###### Abstract  
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. 4G/5G; fronthaul; optical-wireless systems; VLC +
Footnote †: journal: Medical  
(mMTC) applications. According to the release, the technical solutions to be explored include integrated access and backhaul (IAB), industrial Internet of Things (IoT), satellite access, and NR-based access to unlicensed spectrum [7]. Releases 17 and 18 for 5G are currently in production and scheduled to end in 2021, with the goal of further enhancing mobile systems [8]. By the same token, planning of sixth-generation (6G) communications has already begun, focused on providing performance superior to 5G and satisfying future demands as far as 2030. Among the potential technologies for 6G, optical-wireless convergence continues to play an important role for future RANs, whereas terahertz and visible light communications (VLC) have also emerged [9; 10]. In this context, a key area of study is microwave photonics (MWP), which takes advantage of optical and mobile communications for signal distribution in a heterogeneous architecture [11; 12]. MWP techniques have been strongly applied to radiofrequency (RF) generation, processing, detection, transport, and distribution. Notably, radio over fiber (RoF) technology enables transportation and distribution of digital and/or analog signals among central office (CO) and remote radio units (RRUs) via fiber-optic links, giving rise to the fiber-wireless (FiWi) systems [11; 13]. From a C-RAN point of view, HetNet in conjunction with picocell and femtocell implementations allows system coverage and data rate to be enhanced. In C-RAN architecture, the backhaul and fronthaul links connect CO to the core network, typically carrying digital user data and CO to RRUs, respectively. Fifth-generation systems are mostly based on digital radio over fiber (D-RoF) using the common radio public interface (CPRI) protocol, in which radio signals are sampled and directly digitalized into baseband data at the remote radio head (RRH) from the cell site. Millimeter-wave operation requires high-speed A/D (analog-to-digital) and D/A (digital-to-analog) converters, creating a bandwidth bottleneck in uncompressed CPRI. To address this issue, two main network alternatives have been actively investigated. One possibility is functional split (FS), widespread by 3 GPP under e-CPRI. The FS establishes that some network functionalities be carried out at RRHs, thereby alleviating the transmission rate requirements for the 5G fronthaul. Specifically, 3 GPP has defined a series of options for functional split between BBU and RRHs. In short, functional splits decentralize control functionalities in such way as to establish a trade-off between reducing fronthaul throughput and increasing latency. The other option is analog fronthaul (A-RoF), in which the RF wireless signal modulates an optical carrier that is optically distributed. Because there is no high-speed A/D conversion, the system is far less complex and the overall latency is much smaller, dictated essentially by the optical fiber length.