1. Introduction on Massive Machine-Type Communications (mMTC)

With the rise of a new round of scientific and technological revolutions and industrial changes in the world, the emergence of a large number of new applications, new businesses, new fields and new markets pose more requirements and challenges to existing mobile communication technologies [1]. To this end, a cellular communication paradigm shift from the fourth-generation (4G) to the fifth-generation (5G) is provoked [2, 3]. In 2012, the European Union (EU) officially launched the mobile and wireless communications enables for the 2020 information society (METIS) project to conduct research on 5G mobile communication networks. In addition to METIS, the EU has launched a larger research project named 5G-PPP. Moreover, the UK government has established a 5G R&D center with a number of companies at Surrey University.