# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 1 Introduction
An analytical formalism for evaluating the bit error rate (BER) performance of the proposed network is presented, and the design and optimization of a new optical modulator based on graphene aiming at reducing power consumption is addressed. The modulator's ring resonator optimization procedure is carried out by changing its graphene content. The optimized graphene modulator is integrated into the new FSO fronthaul network. To the best of our knowledge, this is the first time reporting on the performance of FSO based on a graphene modulator and spread spectrum codes for use in future mobile networks. The main contributions of this paper are summarized as follows:  
* Proposed high-speed FSO links for mobile fronthaul networks for cell densification or expansion, where the network footprint is critical and the provision of broadband technologies is poor or not feasible. FSO allows for rapid, efficient and temporary broadband services in highly dense areas in a unique fashion. We integrate FSO along with OCDMA-based coding techniques to enable higher security levels at the physical layer. The system integration of OCDMA and FSO not only improves the transmission capacity and security levels, but also supports multiplexing of many data channels simultaneously via spreading codes to enable a centralized architecture which concentrates the data processing and management at the CO premises. This unique system integration is addressed for the first time:
* Design, optimization, and analysis of a graphene-assisted optical modulator for its use in the FSO fronthaul mobile network. The percentage of graphene used in the modulator design and analysis is varied from 25% to 100% with four discrete steps of 25%. The modulator critical coupling condition representing the OFF-state and a \(-3\) dB level as its ON-state is obtained. Numerical results show that this new modulator consumes as little as 4.6 fJ/bit whilst achieving high-speed operation with bandwidth up to 42.6 GHz when surprisingly using only 25% of graphene;
* Development of a FSO fronthaul network based on a centralized architecture approach for consolidating the signal encoding and modulation stages at the CO premises before signal transmission to antennas at the RU side. Signals are transmitted in an asynchronous fashion by means of assigned spectrum spread codes. The latter grants the system higher security levels at the physical layer as the receiver needs beforehand knowledge of the given code to retrieve the transmitted data bits with success. The BER as well as the signal-to-interference-plus-noise-ratio (SINR) performance of the proposed fronthaul network is addressed by taking into account the FSO, OCDMA, graphene modulator, one-dimensional (1D) codes, avalanche photodiode (APD) receiver, APD noise, thermal noise, and cross-talk among transmitted signals. The optimum threshold of the receiver is also addressed. The best BER network performance is achieved by the 25% graphene modulator among the four analyzed scenarios. Analytical results show that the new mobile network accommodates up to 32 antennas upon error-free transmissions, enabling the creation of ultra-dense small cells or cell expansions. This paper is organized as follows. Section 2 describes the fronthaul network which is based on a centralized architecture and employs a graphene modulator and spread spectrum techniques before transmission to remote units via FSO transmitters. Section 3 lays out the design and optimization of the graphene modulator. Section 4 presents the analytical framework used to model the FSO signals through the network for BER performance evaluation. Relevant results regarding the proposed network are discussed in Section 5. Finally, Section 6 presents some concluding remarks.