1. Introduction
The primary goal of the Internet of Things (IoT) is to create a comprehensive computing environment where everyday objects can sense and interact with their surroundings, bridging the physical and digital realms [
1]. The IoT is expected to continue growing further, with forecasts of over 5.5 billion cellular IoT connections by 2027, reflecting its fast expansion [
2]. The rapid development of IoT technology opens up numerous applications that have the potential to transform our daily lives [
3,
4]. These IoT technologies are being leveraged to enhance various aspects of our environment, giving rise to concepts like smart buildings, smart cities, flexible energy infrastructure, and smart agriculture, effectively embedding intelligence within these domains [
5,
6]. In the realm of smart agriculture, for example, the activation or deactivation of an irrigation system can be precisely controlled based on real-time data collected from humidity sensors deployed in remote fields. This automated approach not only optimizes irrigation efficiency but also enables remote monitoring and management of agricultural operations, while simultaneously providing valuable insights for comprehensive evaluation and analysis [
7].
Efficient data transmission is paramount in the expansive realm of the IoT, as it underpins the seamless operation of IoT systems across various domains [
8]. The continuous exchange of data and information between devices, data storage systems, and end-users is crucial for the effective functioning of IoT applications. Current IoT architectures employ a diverse range of communication technologies, encompassing short-range solutions tailored for localized environments like homes and industrial settings as well as long-range solutions capable of covering vast geographic areas [
9,
10]. The introduction of 5G technology marks a pivotal moment in the IoT landscape, addressing the evolving connectivity needs of IoT applications. The standardization of 4G-IoT systems, such as Long-Term Evolution Machine type communication (LTE-M) and NarrowBand Internet of Things (NB-IoT), continued to evolve in 5G standards [
11,
12].
These advancements collectively contribute to a transformative 5G-IoT ecosystem, unlocking a vast spectrum of possibilities for the future of connected devices and services [
13,
14,
15,
16]. The IoT is rapidly evolving, paving the way for innovative scenarios and expanding the range of existing IoT applications, laying the foundation for future 6G-IoT systems [
17,
18]. However, despite the progress made, there are still unique challenges that need to be addressed, particularly in the research area of the Internet of Remote Things (IoRT).
Internet of Remote Things
The Internet of Remote Things (IoRT) extends the reach of the Internet of Things to remote and physically isolated regions where terrestrial and cellular networks are absent. This innovative paradigm broadens the scope of the IoT by connecting the farthest and the most remote corners of the world. However, IoRT deployments often encounter significant challenges, such as limited connectivity and harsh environmental conditions [
18,
19].
Despite these difficulties, there is a growing demand for IoRT applications, as they hold immense potential in various domains, including environmental monitoring, wildlife tracking, disaster relief management, and the delivery of critical healthcare and education services to under-served rural communities [
20]. Thus, a number of specialized IoRT environments are formed, as illustrated in
Table 1. The Internet of Agricultural Things (IoAT), for instance, has the potential to enhance agricultural productivity and income generation in remote places with arable land [
7,
21,
22]. In oceans and seas, the Internet of Maritime Things (IoMT) is also an excellent example of an IoRT environment [
23,
24] where, for example, sensors on ships can transmit vital data, facilitating weather forecasts and enhancing maritime safety [
25]. Specialized IoRT approaches are also required for the Internet of Underground Things (IoUT), which is applicable to subterranean operations such as mining [
26,
27,
28], and the Internet of Underwater Things (IoUwT), which is applicable to marine exploration or aquaculture [
29,
30,
31,
32].
In addition, the Internet of Space, which extends IoT capabilities to satellites and extraterrestrial expeditions, is another prominent example of IoRT deployment [
33,
34]. In the Internet of Battle Things (IoBT), IoT-enabled devices on military assets, equipment, and personnel provide vital information, optimize deployment strategies, and improve situational awareness, thereby enhancing the efficiency and safety of military operations [
35,
36,
37] even in the most remote areas.
Similarly, IoRT technologies are essential for gathering data on wildlife behavior, climate, and glacier melt in the far-flung and harsh Arctic, forming the Internet of the Arctic. These IoT systems enable scientists and researchers to study and comprehend the swiftly transforming Arctic conditions, which is essential for environmental monitoring and the development of strategies to mitigate the effects of climate change [
38]. Hence, IoRT systems are essential for bridging isolated ecosystems in remote environments, enabling communication, data sharing, and intelligent operations.
