  In this letter, we study optimal multicast of tiled 360 virtual reality (VR)
video from one server (base station or access point) to multiple users. We
consider random viewing directions and random channel conditions, and adopt
time division multiple access (TDMA). For given video quality, we optimize the
transmission time and power allocation to minimize the average transmission
energy. For given transmission energy budget, we optimize the transmission time
and power allocation as well as the encoding rate of each tile to maximize the
received video quality. These two optimization problems are challenging
non-convex problems. We obtain globally optimal closed-form solutions of the
two non-convex problems, which reveal important design insights for multicast
of tiled 360 VR video. Finally, numerical results demonstrate the advantage of
the proposed solutions.
