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.

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.

The 5G vision embraces a broad range of applications including the connectivity in underserved and remote areas. In particular, for these applications, satellites are going to play a role in future 5G networks to provide capacity on trains, vessels, aircraft, and for base stations around the globe. In this paper, a 5G edge node concept, developed and evaluated with over-the-air tests using satellites in the geostationary orbit, is presented. The article covers a testbed demonstration study in Europe with a large-scale testbed including satellites and the latest standardization for the network architecture. The main goal of this testbed is to evaluate how satellite networks can be best integrated within the convergent 5G environment. The over-the-air tests for 5G satellite integration in this article are based on a 3GPP Release 15 core network architecture.

The 3rd Generation Partnership Project (3GPP) aims to integrate non-terrestrial networks (NTNs) with terrestrial 5G networks. In this context, prototyping and testing are essential to demonstrate the readiness of new 5G-NTN releases. However, commercial 5G protocol stack implementations are usually costly and closed solutions, which makes it challenging for universities and nonprofit research institutions to adapt and customize them for 5G-NTN testing and development purposes. Software-defined radio testbeds that utilize open-source software implementations are an attractive alternative. In this article, we describe our protocol stack adaptation of an open-source implementation for 5G-NTN based on OpenAirInterface (OAI), which runs on commodity hardware. We evaluate the performance of our 5G-NTN in various layers in real-world environments with a transparent geostationary satellite. We validate our testbed with a variety of Internet applications. These include streaming video, web browsing, and voice over IP. The results indicate that the current implementation utilizing the 5MHz bandwidth is reliable over several hours and achieves both downlink and uplink speeds of 3.6Mb/s while maintaining a typical geostationary latency of approximately 530 ms. The tests demonstrate that our 5G-NTN testbed is an reliable alternative to costly commercial implementations.