I. Introduction
Critical machine-type communications (cMTC), or ultrareliable low-latency communications (URLLC), have been considered as a major use case in the fifth generation (5G) mobile communications [1]. As a candidate 5G technology, the Third Generation Partnership Project (3GPP) is working on evolving the LTE standard to fulfill the requirements of the International Mobile Telecommunications (IMT)-2020 [2]. In this regard, several studies in 3GPP’s Releases 14 and 15 have been conducted to enable cMTC in LTE networks [3]. The proposed enhancements include shortened transmission time intervals (sTTIs) [4], fast uplink access on MAC [5], and support of reduced processing times [4].