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Photonics
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Free Space Optics-Based 6G Non-terrestrial Networks
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Free Space Optics-Based 6G Non-terrestrial Networks

A special issue of Photonics (ISSN 2304-6732).

Deadline for manuscript submissions: closed (20 June 2023) | Viewed by 13075

Special Issue Editors

Dr. Sujan Rajbhandari

E-Mail Website
Guest Editor
School of Computer Science and Electronic Engineering, Bangor University, Bangor LL57 2DG, UK
Interests: optical communication; optical wireless communication; visible light communications; MIMO
Special Issues, Collections and Topics in MDPI journals
Sahil Nazir Pottoo

E-Mail Website
Guest Editor Assistant
Department of Electrical Engineering, Faculty of Engineering Science and Technology, UiT The Arctic University of Norway, 8514 Narvik, Norway
Interests: mobile FSO systems; non-terrestrial networks; 6G communication

Special Issue Information

Dear Colleagues,

The integration of network flying platforms (NFP) into sixth-generation (6G) systems has recently become a hot topic, owing to the rapid progress in non-terrestrial networks (NTN). In this context, free space optical (FSO)-based NTN is growing in importance due to its capability to deliver services anywhere and at any time and also provide coverage in areas that are unreachable by any conventional terrestrial network, including disaster-recovering environments or remote locations. The integrated ground–air–space infrastructure will enable the delivery of futuristic services through FSO-operated communication and flights with UAVs, HAPs, balloons, CubeSats, LEO, MEO, and GEO satellites. Nevertheless, the management and operation of FSO-based mobile platforms require the study and creation of novel solutions for many challenges that will pave the way for several disruptive new applications.

This Special Issue will cover advances in state-of-the-art mobile FSO systems for 6G fronthaul/backhaul connectivity. Authors are encouraged to submit theoretical contributions (new techniques, concepts, surveys, and tutorials), mathematical modeling and numerical simulations, and practical contributions (lab and field experiments, prototypes, and new applications) on FSO-based 6G NTN with UAVs, HAPs, balloons, and satellites. Topics of interest include but are not limited to the following:

  • FSO-based vertical backhaul/fronthaul 6G architecture;
  • Applications of ML/DL in mobile FSO systems;
  • New robust acquisition, tracking, and pointing (ATP) mechanisms for flying FSO platforms;
  • High data rate modulating retroreflector (MRR)-aided mobile FSO systems;
  • Energy consumption models and energy harvesting techniques for flying FSO platforms;
  • Miniature FSO transceiver design based on photonic integrated circuit (PIC);
  • Quantum key distribution (QKD) and orbital angular momentum (OAM) in mobile FSO;
  • All-optical networking;
  • Propagation channel modeling for non-terrestrial FSO systems;
  • Intelligent swarm and connectivity between aerial platforms.

Dr. Sujan Rajbhandari
Guest Editor

Sahil Nazir Pottoo
Guest Editor Assistant

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Photonics is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • free space optics
  • 6G communication
  • non-terrestrial networks
  • network flying platforms
  • aerial communications

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Published Papers (3 papers)

