characterized MTC as a form of data communication between machines in an autonomous manner that does not necessarily require human intervention [128]. MTC denotes two communications scenarios: (1) communication between MTC devices and MTC servers (e.g., in utility smart metering); and (2) direct communication between MTC devices without intermediate server (e.g., IoT) [128]. Although MTC could utilize different types of radio access technologies [129], MTC solutions based on mobile access technologies are of vital importance because cellular MTC offers viable benefits such as mobility, roaming support, robustness against single point of failures, and immediate and reliable data delivery [126, 128]. Moreover, scalability and ease of deployment in cellular MTC can be accomplished via an untethered method. Cellular MTC also excels the ability of connecting devices to the core enterprise systems through a standardized application programming interface (API), in a scalable, real-time, and secure way.
Although MTC offers compelling advantages, it exhibits shortcomings that impact the level of networking and viability of business models. The main challenges are relevant to the diversity of M2M applications and their requirements, energy consumption, and radio resources cost [130]. It also suffers from limited resources in MTC devices (e.g., computation, power resources) and traffic characteristics of MTC applications that are dependent on specific use cases [131]. Such characteristics bring up new technical issues that must be effectively addressed to fully support MTC in cellular systems [126, 130].
1.5.1.1 3GPP MTC Standardization
As mentioned earlier, a number of standard bodies have collaborated on MTC architectures to provide connectivity between shared MTC devices. 3GPP has already specified standardization to promote adaptation and requirements of MTC. There are multiple groups in 3GPP for MTC functions, requirements, and interfaces. The continuous enhancements of 3GPP have appeared in several releases that present ongoing amendments and progresses in standardization works, facilitate introduction of new features, and establish a uniform platform for technologies deployment.
The standardization efforts also focus on optimizing the core network infrastructure to provide efficient delivery of M2M services and minimize operational costs. The first study of 3GPP system architecture (SA1) on MTC was first released in 2007 [132]. In Release 10, 3GPP SA2 put its efforts into identifying MTC communications requirements and system optimizations to address two important challenges in mass-market MTC services: MTC signaling congestion and network overload [133]. Release 10 supports MTC in the Universal Mobile Telecommunications System (UMTS) and LTE core networks. Logical analysis, requirement refinement, and protocol implementation were later introduced in Release 11 [134]. In addition, Release 11 studied network improvements for M2M gateways, P2P communications, co-located M2M devices, and M2M group development. In Release 12, the focus was on identifying key enablers for RF and PHY layer to facilitate LTE deployment in IoT environments [91]. Normative works were pursued in Release 13 to extend MTC coverage and reduce its cost (e.g., bandwidth, transmit power) for cellular IoT deployment. It also focused on identifying multiple categories for new user equipment (UE) [135]. These techniques, namely, NB-IoT and LTE-M, further strengthened in Release 14 and provided novel features such as mobility for service continuity, reduced overhead in network, and support of IoT data in mission-critical use cases [136]. Release 15 and beyond studied additional MTC enhancements for LTE. In 2018, 3GPP foresaw the standardization initiatives and subset of 5G requirements for MTC applications and services.
1.5.1.2 MTC Technical Requirements
MTC is a promising technology for connection of intelligent devices and appliances to the Internet and other networks. Given that 3GPP cellular systems were not primarily designed for machine-type communications, all MTC technical requirements in mobile and cellular technologies should be identified in advance. Some key requirements are as follows:
Low complexity: MTC networks consist of heterogeneous connected devices from multiple vendor equipment and protocols [127]. Hence, a scalable MTC network architecture in a standard format is required to manage system heterogeneity and associated complexity [137]. 3GPP reduces MTC devices complexity by removing the unnecessary features of these devices. For instance, in 3GPP Release 12 and 13, a number of complexity reductions were identified for LTE. Such changes do not impact interoperability with normal 3GPP devices while maintaining IoT requirements.
Increased energy efficiency: A majority of MTC devices are in small size, battery-powered, and located in remote areas. These features imply that recharging and replacement of batteries are infeasible. To prolong the MTC systems’ life cycle, optimization techniques are used to achieve power efficiency in MTC nodes’ sensing and data transmission [138]. These energy-efficient techniques could be applied in the application, network, and link layers.
High coverage: Most industrial applications, such as smart metering and factory automation, require high levels of coverage, and their connectivity model succeeds where nearly the entire network elements are reachable. On the other hand, the large number of network nodes within a cell impacts the achieved QoS. In addition, the extended coverage of the wireless networks in indoor and industrial spaces is challenging and requires large number of base stations that would be very costly. 3GPP proposed a viable approach in Release 12 that improves MTC devices coverage, facilitates a scalable IoT system, and stipulates low complexity without significant increase in overall cost.
Reliability: MTC wireless networks might be unreliable because of interference and noise from adjacent equipment, RF channel fluctuations, and machine interconnections [127]. Given that delivery of sensory data to applications should be reliable in terms of E2E delay [139], some possible solutions such as software reconfiguration of cognitive radios and spatial-temporal redundancy techniques are utilized to improve network reliability. For instance, cognitive radio software reconfiguration enables equipment and terminals to dynamically switch between various wireless modes and adapt to environment changes to decrease RF self-interference and external noise [127]. This technique is exploited in software-defined radio (SDR) systems.
MTC user identification and control: Almost all MTC devices in the market are equipped with subscriber identification module (SIM) that contains crucial information about device profile, identity, and subscribed services. Since network operators support customized MTC services according to the subscription profile, it is essential to regulate access of MTC devices individually and based on the prior defined SIM profiles. MTC user identification impacts decisions of network operators for serving and access of MTC devices. 3GPP defines multiple UE categories in LTE to identify, isolate, and restrict access of MTC devices.
Service enablement and exposure: To ensure connectivity and scalability of MTC systems in emerging industrial IoT, 3GPP requires third-party support to enable required services for MTC. In this context, ETSI and the oneM2M Global Initiative are standardization organizations that collaborate on E2E service enablement. Whereas ETSI focuses on enabling services across servers, gateways, devices, and standard service interfaces, oneM2M develops practical details to tackle requirements of the service layer for M2M communication [140, 141]. Since MTC systems should be able to deal with heterogeneous sensing, it is necessary that application platforms be connected to 3GPP core network via secure interfaces. Additionally, a privacy-preserving approach should be adopted to manage availability of personal information for IoT customers and applications [142].
1.5.1.3 MTC Trade-Off for Different Cellular Generations
Various smart devices are constantly released into IIoT market, and it is difficult to integrate all these parts through communication systems. An important aspect of MTC is to connect devices regardless of type of their cellular and mobile networks, business industries, or machine types. This section briefly reviews different cellular technologies and their trade-offs for MTC deployments.
The 2G family (GSM, GPRS, and EGPRS) is ideal for M2M communication as its power consumption and cost is low. In addition, M2M transmission requires very few bytes that could be effectively handled by 2G. Compared with newer generations, 2G is not spectrum efficient; therefore, its data rate and device management communication are less efficient for the