The sixth generation of mobile communication (6G) systems is recently rising a lot of interest, introducing new futuristic and challenging use cases that will demand much more than just communications to become a reality. Higher throughput, lower latencies, higher number of connections will push the requirement of the future mobile networks to a new level, but also sensing, positioning and imaging will play an important role in the new foreseen use cases. The integration of techniques developed for wireless communications with those conceived for optical links will be essential to provide the infrastructure for the 6G networks. In this context, this paper presents a review on wireless and optical convergent access solutions towards the 6G systems. The manuscript brings the use cases, requirements and enablers for 6G networks including a discussion about the state-of-the-art on THz and sub-THz communications, wireless and optical convergence, visible light communication, integrated and free-space optics, new antenna designs, power-over-fiber deployments and the use of machine learning in the physical layer of future networks. By reviewing the most relevant contributions available in the literature for wireless and optical communications and presenting their main contributions, this paper clearly shows that, more than a technological trend, the convergence of wireless and optical technologies is a fundamental step towards the development of the 6G network infrastructure.

This paper reports two implementations of power-over-fiber (PoF) solutions applied to radio-over-fiber (RoF) and optical wireless communication (OWC) systems, in the context of an industrial environment. We employ a conventional 62.5-µm multimode fiber (MMF) to deliver optical power to different communication links based on RoF, free-space optics (FSO), and visible light communication (VLC) technologies aiming beyond 5G (B5G) and 6G applications. First, a 3.5-GHz 5G New Radio (5G NR) signal is transmitted throughout a 20-km single-mode optical fiber (SMF) link using RoF technology. Regarding the PoF system, a 5-W optical power is transmitted through a 100-m MMF link. A photovoltaic power converter (PPC) and a DC/DC converter are employed to convert the power from the optical to the electrical domain and adjust the voltage level, respectively, with the purpose of energizing a remote RoF module. The attainable optical and electrical power transmission efficiencies (OPTE and PTE) are 80% and 19%, respectively. Posterior, a second PoF system is implemented to power a hybrid RoF/FSO/VLC B5G system, comprising a 200-m MMF and an additional DC/DC converter. Over 10.5 W of optical power is transmitted to feed an electrical amplifier (EA) and a white LED from the VLC link. In this configuration, we achieve 78% and 18.5% of OPTE and PTE, respectively. Furthermore, a performance investigation based on the root mean square error vector magnitude (EVM_RMS) metric is conducted to evaluate the signal using the implemented PoF systems and a conventional electrical power supply. In the first implementation, a throughput of 600 Mbps is achieved with 100-MHz bandwidth without performance degradation, when compared to the conventional-powered RoF system, whereas, in the second implementation, 60-Mbps throughput is achieved when employing the FSO and VLC technologies simultaneously, demonstrating the applicability and potential of the PoF technique for B5G and 6G industrial communications.

We report the experimental implementation of optically-powered wireless sensor nodes based on the power-over-fiber (PoF) technology, aiming at Industrial Internet of Things (IIoT) applications. This technique employs optical fibers to transmit power and is proposed as a solution to address the hazardous industrial environment challenges, e.g., electromagnetic interference and extreme temperatures. The proposed approach enables two different IIoT scenarios, in which wireless transmitter (TX) and receiver (RX) nodes are powered by a PoF system, enabling local and remote temperature data monitoring, with the purpose of achieving an intelligent and reliable process management in industrial production lines. In addition, the system performance is investigated as a function of the delivered electrical power and power transmission efficiency (PTE), which is the primary performance metric of a PoF system. We report 1.4 W electrical power deliver with PTE = 24%. Furthermore, we carry out a voltage stability analysis, demonstrating that the PoF system is capable of delivering stable voltage to a wide range of applications. Finally, we present a comparison of temperature measurements between the proposed approach and a conventional industrial programmable logic controller (PLC). The obtained results demonstrate that PoF might be considered as a potential technology to power and enhance the energy efficiency of IIoT sensing systems.

We report the implementation of a full optically-powered 5G new radio (5G NR) fiber-wireless (FiWi) system based on power-over-fiber (PoF) and radio-over-fiber (RoF) technologies. Our approach enables the simultaneous transmission of a 5G NR signal at 3.5 GHz with bandwidth up to 100 MHz and a 2.2-W optical power signal employing dedicated fiber-optics links. The optical-wireless data link consists of a 12.5-km single-mode fiber (SMF) optical fronthaul followed by a 10-m wireless propagation environment, which is the longest wireless reach reported in literature up to now, regarding optically-powered FiWi systems. The proposed PoF system is able to deliver stable electrical power up to 475 mW, by means of using a 100-m multimode fiber (MMF) link, with the purpose of optically powering a 5G NR remote antenna unit (RAU). An overall power transmission efficiency (PTE) of 23.5% is experimentally demonstrated in a real 5G NR system. Furthermore, the FiWi system performance is investigated in accordance with the $3^{\mathrm rd}$ generation partnership project (3GPP) Release 15 requirements, in terms of root mean square error vector magnitude ( ${\mathrm EVM}_{\mathrm RMS}$ ). The proposed optically-powered 5G NR FiWi system provides 500 Mbit/s throughput with ${\mathrm EVM}_{\mathrm RMS}$ as low as 3.9%, employing 64-quadrature amplitude modulation (QAM) without using optical amplification.

The rapid evolution of wireless communication networks has fueled the demand for high-speed data transmission and low-latency connectivity. As the world looks toward the development of sixth-generation (6G) Networks, the need for innovative power distribution solutions becomes critical. Power-over-fiber (PoF) technology has emerged as a promising candidate for addressing these challenges by enabling the delivery of power through optical fibers. This review paper focuses on the future prospects of PoF technology for 6G Networks, with a particular emphasis on the integration of the radio-over-fiber (RoF) technique. In this context, we explore the application scenarios for radio- and power-over-fiber (RPoF) systems, enabling simultaneous data and power transmission over optical fibers. Furthermore, we discuss the underlying principles of PoF systems, including operation and key components, e.g., high-power laser diode (HPLD), optical fiber, and photovoltaic power converter (PPC), highlighting their key features, performance metrics, and design considerations. A comprehensive analysis of the current state of PoF technology is also presented, focusing on its potential applications in the context of 6G Networks. Furthermore, the main challenges and limitations of RPoF systems are identified, including power transmission efficiency (PTE), optical fiber design and deployment, scalability, capacity, safety, and reliability. Finally, we discuss the potential research directions and technological advancements needed to fully exploit the capabilities of RPoF in the 6G era.