IoT ’ s Tiny Steps towards 5 G : Telco ’ s Perspective

The numerous and diverse applications of the Internet of Things (IoT) have the potential to change all areas of daily life of individuals, businesses, and society as a whole. The vision of a pervasive IoT spans a wide range of application domains and addresses the enabling technologies needed to meet the performance requirements of various IoT applications. In order to accomplish this vision, this paper aims to provide an analysis of literature in order to propose a new classification of IoT applications, specify and prioritize performance requirements of such IoT application classes, and give an insight into state-of-the-art technologies used to meet these requirements, all from telco’s perspective. A deep and comprehensive understanding of the scope and classification of IoT applications is an essential precondition for determining their performance requirements with the overall goal of defining the enabling technologies towards fifth generation (5G) networks, while avoiding over-specification and high costs. Given the fact that this paper presents an overview of current research for the given topic, it also targets the research community and other stakeholders interested in this contemporary and attractive field for the purpose of recognizing research gaps and recommending new research directions.


Introduction
Telecom operators (telcos) had the most power and influence over business in the information and communication technology (ICT) industry during past decades.This dominance was the result of owning and provisioning communication infrastructures which nowadays have become more of a commodity than a luxury.Telco's revenue streams were mainly based on the provisioning of traditional services, such as voice calls and short message services (SMS).Recent work of regulatory agencies and the appearance of alternative service providers have led telecom operators to form the opinion that alternative service providers have conflicting interests and provide competitive services, thus decreasing telco's revenue from traditional services [1].However, the latest econometric analysis presented in [2] has shown that telecom operators and alternative service providers have aligned interests and their collaboration could be favorable to both parties.The interests of alternative service providers and telcos are not inevitably conflicting, since the economic growth of the alternative service providers are positively correlated with telco revenues and vice versa.
The development of a digital society has changed the traditional value chain and introduced new issues for telcos as they seek a way how to monetize new digital services [3].These services are diversified across various domains of industry, such as agriculture, construction, utilities, transportation, healthcare, finance, etc., and delivered to customers through connected devices building thereby the concept of Internet of Things (IoT).
IoT can be defined as an interconnected network of things/objects that are able to interact with each other and cooperate with other things/objects through wireless and wired connections in order to create new services/applications for the benefit of society.In this way, IoT brings almost limitless benefit's, which have the potential to radically change our daily life by saving time and resources while creating possibilities for growth and innovation [4].IoT has disruptive potential in almost every application domain, which has created many challenges that need to be faced when implementing IoT-based solutions, such as prevailing over obstacles generated by the fragmentation of IoT, both in terms of application domains and in terms of technologies.Therefore, recognizing all potential IoT applications while having in mind the development of technology and the requirements of individuals, businesses, and society as a whole is quite challenging.
Telcos now have the opportunity to seize a share of the value that is generated by IoT implementation.The size of this share will depend on telco's role in the value chain that ranges from being a traditional provider of communication infrastructure to being an end-to-end solution provider [3].In order to monetize IoT, telcos will have to address many challenges which can be summarized as follows: (1) strategic challenges relating to decisions on future directions; (2) business challenges relating to successful management, investment, partnerships; and (3) technical challenges relating to changing connectivity and performance requirements.This paper focuses on the technical challenges since connectivity and performance requirements of IoT objects/things cannot be fulfilled using existing cellular networks that limit numerous IoT applications.In order to overcome issues associated with the current cellular networks, new types of technologies are being introduced leading towards the fifth generation (5G) network [5][6][7].
The architecture of the 5G network has to seamlessly integrate the requirements of diverse IoT applications: from delay-sensitive video applications to ultra-low latency, from high-speed entertainment applications in a vehicle to mobility on demand for connected objects and from best effort applications to reliable and ultra-reliable ones for health and safety [8].A full understanding of emerging IoT applications and the variability of their performance requirements can serve telecom operators as input for specifying 5G enabling technologies.These technologies should be flexible and scalable to meet the aforementioned requirements.Since IoT applications sometimes demand extreme requirements, the 5G network must simultaneously satisfy all of them, which can lead to over-specification and high cost.In order to avoid this, telecom operators first have to adequately classify IoT applications to facilitate the selection of 5G enabling technologies being capable to efficiently meet their performance requirements.
The 5G classification concept includes three different service classes [9], i.e., (1) extreme mobile broadband (xMBB); (2) massive machine-type communications (mMTC); and (3) ultra-reliable machine-type communications (uMTC).Nevertheless, this classification concept can be considered insufficient to properly select 5G enabling technologies to meet the diverse requirements of IoT applications.Furthermore, it will be challenging to classify the emerging IoT applications given that there is many criteria for their categorization.The definition of the IoT domain in many cases overlaps with the definition of IoT application, which may at the same time belong to another IoT domain.For example, Smart Buildings may either be considered as a standalone IoT application or as an element needed to form a Smart City application domain.IoT applications are usually classified according to spheres of human life [4,[10][11][12] or performance requirements [9,[13][14][15].However, these classifications are not suitable for telecom operators which have to fulfil the performance requirements of particular IoT applications, since it is difficult to choose the most important one from a class containing a wide range of IoT applications.
Telecom operators can easily specify performance requirements for current communication services since traffic patterns generated by these services are driven by predictable activities, such as making calls, receiving email, surfing the web, and watching videos.However, traffic patterns generated by emerging IoT applications are driven by less predictable activities.Hence, the idea is to determine a common set of activities based on customer service requests which describe the reasonably foreseeable function/purpose of emerging IoT applications.Then these activities can be used by telcos as a new classification criterion of IoT applications.The assumption is that such a classification can help telecom operators meet performance requirements of future IoT applications and select appropriate 5G enabling technologies.Therefore, the aims of this paper are to classify IoT applications according to relevant activities, specify and prioritize the performance requirements of each IoT application class, and consider enabling technologies used to accomplish specified demands on the radio access part of 5G networks, all according to telco's perspective.In addition, the intention is to provide a literature review which covers 258 references dominantly published in the period between 2012 and 2017.As this paper provides a state-of-the-art review of IoT applications, their performance requirements and 5G enabling technologies, it can also be of use to the research community and other stakeholders interested in this contemporary and attractive field in order to recognize the research gaps and recommend new research directions.
The remainder of the paper is thusly organized.In order to answer research questions, Section 2 describes the methodological approach in regard to the conducted research.Section 3 identifies a common set of activities as a new classification criterion for IoT applications and assigns them the relevant 5G service classes.This facilitates the determining of performance requirements of activity-based IoT application classes, which are summarized and prioritized in Section 4. The enabling technologies used to accomplish these requirements in 5G networks are contemplated in Section 5. Finally, Section 6 discusses the outcomes of the paper by clarifying the contribution of our research in identifying open issues for future work, while Section 7 concludes this paper.

Research Approach and Design
The main objectives of this paper can be summarized as follows: (1) to identify activities relevant to IoT customer service requests and use them as a new classification criterion of IoT applications; (2) to specify and prioritize performance requirements of such IoT application classes; (3) to analyze relevant literature in order to provide insight into 5G radio technologies used to fulfil the requirements of activity-based IoT application classes; (4) to recognize research gaps and directions.
The research questions posed in this study are: (1) Which activities can be used as classification criteria for IoT applications?(2) What are the performance requirements of such IoT application classes?(3) Which enabling technologies in the radio access part of the 5G network can be used to meet the requirements of IoT application classes?
Investigation of different criteria for the classification of IoT applications underlines to what extent various classification concepts have been covered by existing literature and provides a basis for the introduction of a new classification criterion called the activity.We thus surveyed and compared different studies contributing to the understanding of the scope and classification of a wide range of IoT applications.This approach has been motivated by the challenge to propose an activity-based classification of IoT applications, the determination and prioritization of performance requirements of such IoT application classes and the review of 5G enabling technologies used to meet them.These findings can be utilized by telecom operators and other interested parties (e.g., the research community, software network function providers, network infrastructure manufactures, etc.) depending on their interest and potential to utilize these findings towards 5G implementation and commercialization.
The methodological approach to research conducted in this paper is illustrated in Figure 1 and includes four phases.
Phase I included searching, identifying, and extracting papers from three categories, i.e., IoT in 5G service classification, IoT in 5G performance requirements and IoT in 5G enabling technologies.The keywords used to search relevant scholar databases are shown in Figure 2.This search has resulted in the selection of 258 recently published papers, which can be categorized into three groups (i.e., review papers, technical papers, research effectiveness), as shown in Table 1.Review papers summarize the status of knowledge and outline future directions in a given area of research.Technical papers describe the process, progress, or result of research, whereas research effectiveness directly provides an answer to the research question raised for this study.The reference distribution by year of publication is shown in Figure 3a, while the total percentage of references per publication type is presented in Figure 3b.summarize the status of knowledge and outline future directions in a given area of research.Technical papers describe the process, progress, or result of research, whereas research effectiveness directly provides an answer to the research question raised for this study.The reference distribution by year of publication is shown in Figure 3a, while the total percentage of references per publication type is presented in Figure 3b.summarize the status of knowledge and outline future directions in a given area of research.Technical papers describe the process, progress, or result of research, whereas research effectiveness directly provides an answer to the research question raised for this study.The reference distribution by year of publication is shown in Figure 3a, while the total percentage of references per publication type is presented in Figure 3b.