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Research

45 pages, 2293 KiB  
Article
Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
by Mohammed Elamassie and Murat Uysal
Photonics 2023, 10(11), 1210; https://doi.org/10.3390/photonics10111210 - 30 Oct 2023
Cited by 13 | Viewed by 5369
Abstract
The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended [...] Read more.
The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended coverage and operational altitude requirements. NTN nodes have the potential to support both wireless access and backhauling. While the onboard base station provides wireless access for the end users, the backhauling link connects the airborne/space-borne base station to the core network. With its high data transmission capability comparable to fiber optics and its ability to operate in the interference-free optical spectrum, free space optical (FSO) communication is ideally suited to backhauling requirements in NTNs. In this paper, we present a comprehensive tutorial on airborne FSO backhauling. We first delve into the fundamentals of FSO signal transmission and discuss aspects such as geometrical loss, atmospheric attenuation, turbulence-induced fading, and pointing errors, all of which are critical for determining received signal levels and related link budget calculations. Then, we discuss the requirements of airborne backhaul system architectures, based on use cases. While single-layer backhaul systems are sufficient for providing coverage in rural areas, multi-layer designs are typically required to establish connectivity in urban areas, where line of sight (LoS) links are harder to maintain. We review physical layer design principles for FSO-based airborne links, discussing both intensity modulation/direct detection (IM/DD) and coherent modulation/coherent demodulation (CM/CD). Another critical design criteria for airborne backhauling is self-sustainability, which is further discussed in our paper. We conclude the paper by discussing current challenges and future research directions. In this context, we discuss reconfigurable intelligent surfaces (RIS) and spatial division multiplexing (SDM), for improved performance and an extended transmission range. We emphasize the importance of advanced handover techniques and scalability issues for practical implementation. We also highlight the growing role of artificial intelligence/machine learning (AI/ML) and their potential applications in the design and optimization of future FSO-based NTNs. Full article
(This article belongs to the Special Issue Free Space Optics-Based 6G Non-terrestrial Networks)
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18 pages, 1926 KiB  
Article
Securing Non-Terrestrial FSO Link with Public Key Encryption against Flying Object Attacks
by Daniel Hicks, Fatma Benkhelifa, Zahir Ahmad, Thomas Statheros, Osama Saied, Omprakash Kaiwartya and Farah Mahdi Alsallami
Photonics 2023, 10(8), 884; https://doi.org/10.3390/photonics10080884 - 29 Jul 2023
Cited by 4 | Viewed by 2401
Abstract
Free Space Optical (FSO) communication has potential terrestrial and non-terrestrial applications. It allows large bandwidth for higher data transfer capacity. Due to its high directivity, it has a potential security advantage over traditional radio frequency (RF) communications. However, eavesdropping attacks are still possible [...] Read more.
Free Space Optical (FSO) communication has potential terrestrial and non-terrestrial applications. It allows large bandwidth for higher data transfer capacity. Due to its high directivity, it has a potential security advantage over traditional radio frequency (RF) communications. However, eavesdropping attacks are still possible in long non-terrestrial transmission FSO links, where the geometry of the link allows foreign flying objects such as Unmanned Aerial vehicles (UAVs) and drones to interrupt the links. This exposes non-terrestrial FSO links to adversary security attacks. Hence, data security techniques implementation is required to achieve immune FSO communication links. Unlike the commonly proposed physical layer security techniques, this paper presents a lab-based demonstration of a secured FSO communication link based on data cryptography using the GNU Radio platform and software-defined radio (SDR) hardware. The utilized encryption algorithm (Xsalsa20) in this paper requires high-time complexity to be broken by power-limited flying objects that interrupt the FSO beam. The results show that implementing cryptographic encryption techniques into FSO systems provided resilience against eavesdropping attacks and preserved data security. The experiment results show that, at a distance of 250 mm and laser output power of 10 mW, the system achieves a packet delivery rate of 92% and transmission rate of 10 Mbit/s. This is because the SDR used in this experiment requires a minimum received electrical amplitude of 27.5 mV to process the received signal. Long distance and higher data rates can be achieved using less sensitive SDR hardware. Full article
(This article belongs to the Special Issue Free Space Optics-Based 6G Non-terrestrial Networks)
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17 pages, 3410 KiB  
Article
Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems
by Manuel M. Freitas, Marco A. Fernandes, Paulo P. Monteiro, Fernando P. Guiomar and Gil M. Fernandes
Photonics 2023, 10(4), 394; https://doi.org/10.3390/photonics10040394 - 2 Apr 2023
Cited by 8 | Viewed by 3806
Abstract
The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous [...] Read more.
The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized. Full article
(This article belongs to the Special Issue Free Space Optics-Based 6G Non-terrestrial Networks)
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