  A 360 virtual reality (VR) video, recording a scene of interest in every
direction, provides VR users with immersive viewing experience. However,
transmission of a 360 VR video which is of a much larger size than a
traditional video to mobile users brings a heavy burden to a wireless network.
In this paper, we consider multi-quality multicast of a 360 VR video from a
single server to multiple users using time division multiple access (TDMA). To
improve transmission efficiency, tiling is adopted, and each tile is
pre-encoded into multiple representations with different qualities. We optimize
the quality level selection, transmission time allocation and transmission
power allocation to maximize the total utility of all users under the
transmission time and power allocation constraints as well as the quality
smoothness constraints for mixed-quality tiles. The problem is a challenging
mixed discrete-continuous opti-mization problem. We propose two low-complexity
algorithms to obtain two suboptimal solutions, using continuous relaxation and
DC programming, respectively. Finally, numerical results demonstrate the
advantage of the proposed solutions.
