# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 1 Introduction
### Related Work  
A hybrid FSO and passive optical network (PON) as mobile fronthaul for the development of ultra-dense small cells was proposed in Ref. [27]. Such a hybrid network architecture is interesting, since optical fiber deployment as a fronthaul for every small cell is not feasible due to the high-costs or physical constraints. Nevertheless, the performance evaluation of the network is limited to the latency and considers a time wavelength-division multiplexing scheme which requires end-to-end synchronism throughout the network, which adds complexity to the system. A hybrid network based on FSO and PON was addressed in Ref. [85]. However, such a hybrid network also considers end-to-end signal synchronization, which poses significant challenges in real deployment as any minor time deviation would result in substantial data bit losses at the DU side. Additionally, hybrid FSO and PON rely on existing optical fiber infrastructures that are supposed to be already deployed and available as fiber-to-the-antenna. Accordingly, this suits a limited networking scenario, and it might not be the case for many fronthaul scenarios. In Ref. [86], a bipolar-OCDMA technique based on spectral amplitude coding is developed for signal transmissions in FSO channels. Further, a hybrid FSO and optical fiber system based on OCDMA was experimentally addressed in Ref. [28]. The system supports 10 Gb/s transmissions and is considered highly secure. However, the system supports only a single link. A network flying platform based on FSO technology for providing services under a vertical fronthaul framework was proposed in Ref. [14]. The platform is particularly interesting for ultra-dense heterogeneous small cells. Nonetheless, the signal synchronization among the elements of the flying platform poses some real complexities and challenges. In Ref. [15], an overview of FSO systems for beyond 5G based on transceiver design and integration with the millimeter wave and terahertz technologies was given. The latter developed a network based on hybrid and mixed FSO, millimeter wave and terahertz technologies. Traditional FSO networks have been demonstrated in several works. For example, a 400 Gb/s FSO was experimentally demonstrated in Ref. [87], while over 800 Gb/s was shown in Ref. [88], and 1 Tb/s was achieved under a single-wavelength FSO system in Ref. [89]. Lastly, a FSO link at 1.28 Tb/s was achieved with the help of multiple wavelengths in Ref. [90]. The integration of multiple wavelengths along with spatial multiplexing resulted in an aggregate bit rate of 14 Tb/s over a 220 m FSO link [91]. Moreover, a FSO transceiver based on a novel, intelligent, lens-based optical beam stabilization technology has recently been demonstrated in Ref. [92]. A new scheme to enhance security based on non-orthogonal multiple access and physical layer coding hasbeen proposed in Ref. [93]. Meanwhile, the uplink transmission of a hybrid FSO and radio frequency network was evaluated against eavesdroppers in Ref. [94]. On the other hand, the feasibility of using FSO technology for flexible architectures of optical fronthaul networks was demonstrated in Ref. [95]. An optical frequency comb for the generation and distribution of low-phase noise signals in 5G fronthaul was used in Ref. [96], but with the main focus on optical distribution networks based on fibers. A recent and comprehensive review of a potential convergence of wireless and optical solutions toward 6G networks was given by Ref. [97]. An experimental performance investigation of a laboratory-controlled FSO setup was carried out in Ref. [98]. The experiments were performed in a chamber for a 5 m link at 10 Gb/s and demonstrated performance improvements when a cyclic redundancy check technique was employed. In contrast, experimental results of a triple-hop relay-based all-optical FSO system with an amplify-and-forward scheme for atmospheric turbulence channels was demonstrated in Ref. [99]. The experiment addressed system channels under different atmospheric regimes for data rates up to 10 Gb/s.