Reference Type Reference Number
Review paper [1,2,10,12, Technical paper [8, Research effectiveness [3][4][5][6][7]9,[11][12][13]29,52, (a) (b) Phase II included the analysis and comparison of papers related to the category of IoT in 5G service classification.Papers were first sorted according to the existing classification criteria of IoT applications in order to underline their deficiencies from telco's perspective.Since emerging IoT applications generate traffic driven by less predictable activities, specifying their performance requirements and enabling technologies is not an easy task from telco's point of view.Therefore, we have proposed a common set of activities to serve as classification criteria of IoT applications.Four activities were selected based on IoT customer service requests denoting the function/purpose of existing IoT applications.According to the authors' knowledge, existing IoT applications can be grouped around following activities: ticketing, tracking, monitoring, and managing/controlling.Such a classification makes it easier for telecom operators to associate particular IoT applications to the relevant 5G service class.Finally, Phase II has resulted in the proposal of an activity-based classification of IoT applications which was associated with the 5G service classification.
Phase III included the analysis of papers related to the category of IoT in 5G performance requirements.Papers were grouped according to eight key performance indicators, i.e., data rate, mobility, latency, connection density, reliability, positioning accuracy, coverage, and energy efficiency.These performance requirements were prioritized indicating high, medium, and low importance of each requirement for specific activity-based IoT application classes proposed in Phase II.Finally, Phase III resulted in the identification of highly important performance requirements for each activity-based IoT application class which can be utilized by telcos to identify 5G enabling technologies used to meet them.
Phase IV provided a review of papers from the category of IoT in 5G enabling technologies.Papers were first sorted according to eight technological groups used in the radio access part of 5G networks, i.e., wide and flexible bandwidth technology, advanced modulation and coding, duplexing, multiple access and waveform, advanced interface management, access architecture related radio technologies, energy related technologies, and other technologies.These technologies have been discussed in terms of their possibility to meet the high priority performance requirements of mMTC activity-based IoT application classes, since this area is more mature from telco's perspective.The results of Phase III were reflected in a proposal of technologies in the radio access part of 5G networks that can be used to meet the high priority performance requirements of the mMTC activity-based IoT class, as well as the identification of research gaps and directions for future work.

IoT in 5G Service Classification
As mentioned in the Introduction, various challenges awaiting telcos can be identified on the basis of the role that they will play in the IoT value chain.Regardless of this role, telcos will face many technical challenges, such as the necessity for global deployment, the need for rapid infrastructure scaling, unpredictable IoT application behavior, etc.Even now, existing IoT applications accelerate the growth rate of traffic driven by less predictable activities which require new strategies towards 5G networks.These networks will be the key enabler for IoT applications by providing a unified Phase II included the analysis and comparison of papers related to the category of IoT in 5G service classification.Papers were first sorted according to the existing classification criteria of IoT applications in order to underline their deficiencies from telco's perspective.Since emerging IoT applications generate traffic driven by less predictable activities, specifying their performance requirements and enabling technologies is not an easy task from telco's point of view.Therefore, we have proposed a common set of activities to serve as classification criteria of IoT applications.Four activities were selected based on IoT customer service requests denoting the function/purpose of existing IoT applications.According to the authors' knowledge, existing IoT applications can be grouped around following activities: ticketing, tracking, monitoring, and managing/controlling.Such a classification makes it easier for telecom operators to associate particular IoT applications to the relevant 5G service class.Finally, Phase II has resulted in the proposal of an activity-based classification of IoT applications which was associated with the 5G service classification.
Phase III included the analysis of papers related to the category of IoT in 5G performance requirements.Papers were grouped according to eight key performance indicators, i.e., data rate, mobility, latency, connection density, reliability, positioning accuracy, coverage, and energy efficiency.These performance requirements were prioritized indicating high, medium, and low importance of each requirement for specific activity-based IoT application classes proposed in Phase II.Finally, Phase III resulted in the identification of highly important performance requirements for each activity-based IoT application class which can be utilized by telcos to identify 5G enabling technologies used to meet them.
Phase IV provided a review of papers from the category of IoT in 5G enabling technologies.Papers were first sorted according to eight technological groups used in the radio access part of 5G networks, i.e., wide and flexible bandwidth technology, advanced modulation and coding, duplexing, multiple access and waveform, advanced interface management, access architecture related radio technologies, energy related technologies, and other technologies.These technologies have been discussed in terms of their possibility to meet the high priority performance requirements of mMTC activity-based IoT application classes, since this area is more mature from telco's perspective.The results of Phase III were reflected in a proposal of technologies in the radio access part of 5G networks that can be used to meet the high priority performance requirements of the mMTC activity-based IoT class, as well as the identification of research gaps and directions for future work.

IoT in 5G Service Classification
As mentioned in the Introduction, various challenges awaiting telcos can be identified on the basis of the role that they will play in the IoT value chain.Regardless of this role, telcos will face many technical challenges, such as the necessity for global deployment, the need for rapid infrastructure scaling, unpredictable IoT application behavior, etc.Even now, existing IoT applications accelerate the growth rate of traffic driven by less predictable activities which require new strategies towards 5G networks.These networks will be the key enabler for IoT applications by providing a unified infrastructure capable of meeting the high variability of their performance requirements [8,16].Various stakeholders have recently described what 5G networks may be and grouped the major types of 5G services into three different classes: (1) xMBB-Requiring extremely high data rates and low-latency communication; (2) mMTC-Requiring scalable connectivity for an extremely large number of devices; and (3) uMTC-Requiring ultra-reliable low-latency and resilient communication [198].This 5G service classification can be used by telcos to identify performance requirements of various IoT applications, and, as such, was used in this research study.However, emerging IoT applications will have extreme requirements, so an analysis based solely on 5G service classifications may not be sufficient.Therefore, telcos need a more precise classification of IoT applications which is an important precondition for meeting their diverse requirements.

Existing Classifications of IoT Applications
In order to meet the requirements of a wide range of IoT applications, they have to be classified in an appropriate manner.The existing approaches to the classification of IoT applications are summarized in Table 2. IoT serves different user categories, including individuals, businesses, and society as a whole, and may span through a broad range of application domains [4,[10][11][12], such as transportation and logistics, healthcare, smart environment, personal and social, futuristic applications, food sustainability, smart living, smart manufacturing, smart energy, smart city, etc.These application domains are created to be human-centric, which means that they cover different domains of human life.IoT applications belonging to these domains have diverse requirements for 5G networks, and should not be treated equally.Therefore, the acceptance of these domains as a classification criterion raises the issue of assigning IoT application to a particular domain.Moreover, the emergence of new applications of IoT may require defining new application domains, which makes this classification inappropriate from telco's perspective.
The M2M applications may be classified by the mobility and the amount of dispersion that needs to be supported into four categories [13]: (1) fixed and concentrated; (2) fixed and dispersed; (3) mobile and concentrated; and (4) mobile and dispersed.However, the mobility and amount of dispersion present very rough classification criteria and cannot meet the precise network requirements of individual IoT applications.
Additionally, M2M applications can be grouped according to delay tolerance into four categories [14]: (1) elastic (delay tolerant); (2) hard real-time (delay constraint); (3) delay-adaptive (delay sensitive but tolerant); and (4) rate-adaptive application (adjust their transmission rates according to available radio resource).However, the main drawback of this classification is the lack of consideration of other IoT application requirements except delay tolerance, although its importance as a classification criterion has been recognized.
According to data reporting mode, the M2M applications can be classified into five categories [91]: (1) time-driven; (2) query-driven; (3) event-driven; (4) continuous-based; and (5) hybrid-driven.This classification is specific for the former IoT concept, which is narrower in nature than the definition adopted in this study.
Another study considered the reliability, availability, and end-to-end latency in order to classify IoT applications into two groups [15]: (1) monitoring-based and mission-critical; (2) monitoring-based and non-mission critical; (3) control-oriented and mission-critical; and (4) control-oriented and non-mission critical.Monitor-based IoT applications periodically collect sensor data from smart objects and transmit them.The majority of monitoring-based IoT applications are not mission-critical.Control-oriented IoT applications use sensor data to control actuators in real-time, and rely on mission critical communication.This classification is based on multiple criteria and represents the precursor of the Mobile and wireless communications Enablers for Twenty-twenty Information Society (METIS) 5G service classification.Data reporting mode (1) Time-driven (2) Query-driven (3) Event-driven (4) Continuous-based (5) Hybrid-driven [198] Reliability, availability, and end-to-end latency (1) Monitoring-based and mission critical (2) Monitoring-based and non-mission critical (3) Control-oriented and mission critical (4) Control-oriented and non-mission critical [15] Characteristics and requirements mMTC and uMTC [12] Legend: IoT (Internet of Things), mMTC (massive Machine Type Communication), uMTC (ultra-reliable Machine Type Communication).
Although it spans through a wide range of different applications, MTC can be divided in two main categories, i.e., massive and ultra-reliable MTC, which depend on their characteristics and requirements [89].As mentioned above, this categorization is a part of 5G service classification which was used in this study as a basis to identify performance requirements of IoT applications and will be described in more detail in Section 4. Massive MTC (mMTC) typically involves a very large number of devices (tens of billions [9]), such as sensors, actuators, and similar devices [12], different in complexity and cost [9], and with varying quality of service (QoS) requirements.These devices should be of very low cost with very low energy consumption, enabling very long battery life [12].At the same time, the amount of data generated by each device is normally very small, and very low latency is not a critical requirement [12].Ultra-reliable MTC (uMTC) requires very high reliability and availability, and very low latency [9,12].Low device cost and energy consumption are not as critical as they are for mMTC applications [12], and the number of devices and required data rates are relatively low [198].
The vision of a pervasive IoT requires the integration of various domains into a single and unified domain.It addresses the enabling technologies needed for these domains while taking into account the elements that form the third dimension like security, privacy, trust, and safety [11].The current classification criteria and IoT application classes do not clearly differentiate IoT domains from IoT applications.IoT domains are usually viewed as a specific area of IoT applications that support a huge variety of use cases across industries impacting businesses and customers.Therefore, IoT domains overlap with IoT applications within existing IoT classifications.Moreover, IoT classification based on certain QoS parameters is difficult to apply from telco's perspective, because it is challenging to choose the most important parameter for the wide range of IoT applications.However, these problems need to be solved for upcoming 5G networks.