This survey investigates the application of non-terrestrial networks to connect remote IoT systems within the framework of the Internet of Remote Things. The primary objectives are to identify the scope of Non-Terrestrial Networks (NTNs), classify technological and energy efficiency challenges, and discuss open issues towards 6G-IoT. Hence, the main contributions of the present study are:
A thorough analysis of various NTN architectures, including satellite networks operating in different orbits, UAV-based networks, and hybrid NTNs, that can address the limitations of current terrestrial networks. Each NTN type is discussed in detail, outlining its strengths, limitations, and potential applications.
Identifies and discusses key challenges in integrating NTNs into the IoRT ecosystem. It further outlines seven important research objectives for NTN integration in IoRT, ranging from energy efficiency and cost-effective deployment to enhanced coverage, throughput, data transmission reliability, and timely data acquisition.
Delving into the research field of NTN-based IoRT, this study dissects three key approaches: satellite, hybrid satellite-UAV, and UAV-based systems. By further analyzing each category through the lens of individual research objectives, it unveils a comprehensive overview of this dynamic landscape.
Exploring the future of IoRT, this study examines research trends and challenges while also spotlighting promising technologies. Focusing on the 3GPP (3rd Generation Partnership Project) standardization roadmap, it highlights the crucial role of upcoming research and standardization efforts in bridging remote connectivity gaps and paving the way for energy-efficient IoRT environments in the 6G era.
2. Related Work and Survey Scope
In order to establish clear research objectives for the present study, we conducted a comprehensive review of previous studies in the relevant research literature. Below, we present our key findings that emerged from this review, which are also summarized in
Table 2.
The study in [
39] provides a comprehensive overview of satellite communication systems for the IoT. However, it overlooks two crucial aspects: energy efficiency considerations in NTNs and the integration of Unmanned Aerial Vehicles (UAVs) in the IoRT framework, which are essential for the successful implementation of IoRT solutions. Similarly, while [
40] while provides a comprehensive overview of the Low Earth Orbit (LEO) satellite-based IoRT architecture, the authors do not examine energy efficiency issues or the employment of UAVs in the IoRT environment. The study in [
41] proposes integrating IoT, 5G, UAVs, and satellites to address IoT deployment challenges and overcome terrestrial infrastructure limitations, such as limited coverage and capacity. Nevertheless, while it provides a general architectural framework, it does not adequately address the challenges of integrating UAVs, satellites, and terrestrial 5G infrastructure, nor does it address the energy management complexities of NTNs.
The authors of the article [
42] explore the potential of Artificial Intelligence (AI) techniques, including Machine Learning (ML) and Deep Learning (DL), for enabling ultra-reliable and low-latency communications (URLLC) and ubiquitous interconnectivity in the NTN-based Industrial Internet of Things (IIoT). However, the study primarily focuses on AI-based solutions and does not fully address the diverse range of challenges presented by IoRT use cases. In [
43], the authors propose a simplified approach to analyzing Hybrid Satellite–Terrestrial Networks (HSTNs) by introducing three fundamental cooperative models, providing a survey of the state-of-the-art technologies for each model, and outlining prospective research directions. Nevertheless, their research specifically targets a subset of possible IoRT architectures.
The merits and drawbacks of current satellite-based IoT solutions for remote regions are highlighted in [
44], which examines a variety of architectures and technical approaches. This study does not adequately address the diverse energy efficiency challenges associated with NTNs, and while it focuses on satellite-based IoT architectures, it does not consider the potential use of UAVs in the context of IoRT. Kua et al. in [
45] provide a comprehensive overview of the potential benefits and challenges of using IoT and space-based technologies for future space exploration missions. While detailed, this study is focused on IoT for space and, therefore, does not consider all the range of IoRT scenarios and the respective challenges they pose. The study in [
24] explores the need for hybrid satellite–terrestrial maritime networks, highlighting technologies that enhance efficiency, expand coverage, and support specialized maritime services. This study primarily focuses on the IoT implementations for maritime environments and thus does not comprehensively address the diverse landscape of IoRT scenarios.
