# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 1 Introduction  
Sixth-generation (6G) telecommunications technologies have recently emerged as a potential candidate to support the vast volume of data traffic required by today's modern and digital society. Mobile data traffic is growing exponentially every year, along with the number of mobile subscribers worldwide [1]. Mobile systems are expected to handle the increases in global traffic volume, presently doubling every two years [1]. Furthermore, over two billion global 5G users are expected by the end of 2025 [1]. As a consequence, the existing cellular networks will not be able to meet such traffic demands and predicted energy consumption. Next-generation mobile networks will further deal with the changing behavior of high-bandwidth data traffic applications that will eventually be part of the commercial push for new mobile solutions and networking upgrades. Moreover, key performance indicators [2] suggest that beyond 5G mobile systems should support a density of 10 million connections/km\({}^{2}\), 500 exabyte/month traffic, and user data ratesreaching 1 Gb/s for a few devices and 100 Mb/s for tens of thousands of users highly concentrated in hot-spot areas such as, for example, football stadiums. The densification of mobile cells is a promising way for operators to keep continuously evolving their network's capacity. The connectivity between central offices (CO) and remote units (RU), which is known as mobile fronthaul, accounts for an important segment of mobile network cells. The creation of ultra-dense networks by deploying mobile cells will play a key role in beyond 5G networks. In fact, this sort of network architecture is expected to reduce the capital and operational costs, as reported in Ref. [3]. Among many fronthaul technologies for deploying a mobile network [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28], free-space optics (FSO) have gained momentum mainly due to the progress in the laser-based transmission stage [29]. Laser technologies for high-speed optical telecom are becoming mature, scalable, and cost-efficient [30]. In this fashion, many lasing technologies are commercially available and enabling many interesting networking scenarios. Further, FSO has recently been proposed as a solution for many different emerging applications such as high-speed rail connectivity [31; 32; 33], high altitude platforms [34], and optical satellite communications [35; 36; 37] to name a few. In addition, FSO can be seen as a prospective solution for rapid and cost-efficient creation of new mobile cells to support high-bandwidth services where the provision of broadband technologies has not been feasible so far. The elegancy of this wireless technology to create rapid, efficient and temporary broadband services in highly dense areas is quite unique compared to its baseline counterpart based on optical fiber. Although optical fibers are also used in some scenarios of beyond 5G networks, the lack of their deployment flexibility, especially in highly dense urban areas and associated high-costs, pose some real challenges that call for alternative technological solutions [15]. The need for the fast and efficient broadband service provision to support the traffic supply and demand in ultra-dense urban areas grants FSO technologies a potential to become a viable alternative for the fronthaul mobile cells densification. The large interest in FSO [38; 39; 40; 41; 42; 43; 44] is mainly due to its immunity to the radio interference, license-free operation, and optical broadband availability which render this wireless technology as a strong candidate for the deployment of broadband mobile fronthaul systems. FSO has higher security levels than have the main competing technologies such as optical fibers [45; 46; 47], albeit with some vulnerabilities to eavesdroppers as suggested in Ref. [48]. Security has been a major concern for many applications in communication systems [48].