Activity-Based Classification of IoT Applications
The aforementioned issues motivated us to propose a new approach to the classification of IoT applications.This approach is based on "the activity", which primarily characterizes specific IoT application.In this sense, the activities are defined as new classification criteria, which denote a main function/purpose of specific IoT application observed from telco's point of view.Telecom operators observe the technical challenges of IoT through systems, tools, devices, and platforms because their availability and integration complexity determines the opportunity to capture a share of the value that is generated by IoT implementation.Based on the literature review, it was found that the most commonly mentioned terms in this sense were related to: (1) ticketing system [201]; (2) monitoring: devices [199,[201][202][203][204]/services [199,201,202,205,206]/tools [207]/systems [11,12,199,201,205,206,[208][209][210]/ data [95,201]/framework [12,201]/solutions [20,205]/networks [211]/process [7]/activity [203,204,207]; (3) tracking technologies [4]/devices [204]/systems [204]/applications [11,210]/system [201]/services [202,205]; (4) managing/controlling applications [15,210,212]/operation [211]/services [4,208]/tools [208,211]/devices [204]/system [95,198,204,210]/concepts [211]/solutions [95]/platforms [95,199].This has led us to identify four activities, i.e., ticketing, monitoring, tracking, and managing/controlling, as new classification criteria of IoT applications.According to the authors' knowledge, it was found that these activities can cover reasonably foreseeable functions/purposes of IoT in existing application domains.Using activities as classification criteria, telcos can specify IoT application performance requirements more easily, which are in that case dictated by the specific activity, not by the IoT application domain, and determine enabling technologies in the radio access part of 5G networks.
In addition, our classification approach allows better service differentiation and service delivery closer to customer expectations.According to BH Telecom's experience, customers typically come up with the following service requests when it comes to IoT: (1) they need to track their products and determine products' distribution across different regions based on their own data analysis; (2) they want to monitor their products since they do not have a department for supervision and analysis; (3) they need information panels about their outlets and working time to be downloaded by a scanning tab-ticket; (4) they want to manage/control their products according to market needs.This way of expressing customer needs has inspired us to propose the activity-based classification of IoT applications as it allows telcos to define performance requirements more precisely, and thereby improve the customer experience.
The activity-based classification of IoT applications is presented in Table 3.Each activity is associated with the application domain where that activity may be applied.The requirements of identified activities and associated application domains are then mapped to 5G service classes in order to be further able to identify their performance requirements and enabling technologies necessary to meet them.For example, according to Table 3, the managing/controlling activity can be realized in several domains, such as healthcare, food, energy, transportation and logistics.For each domain, we have identified an application example, such as remote surgery [213] in the healthcare domain, food processing facilities [96] in the food domain, energy distribution [12] in the energy domain and traffic/driving [12,213] in the transportation and logistics domain.According to diverse performance requirements of these IoT applications [214], managing/controlling activity in healthcare, food, transportation, and logistic domains can be associated with the uMTC service class, while the energy domain can be mapped to the mMTC service class.A more detailed description of performance requirements for each activity-based class of IoT applications is provided in Section 4, while in Section 5 it is discussed how these requirements can be accomplished in the radio access part of 5G networks.Smart food/water monitoring uMTC Food processing facilities [96] Smart Transportation and Logistics uMTC Traffic, driving [12,213] Smart Energy mMTC Energy distribution [12,98] Legend: 5G (Fifth Generation), POS (Point Of Sale), mMTC (massive Machine Type Communication), uMTC (ultra-reliable Machine Type Communication).
As such, the activity-based classification of IoT applications can be used for the creation of new business models, which represent the stakeholder's plan to generate revenue and make a profit from operations, and thereby include many components and functions of the business [21].However, there is no common opinion which components constitute a business model.The business model architecture can be illustrated by four dimensions [22]: (1) who, identifying the definition of the target customer as one central dimension in designing a new business model; (2) what, describing what is offered to the target customer; (3) how, referring to the construction and distribution of the value proposition; (4) value, explaining why the business model is financially viable.Answering these four questions allows the creation of IoT business models.According to this business model definition, the activity-based classification of IoT applications affects the who and the how dimensions in the following manner.In terms of the who dimension, it directly allows customer segmentation according to considered activities as a way to express their requirements (e.g., customers that require tracking or monitoring of their products).Being aware of the current customer requirements, the proposed classification of IoT applications around four activities (i.e., ticketing, monitoring, tracking, and managing/controlling) can be considered complete.At the same time, the activity-based classification of IoT applications is flexible since additional activities as classification criterion can be concerned with emerging customer requirements.On the other hand, the activity-based classification of IoT applications indirectly impacts the how dimension of new business models which among others includes relevant resources and capabilities in the focal stakeholder's internal value chain.This indicates that the proposed activity-based classification of IoT applications can be easily applied to IoT business models.
Various categories of IoT business models can be identified according to eight IoT architectural layers [23], i.e., collaboration and processes layer, application layer, service layer, abstraction layer, storage layer, processing layer, network communication layer, physical layer.Along with IoT architectural layer, the IoT business model needs also to address the IoT value proposition [24] and IoT stakeholders that can participate in more than one layer.Based on their role in the IoT ecosystem, telecom operators usually take a part in the network communication layer.In this regard, they follow four evolutionary business models [23]: (1) selling connectivity services only; (2) selling third-party products; (3) selling internal products; (4) providing a broad menu of IoT products.Each of these IoT business models includes traditional telcos strength, i.e., connectivity which determines the how dimension of the IoT business model.In this sense, our activity-based classification of IoT applications indirectly allows the construction and distribution of performance as IoT value proposition using enabling technologies as IoT resources in telco's internal value chain.

Summary of IoT in 5G Service Classification
The previous discussion has shown that the classification of IoT applications is a complex task due to their numerosity and diversity.The existing classifications of IoT applications pose some drawbacks that can be summarized as follows.The domain-based classification does not allow clear differentiation between IoT domains and IoT applications due to either an imprecise classification criterion or the diversity and unpredictability of IoT applications.On the other hand, QoS-based classifications need to identify the common and most important performance metric for a broad range of IoT applications.Therefore, on the basis of the literature review and IoT customer service requests, we have proposed a new approach to the classification of IoT applications.It is based on the activity as new classification criterion, which denotes a main function/purpose of specific IoT application observed from telco's point of view.According to the authors' knowledge, four activities, i.e., ticketing, monitoring, tracking, and managing/controlling, have been identified to cover reasonably foreseeable functions/purposes of existing IoT applications.The resulting activity-based IoT application classes have been associated with 5G service classes in order to determine and prioritize their performance requirements as described in next section.Finally, the proposed classification of IoT applications was discussed in terms of its completeness, flexibility, and applicability to new business models.