The study [
46] explores the integration of UAVs into Wireless Sensor Networks (WSNs). The authors discuss the performance and capabilities of UAVs as communication nodes, examine architectural aspects and emerging technologies within UAV-enabled WSNs (U-WSNs), and shed light on crucial factors that influence the design of U-WSNs. The study’s scope is limited to UAVs, and thus, it does not investigate satellite systems within the framework of IoRT. In [
47], the authors examine the growing potential of NTNs in 5G and beyond networks, particularly when combined with terrestrial networks. The survey covers various aspects, including services, architectures, technological enablers, and challenges associated with NTN integration. Although NTNs are explored in this study, they are not evaluated in the IoRT context; hence, the associated challenges are not investigated.
Furthermore, while the study in [
48] provides a comprehensive overview of the rapidly evolving landscape of wireless technologies, its focus on the broader IoT domain limits its ability to delve into the specific details and challenges of IoRT connectivity. The paper’s coverage of key performance indicators such as scalability, energy efficiency, reliability, and low latency is valuable, but its broad scope hinders a thorough examination of these factors within the unique context of IoRT applications. Additionally, the paper’s discussion of IoRT connectivity scenarios, particularly those involving satellite communication, has limitations due to its overarching focus on IoT as a whole.
Moreover, the study in [
49] explores resource management for integrated space–air–ground–sea 6G networks incorporating UAVs and satellites, providing valuable insights. However, the study takes a general approach to these networks and does not specifically address the intricacies of IoRT environments. Moreover, it lacks adequate emphasis on energy-efficient strategies, particularly in utilizing Non-Orthogonal Multiple Access (NOMA) schemes and applying machine learning techniques such as Deep Reinforcement Learning (DRL).
Finally, the study in the work [
50] offers a thorough analysis of challenges in satellite IoT networks, focusing on broad connectivity, extensive geographical coverage, mobility, and power consumption limitations. However, its focus remains exclusively on satellite NTNs, omitting any discussion of hybrid satellite-UAV and pure UAV connectivity approaches in IoRT environments. Furthermore, the study lacks a comprehensive analysis of energy-efficient approaches, especially through the utilization of machine learning techniques.
Survey Scope
Therefore, as evident from the preceding discussion, a significant research void exists in the relevant literature, defining the domain that this current work aims to address: the realm of connectivity within the IoRT context, with a particular focus on the crucial aspects of energy efficiency and sustainability. Regarding the paper structure,
Section 3 provides a detailed description of the characteristics of non-terrestrial networks, laying the foundation for understanding their significance in the context of IoRT. Moving forward,
Section 4 identifies and clarifies the key challenges involved in integrating NTNs within IoRT systems. To address these challenges,
Section 5,
Section 6 and
Section 7 present comprehensive surveys of IoRT systems based on Space-borne NTN Networks, Aerial NTN Networks, and Hybrid NTN Networks, respectively. Each survey investigates the unique objectives, characteristics, and challenges associated with these NTN implementations in IoRT.
Section 8 examines future research trends and challenges in IoRT, while
Section 9 focuses on the 3GPP standardization roadmap, emphasizing the importance of research and standardization efforts in both bridging remote connectivity gaps and fostering energy-efficient IoRT environments in the 6G landscape. Finally,
Section 10 concludes the paper by summarizing our work and identifying future research directions.
3. Non-Terrestrial Networks
Current terrestrial networks face limitations in providing extensive wireless coverage to remote regions, adequate availability and reliability, and resilience to natural and man-made disasters. Non-terrestrial base stations, on the other hand, offer several advantages over their terrestrial counterparts. Their ability to be quickly deployed in emergency situations is crucial for disaster response, environmental monitoring, and search-and-rescue operations [
51]. Additionally, their elevated positioning above the ground enhances radio link quality, reducing signal attenuation and interference from terrestrial obstacles. In essence as shown in
Figure 1, the integration of terrestrial and non-terrestrial networks provides a 3D strategy for network coverage, paving the way for seamless and reliable services in remote areas, high-altitude locations, and deep-sea environments [
52,
53,
54].
Space-Borne Networks Based on Satellites
Satellites operate in different orbits, each offering distinct advantages and limitations. The choice of orbit depends on the specific requirements of the IoT application. Geostationary Earth Orbit (GEO) satellites maintain a fixed position over the Earth, providing broad coverage but suffering from high latency and signal attenuation. LEO satellites, on the other hand, operate at lower altitudes (500–2000 km), minimizing latency and attenuation but requiring a larger constellation for global coverage [
55,
56]. Medium Earth Orbit (MEO) satellites offer a balance between coverage and latency, with a smaller constellation than LEO but a higher latency. Very Low Earth Orbit (VLEO) satellites, operating at altitudes below 200 km, promise ultra-low latency and attenuation but require advanced propulsion systems due to increased atmospheric drag and suborbital launch capabilities [
57]. LEO and VLEO orbits are commonly utilized for deploying Cubesats, which are small, standardized satellites with a cube-shaped form factor [
58,
59]. CubeSats are often employed for educational and scientific purposes due to their compact size and lower launch costs. Nanosats, a broader category of small satellites that includes CubeSats but extends to slightly larger satellites, are suitable for a wider range of applications [
60].
