Non-geostationary orbit (NGSO) satellites are anticipated to support various new communication applications from different industries. NGSO communication systems are known for a number of key features such as lower propagation delay, smaller size, and lower signal losses in comparison to the conventional geostationary orbit (GSO) satellites, which can potentially enable latency-critical applications to be provided through satellites. NGSO promises a significant boost in communication speed, and thus, tackling the main inhibiting factors of commercializing GSO satellites for broader utilisation.

However, there are still many NGSO deployment challenges that need to be adequately addressed in order to ensure seamless integration not only with GSO systems but also with terrestrial networks. These unprecedented challenges are identified in this chapter, including coexistence with GSO systems in terms of spectrum access and regulatory issues, satellite constellation and architecture designs, resource management problems, and user equipment requirements. Furthermore, future research challenges inspired by utilising NGSO systems to advance satellite communications within versatile applications are also provided.

The last years have seen an unprecedented demand for improved broadband connectivity, near-zero latency services, and ultra-reliable and heterogeneous communications. Such a trend is expected to further increase in the near future, with forecasts of 5.3 billion Internet users and 14.7 billion machine-to-machine (M2M) connections by 2023 [1]. The evolution of 5G into beyond 5G (B5G) and 6G networks aims at responding to this increasing need for ubiquitous and continuous connectivity services in all areas of our life: from education to finance, from politics to health, from entertainment to environment protection.

The relentless pursuit of continuous, global, broadband connectivity has recently intensified the interest in low-Earth orbit (LEO) satellite mega-constellations, which are capable of delivering massive throughput. This and the insatiable demand for low-cost, low-complexity user terminals conspire to cause analog radio-frequency (RF) components to exceed their tolerance limits. Advanced digital technological solutions are explored in this chapter to minimize the strong and frequency-selective in-phase/quadrature (I/Q) imbalance introduced by analog frequency-conversion circuits when signaling at multiple Giga (G)-Baud rates. Specifically, two characterization models of analog RF impairments are provided when frequency offset is present. Novel digital compensation algorithms with immunity to frequency offset are presented and categorized into equalization with image-rejection capability and image cancelation. Adaptive techniques are utilized to obtain compensation coefficients in an iterative manner using stacked construction, while the pursued coefficients are independent of frequency offsets. These methods are useful when using known data samples for initial factory calibration or in a decision-directed mode during field recalibration. Extensive computer simulations reveal that the proposed compensators provide lossless attenuation of the imbalance-induced, frequency-selective image in the presence of frequency offsets.

The task of multiple access strategy is to link users as flexible as possible with limited spectrum and power resources as far as possible. The design and implementation of appropriate multiple access scheme are of great importance for satellite networks since their on-board resource is severely limited while the service requirements of terminal devices are ever-increasing, especially in rural areas. In this case, novel multiple access schemes, such as non-orthogonal multiple access (NOMA) and enhanced ALOHA, have been introduced in non-geostationary orbit (NGSO) communication to further improve resource utilization efficiency in recent years. This chapter focuses on these two novel access schemes which are relevant for future satellite systems.

Section 7.1 introduces and compares three commonly used orthogonal multiple access (OMA) schemes. Section 7.2 is devoted to the performance analysis of a NOMA-based NGSO satellite system, where the key points of the NOMA scheme are specifically addressed. Section 7.3 focuses on the performance improvement brought by the enhanced ALOHA. Finally, the last section summarizes the content of this chapter.

The success of new NGSO constellations relies on a number of financial and technological issues, one of them being the use of ISLs between the satellites of the constellations. Thanks to ISL, the performance of the NGSO system is improved in terms of the number of ground station sites, end-to-end latency, capacity, service area and security. Moreover, the use of space systems for 5G is fostered by implementing ISL as a distant gateway station that can be reached from satellites located out of its field of view. In fact, most of the current NGSO system for broadband access makes use of ISL as an intrinsic part of the system. In addition, ISL facilitates diverse communication architectures and can be used in other-than-communication missions, such as data collection space systems, fractionated spacecrafts or to support scientific missions.

However, the introduction of ISLs in NGSO satellites has a large effect on the platform subsystems, communication architecture, and system operation. As shown in section 8.4, a number of complex technical and operational challenges arise in the presence of ISL.