IoT in 5G Performance Requirements
This section provides an insight into performance requirements of activity-based classes of IoT applications proposed in Section 3. The analysis is based on eight key performance indicators identified in [5,207] as shown in Table 4: data rate, mobility, latency, connection density, reliability, positioning accuracy, coverage, and energy efficiency.These performance indicators are usually well described for specific IoT applications.However, one of the main challenges of 5G is to support a variety of performance requirements for numerous IoT applications in a flexible, reliable, and cost-effective way [15].Hence, there is a need for a comprehensive understanding of these requirements for activity-based IoT application classes.The 5G service classification defined the performance requirements for mMTC and uMTC [9].We have assigned these requirements to the activity-based IoT application classes introduced in Section 3 for the purpose of proposing priorities of each requirement for a specific class.Three levels of priorities (high, medium, low) are associated with the performance requirements of activity-based classes of IoT applications as shown in Table 4.The prioritization of performance requirements is inspired by analysis of related work undertaken in [213].Some activity-based classes of IoT applications may demand optimization of multiple performance requirements.Table 4 illustrates the main differences between activity-based IoT application classes, and therefore, the need for a 5G network that enables support of optimal configurations for a variety of, sometimes opposite, requirements.For example, the mMTC tracking-based IoT application class requires support for high mobility, high positioning accuracy, and high connection density, while the mMTC monitoring-based IoT application class also requires high connection density but low mobility and low positioning accuracy.A more detailed description of considered performance requirements is contained in the following subsections.

Data Rate
Data rate is the most important evaluation factor for generations of wireless communication networks [6].It is contemplated in two ways: (1) peak data rate-defined as the maximum achievable data rate by the user; and (2) minimum guaranteed user data rate-defined as the minimum experience data rate by the user [16].New mobile technologies are primarily driven by users' needs for higher data rates, as discussed in [8,16,99,215,216].The expected values in 5G networks are 10 Gbps for minimum peak data rate and 100 Mbps as minimum guaranteed user data rate [5].High data rate requirements are mainly posed by xMBB related use cases [9] like hotspots [214] or dense urban areas where in 95% of locations and time experience data rate by user should be 300 Mbps in downlink and 60 Mbps in uplink [100].High data rate is also important in some activity-based classes of IoT applications, as shown in Table 4. Medium (e.g., monitoring of parking spaces) to high (e.g., monitoring of home video) data rate is needed in cases of mMTC monitoring activities.High data rate can be achieved by using a millimeter wave (mmWave) spectrum to share multi-gigabit data in the surrounding environment (i.e., mMTC monitoring with high data rate requirement) and to recognize an object via cloud in real time to find the optimal driving strategy instantaneously for IoT-autonomous vehicles application [101].Data rates generated during transmissions of tracked medical assets, transport fleets, and ticked point of sale (POS) terminals is low (e.g., medium importance for ticketing and tracking activities in mMTC).In addition, low data rates are needed in uMTC monitoring and controlling (e.g., monitoring of health condition, control of driving).

Mobility
Mobility is defined as relative velocity between the receiver and the transmitter [16].The applications of IoT pose very diverse requirements for mobility in 5G networks, which range from static to high mobile, even up to 500 km/h [16,102,207].Use cases in which except ultra-high mobility, ultra-high traffic volume density, and ultra-high connection density are needed may be quite challenging for 5G networks [215], like V2X communication [5].High mobility is a very important requirement for mMTC tracking activity (e.g., tracking of assets in high speed trains [214]).Moreover, the support for high mobility is needed in uMTC management activities, if the monitor/control object is moving (e.g., high speed trains [207]).Low mobility is needed for ticketing, monitoring, and managing/controlling activities in the mMTC class, as shown in Table 4.This is the reason why 5G networks should not assume mobility for all devices and services but rather provide mobility on demand [8].

Latency
Latency requirements are usually expressed in terms of end-to-end (E2E) latency perceived by the end user [8].5G networks should enable "zero latency" [103] represented by the millisecond level of E2E latency [16,26,27,213] through significant enhancements and new technology in architecture aspects [6,28,214], such as device-to-device (D2D) communication [5].The IoT applications associated with uMTC monitoring and managing/controlling activities require low latency, as they tend to be real time.Required latency levels depend on the particular IoT application [8,16,28], being the narrowest in uMTC managing/controlling activity with a value that should not exceed 1 ms [8].For example, the tolerable delay for use case mobile health [215] and remote surgery application is in order of sub-milliseconds [213].For V2X communications, latencies should be ultra-low for some warning signals [8].

Connection Density
Connection density is defined as the number of connected devices per unit area [16].It can usually be expressed in terms of an extremely high number of simultaneous active connections, such as 1 million connections per square meter [207,215], or 10 to 100 times higher number of connected devices [29].The performance values of connection and traffic density for various 5G services are listed in [8,212].This performance requirement is highly correlated with identified activity-based classes of IoT applications, since it is the main characteristic of mMTC.High device density brings the need for rapid internet protocol version 6 (IPv6) deployment, and high quality security algorithms and techniques, which lead to new system design and implementation, as described in [104].

Reliability
Reliability is the maximum tolerable packet loss rate at the application layer [105].5G must bring a reliability of 99,999% [27,30,31,212], or higher for specific use cases [212], (e.g., tele-protection in a smart grid network [207] or driverless cars [106]).Reliability is the main characteristic of uMTC monitoring and managing/controlling activities.Reliability will be a particularly challenging task in high-speed trains because of speed, load, and cell distance [16,107].

Position Accuracy
Position accuracy is the maximum positioning error tolerated by the application [105].5G should ensure accurate positioning of the device outdoors [108] with accuracy from 10 m to <1 m on 80% of occasions and better than 1 m in indoor deployment [212].Accuracy positioning is very important in uMTC monitoring-based activities (e.g., monitoring remote cameras), and uMTC managing/controlling-based activities (e.g., driving) [213].Moreover, the mMTC tracking-and ticketing-based activities pose high performance requirements in terms of position accuracy.

Coverage
Coverage requirements in 5G should provide connectivity anytime and anywhere with a minimum user experience data rate of 1 Gbps [32].However, the perception of 100% coverage is rather a technical decision than a business one [33], which could be extended through ultra-cell deployment [31] and vehicle-to-infrastructure (V2I) communications [30].Almost every activity-based IoT application class requires very high levels of coverage (99,999% availability) [207].Total coverage will enable new unmanned aerial vehicles (UAV) to use single network connection, instead of connection steering mechanism, as one described in [109].

Energy Efficiency
Energy efficiency is defined as the number of bits that can be transmitted per joule of energy [216].Compared with current wireless technologies, the energy efficiency (measured in b/J) of the 5G network may need to be improved by a factor of 1000 [4,7,34,110,217].High energy efficiency is important in case of ticketing-and in some cases of tracking-based activities (e.g., smart industry [205] or implantable medical devices [35]).Other activities, such as both mMTC and uMTC monitoring-and managing/controlling-based activities (e.g., health condition [205]) and some tracking-based activities (e.g., sports wearables [205]) require medium energy efficiency.Energy efficiency is very important design objective for the reduction of operating costs of telecom operators, as well as for minimizing the environmental impact of the wireless domain [217].On a higher layer of the network protocol stack, adaptive base station switch on/off algorithms use renewable energy sources to save energy [218] along with an energy scheduler, as in the with heating, ventilation and air conditioning (HVAC) [111].However, at the physical layer, adaptively switching off unused carriers is a key strategy that can be used to save energy from the radio-frequency (RF) transceiver chain of base stations [218].

Spectrum Efficiency
Spectrum efficiency is defined as the data throughput per unit of spectrum resource per cell or per unit area (bps/Hz/cellar bps/Hz/km 2 ) [216].In order to achieve network sustainability, required for 5G networks [216], spectrum efficiency needs to be improved 3-5 times [20,32,216].Minimum peak spectrum efficiency is 30 bps/Hz for downlink and 15 bps/Hz for uplink [32].This is mostly important for the xMBB services class [212,214].Since this performance requirement is not relevant for activity-based IoT application classes which are associated with the mMTC and uMTC service class, it has not been further considered herein.

Summary of IoT in 5G Performance Requirements
Activity-based classes of IoT applications are associated with performance requirements of 5G service classes, i.e., mMTC and uMTC.On the basis of the literature review, each activity-based class (ticketing-, tracking-, monitoring-, and managing/controlling-based class) is associated with mMTC service class, while the monitoring-and managing/controlling-based classes are the only ones linked to the uMTC service class.According to the authors' best knowledge, there is no available literature concerning the uMTC ticketing-and tracking-based classes of IoT applications or such IoT customer service requests.
Activity-based IoT application classes pose many performance requirements, which have been discussed in terms of data rate, mobility (speed), latency, connection density, reliability, positioning accuracy, coverage, and energy efficiency.These performance requirements have been assigned three-level priorities (high, medium, low) for the purpose of facilitating identification of the enabling technologies used to fulfill them.Based on the literature review, it was found that each activity-based IoT application class poses high requirements in terms of connection density except the uMTC managing/controlling-based IoT application class.Moreover, ticketing-and tracking-based IoT application classes have high requirements in terms of positioning accuracy, coverage, and energy efficiency.Additionally, positioning accuracy is a highly important performance requirement for the uMTC managing/controlling-based IoT application class, whereas coverage is important for both uMTC and mMTC monitoring-based IoT application classes.Finally, latency and reliability represent highly important performance requirements for uMTC monitoring-and managing/controlling-based IoT application classes.A deep and comprehensive understanding of performance requirements of each activity-based IoT application class may facilitate the selection of 5G enabling technologies needed to meet them as described in the following section.