Airborne Networks Based on Unmanned Aerial Vehicles
Unmanned Aerial Vehicles (UAVs) offer a promising solution for expanding wireless coverage in challenging environments and beyond terrestrial infrastructure. They can serve as mobile base stations or relay nodes, bridging connectivity gaps and extending coverage to remote or isolated areas. This flexibility addresses the limitations of traditional IoT communication technologies, which often struggle in harsh or geographically constrained conditions [
61,
62]. UAVs can seamlessly integrate into existing terrestrial networks, effectively utilizing existing base stations without the need for additional infrastructure deployment. The altitude at which UAVs operate determines their role in the network: low-altitude platforms (LAPs) operate at altitudes of up to approximately 17 km, providing localized coverage and relaying data between terrestrial networks and remote areas. High-altitude platforms (HAPs), operating at altitudes of up to 25 km, offer broader coverage and can serve as central nodes for large-scale wireless systems [
46].
Hybrid Non-Terrestrial Networks
Hybrid Non-Terrestrial Networks (Hybrid NTNs) refer to advanced communication infrastructures that seamlessly integrate multiple non-terrestrial technologies to create a unified and resilient network. This integration typically involves combining satellite communication systems with UAVs, either LAPs or HAPs [
63]. The goal is to leverage the unique strengths of each component to enhance network performance, coverage, and flexibility. Satellites form the backbone of the network, providing global coverage and enabling long-range data transmission. UAVs and HAPs supplement this backbone infrastructure with dynamic and mobile capabilities, allowing for targeted coverage, rapid deployment, and persistent communication in specific areas. This synergistic combination of global reach and agility ensures the network’s ability to adapt to changing communication requirements and effectively serve remote and dynamic environments [
64,
65].
In conclusion, the utilization of non-terrestrial networks presents significant potential for expanding communication capabilities, enhancing coverage, facilitating the distribution of computing resources, reducing data processing delays, and establishing local IoT networks that can adapt to diverse Quality of Service (QoS) demands [
66]. These platforms are characterized by their rapid and flexible development, which further enhances their ability to adapt to remote environments. In this context, the implementation of NTNs can facilitate the realization of different scenarios and services related to the IoRT ecosystem [
47]. This involves providing support for demanding applications, including remote infrastructure control, monitoring, data collection, and connectivity, specifically in environments characterized by their remoteness, such as the Arctic, maritime regions, and rural areas.
4. Key Challenges in the Integration of NTNs in IoRT
The Internet of Remote Things represents a groundbreaking advancement in IoT technology, extending its reach to connect remote and resource-constrained devices beyond the limitations of traditional terrestrial networks. Leveraging NTNs such as satellites, UAVs, and hybrid combinations of these platforms enables the collection, transmission, and processing of data from remote locations, unlocking a vast array of applications in diverse fields.
Figure 2 illustrates a generic IoT system model for interfacing remote IoT systems to backbone communication infrastructure.
Despite significant advancements in Non-Terrestrial Networks, integrating them into the Internet of Remote Things presents notable challenges, as shown in
Figure 3. The utilization of space and aerial channels in NTNs, poses unique challenges in implementing underlying communication across diverse settings such as oceans, maritime zones, the Arctic, and rural areas. While enhancing the quality of space-aerial channels holds promise for extensive coverage and increased throughput, it confronts challenges related to signal propagation, atmospheric conditions, and latency. Hence, developing innovative approaches is essential to ensuring uninterrupted and reliable communication [
67].
Furthermore, data collection in geographically isolated areas presents a distinctive set of challenges in ensuring timely and reliable data transmission and analysis. The interplay between spatial and temporal dynamics in these remote ecosystems can be intricate [
68]. For instance, in a remote forest, the distribution of plant species may be influenced by geographic factors such as elevation and soil type as well as temporal factors like seasonal changes in temperature and precipitation. Due to the complexity of remote ecosystems, traditional methods of data collection, transmission, and analysis are often inadequate. Novel approaches are required, and new technologies and protocols are being developed to address these challenges [
69].
