Previous decades stood witness to the remarkable achievement of wireless technologies in terms of connecting people all over the world. Recently, there has been growing interest from both academia and industry in another direction – to provide ubiquitous connectivity among machines, autonomous devices, and so forth. Such a paradigm shift from human-type communications (HTC) toward machine-type communications (MTC) is mainly driven by the need to create an Internet of Things (IoT), which holds the promise to revolutionize the way we live and work through seamless interactions between many devices. With the increasing popularity of intelligent transportation, smart city, etc., it is envisioned that the number of IoT devices will reach 75 billion by 2025, which is much larger than the number of the mobile phone users. To provide wireless connectivity to such a large number of devices by the time IoT comes to fruition, the third-generation partnership project (3GPP) has identified massive machine-type communications (mMTC) as one of the three main use cases of the fifth generation (5G) wireless systems.

Compared to its HTC counterpart, mMTC gives rise to new communication models and associated challenges that need to be addressed. For example, high-speed downlink communication is prioritized in HTC, while low-rate uplink communication, e.g., the collection of small payload measurements from a massive number of smart meters, is of utmost importance to mMTC. Therefore, short packet transmission techniques should be investigated in mMTC, where the overhead arising from the control signals needs to be significantly reduced since their size can be similar to that of the data packets. Moreover, the contention-based random access schemes that are widely used for HTC, e.g., ALOHA, may lead to a vast number of collisions in mMTC when there are thousands of IoT devices competing for the access grant. This calls for innovative access strategies, combining random access and scheduling that are tailored to mMTC. Furthermore, novel solutions, such as network slicing of wireless resources, are required to enable the co-existence of HTC and mMTC in future cellular networks.

This Special Issue invites original research and review articles discussing original algorithms, theories, and applications of Massive Machine-Type Communications for IoT.

Potential topics include but are not limited to the following:

• New opportunities/challenges/use cases for mMTC in 5G and beyond
• Traffic patterns and communication models for massive IoT
• Information/communication theory for mMTCCellular IoT networks
• Low power wide area networks and spectrum sharing
• Short packet transmission techniques in mMTC
• Grant-free random access and unsourced random access schemes
• Joint user activity detection and channel estimation
• Machine learning for mMTCPhysical layer security for mMTC
• Network slicing for coexistence of mMTC and HTC
• Orthogonal and nonorthogonal multiple access schemes
• Massive MIMO (multiple-input multiple-output) and cell-free MIMO for mMTC
• Downlink aspects of massive IoT systems