I. Introduction
360-degree videos have received more and more attentions from the public in recent years, as they provide immersive scenes for users, increasing the Quality of Experience (QoE) of both video-on-demand and live-video streaming [1]. Many large-scale commercial video service providers such as YouTube, iQIYI and Hotstar have provided services on 360-degree video streaming. These emerging video services help attract new consumers, leading to a potential of great profits for the video streaming business.