Moreover, efficient resource allocation is always a paramount challenge, driving the development of advanced techniques to optimize resource distribution on satellite, UAV, or hybrid platforms [
70]. This necessitates addressing obstacles related to scalability, prioritization, and adaptability in resource allocation. Energy efficiency especially poses a critical challenge, demanding sustainable and efficient energy management strategies for NTNs operating in remote and challenging environments [
71]. Tackling these issues is crucial for the seamless integration of NTNs into the IoRT ecosystem, enabling efficient communication across a wide range of applications in remote and challenging conditions [
72]. In our classification of papers on non-terrestrial networks for the IoRT, we identified seven important objectives. While specific objectives might overlap and impact one another, we centered on each work’s main research focus.
Energy Efficiency: NTNs play a crucial role in extending the lifespan and reducing the operational costs of IoT devices, especially in remote or resource-constrained environments. Research efforts aim to optimize NTNs to minimize energy consumption while maintaining network performance.
Cost-Efficient Deployment/Operation: The deployment and operation of NTNs, particularly in large-scale IoT deployments, must be cost-effective. Researchers explore strategies to reduce infrastructure costs, optimize network utilization, and minimize maintenance expenses.
Enhanced Coverage and Availability: NTNs provide an opportunity to extend network coverage beyond the limitations of terrestrial networks, ensuring reliable connectivity in challenging terrains and remote areas. Research focuses on improving NTN coverage, overcoming obstacles, and maintaining connectivity in harsh conditions.
Enhanced Throughput/Transmission Rate: As IoT data volumes continue to grow, NTNs need to handle high-speed data transmission to meet the demands of applications. Researchers investigate techniques to increase NTN throughput and support efficient data transfer.
Enhanced Data Transmission Reliability: Reliable data transmission is paramount, especially for mission-critical IoT applications. NTN research addresses factors such as interference, fading, and signal attenuation to ensure data integrity and minimize packet loss.
Enhanced and Timely Data Acquisition: IoT devices generate vast amounts of data at frequent intervals. Timely data acquisition is crucial for proper processing and analysis. NTN research explores methods to optimize data collection, reduce delays, and ensure timely data delivery, often leveraging advanced Medium Access Control (MAC) protocols to optimize channel access.
Enhancement of Edge Computing Capabilities: Edge computing brings processing power closer to IoT devices, reducing latency and enabling real-time decision-making.
By addressing these objectives, NTNs for IoRT are poised to revolutionize the way data are collected, processed, and utilized, enabling a wide range of applications in remote and isolated environments.
9. Future Outlook toward 6G-Era
Despite the transformative impact of 5G networks on user experiences and IoT applications, their limitations, particularly in remote or isolated regions, are undeniable. These areas lack terrestrial network coverage, hindering seamless access and service provision for 5G-enabled IoT applications. In response to this challenge, ongoing research and scientific trends highlight the need for continuous 5G standard evolution, particularly in the realm of non-terrestrial networks. By seamlessly integrating these networks with their terrestrial counterparts, we pave the way for the emergence of futuristic 6G networks [
137].
This shift in focus aims to address the limitations of current IoT connectivity and unlock new opportunities for remote applications [
138,
139]. As we look towards the 2030s, the advent of 6G networks is poised to overcome these obstacles, fulfilling the fundamental need for ubiquitous connectivity [
140,
141]. Research and industry are actively exploring the capabilities and requirements of 6G, with ultra-high frequency communications, artificial intelligence, edge computing, and non-terrestrial networks at the forefront [
142,
143,
144]. These elements together will construct a fully networked 6G world, empowering remote IoT applications and emerging services. A 6G network provides the vision of a unified environment that seamlessly connects the globe, bridging the divide between remote and accessible areas [
145,
146,
147].
9.1. NTNs Vision and The Forthcoming Energy-Awareness
Figure 4 illustrates the various stages of 5G network upgrades towards 6G-IoT as per the 3GPP Release cycles. Anticipating additional technical advancements, the evolution from 5G to 5G-Advanced standards is expected to address the limitations of non-terrestrial networks, particularly in areas with no terrestrial coverage. The ongoing development of 5G technology aims to enhance performance and cater to emerging use cases.
