To satisfy the explosive access demands of Internet-of-Things (IoT) devices, various kinds of multiple access techniques have received much attention. In this article, we investigate the multicast communication of a satellite and aerial-integrated network (SAIN) with rate-splitting multiple access (RSMA), where both satellite and unmanned aerial vehicle (UAV) components are controlled by network management center and operate in the same frequency band. Considering a content delivery scenario, the UAV subnetwork adopts the RSMA to support massive access of IoT devices (IoTDs) and achieve desired performances of interference suppression, spectral efficiency, and hardware complexity. We first formulate an optimization problem to maximize the sum rate of the considered system subject to the signal-interference-plus-noise-ratio requirements of IoTDs and per-antenna power constraints at the UAV and satellite. To solve this nonconvex optimization problem, we exploit the sequential convex approximation and the first-order Taylor expansion to convert the original optimization problem into a solvable one with the rank-one constraint, and then propose an iterative penalty function-based algorithm to solve it. Finally, simulation results verify that the proposed method can effectively suppress the mutual interference and improve the system sum rate compared to the benchmark schemes.

Supporting IoT With Rate-Splitting Multiple Access in Satellite and Aerial-Integrated Networks

Terrestrial communication networks mainly focus on users in urban areas but have poor coverage performance in harsh environments, such as mountains, deserts, and oceans. Satellites can be exploited to extend the coverage of terrestrial fifth-generation networks. However, satellites are restricted by their high latency and relatively low data rate. Consequently, the integration of terrestrial and satellite components has been widely studied to take advantage of both sides and enable the seamless broadband coverage. Due to the significant differences between satellite communications (SatComs) and terrestrial communications (TerComs) in terms of channel fading, transmission delay, mobility, and coverage performance, the establishment of an efficient hybrid satellite–terrestrial network (HSTN) still faces many challenges. In general, it is difficult to decompose an HSTN into a sum of separate satellite and terrestrial links due to the complicated coupling relationships therein. To uncover the complete picture of HSTNs, we regard the HSTN as a combination of basic cooperative models that contain the main traits of satellite–terrestrial integration but are much simpler and thus more tractable than the large-scale heterogeneous HSTNs. In particular, we present three basic cooperative models, i.e., model ${X}$ , model ${L}$ , and model ${V}$ , and provide a survey of the state-of-the-art technologies for each of them. We discuss future research directions toward establishing a cell-free, hierarchical, decoupled HSTN. We also outline open issues to envision an agile, smart, and secure HSTN for the sixth-generation ubiquitous Internet of Things.

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5G Embraces Satellites for 6G Ubiquitous IoT: Basic Models for Integrated Satellite Terrestrial Networks

Terrestrial communication networks mainly focus on users in urban areas but have poor coverage performance in harsh environments, such as mountains, deserts, and oceans. Satellites can be exploited to extend the coverage of terrestrial fifth-generation (5G) networks. However, satellites are restricted by their high latency and relatively low data rate. Consequently, the integration of terrestrial and satellite components has been widely studied, to take advantage of both sides and enable seamless broadband coverage. Due to the significant differences between satellite communications (SatComs) and terrestrial communications (TerComs) in terms of channel fading, transmission delay, mobility, and coverage performance, the establishment of an efficient hybrid satellite-terrestrial network (HSTN) still faces many challenges. In general, it is difficult to decompose a HSTN into a sum of separate satellite and terrestrial links due to the complicated coupling relationships therein. To uncover the complete picture of HSTNs, we regard the HSTN as a combination of basic cooperative models that contain the main traits of satellite-terrestrial integration but are much simpler and thus more tractable than the large-scale heterogeneous HSTNs. In particular, we present three basic cooperative models, i.e., model X, model L, and model V, and provide a survey of the state-of-the-art technologies for each of them. We discuss future research directions towards establishing a cell-free, hierarchical, decoupled HSTN. We also outline open issues to envision an agile, smart, and secure HSTN for the sixth-generation (6G) ubiquitous Internet of Things (IoT).