IoT in 5G Enabling Technologies
Activity-based classification of IoT applications proposed in Section 3 allows telecom operators to identify performance requirements of each class relying on 5G service classification, as discussed in Section 4.However, these performance requirements cannot be satisfied for many IoT applications with current cellular (2nd, 3rd, and 4th generation) network technologies, since they limit their potential due to many issues [3], i.e., protocol implementation complexity, poor coverage in non-urban environments, high cost of networking equipment and data transmission.In order to overcome these issues, many dedicated communication technologies are being installed.
From the very beginning of IoT, many proprietary technologies, such as radio-frequency identification (RFID), wireless highway addressable remote transducer (WirelessHART) or Z-Wave, have first appeared, and then, more generic ones, such as Bluetooth, IEEE 802.15.4,IPv6 over low-power wireless personal area networks (6LoWPAN).However, none of these technologies have become a market leader mainly because of technology shortcomings and business model uncertainty [219].Hence, new solutions, such as low-power wireless fidelity (LP Wi-Fi), low-power wide area (LPWA) or several improvements for cellular M2M communications have become serious candidates for IoT implementation.LP Wi-Fi is an IEEE 802.11ah standard designed to extend the application area of Wi-Fi networks in order to meet IoT requirements (i.e., large number of devices, large coverage range, energy constrains).First performance studies indicate that this standard will support a broad range of M2M scenarios with a required QoS level, and enable scalable and cost-effective solutions.In addition, LPWA networks have been deployed for some time in the form of many different proprietary solutions (e.g., Amber Wireless, Coronis, Huawei's CIoT, LoRa, M2M Spectrum Networks, Sigfox, Weightless, etc.), but only the LoRa Alliance, Sigfox and Weightless are involved in LPWA standardization activities.Despite some drawbacks which are mainly related to use of an unlicensed spectrum, LPWA networks are expected to become a key enabler for IoT deployment in early market rollouts and for limited IoT applications [219].
The appearance of these IoT communication technologies was considered as competition to current cellular networks from the telco's point of view from the start.But, in the meanwhile, telcos have realized that the aforementioned communication technologies can be utilized to meet the changing connectivity and performance requirements of IoT applications.Therefore, many dedicated communication technologies have already been deployed in various IoT applications.For example, Orange, Swisscom and South Korea (SK) Telecom have built nationwide networks based on LoRa [3], whereas Deutsche Telekom (DT), Vodafone and all three Chinese operators have completed the rollout of narrowband IoT (NB-IoT), as another LPWA standard utilizing existing long term evolution (LTE) networks [36].In this context, standardization and interoperability becomes critical because there is a need to consider a broad range of connectivity solutions as presented in Figure 4 [37].One of those solutions includes satellite technologies which integrated with 5G radio technologies form the 6th generation (6G) standard for providing global coverage [38].In this regard, standardization activities of 5G and beyond are being undertaken by several standards bodies, such as the Institute of Electrical and Electronics Engineers (IEEE), the 3rd Generation Partnership Project (3GPP), the Internet Engineering Task Force (IETF), ITU Radiocommunication Sector (ITU-R), and ITU Telecommunication Standardization Sector (ITU-T).An overview on these standards bodies and their effort to develop communication standards for 5G and beyond are provided in [112].Due to the envisioned 5G applicability, these standards have defined an air interface that can significantly improve the performance, and network architectures that allow the deployment and coexistence of various 5G technologies.A detailed discussion of these technologies regarding both technological and standardization aspects indicates that 5G can be perceived as the main driving force for enabling the vision of a truly global IoT [219].
In this sense, the technologies described in this section will allow 5G networks to form a unified communication infrastructure for the realization of a wide range of IoT applications.These technologies will bring performance enhancement which will trigger a complete revolution in almost all spheres of human life creating a new "everything connected" era.Future 5G networks in such an era will face a serious problem regarding a huge number of different service types [220].To meet the diverse requirements for a huge number of IoT applications, 5G introduces the concept of network slicing to offer programmable network instances [220].Using network slicing, multiple independent In this regard, standardization activities of 5G and beyond are being undertaken by several standards bodies, such as the Institute of Electrical and Electronics Engineers (IEEE), the 3rd Generation Partnership Project (3GPP), the Internet Engineering Task Force (IETF), ITU Radiocommunication Sector (ITU-R), and ITU Telecommunication Standardization Sector (ITU-T).An overview on these standards bodies and their effort to develop communication standards for 5G and beyond are provided in [112].Due to the envisioned 5G applicability, these standards have defined an air interface that can significantly improve the performance, and network architectures that allow the deployment and coexistence of various 5G technologies.A detailed discussion of these technologies regarding both technological and standardization aspects indicates that 5G can be perceived as the main driving force for enabling the vision of a truly global IoT [219].
In this sense, the technologies described in this section will allow 5G networks to form a unified communication infrastructure for the realization of a wide range of IoT applications.These technologies will bring performance enhancement which will trigger a complete revolution in almost all spheres of human life creating a new "everything connected" era.Future 5G networks in such an era will face a serious problem regarding a huge number of different service types [220].To meet the diverse requirements for a huge number of IoT applications, 5G introduces the concept of network slicing to offer programmable network instances [220].Using network slicing, multiple independent and dedicated network instances can be created within the same infrastructure to run services that have completely different requirements for latency, reliability, throughput, and mobility [220].This technology primarily targets a portion of the 5G core network, but also indicates that 5G radio access network (RAN) functionalities can be different for each network slice [221].To deploy network slices, network functions need to be enabled on an on-demand basis, which has driven the use of virtualization and softwarization in 5G core network [221].Network function virtualization (NFV) is a paradigm that enables that 5G network functions to run in a virtual environment instead of dedicated hardware [221].The software define networking (SDN) paradigm facilitates isolation of network slices avoiding the traffic of one slice affecting the performance of another slice [221].These two paradigms make the network much more dynamic, agile, on-demand, and flexible [222,223].Since the aforementioned technologies are used to deal with a variety of 5G applications, they will not be further discussed.This paper will focus on technologies used to meet performance requirements for specific applications (inter slice performance) which are implemented in the radio access part of 5G networks.
These technologies are briefly described and discussed in terms of their advantages, disadvantages, and research gaps.Given the fact that the realization of particular technology affects multiple performance requirements, tiny steps towards its implementation will allow telcos to deploy 5G networks, and thereby provide a performance improvement in terms of more capacity, lower latency, more mobility, more position accuracy, increased reliability, and availability [6,214,224].In other words, 5G networks will accommodate many more users and devices while delivering more data [113,114] to each user requiring high data rates [108] in a more energy-efficient way [115,116].
Table 5 summarizes considered radio technologies used to implement future 5G networks being capable of achieving performance requirements identified in Section 4. On the basis of the literature review, one may conclude that the 5G radio access network is crowded with multiple technologies, and there seems to be a duplication of technologies, all eager to grab telco's attention and convince them to buy into the particular choice.As Table 5 shows, multiple technologies and techniques can be used to meet each performance requirement.Since the mMTC activity-based IoT classes are more mature, we have primarily focused on technologies used to fulfil their high performance requirements (i.e., connection density, positioning, and coverage).We have selected 5G enabling technologies considered from telco's perspective as the most representative and promising candidates to meet these requirements.The implementation of these technologies will also significantly affect the fulfillment of uMTC service class requirements, which will lead to complete automation in all spheres of human life.Therefore, the following subsections will discuss these technologies in more detail with the final goal of identifying research gaps and providing recommendations for future work, which are summarized in Table 6.[ 55,197] Multiple access and waveform New waveform (1) Performance of SIC cancelation or filtering on f-OFDMA; (2) balance of time and frequency dispersion and design an efficient filter prototype for UFMC.