The inherent versatility of 5G lays a robust foundation for enabling NTNs. Given the complexity of NTNs, especially satellite communication networks, a holistic approach is necessary for their design. Across multiple 3GPP releases, considerable attention has been dedicated to NTN design [
72], signaling a commitment to making 5G from space a reality. Activities beyond standards are deemed essential for integrating non-terrestrial networks with IoT [
41]. Subsequent 3GPP upgrades are poised to unlock possibilities for servicing remote locations.
9.2. NTNs in 3GPP Releases 18 and 19
Enhancements in 3GPP Release 18 (Rel-18) for NTNs encompass support for frequencies above 10 GHz, integration with 3GPP New Radio (NR), improved mobility management, handover procedures, power-saving features, and enhanced security and reliability [
12,
148]. These improvements are projected to facilitate diverse use cases, including global broadband connectivity, support for remote and underserved areas, disaster relief, connected vehicles, drones, and Industrial IoT. Furthermore, Rel-18 is expected to introduce the option to disable Hybrid Automatic Repeat Request (HARQ) feedback for NTN IoT by default, which can significantly improve throughput for devices with sporadic data traffic.
In the context of Release 19 (Rel-19), support for satellite-based 3GPP communication implies the availability of a backhaul network between the radio access node and the core network, as well as the wireless device’s ability to utilize existing Global Navigation Satellite System (GNSS) functionality [
149]. For the Rel-19 discussion, 3GPP will explore scenarios involving discontinuous backhaul connections. For example, a satellite could regularly orbit Earth and receive data from a location without direct transmission. In this scenario, it is necessary for satellite communication to have hold-and-forward capabilities.
While devices may use a 3GPP-based satellite communication architecture to determine their location in certain scenarios, the primary driver behind the upcoming 3GPP specifications for NTNs is the development of direct links with both user equipment (UE) and IoT devices.
In addition, Rel-19 is projected to integrate support for ambient power-enabled remote IoT devices, enabling them to harvest energy from their surroundings to power their operations. Apart from this, the Hold-and-Forward (HF) feature, which is also expected and being studied in Rel-19, is designed to overcome the issues of latency and reliability that are commonly encountered in satellite communications as it enables the satellite to temporarily hold data packets prior to their transmission to the ground station. Consequently, the hold-and-forward technique plays a crucial role in guaranteeing the integrity of data in challenging and remote conditions by storing packets that could potentially be lost as a result of interference or obstacles.
The scientific community has consistently emphasized that adherence to the 3GPP standards for NTNs is crucial to the successful implementation of future enterprise strategies [
150]. This recognition of NTN’s significance as a key work item and research area within the 3GPP has attracted a growing number of companies seeking to participate in this burgeoning field. However, the organization is actively exploring potential collaborations with emerging space companies and established corporate entities to harness the potential of 3GPP-based NTNs and expand their reach.
Energy Efficiency Challenges
In 3GPP Rel-18, significant focus has been directed towards enhancing the energy efficiency of NTNs [
12]. These enhancements are intended to reduce power consumption in NTN base stations and user UEs while maintaining or even improving overall performance. One noteworthy improvement involves refining beamforming techniques, enabling more precise targeting of signals from NTN base stations towards UEs. This targeted signal delivery minimizes the power required for data transmission and reception. Furthermore, introducing new power-saving modes for NTN UEs is another crucial addition. These modes allow UEs to conserve power during periods of inactivity or when data transmission or reception is not actively in progress. In addition, network-assisted power management is incorporated to assist UEs in effectively managing power consumption. The network will provide valuable information about traffic conditions and available energy sources, empowering UEs to optimize their power usage efficiently [
151].
Furthermore, leveraging AI and ML has emerged as a pivotal strategy for enhancing energy efficiency in NTNs. AI and ML algorithms are employed to dynamically adjust critical parameters such as beamforming, power levels, and network topology, leading to significant energy savings [
152]. As recognized by 3GPP, energy efficiency will be a major focus of Rel-19, with its importance reflected in its consideration as a service criterion. Telecommunications operators are actively pursuing strategies to reduce energy usage in network infrastructure and wireless devices. Optimizing power consumption in the network equipment is paramount for achieving cost efficiency and reducing operating costs [
153]. However, it is crucial to strike a balance between power reduction and maintaining QoS for end-users.
