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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.

This work reports a common indoor experimental coexistence performance analysis among 5G New Radio (5G NR), 4G Long Term Evolution Advanced Pro (LTE-A Pro) and Narrowband Internet of Things (NB-IoT), using co-channel and adjacent channels in the 700 MHz Band. The required frequency offset for the 4G and 5G generations simultaneous operation has been evaluated, as a function of the modulation scheme in order to minimize the adjacent channel interference. A suitable management of the numerology and allocating time-frequency resources, configuring CORESET, enables a peaceful coexistence, as well as high performance for all evaluated systems. Experimental results demonstrate an efficient spectrum sharing of 10 MHz-bandwidth for a 5G NR, an LTE-A Pro and three NB-IoT downlink carriers, aiming to take advantage of 700 MHz propagation aspects for the current 4G and future 5G. Constellation and error vector magnitude (EVMRMS) in accordance to the 3GPP requirements reinforce the successful 4G and 5G spectrum refarming implementation.

The fifth-generation of mobile network (5G) and beyond requires a radio access network (RAN) update in order to cope with the incoming increase of wireless data traffic and new applications. In this context, we propose an efficient optical-wireless architecture applied to the non-standalone (NSA) 5G new radio (NR) framework. Several distinct electrical- and optical-based fronthaul configurations combining free-space optical (FSO), wireless links, and radio over fiber (RoF) techniques were implemented and properly analyzed for selection according to network operator deployment requirements. In addition, visible light communication (VLC) was investigated as a future access network technology when immunity to electromagnetic interference is paramount. Experimental results demonstrated fourth-generation of mobile network (4G) and 5G coexistence at Gbit/s throughput and error vector magnitude (EVM) in accordance with 5G NR Release 15.

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