I. Introduction
The explosive growth of mobile multimedia and social networking applications poses challenging multiple access and quality of service (QoS) requirements for 6G [1]. NOMA as a significant technology holds the promise to meet massive machine type communications (mMTC) application scenarios. Hard delay constraint is considered as the critical performance metric of QoS for ultra-reliable and low latency communications (URLLC). URLLC and mMTC are the key functions enabled by 5G. In 6G, an even more critical mission is to rely on URLLC in mMTC [2]. In other words, the scheduling policy to minimize the delay violation probability for NOMA is desired.