Access architecture related radio technologies
Advanced small cell (1) Expect of wireless backhauling on user experience; (2) exploitation of location data and fingerprints in optimizing small cell discovery in terms of time and energy-efficiency; (3) interference management when integrating D2D and small cells.[34,80] Enhanced wireless backhaul (1) TDD multi-flow coordination schemes to avoid bottlenecks in the downlink backhaul; (2) Backhaul aware association in ultra-dense deployment; (3) reliability and security of the backhaul.[34,88,198] Moving network

Wide and Flexible Bandwidth Technology
mmWave band communication and large-scale antennas are promising technologies for future 5G networks.The mmWave band covers frequencies from 30 GHz to 300 GHz [202], and from an industry and wireless academia point of view, it is a unique solution for solving 5G capacity requirements [96].mmWave band communications will provide high data rates [7,202,[225][226][227] utilizing a much larger spectrum bandwidth that can reach up to 5 GHz [39] and by using directional antennas and high attenuation [40].To provide sufficient antenna gain mmWave requires implementation of large-scale antennas at the transmitter and receiver side [96].Deployment of large antenna arrays with mmWave will also bring high spectral efficiency, high throughput and channel gain [7].Despite all of its advantages, mmWave needs line of sight (LOS) operation [41], while the effective communication distance of mmWave signals is within 200 m due to the propagation characteristic of this frequency band [29].Open problems associated with mmWave include three dimensional (3D) channel modeling, dynamic power control, user scheduling and congestion control, hardware limitation and the adaptive beam-steering technique, as described in [7].The problem of high power consumption of a large number of antennas in an array [7], high efficiency low complexity adaptive antenna array processing algorithms [29] and innovative hardware architecture of large-scale antenna transmitters [29] still remain unsolved.A proposal of design guidelines in architectures and protocols for mmWave communications is presented in [42].It is demonstrated that new mmWave technologies, which are under investigation for 5G communications systems, will be able to provide indoor centimeter (cm)-accuracy localization in a robust manner, ideally suited for Assisted Living (AL) [228].
Heterogeneous multi-radio access technologies (multi-RAT) integration is specific to 5G networks radio design that coexists with existing networks.Since 5G networks will not be developed to replace current wireless networks, but rather to advance and integrate existing network infrastructures with new ones [217], we refer to 5G as a heterogeneous network.Multi-RAT is defined as the capability of a mobile network to support multiple radio access technology with seamless interworking among them [229].When deployed in heterogeneous networks (HetNets), with traffic offloading among different RATs, multi-RAT improves capacity [230], supports better communication rates [34,[117][118][119]231,232], better energy efficiency [34,[117][118][119]231,232], and ensures seamless connectivity with higher QoS [230].In designing a heterogeneous RAT, researches are finding solutions for cell-association and traffic-offloading algorithms [217].
Cognitive radio with spectrum sharing is a new software defined technology, which is expected to improve the utilization of the congested radio frequency (RF) spectrum [43].In 5G networks, it is used for designing multi-tier architectures, removing interference among cells, and minimizing energy consumption in the network [44][45][46][47]120,233].Moreover, a spectrum sharing technique can be used along with the CR technology to integrate the 5G spectrum [48,121].During the practical implementation of the CR and spectrum sensing (SS) technique researchers had to design cooperative frameworks, choose cooperative secondary users and transmit cooperative information during the spectrum sensing, further developing the framework and algorithms for the group handoff of secondary users and enhancing security [29].Researchers are already working on designs of protocols for different IoT application, based on cognitive radio, and some of them are presented in [49][50][51]122].Technologies in this group are used together to optimize 5G performance requirements (e.g., a prototype of mmWave integrated HetNet in [123] and HetNet that incorporates massive multiple-input and multiple-output (mMIMO) and mmWave technologies [217]).

Advanced Modulation and Coding
This technological group involves advanced modulation (i.e., frequency and quadrature amplitude modulation (FQAM)) and advanced channel coding schemes.
Advanced modulation schemes group includes FQAM, Amplitude and Phase Shift Keying (APSK), Unitary Space-Time Modulation (USTM), Spatial Modulation (SM), Wave Modulation (WAM), and Orthogonal Time Frequency and Space (OTFS).The FQAM is a combination of frequency shift keying (FSK) and quadrature shift keying (QAM) [124][125][126].This modulation can achieve a higher transmission rate for cell edge users reducing interference at cell edge [52,[234][235][236].FQAM also improves energy efficiency, which makes it adequate for MTC devices with stringent energy consumption requirements [124].Due to performance advantages in terms of Frame Error Rate (FER) [53,236], FQAM is an ideal candidate for services with high coverage and reliability requirements [237].APSK is another modulation which draws a lot of attention.It is shown that its main performance gain (i.e., to achieve a channel capacity very close to Shannon's) relies on advanced channel coding and demodulation algorithms [237].This adds more complexity at the APSK transmitter and receiver.Another modulation, which does not need Channel State Information (CSI) to enable high throughput, is called USTM [237].USTM and its extension (see [238]) is very useful for 5G services with high mobility [127] or latency and reliability constraints [239].Other modulation schemes involve SM [128], and proprietary WAM and OTFS [237].On the basis of the foregoing, researches have analyzed and compared many modulation forms, but still have not found a practical guide on how to choose modulation in any of the 5G use cases.
Advanced channel coding techniques are used for correcting the communication errors caused by noise, interference, and poor signal strength [129].Authors in [54,129,130] compared turbo, low density parity check (LDPC) and polar codes in decoders in contrast to 5G requirements: (1) maturity; (2) throughput and latency; (3) error correction capability; (4) flexibility; (5) computation complexity; (6) interconnect complexity; (7) high-performance flexible implementation complexity; and (8) backward compatibility.This comparison showed that turbo codes hold the greatest promise for offering high performance throughputs, latencies and error correction capabilities, as well as high degrees of flexibility at the lowest implementation complexity [129] in most 5G use cases.However, further work is needed to implement a decoder based on this code followed by detail analysis.

Duplexing
This technological group includes in-band full duplexing (FD) and dynamic time division duplexing (TDD).
In-band FD or simultaneous data transmission and reception will provide a 1000-fold increase in throughput [131,240], double spectral efficiency [132,240], and reduce the air interface delay [240].The central research problem for the practical implementation of an in-band full-duplex radio is the attenuation of the self-interference signal by an adequate amount [132].For the practical implementation of an in-band full-duplex radio, many aspects of network design and management need to be restructured, where terminals antenna and circuit design and the development of theoretical foundation are in focus [55].One practical implementation of the full-duplex radio is shown in [133].Authors in [134] commented that for the design of a full-duplex radio, it is necessary to unify researches from three domains, i.e., RF circuit and system design, digital signal processing and networking.
Dynamic TDD is the predecessor of FD transmission technology and a candidate for 5G [133,135].It represents a scheduling technique in which every base station (BS) is free to choose its own uplink/downlink (UL/DL) split [136][137][138][139].This technique is used to adapt the allocation of network resources to variable traffic requirements [139,140], often found in ultra-densely deployed networks [141].The dynamic TDD can significantly increase bandwidth efficiency [142] and provide higher throughput and low latency [141].However, it is characterized by the severe co-channel interference (CCI) [141][142][143].Dynamic TDD could be used in combination with D2D communication and in the self-backhauling scenario, as described in [55].

Multiple Access and Waveforms
Multiple access techniques are becoming an important technology in 5G because of their ability to support mMTC activity-based IoT classes with urgent deploy demand [144].They include several non-orthogonal multiple access (NOMA) forms: multi-user shared multiple access (MUSA) [145][146][147][148], resource spread multiple access (RSMA) [149], sparse code multiple access (SCMA) [150][151][152], pattern division multiple access (PDMA) [153][154][155], interleave-division multiple access (IDMA) [156,157], and NOMA by power domain [158].The NOMA is a radio access technology design for enabling greater spectrum efficiency [56,[159][160][161][162]207], higher cell-edge throughput, relaxed channel feedback, and low transmission latency [99].NOMA can be employed to enhance user fairness and to support massive connections with diverse QoS requirements [163].In NOMA, there are several challenges and open issues, such as dynamic user pairing, the impact of transmission distortion, the impact of interference, resource allocation, NOMA with multiple antennas, heterogeneous networks, outage probability analysis, practical channel model, uniform fairness, NOMA with antenna selection, carrier aggregation and other challenges as discussed in [216].There are still many challenging issues for SCMA, which need to be solved in future work.For example, there are several open issues in SCMA transceiver design [57], the optimization of algorithms for user grouping and power allocation [164], and further enhancement of the SCMA and MIMO combination.

Advanced Interface Management
This technological group includes receiver advances in terms of simultaneous non-unique decoding (SND) and sliding-window superposition coding (SWSC).
SND follows a rule that implies that each receiver attempts to recover the code words from intended and interfering senders [248].The combination of advanced receivers and joint scheduling provides an improvement of over 50% in cell edge throughput without sacrificing the cell average throughput [5].This gain demonstrates that if 5G networks incorporate advanced interference management, they will provide a virtually edgeless end-user experience [5].
SWSC [61,62] combines the theory concept from superposition coding without rate splitting [63], block Markov coding [166,167], successive cancellation decoding [63,64,168], and sliding-window decoding [65,169,249].The sliding-window coded modulation (SWCM) aims to mitigate inter cell interference at the physical layer by achieving simultaneous decoding performance with point-to-point channel codes, low-complexity decoding, and minimal coordination overhead [170].The realization of the theoretical concept to the practical transmission coding scheme is an important research direction in SWSC [5].

