out D2D transmission.

3. The conclusion

D2D can be used to transmit data directly between users. Resource sharing between adjacent users can provide better user experience. Aiming at the characteristics of HIGH bandwidth, low latency and high reliability of XR, we propose a D2D based XR content transmission mechanism in 5G scenario. This mechanism uses MEC to cache part of XR content to the edge in advance. Based on D2D communication, XR users can transmit XR content to nearby XR users or receive XR content transmitted by nearby XR users. In short, using D2D communication to improve the efficiency of XR video transmission and ensure the USER's QoE is a feasible solution for future XR application scenarios.

References:

[1]J. Chakareski. Vr/Ar Immersive Communication:  Caching, Edge Computing, and Transmission Trade-Offs[C]. 2017 ACM Workshop on Virtual Reality and Augmented Reality Network. Los Angeles, CA, USA, 2017: 36-41.

[2]Y. Liu, J. Liu, A. Argyriou, S. Ci. MEC-Assisted Panoramic VR Video Streaming Over Millimeter Wave Mobile Networks[J]. IEEE Transactions on Multimedia, 2018, 21(5): 1302-1316

[3]Feng L , Yang Z , Yang Y , et al. Smart Mode Selection Using Online Reinforcement Learning for VR Broadband Broadcasting in D2D Assisted 5G HetNets[J]. IEEE Transactions on Broadcasting, 2020,  66 (2) : 600-611.

[4]Zhan, Cheng, Wen, et al. Efficient Video Transmission in D2D Assisted Mobile Cloud Networks[J]. China Communications, 2016.

[5]Zhang Z , Zhang Q , Zeng T , et al. Modeling and Analysis of Dynamic Social Ties in D2D Collaborative Video Transmission[J]. Discrete Dynamics in Nature and Society, 2020, 2020.

[6]Xiang P , Shan H , Zhang Z , et al. NOMA based VR Video Transmissions Exploiting User Behavioral Coherence[C]// 2020 IEEE Wireless Communications and Networking Conference (WCNC). IEEE, 2020.

[7]Konrad R , Dansereau D G , Masood A , et al. SpinVR: towards live-streaming 3D virtual reality video[J]. Acm Transactions on Graphics, 2017, 36(6CD):209.

[8] Pan Linghui. Research on wireless Return Journey Network Performance based on 5G Multimedia Service [D].

[9]Prasad A , Uusitalo M A , Navratil D , et al. Challenges for Enabling Virtual Reality Broadcast Using 5G Small Cell Network[C]// IEEE WCNC Workshops (WCNCW). IEEE, 2018.