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The Internet of Things (IoT) has been a promising communication paradigm that involves sensors, microcontrollers, and transceivers for an efficient communication and computation system. The infrastructure and the applications shall enable and improve the intelligent management of our city service, workspace, and daily life. This article aims at the future 6G vision of IoT, and discusses the convergence with the Radio-over-Fiber (RoF) system. Comparing with the IoT services included in the 5G deployment, 6G IoT exploits high-density heterogeneous devices involving extremely high capacity, supporting much more robust system architecture and artificial intelligence (AI)-based smart algorithms. The RoF is one of the most promising enablers for the outstanding characters of flexibility and efficiency of 6G IoT systems. This article first introduces the IoT envolution roadmap from 5G toward 6G and the potency of optic fiber and RoF technologies. Then, we present the rapidly expanding RoF market and compatible technologies related to IoT-RoF convergence with the discussion on the current outstanding works in multiple dimensions. Finally, we investigate the challenges ahead for the future RoF supported 6G IoT system and the emerging technology solutions.

Toward 6G Internet of Things and the Convergence With RoF System

With the increasing global communication demands and the development of Internet of Things (IoT), extending the connectivity to rural and remote areas has become imperative for future networks. The sixth-generation (6G) network is expected to provide heterogeneous services and seamless network coverage for everyone and everything. Combining the advantages of both satellite and terrestrial networks, the integrated satellite-terrestrial network architecture is promising to provide global broadband access for all types of users, which has drawn much attention from both the academia and industry. In this article, we present a comprehensive survey of the state-of-the-art of integrated satellite-terrestrial networks toward 6G. First, an executive classification and summary of the integration architecture is presented from network design to performance optimization. Then, typical applications of the integrated satellite-terrestrial network are discussed based on the architecture. By considering the unique characteristics of the two networks, main challenges are pointed out when performing integration, such as the long propagation delay, complex link conditions, and high dynamics of the network topology. Finally, some promising future techniques are explored from the perspective of the integrated architecture. A detailed survey of the potential integration architectures is of great importance to enable more flexible network design and construction in future 6G networks. This article will provide a valuable guideline on future research and development of integrated satellite-terrestrial networks. 

Integrated Satellite-Terrestrial Networks Toward 6G: Architectures, Applications, and Challenges

With the ubiquitous deployment of Internet-of-Things (IoT) devices and applications, satellite-aerial-terrestrial integrated network (SATIN) is regarded as a promising candidate to facilitate global IoT with diverse services. To accommodate the explosive demands of IoT within crowded spectrum, we focus on the cognitive radio based SATINs where spectrum sharing is enabled among satellite, aerial, and terrestrial network segments, and devote to efficient resource management. Specifically, by reviewing main challenges that the characteristics of cognitive SATINs and the requirements of IoT bring to resource management, we analyze the potential of incorporating cooperation into cognitive SATINs for future intelligent IoT. Then, we propose a cooperative beamforming scheme to facilitate secure and energy-efficient IoT communications under limited energy for open channel environments in cognitive SATINs. Simulation results validate the superiority of cooperation resource management in cognitive SATINs towards IoT.

/pdf/cooperative-resource-management-for-cognitive-satellite-29dbsirzvu.pdf

Cooperative Resource Management for Cognitive Satellite-Aerial-Terrestrial Integrated Networks Towards IoT

Common signals in public channels of cellular systems are usually transmitted omnidirectionally by the base station (BS) to ensure cell-wide coverage. In this paper, two categories of omnidirectional space-time block codes (STBCs) are proposed to broadcast the common information omnidirectionally in three-dimensional (3D) massive multiple-input multiple-output (MIMO) systems. By utilizing Zadoff-Chu sequences, we propose a discrete omnidirectional STBC, through which constant received signal power at discrete angles is achieved while guaranteeing equal-power transmission per antenna. Then, by utilizing two-dimensional orthogonal complementary codes (2D-OCCs), we also propose a consecutive omnidirectional STBC, which is insensitive to the number of BS antennas, and can realize constant received sum power at any angle through one STBC transmission. Besides, through theoretic analyses, we also demonstrate that all the proposed STBC designs can achieve full diversity order. Simulation results verify the effectiveness of the proposed omnidirectional STBC designs.

