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 same wireless bandwidth. Furthermore, spectrum sharing is not a viable option for 2G.
The 3G family (UMTS and HSPA) supports the data rate of 1–3 Mbps that exceeds the requirements of most M2M applications. For automotive M2M applications that need a broad range of data rates, 3G is an appropriate wireless network. Compared with 2G, devices, network equipment, and connectivity are more expensive and less power efficient.
4G technologies (LTE and LTE-A) have an “all IP” technology that makes network infrastructure deployments simpler and less expensive than older cellular networks. It offers improved spectral efficiency, greater longevity, bandwidth flexibility, and scalability, all of which meet the requirements for MTC applications. To cope with the increased complexity of the protocols in 4G, high-performance processors in the radios are required. This leads to higher cost in 4G and makes large-scale M2M deployments difficult.
5G technology supports the requirements of MTC design as the forefront of IoT by offering lower cost, better power efficiency, and increased data rate for both terminals and systems. It also offers minimum latency for delay sensitive applications, massive MTC access, and seamless integration of IoT devices, all without QoS deterioration [143]. A number of 5G features also fit well with the M2M path, namely, service creation, service provisioning, and dense deployments.
1.5.2 LTE Features Enhancement
LTE is introduced in 3GPP Release 8 [144]. The foundation of the LTE network is an IP network architecture used for mobile, fixed, and portable broadband access. The all-IP architecture of LTE enables new converging services based on the IP multimedia core subsystem. To achieve high data rates, LTE utilizes a combination of orthogonal frequency division multiple access (OFDMA), multiple-input, multiple-output (MIMO) antenna systems, scalable bandwidth, higher order modulation schemes, and spatial multiplexing in the downlink. LTE works in two modes: time division duplex (TDD) and frequency division duplex (FDD). In 3GPP Release 10, LTE Advanced was introduced as a more advanced version of LTE [145].
LTE enhancements have been included in subsequent releases of LTE standard for different LTE-based devices (i.e., Cat-M, Cat-0) to meet the requirements of M2M/IoT devices [146]. The new versions are called LTE-eMTC, or LTE-M by 3GPP, and promise new generation of devices with lower cost, ubiquitous coverage, and ultra-low battery life.
1.5.3 4G Features Enhancement
The emerging IoT environment is composed of various types of wireless access nodes that require seamless connectivity [147]. The 4G network exploits interoperability and integration between multiple radio access networks (RAN) and radio access technologies (RAT) to create a more solid heterogeneous networking paradigm. This section briefly summarizes the evolution of 4G and its features that accommodate connectivity in IoT.
Coverage extension: Relaying is a key radio access technology in 4G that extends base station range beyond its coverage area [148]. A relaying service is composed of chains of relaying nodes that adopt a suitable transmission scheme (and spectrum) based on the required latency and reliability [149]. With reference to IoT systems, the adoption of relaying services decreases network overload, leading to improved network scalability. It also alleviates the single point of failure issue and provides some mean of fault-tolerant communications in IoT systems.
Enhanced data rate and throughput: 4G networks utilize the concept of licensed and unlicensed carriers’ aggregation, where control-related traffic and non-critical transmissions are sent via licensed and unlicensed bands, respectively. Such techniques can be beneficial for scalable IoT systems that require high throughput.
Power saving: As elaborated in 3GPP Release 12, frequent link quality measurements at the device side are a main source of energy consumption in networks. To address this issue, the power-saving mode was introduced as a viable solution to manage data transmission. In power-saving mode, a device could transmit uplink data at any time. However, for downlink communication, the device is reachable only either when it is active in uplink or is at configurable time instances.
RAN as a service: RAN utilizes radio resource virtualization to create various virtual functions and to expose them via cloud platforms to distribute networks functionalities and management [150]. Also known as RAN as a service, it improves the flexibility of the communication infrastructure and allows IoT systems to self-heal and self-configure.
Device-to-device (D2D) communication: It entails the possibility of data exchange between two devices in the unlicensed band without involving base stations or with just its partial aid [151]. In this technology, devices serve as mobile relays to communicate in IoT environment.
1.5.4 5G Features Enhancement
The 5G cellular network provides advanced wireless connectivity for various use cases and vertical industries and paves the way for Industry 4.0. It exploits different technologies such as network slicing, network function virtualization (NFV), software defined network (SDN), and mobile edge computing (MEC) to pre-allocate resources for both communications and computing [152]. The broadband capability of 5G mobile network facilitates direct and seamless wireless communication from the field level to the cloud and enables new operating models without redesigning the production line for smart