Access Architecture Related Radio Technology
This technological group includes ultra-densification, enhanced wireless backhaul, moving networks, and D2D communication.
Advanced small cell [66,123] deployment is considered to be one of the key enablers for achieving many of the requirements currently envisioned for 5G [67], with higher data rates [68,171,250] and more capacity [171][172][173][174], as most important.Higher data rates and smaller battery consumption can be achieved using short distances in the small cell [69].Solving the capacity and data rate challenge with network densification could be very expensive in terms of equipment, maintenance, and operations [200].With denser cell deployment come significant challenges in the design of a high-performance backhauling system for RAN and the impact of backhaul on radio resource management (RRM) [70].However, there are in existence many interesting topics to be further investigated which can be grouped around several problem areas [34]: user association, backhauling, interference management, energy efficiency, and propagation modelling.
Moving networks (MN) are the combination of multi-hop and vehicular communications concepts [69,71].The MN deploys one or several moving relay node(s) (MRNs) [175,176,251] on vehicles that form their own cell(s) inside the vehicle to serve vehicular users [72].Challenges in using MRN are efficient backhauling, design of efficient resource allocation and interference management techniques, as well as mobility management schemes to exploit the benefit of group handovers for vehicular user equipment (UE) devices served by the same MRN [72].The main research challenges related to MN concept are associated with complexity management due to the mobility of access points and providing a high-rate wireless backhaul link from the moving cell to the fixed network [69].
Enhanced wireless backhaul is one of the main challenges in hyper-dense 5G networks [34,217].The performance of wireless backhaul is dictated by the environment and traffic profile of the intended use case [9].In UDN except mmWave, new approaches are needed, e.g., interface aware routing and intelligent resource allocation [9], as these links may occupy part of the spectrum used in the access network [73].Latency and reliability of this link are important issues to be considered being prone to blocking and fading [69].In any case, further research is necessary to explore TDD multi-flow coordination schemes to avoid bottlenecks in the downlink backhaul [9].The guidelines for deploying future 5G wireless backhaul networks in economical and highly energy-efficient ways are provided in [74].
D2D communication is defined as a direct route of data traffic between spatially closely located mobile UE [75,177,178].The advantages of D2D communication compared with the traditional cellular method include the reduction of transmission latency [73,75] and power consumption [75], improvements in coverage [71,76,179], spectral and energy efficiency [77], and throughput, when power control and resource allocation methods are used [78].The use of D2D communications has an overall positive impact on system capacity in cellular environments.D2D communication brings challenges and complexities related to interference management [79,80], QoS requirements [79], and resource allocation [34,215].The design of D2D direct communication link is still a hot topic [29].

Energy Related Technologies
IoT applications span a broad range of domains including home automation, healthcare, surveillance, transportation, smart environments, etc.One of the most important obstacles for implementing such an impressive scheme is supplying adequate energy to operate the network in a self-sufficient way without compromising QoS [180].Since energy efficiency is of most importance to battery constrained IoT devices, researches have focused their work on the development of the device energy saving mechanisms.
Energy harvesting is a new paradigm which uses solar, thermal, wind and kinetic energy sources to power sensor nodes and consequently prolong network lifetime [181].Among different energy harvesting methods, wireless energy harvesting (WEH) has proven to be one of the most promising solution for energy aware IoT devices, because of its simplicity, ease of implementation, and availability [180].This concept will improve some of the 5G high network communication requirements, like reliability of IoT communications, as presented in [182].Authors in [252] have shown that the integration of social awareness and energy harvesting in D2D communication results in higher system capacity.Network architectures proposed in [100,253] have shown better energy efficiency in 5G wireless network.Researchers could work on improving this architecture and integration with various 5G technologies in the future.
UAV implies a flying vehicle without the driving presence of a human pilot to control it on-board [113].Drones are a possible example of UAVs, although many other robotics related applications could be part of this application group [113].UAVs can play a vital role in IoT scenarios where devices are unable to transmit over a long distance due to their energy constraints.In this case UAVs play the role of moving aggregators which fly toward one IoT devices, collect the data, and transmit it to other devices [81].In [183] authors presented UAV-based floating relay (FR) for cell dynamic and coverage improvement.Deployment of unmanned aerial base stations in 5G heterogeneous architecture can improve throughput [254], coverage [183][184][185]254], connectivity [184,185], and 5th percentile spectral efficiency [254].On-demand wireless systems with low-altitude UAVs are in faster to deploy, more flexibly reconfigured, and likely to have better communication channels due to the presence of short-range line-of-sight links [186] when compared to terrestrial communications or those based on high-altitude platforms.The utilization of highly mobile and energy-constrained UAVs for wireless communication introduces many new challenges [186], some of them are listed in [82].To effectively use UAVs for IoT, several challenges must be addressed such as optimal deployment, mobility and energy-efficient use of UAVs [81].

Other Technologies
Along with the aforementioned technologies, a lot of research attention is devoted to technologies, such as mMIMO, visible light communication (VLC), SIC, vehicle-to-everything (V2X), etc.
mMIMO technology [73,123] promises significant gains in data rate and link reliability [187], reduces latency and energy [13], simplifies media access control (MAC) layer [13], shows robustness against intentional jamming [13], unintended man-made interface [83], and increases capacity [127,198] due to spatial multiplexing [123].Reliable links are provided by benefiting from spatial diversity and the mitigating effects of fast fading, beamforming, and zero forcing caused by multi-user interference [203].The mMIMO can be exploited to extend the coverage of higher frequency bands by relying on beamforming gains [200].Other specific benefits of mMIMO system are: increased capacity 10 times or more with simultaneous improvement in radiated energy efficiency in the order of 100 times, the possibility to use inexpensive, low-power components, the reduction of latency on the air interface, and multiple access layer simplification [83].The low complexity mMIMO uplink scheme for IoT lightweight devices is presented in [188].The performance of practical mMIMO system needs to be investigated, since the research community pays attention to analyzing the performance of mMIMO in an ideal channel state information assumption [73].
VLC is a growing technology for short range, high capacity LOS optical links [189].It uses a visible range of the electromagnetic spectrum (370-780 nm) which provides data transmission and room illumination using light emitting diodes (LEDs).Prominent features of VLC are an abundant license-free spectrum, the ability to provide multiple gigabit-per-second data rates, low energy consumption, and low implementation costs [190].VLC is sensitive to sunlight and is not able to work long range without LOS.Since VLC coverage is LOS limited [84,191], this is known as LOS blocking [192].With poor performance in non-line-of-sight scenarios, VLC networks fail to provide convenient UL coverage at the current state-of-the-art, and each AP illuminates only a small confined cell compared to cellular RF networks [192].VLC applications are expected to include IoT, wireless Internet access, and vehicle-to-vehicle (V2V) communications, broadcast from LED, M2M communications, positioning systems, and navigation.The problem of how to provide mobile applications over VLC is still quite an open one [189].
SIC implementation enables low-latency applications in a cost effective manner [132].SIC can be used to increase link capacity, spectrum virtualization, any-division duplexing, novel relay solutions, and enhanced interface coordination [132].Practical SIC implementation is still in the investigation phase.
V2X is the communication between a vehicle and everything with which it might interact (e.g., other vehicles, traffic operators and service providers).It is used in conjunction with D2D for coverage extension and latency reduction in 5G networks [255].V2X communication might play a vital role in terms of implementing smart and efficient traffic solutions [193], improving road safety, traffic efficiency, and driver convenience.To reduce traffic congestion one may use an intelligent route management system based on V2X communication [7].Since it has big potential in autonomous driving application, the localization of vehicles in dense urban areas is difficult, so several algorithms are proposed to solve the problems [194].Many V2X applications are described in existing literature [85,195].However, there is still a need to design vehicular mobility management strategies in next UDN networks and to find a way to enhance existing connected vehicle services with V2X [86].
These technologies are highlighted as the main 5G driver in existing literature [5][6][7]96,207].Authors compare these technologies and try to implement them through prototypes, or test the combination of these technologies.However, other technologies exist, such as cloud-RAN [7], Coordinated Multi-Point (CoMP), White Space Spectrum [208], which are not in the scope of this paper.

