Positioning, Navigation, and Timing (PNT) is an essential service for modern societies, their industries, and governmental organizations. Mobility, logistics, and agriculture, among others, depend heavily on reliable PNT and will do so even more in the future. However, the predominant Global Navigation Satellite Systems (GNSS) are highly susceptible to jamming and spoofing, a threat that has increased in the recent years. As a result, there is a growing need for a robust, independent PNT backup system. A promising approach to meet this demand is to use the communication signals of satellites in Low Earth Orbit (LEO) as signals of opportunity (opportunistic LEO-PNT). In recent years, opportunistic LEO-PNT has gained relevance in academic research due to the emergence of megaconstellations. This survey provides an holistic overview of opportunistic LEO-PNT and an exhaustive review of the academic work in the field. Error sources and challenges with respect to the development of operational systems are evaluated, and the state of the art performance is analyzed. System-level strategies to mitigate the error sources and challenges are identified, including the combination of opportunistic LEO-PNT with GNSS or other sensors, or the use of base stations. Future research directions, such as the investigation of non-accuracy related KPIs, the required receiver hardware, or the use in low SNR scenarios, are derived.

Direct connectivity in L/S frequency bands between satellites and common mobile terrestrial user equipment (UE), such as smartphones, is an essential feature for future 6G non-terrestrial networks. The technical trend in closing the link between the communication endpoints is to develop large phased antenna arrays to be launched in LEO orbit. Satellite swarms represent an innovative and promising approach. Swarms are composed of several small and lightweight satellites organized in a free-flying formation (i.e., wireless connected) or a tethered formation (i.e., wired connected) creating a distributed phased antenna array. It has the potential to provide an enhanced gain, narrower beam width and lower launch/build costs compared to conventional single satellite systems with large phased antenna arrays. The first objective of this work is the design of swarm-based antenna arrays, in which the impact of key parameters such as the number of satellites in the swarm, their reciprocal distance and the array geometry, is thoroughly analyzed. It is shown that the undesired phenomenon of grating lobes can be mitigated via optimized array geometries and a new geometry named the enhanced logarithmic spiral array (ELSA) is presented. The second objective of this work is the identification of the most important research directions and system design aspects for the swarm system. In particular, it is shown that tethered swarms with ELSA geometries, innovative deployable structures and very small satellites can foster the deployment of swarms in future satellite systems.

In satellite communication downlinks, physical layer security is challenging to achieve due to their broadcasting nature and Line-of-Sight channel characteristics. This paper provides a precoding algorithm to secure the downlinks of multiple users against multiple eavesdroppers with optimization of the minimum secrecy capacity. By the use of artificial noise, a positive secrecy capacity is achievable even if the number of eavesdroppers is higher than the number of beams. We demonstrate that a multiple-reflector antenna design provides a significantly higher secrecy and throughput performance when compared to a single-reflector design due to additional degrees of freedom, exhibited by the signal phases. The total vulnerability region is introduced as a new figure of merit with respect to unidentified eavesdroppers.

Satellites enable the provision of global 5G coverage utilizing a customized protocol stack for direct access of 5G devices. This paper reports the results of a field trial based on an Open Source Software (OSS) 5G protocol stack modified for Satellite Communications. 5G New Radio (NR) functions have been adapted to support 3GPP Non-Terrestrial Networks (NTN) standardization for Release 17. Some of the currently discussed enhancements for 5G New Radio (NR) direct access for NTNs were evaluated in this study utilizing a geosynchronous satellite system. Non-Terrestrial Networks (NTN) devices and base station were software-defined solutions communicating via satellite through a standardized 5G NR waveform. The results show that Open Source Software (OSS) implementations running on general purpose platforms greatly support the research and standardization process for 5G-Beyond NTNs. New functions related to NTNs can be evaluated at an early stage over channel emulators or real world satellites to highlight the capabilities of space based 5G-Beyond networks. Therefore, OSS can be a starting point to turn the vision of 6G protocols on-board the satellite into reality.

With sum throughputs exceeding 1 Tbit/s, nextgeneration high throughput satellites will play a key role in 5G networks for applications including backhauling or broadband in-flight connectivity. Such satellites require the deployment of tens of Q/V-band feeder links to support the data traffic in their uplink. Meanwhile, geographical restrictions and cost considerations significantly constrain the search for gateway deployment sites. Diversity strategies are, furthermore, necessary to cope with strong rain fades. In this work, multiple-input-multiple-output (MIMO) feeder links are proposed to address these challenges. Using two active gateway antennas per feeder link, a MIMObased solution enables to support twice the user link bandwidth per link compared to the state-of-the-art. On-ground and onboard processing strategies for such MIMO links are introduced, and the link outage probabilities are determined. An extension of smart diversity schemes to MIMO-based architectures is also presented to guarantee a sufficient robustness against rain fades. An analysis of the achieved uplink CINR demonstrates the superiority of the proposed concept and provides guidelines for an efficient design.