Contents 1. Introduction
5G is paving the way for the transformation and digitalization of key industry sectors like healthcare, smart cities, transportation, entertainment, agriculture, manufacturing, and others. However, it is challenging to simultaneously meet all the networking requirements since typical applications cover real-time high-resolution videos and holographic experiences demanding massive amounts of data, with uninterrupted service everywhere even at high mobility [1]. The key performance indicators (KPIs) of 5G for download and upload speeds are 100 Mbps and 50 Mbps, respectively [2]. However, the long-term requirements are substantially higher than that to fully support applications like the eXtended Reality (XR) and digital twins. Thus, new solutions that can provide higher data rates and a true ubiquitous service are needed for the next generation mobile networks, i.e., the 6G. In contrast to traditional deployments where the antennas are co-located in a single array in the center of the cell, the lately introduced distributed multiple-input multiple-output (D-MIMO) networks are equipped with geographically distributed antennas that can operate jointly, synchronously, and coherently to serve users. In such a way, D-MIMO can increase considerably the data rates and coverage, however, it is more complex and costly to deploy [3]. As D-MIMO requires synchronization of the fronthaul links of the spatially distributed antennas to realize coherent joint transmissions (CJTs), a cost-efficient solution to realize CJT is to use analog fronthaul links [4] in combination with centralized processing [5]. Free-space optical (FSO) links are a prospective alternative to fiber optics that fulfil the preceding requirements facilitating the deployment of D-MIMO networks. Recently, it has been shown that mid-infrared (IR) free-space optics (FSO) [6], specifically in the Long-wave IR (LWIR) band (8-12 µm), contain advantageous characteristics for terrestrial applications. It offers broader spectrum and lower atmospheric attenuation than the sub- THz/THz bands, whereas it is less sensitive to atmospheric turbulence and particle scattering than the traditional near- IR wavelengths used for fiber-optic telecom systems [7].