9.3. Future 3GPP Standard Releases
Future versions beyond 3GPP Rel-18 and Rel-19 pave the way for future technical upgrades of 5G-NTN on a physical level, as well as a reconfiguration of 5G networks by 2030, when the first version for 6G networks will be released. This direction necessitates substantial investigation and is open to scientific testing and simulations by the scientific community. The need for improvements in energy efficiency and the limited availability of spectrum require further enhancements.
To tackle these challenges, 3GPP Release 20 (Rel-20) initiatives will address them by introducing innovative technologies such as extended discontinuous reception (eDRX) and millimeter wave (mmWave) bands [
150,
154,
155,
156]. The eDRX is a power-saving technology that enables NTN devices, like satellites, to enter periodic low-power sleep mode while maintaining continuous network communication. This is crucial for satellites that have limited energy resources, as it helps to increase their operational lifespans and decrease expenses. 3GPP specifications for mmWave bands for NTNs introduce novel channel models, modulation and coding schemes, and beamforming techniques. These advancements are specifically designed to optimize the propagation characteristics and interference environment of NTNs. As a result, they provide improved bandwidth, reduced susceptibility to interference, and the potential for high data rates and low latency [
155].
In the direction of the space bands, specific advancements were made for L-band and S-band in the context of NTNs. For L-band, the standardization included Single Carrier Modulation and Coding Schemes (SC-MCS), providing enhanced spectral efficiency and lower latency compared to conventional multi-carrier modulation. Additionally, new channel models and interference mitigation techniques tailored for L-band NTN links were introduced [
157]. In the case of S-band, the focus was on optimizing Dual Polarized (DP) and Quadrature Polarized (QP) antenna systems, improving resource allocation, and modulation schemes. The standardization also included beamforming techniques to concentrate transmitted signals, thereby extending the capacity and range of S-band NTN links. These standardized improvements in the L-band and S-band, driven by 3GPP Rel-20, are poised to significantly enhance the capabilities of NTNs, fostering innovations in areas like maritime connectivity, aerial broadband, and disaster relief [
157,
158].
In conclusion, 3GPP Rel-20 and Release 21 (Rel-21) mark a pivotal transition towards the next generation of the Internet of Things, commonly referred to as 6G-IoT. These releases signify significant progress in Non-Terrestrial Networks, introducing millimeter wave (mmWave) bands, enhancing L-band and S-band, and, notably, introducing network-wide energy-saving modes. Rel-20’s mmWave bands provide expanded bandwidth, which is crucial for supporting emerging applications like video streaming, while advancements in L-band and S-band improve spectral efficiency and reduce latency, fostering wider coverage in remote areas. Building upon Rel-20, Rel-21 introduces energy-saving modes, enabling NTN devices to operate more sustainably.
The synergistic impact of these advancements positions 6G-IoT as a transformative force with the potential to revolutionize the connectivity and quality of experience (QoE) of users in remote areas and utilize services upon the remote IoT, including remote ultra-high precision agriculture, remote environmental condition monitoring, and remote supply chain optimization.
10. Conclusions and Future Work
In conclusion, this survey paper emphasizes the importance of non-terrestrial networks in the Internet of Remote Things ecosystem. These networks overcome the limitations of traditional terrestrial networks, enabling reliable connectivity in geographically isolated areas and challenging environments. The paper identifies seven key objectives for NTN research: energy efficiency, cost-effective deployment/operation, enhancement of coverage and availability, enhancement of throughput/transmission rate, enhancement of data transmission reliability, enhanced and timely data acquisition, and enhancement of edge computing capabilities. By addressing these objectives, NTNs for IoRT have the potential to revolutionize the way data are collected, processed, and utilized, enabling a wide range of applications in remote and isolated environments.
However, to fully unlock this potential, further research is necessary. Building upon the findings of this survey, our future work will focus on two key areas:
Investigating the impact of distance, network resources, and aerial platform types on the feasibility and performance of remote IoT connectivity via NTNs. This will involve developing models to predict network performance and identify optimal configurations for different deployment scenarios.
Exploring energy-efficient communication protocols and developing power-aware routing algorithms that leverage the unique characteristics of NTNs to minimize energy consumption. This may involve integrating renewable energy sources into remote IoT devices and quantifying the environmental impact of different IoRT deployment scenarios.
By addressing these research areas, we aim to contribute to the development of a sustainable and efficient IoRT ecosystem, unlocking the full potential of NTNs.