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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 propose and report the implementation of a multiband and photonically amplified fiberwireless (FiWi) Xhaul based on radio over fiber (RoF) technology and four-wave mixing (FWM) nonlinear effect, aiming 5G applications. The proposed ultra-wideband approach enables to simultaneously transport and amplify multiple radiofrequency (RF) signals through optical links, which might be employed as backhaul, midhaul or fronthaul of cellular systems. The FWM effect, originated from the use of 35m highly nonlinear fiber piece, gives rise to RF gain, when compared to conventional RoF (CRoF) systems. We demonstrate our technique allows replacing two conventional RF amplifiers with enhanced digital performance and/or significantly increasing the system throughput in 2.4 times, attaining 12 Gbit/s. Furthermore, a dual-band (7.5 and 28.0 GHz) wireless fronthaul, preceded by a 12.5-km optical midhaul, illustrates the multiband and photonically amplified FiWi Xhaul, by means of providing performance in terms of root mean square error vector magnitude (EVM RMS) in accordance to the 3GPP recommendations and at low phase noise level. INDEX TERMS 5G networks, four-wave mixing, microwave photonics, optical-wireless networks.

We experimentally demonstrate the use of optical frequency combs (OFCs), generated by a photonic integrated circuit (PIC), in a flexible optical distribution network based on fiber-optics and free-space optics (FSOs) links, aimed at the fifth generation of mobile network (5G) Xhauls. The Indium Phosphide (InP) monolithically integrated OFC is based on cascaded optical modulators and is broadly tunable in terms of operating wavelength and frequency spacing. Particularly, our approach relies on applying the PIC in a centralized radio access network (C-RAN) architecture, with the purpose of optically generating two low-phase noise mm-waves signals for simultaneously enabling a 12.5-km of single-mode fiber (SMF) fronthaul and a 12.5-km SMF midhaul, followed by a 10-m long FSO fronthaul link. Moreover, the demonstrator contemplates two 10-m reach 5G wireless access networks operating in the 26 GHz band, i.e. over the frequency range 2 (FR2) from the 5G NR standard. The proposed integrated OFC-based 5G system performance is in accordance to the 3rd Generation Partnership Project (3GPP) Release 15 requirements, achieving a total wireless throughput of 900 Mbit/s.