I. Introduction
As one of the most important application scenarios in the sixth generation (6G) wireless communication, massive machine type communications (mMTC) aims to support massive connectivity and sporadic short-packet transmissions in diversified Internet of Things (IoT) applications, such as smart city, smart factory, and smart home, etc., [1] and [2]. It is envisioned that the potential deployment density of MTC devices will reach 107/km2 in the future 6G communication system, which inevitably brings severe challenges to the design of the multiple access mechanism [3], [4].