Omnidirectional Space–Time Block Coding Design for Three-Dimensional Massive MIMO Systems

By providing interactive broadband services to geographical areas underserved by terrestrial infrastructure, multi-beam satellite systems play a central role in future wireless communications. Targeting the terabit throughput requirements in satellite communications, we introduce a cognitive radio-based high-throughput satellite (HTS) system architecture where full frequency reuse is employed among beams. Moreover, by analyzing the characteristics of the considered architecture, we discuss the design challenges of radio resource management in cognitive HTS systems exposed to both intra-system and inter-system co-channel interference. Furthermore, to combat interference with low overhead, we propose a generic interference-aware resource management framework based on joint spatial division and multiplexing (JSDM). Under this framework, user grouping along with two-stage precoding is studied to achieve substantial improvement in the overall system throughput. Finally, some future research directions and challenges are also given.

https://www.mdpi.com/1424-8220/20/1/197/pdf

Interference-Aware Radio Resource Management for Cognitive High-Throughput Satellite Systems.

Common signals in public channels of cellular systems are usually transmitted omnidirectionally from the base station (BS). In recent years, both discrete and consecutive omnidirectional space–time block codings (STBC) have been proposed for massive multiple-input multiple-output (MIMO) systems with a uniform linear array (ULA) configuration to ensure cell-wide coverage. In these systems, constant received signal power at discrete angles, or constant received signal sum power in a few consecutive time slots at any angle is achieved. In addition, an equal-power transmission per antenna and a full spatial diversity can be achieved as well. In this letter, by utilizing the property of orthogonal complementary codes (OCCs), a new consecutive omnidirectional quasi-orthogonal STBC (QOSTBC) design is proposed, in which constant received sum power at any angle can be realized with the equal-power transmission per antenna through one STBC transmission, and a higher diversity order of four can be achieved. Moreover, the proposed design can be further extended to the uniform planar array (UPA) configuration with the two-dimensional OCCs.

Omnidirectional Quasi-Orthogonal Space–Time Block Coded Massive MIMO Systems

Eigen-beamforming is an optimal beamforming scheme for full-dimension multi-input multi-output (FD-MIMO). However, it suffers from high complexity of the singular value decomposition (SVD) of the 3D channel matrix. In this paper, we propose a low complexity dual layer beamforming scheme to exploit both the horizontal and vertical degrees of freedom. The proposed scheme first segments the channel matrix into several sub-channel matrices. Then, by selecting the vertical and horizontal beamforming from the sub-channel matrices respectively, the proposed scheme can reduce the complexity, while achieving the performance approximate to the optimal eigen-beamforming scheme. Moreover, to enable practical implementation, a hybrid codebook design is proposed which exploits the structure of the dual layer beamforming to reduce feedback overhead. According to the different correlation characteristics in horizontal and vertical direction, we design different codebooks for the horizontal and vertical beamforming. Simulation results demonstrate that the proposed scheme can effectively improve the spectral efficiency of FD-MIMO systems with limited feedback, especially for the cell edge users.

Can Liu

Papers

Cooperative Resource Management for Cognitive Satellite-Aerial-Terrestrial Integrated Networks Towards IoT

Omnidirectional Space–Time Block Coding Design for Three-Dimensional Massive MIMO Systems

Interference-Aware Radio Resource Management for Cognitive High-Throughput Satellite Systems.

Omnidirectional Quasi-Orthogonal Space–Time Block Coded Massive MIMO Systems

Dual layer beamforming with limited feedback for full-dimension MIMO systems