I. Introduction
Point cloud data, which mostly are collected from Lidar, can represent the 3D structure of the scene. Point cloud processing has been widely studied for 3D scene understanding, and has been used in many fields, including autonomous driving, virtual enhancement, and reality enhancement. Point cloud semantic segmentation is one of the basic point cloud processing tasks, which aims to predict the label of each point. However, the unordered and unstructured characteristics of point clouds make the semantic segmentation of point clouds a challenging task.