Summary of IoT in Enabling Technologies
Performance requirements of activity-based IoT application classes have served to identify the enabling technologies in the radio access part of 5G networks, which are summarized in Table 5. Various 5G enabling technologies have been sorted into eight technological groups: 1.
Wide and flexible bandwidth technology: mmWave band communication and large-scale antennas, heterogeneous multi-RAT integration, cognitive radio, and spectrum sharing; 2.
Advanced modulation and coding: advanced modulation schemes, advanced channel coding; 3.
Multiple access and waveform: multiple access, new waveforms; 5.
These technologies have been discussed in terms of their possibilities to meet performance requirements of activity-based IoT application classes and identify research gaps and directions, which are summarized in Table 6.
We have highlighted technologies only used to satisfy high priority performance requirements of mMTC activity-based IoT application classes, i.e., connection density, positioning accuracy, and coverage.We have focused on these technologies from telco's perspective, since the area of mMTC activity-based IoT classes is more mature, as its development has already started within LTE, while uMTC poses new research questions to be answered in order to achieve unprecedented levels of reliability needed for new applications in 5G.
The new waveform technology may be used to fulfil the requirements of almost every activity-based IoT application class in terms of connection density.This technology affects the connection density by definition.Analysis of different modulation formats indicates that f-OFDM is the most suitable candidate for IoT applications with high connection density requirements.In this regard, a deeper and more comprehensive analysis of f-OFDM is needed in the context of different mMTC activities (i.e., ticketing, tracking, monitoring, and managing/controlling).The result of the analysis to be performed should be the discovery of an optimal waveform for each activity-based IoT class which has to be integrated with other 5G service classes.
Large scale antennas and advanced small cell technologies can be used to satisfy the requirements of ticketing-, tracking-, and uMTC managing/controlling-based IoT application classes in terms of positioning accuracy.Fulfilling these performance requirements is rather a business than a technical decision.The size of the 5G cell must be planned to meet the desired position accuracy since it is very important for future IoT applications, such as automated driving, remote surgery, robotics, taxis, etc.
Wide and flexible bandwidth technologies, in-band full duplex technologies, access architecture related radio technologies, and mMIMO can be used to achieve the high requirements of ticketing-, tracking-, and monitoring-based IoT application classes in terms of coverage.The combination of these technologies should be tested to find the optimal solution to meet these performance requirements.Since small cell deployment has an impact on required position accuracy, they should serve as a basis for achieving the desired level of coverage.Along with small cell deployment, mMIMO and mmWave technologies are present in almost every 5G prototype.
Since this section discusses radio technologies used to accomplish performance requirements of mMTC activity-based IoT application classes, it can serve telecom operators and other interested parties depending on their interest and potential to utilize these findings towards IoT implementation and monetization.It describes each technology in terms of definition, advantages, disadvantages, and the possible impact on the performance requirements of a broad range of IoT applications that drive the deployment of 5G networks.Moreover, this section can be useful to the research community interested in this attractive field to address recognized research gaps and directions.

Discussion
Telecom operators have the opportunity to capture a share of the revenue that is generated by IoT implementation depending on their role in the IoT value chain.Regardless of their role, telcos have to face many technical challenges in order to meet the changing connectivity and performance requirements.Since current cellular networks limit numerous IoT applications, new technologies are being introduced leading towards 5G networks.Therefore, telcos have to focus on deploying these network technologies in order meet the changing requirements necessary to achieve success in IoT.
In this regard, writing this paper was motivated by the challenge of providing an enhanced understanding of the scope and classification of the broad range of IoT applications in order to determine and prioritize their performance requirements with the goal of specifying the enabling technologies towards 5G networks.The aim has been to propose a new classification of IoT applications, define and prioritize the performance requirements of such IoT application classes, and give insight into state-of-the-art technologies used to meet these requirements from telco's point of view.The motivation that led to the focus of this paper being on IoT applications classification, performance requirements, and 5G enabling technologies could be explained by telco's need for added value from IoT services.Hence, an analysis of IoT customer service requests inspired us to propose an activity-based classification of IoT applications as it can, according to authors' best knowledge, has allowed telcos to more precisely specify their performance requirements and 5G enabling technologies, thereby improving customer experience.
In this regard, the paper fulfilled the following four objectives: (1) the identification of activities relevant to IoT applications and their usage as a new criterion for IoT application classification; (2) the specification and prioritization of performance requirements of such IoT applications classes; (3) the analysis of the radio technologies used to accomplish IoT application requirements; and (4) the identification of the research gaps and the recommendation of new research directions.
Through fulfilling these objectives, we reviewed literature from the fields of IoT in 5G service classifications, IoT in 5G performance requirements, and IoT in 5G enabling technologies.Since the first aim of this paper was to propose a new classification of IoT applications, the existing approaches have been summarized to serve as a basis to identify their drawbacks and formulate the appropriate solution to overcome them.In this context, we proposed a new approach to IoT applications classification, which was based on the activity as a new classification criterion denoting the main function/purpose of specific IoT application.This approach enabled a clearer and more precise positioning of particular application in the IoT application spectrum, as well as the determination of performance requirements and enabling technologies from telco's point of view.The activity-based IoT application classification facilitated the specification of the performance requirements and determination of technologies which enable these requirements to be fulfilled.In this context, the proposed approach served as a basis for the simplification of the realization of particular IoT application.For example, although the monitoringand tracking-based applications seem to be similar, they set different performance requirements on the network, thereby allowing the proposed approach to precisely grade the IoT applications from the same IoT application domain.
Furthermore, we associated the activity-based IoT application classes with the 5G service classes, i.e., mMTC and uMTC, in order to specify and prioritize their performance requirements.It was determined that almost each activity-based IoT application class poses high requirements in terms of connection density, whereas ticketing-based and tracking-based IoT application classes additionally require high positioning accuracy and coverage.In order to analyze these performance requirements, we have summarized the radio technologies used to implement the future 5G networks.The focus was on the technologies used to meet the aforementioned performance requirements of mMTC activity-based IoT application classes, since mMTC is more mature, as its deployment has already started, while uMTC requires further research achieve incomparable levels of reliability needed to enable new applications in 5G.
The analysis showed that new waveform technology can be used to meet the requirements in terms of connection density; large-scale antennas and advanced small cell technologies can be used for the purpose of satisfying the requirements in terms of positioning accuracy; while wide and flexible bandwidth technologies, in-band full duplexing technologies, and access architecture related radio technologies (i.e., advanced small cell, enhanced wireless backhaul, moving network, and D2D communication) can be used to achieve high requirements in terms of coverage.In addition, the conducted research study allowed us to highlight a number of open research issues that could serve the research community and other stakeholders interested in this contemporary and attractive field.In this context, we have recognized research gaps and directions which mostly relate to network redesign and optimization in order to accommodate large-scale IoT applications.

Conclusions
The IoT paradigm has the potential to revolutionize all areas of daily life of individuals, businesses, and society as a whole.Telcos enjoy a central role in the paradigm of IoT because of owing communication infrastructure which is exposed to the numerous technical challenges due to changing connectivity and performance requirements of various IoT applications.These requirements cannot be met with the current cellular networks which create the need to introduce new types of technologies leading toward 5G networks as the main driver for enabling numerous IoT applications.In this context, this paper proposed the activity-based classification of IoT applications and specification of their performance requirements in order to identify 5G radio technologies used to meet them, all from telco's perspective.Activity-based classification of IoT applications indirectly allowed the construction and distribution of performance, as IoT value proposition, while using 5G enabling technologies, as IoT resources in telco's internal value chain.In this context, high performance requirements of each activity-based IoT application class (i.e., connection density, positioning accuracy, coverage) served as a basis to analyze various enabling technologies in radio access part of 5G network in terms of advantages, disadvantages, and research gaps.
On the basis of the conducted analysis, we concluded that the following technologies can meet the high performance requirements of mMTC activity-based IoT application classes: new waveform technology (in terms of connection density), large scale antennas (in terms of positioning accuracy), wide and flexible bandwidth technologies, in-band full duplexing technologies, and access architecture related radio technologies (in terms of coverage).According to the identified research gaps, one may conclude that the optimal solutions for each technology are still in its infancy.This implies that any 5G enabling technology should be first implemented and tested through prototype construction before the deployment of specific IoT application.Thereafter, the technology combination that meets the performance requirements of each activity-based IoT class should first be found.Finally, the practical implementation of different technological combinations may further lead to a deep and comprehensive analysis of QoS and QoE in the context of IoT applications.

Figure 2 .
Figure 2. Keywords for searching relevant databases.

Figure 2 .
Figure 2. Keywords for searching relevant databases.

Figure 2 .
Figure 2. Keywords for searching relevant databases.

Figure 3 .
Figure 3. Reference distribution by (a) year of publication; (b) type of publication.

Figure 3 .
Figure 3. Reference distribution by (a) year of publication; (b) type of publication.
performance requirements of IoT applications.Therefore, many dedicated communication technologies have already been deployed in various IoT applications.For example, Orange, Swisscom and South Korea (SK) Telecom have built nationwide networks based on LoRa[3], whereas Deutsche Telekom (DT), Vodafone and all three Chinese operators have completed the rollout of narrowband IoT (NB-IoT), as another LPWA standard utilizing existing long term evolution (LTE) networks[36].In this context, standardization and interoperability becomes critical because there is a need to consider a broad range of connectivity solutions as presented in Figure 4[37].One of those solutions includes satellite technologies which integrated with 5G radio technologies form the 6th generation (6G) standard for providing global coverage[38].

Figure 4 .
Figure 4. IoT performance requirements and enabling technologies.

Figure 4 .
Figure 4. IoT performance requirements and enabling technologies.

Table 3 .
Activity-based classification of IoT applications.

Table 4 .
Activity-based classes of IoT applications-performance requirements.

Table 6 .
5G enabling technologies-research directions.Redesign of network and management; (2) antenna and circuit design and development of the theoretical foundation;(3) analyze the throughput of a network of randomly deployment terminals sing stochastic geometry; (4) characterize the capacity advantage due to IFDB in various network scenarios; (5) guidelines to practical design: coding, modulation, power allocation, beamforming, channel estimation, equalization, digital interference cancellation and decoding, (6) design of a MAC layer.