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 the integration and experimental performance analysis of a GFDM-based 5G transceiver in a gigabit passive optical network (GPON), using radio over fiber technology. The proposed architecture enables to simultaneously transport two 5G candidates RF signals through an active GPON under real channel conditions. One signal is generated by a GFDMbased 5G prototype transceiver at 735 MHz, whereas the second one is synthetized by a vector signal generator at 26 GHz. A dualdrive Mach-Zehnder modulator has been utilized in the optical line terminal to modulate both signals, with the purpose of mitigating the interference between them. Particularly for the GFDM-based 735 MHz signal, a modulation error ratio (MER) of 40 dB has been obtained at RF-driven signal up to -9 dBm. Furthermore, the use of a digital predistortion scheme has been efficiently employed to reduce the impact of the nonlinear distortions and enhance MER. The 26-GHz RF signal, aimed for the 5G millimeter wave band, has been investigated as a function of error vector magnitude (EVM) for bitrates up to 1 Gbit/s. EVMRMS of 2.18% and 5.70% have been obtained for 100 Mbit/s and 1 Gbit/s, respectively. Finally, the latency and throughput measurements of the baseband signal originally running over GPON have shown no significant penalties.

This article reports the implementation and experimental performance investigation of a DSP-based flexible-waveform fiber-wireless (FiWi) system for 5G enhanced mobile broadband (eMBB) and new vertical applications. Our radio over fiber-based fronthaul solution uses a wavelength-division-multiplexing passive optical network (WDM-PON) infrastructure, from a commercial Internet service provider, to enable 5G operation over multiple frequency bands, including: a DSP and flexible waveform-based signal at 788 MHz, which can be set as generalized frequency division multiplexing (GFDM) or filtered orthogonal frequency division multiplexing (F-OFDM); 5G new radio (NR) signals at 3.5 or 26 GHz in accordance to 3GPP Release 15; an additional vector 26 GHz signal with bandwidth of up to 800 MHz. The DSP-based functionality provides digital pre-distortion (DPD), besides the real-time waveform generation. Experimental results demonstrate 4.41 Gbit/s total throughput in the air in accordance to the 3GPP requirements, as well as an innovative low-latency M2M application based on PROFINET standard.

This work reports the development of two high-performance waveguide-based antenna arrays for 5G cellular networks, operating in the underutilized millimetre wave (mm-wave) frequency spectrum. Two different scenarios of mm-wave communications are proposed for illustrating the applicability of the proposed arrays, which provide specific radiation patterns, namely, 12 dBi gain omnidirectional coverage in the 28 GHz band and dual-band sectorial coverage using the 28 and 38 GHz bands with gain up to 15.6 dBi. Numerical and experimental results of the array reflection coefficient, radiation pattern, and gain have been shown in an excellent agreement.

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 35-m 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 2.72 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 (EVMRMS) in accordance to the 3GPP recommendations and at low phase noise level.

This work reports the concept and implementation of a dual-band wireless fronthaul (FH), using a frequency selective surface (FSS)-based focal-point/Cassegrain antenna, assisted by an optical midhaul (MH). Our innovative fiber-wireless architecture enables to achieve simultaneous and extended-reach transmission over two distinct bands, namely C- and Ka-bands. The dual-band wireless FH is ensured by employing a dual-band antenna, consisting of a conventional main reflector (paraboloid) and a subreflector based on a frequency selective surface, which allows integrating a focal-point and a Cassegrain system in the same structure. It presents gain from 30 to 39.4 dBi and bandwidth from 1.1 to 4.3 GHz at 7.45 and 28 GHz, respectively. The optical MH relies on applying a dual-drive Mach-Zehnder modulator for simultaneously modulating both RF signals at the same wavelength and distribute them through a 25 km fiber optical link. Finally, the RF carriers at 7.5 and 28 GHz are radiated by the FSS-based antenna over dozens of meters wireless link. Experimental results demonstrate throughput up to 18 Gbit/s with error vector magnitude in accordance to the 3GPP Release 15 requirements, giving rise to a potential technological solution for increasing the system range, as well as reducing capital expenditure costs, footprint, weight and complexity.