ISL in broadband constellations can be RF or optical. Each of these technologies has pros and cons, so the selection of one of them must consider systems aspects like mission concept, platform requirements, communication link needs and technology maturity. In the case of RF ISLs, the roadmap should be focused on the design of PAs with higher efficiency in the ISL bands as well as advancing in antenna aperture with beam steering capabilities. For optical ISLs, as the links are free from limiting atmospheric effects, one critical aspect is the ATP subsystem to align the laser beam in the presence of vibrations of the platform. With both technologies, lower mass and power requirements for the ISL terminal are required.

Finally, the chapter presents a case study where requirements for the design of a radio ISL in an NGSO constellation formed by small satellites. The case study concludes with the presentation of prototype of an ISL antenna operating in E band which meets the requirements of mass, losses, beam scanning, and integration with the platform. The measurement results of the proof of concept reinforces the idea that integration of ISL in NGSO constellations, even with small satellites, thanks to the evolution of RF and optical technology achieved since the first NGSO systems were proposed in the 1990s.

In this chapter, the DL transmit design with sCSIT was investigated for massive MIMO LEO SATCOM systems. First, the DL massive MIMO LEO satellite channel model was derived, where the satellite and the UTs are both equipped with UPAs. Then, it was shown that the single-stream precoding for each UT is able to maximize the ergodic sum rate for the linear transmitters. To reduce the computational complexity, another transmit design was formulated by using an upper bound on the ergodic sum rate, for which the optimality of single-stream precoding also holds. Moreover, it was revealed that the design of precoding vectors can be simplified into that of scalar variables. The effectiveness and the performance gains of the proposed DL transmit designs were verified via the simulation results. There are still many potential challenges for future high-throughput LEO SATCOM systems, which are briefly summarized as follows, e.g., low-complexity hybrid precoding, real-time resource management, multiple satellite cooperation, the coexistence of NGSO and GSO satellites.

Beyond 5G and 6G networks are expected to meet ambitious performance parameters of coverage, data rates, latency, etc., with the objective of exploiting as many as possible physical network resources, such as capacity, to get the maximum achievable performance out of them. In this context, the smart and efficient use of available resources, as well as ubiquitous and continuous coverage provided by satellite networks, have become a must. Network virtualization (NV) has been proved to be a key enabling technology to fulfill the challenging requirements of the upcoming telecommunication networks. NV is based on algorithms that can instantiate virtualized services on the substrate infrastructure, optimizing the embedding, according to a specific objective. This kind of algorithms is known as virtual network embedding (VNE). The aim of this chapter is to focus on two main aspects of the VNE. First, an efficient parallel approach for the VNE problem is considered. More precisely, the aim is to show how a parallel computation for the resource mapping allows to further improve the performance of the algorithm. Second, the chapter introduces a practical implementation of the VNE algorithm in a software defined networking (SDN)-based testbed. An experimental testbed to support the VNE algorithm for non-geostationary orbit (NGSO) constellations is presented. The laboratory testbed has been developed and validated, consisting of a Mininet-based simulator, a Ryu SDN controller with an end-to-end (E2E) traffic engineering (TE) application for the virtual networks (VNs) establishment, and the VNE algorithm implemented in MATLAB^®.

This chapter investigates the anti-jamming routing selection problem for the NGSO satellites system. First, the anti-jamming routing selection problem between users and smart jammers is formulated as a hierarchical Stackelberg anti-jamming routing game. Second, DRLR is proposed to obtain an available routing subset. Based on this subset, FRA is proposed to make a quick anti-jamming decision. The jammer can automatically adjust the targeted node and jamming power according to the jamming effect, and the user utilizes DRLR and FRA algorithm to actively explore the dynamic network, empirically analyze the jammer's strategies and adaptively make an anti-jamming decision. Finally, the simulations have proven that the proposed algorithm has better performance than existing approaches, and the anti-jamming policies converge to the Stackelberg equilibrium.

Higher broadband speed, lower latency, and expanded coverage are the key characteristics behind the popularity of non-geostationary orbit (NGSO) satellite constellations. While NGSO satellite constellations for broadband are in the initial stages of development and deployment, it is expected that in the coming decades, we will witness a substantial increase in the number of NGSO satellites launched to space. In this book, we have provided an overview of the main uncertainties pose by such imminent deployment and for a successful and efficient operation of such megaconstellations. Some of them include their coexistence and/or integration with legacy satellites and terrestrial wireless communication systems, flexible radio resource allocation and interference management, constellation design and reliability, etc. These open challenges crucially interconnected puzzle pieces that shall fit together to unleash the full potential of NGSO communication systems.