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, powerover-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.

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 opticallypowered 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 singlemode 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 rd generation partnership project (3GPP) Release 15 requirements, in terms of root mean square error vector magnitude (EVM RMS ). The proposed optically-powered 5G NR FiWi system provides 500 Mbit/s throughput with EVM RMS as low as 3.9%, employing 64-quadrature amplitude modulation (QAM) without using optical amplification.

The millimeter-waves band will enable multi-gigabit data transmission due to the large available bandwidth and it is a promising solution for the spectrum scarcity below 6 GHz in future generations of mobile networks. In particular, the 60 GHz band will play a crucial role in providing high-capacity data links for indoor applications. In this context, this tutorial presents a comprehensive review of indoor propagation models operating in the 60 GHz band, considering the main scenarios of interest. Propagation mechanisms such as reflection, diffraction, scattering, blockage, and material penetration, as well as large-scale path loss, are discussed in order to obtain a channel model for 60 GHz signals in indoor environments. Finally, comparisons were made using data obtained from a measurement campaign available in the literature in order to emphasize the importance of developing accurate channel models for future wireless communication systems operating in millimeter-waves bands.

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