## 2 Related Work  
As an example of the service requiring both multicast and unicast transmissions, we consider a multi-layer VR service. In this service, the base layer may provide the standard quality, and each enhancement layer contains additional data. The new user always requests the base layer. Then, the quality of experience (QoE) can be improved by adding the enhancement layers to the lower one [6]. However, the latter capability of the service is heavily affected by the mmWave NR BS capacity provisioning the service. We also note that additional layers may have different purposes. For example, instead of enhancing quality, it may deliver different angles/perspectives as requested by the user in an on-demand way. To reduce bandwidth consumption when delivering multi-layer services, the authors [7] propose a multi-level SVC coding algorithm for 360-degree video. According to it, a base layer of video is provided to all the UEs, and higher quality is available at users' requests. Thus, instead of losing the user in case of incorrect prediction of being in the viewpoint, SVC only asks for improvement of video quality. This algorithm reduces the likelihood of video freezing and also minimizes the response delays to the changing position of VR displays located on users' heads, however, performance evaluation of this algorithm is not provided. The study of multicast opportunities for efficient 360 VR video transmission is performed in [8]. The authors build a mathematical model that shows the impact of multicast transmission capabilities on improving transmission efficiency with and without transcoding. Particularly, they demonstrate the importance of using transcoding-enabled multicast capabilities in the user transcoding case. Another approach based on multi-session multicasting is presented in [9]. A grouping algorithm allows to achieve better spectral efficiency and obtain an optimal resource allocation using convex optimization. Their algorithm provides the opportunity to create sessions with many tiles of different quality, therefore, the number of sessions is equal to the number of user groups. However, at the same time, users can request several multicast sessions, so the number of multicast sessions and multicast groups can be different. The interesting solution to improve the quality of the provided video with low latency is provided in [10]. In order to provide high throughput along with limited latency for 360 VR streaming, it is proposed to use a multicast scheme at the physical layer, which takes into account changes in content and the location of users, which is especially significant for communication in the millimeter-wave band. In [11], the authors consider the problem of intensive use of VR video bandwidth. The authors noted that such video content is difficult to broadcast with an acceptable level of quality. This article proposed a bandwidth-efficient, adaptive 360 VR video streaming system using a divide and conquer approach. The video was spatially split into multiple chunks during encoding and packaging using MPEG-DASH SRD to describe the spatial relationships of the chunks in 360 degrees. Also, the authors have set clear priorities for fragments in the field of view (FoV). To describe such tiled representations, the authors extended the MPEG-DASH SRD to the 3D space of 360 VR video. The results of this evaluation paper show bandwidth savings of up to 72% when streaming 360 VR videos with little negative impact on quality when compared to other scenarios that did not use onboarding schemes. In [12], the authors outline that multicast transmission is the most suitable option for providing video content to a large audience. This article proposes various schemes for efficiently grouping and allocating resources for multicast transmission in LTE, and the authors propose heuristic algorithms for both of these tasks. In addition, an algorithm based on simulated annealing was formulated to approximate the optimal resource allocation.