# Beyond 5G Fronthaul Based on FSO Using Spread Spectrum Codes and Graphene Modulators
## 2 Novel Network Architecture Design  
Mobile fronthaul networks support a large number of antennas over small mobile cells to increase data rates, coverage, and data traffic support. As more and more 5G networks roll out [1], energy-efficient fronthaul technologies are becoming a desired answer for improving the networks' energy consumption and OPEX [60]. As an example, the power consumption of a (micro) cell can reach up to 300 kW [108]. Thus, future generation of networks will therefore need to increasingly rely on energy-efficient multiplexing schemes, network devices, and disruptive approaches. The proposed next-generation FSO is one such approach. By using the FSO approach, one can simultaneously transmit data signals to many antennas from a centralized location via FSO-to-the-antenna, as illustrated in Figure 1.  
The network consists of data transmission bits between the BBU at the CO and a cell site RU, where the centralized BBU serves multiple RU. Several RUs are connected to the BBU with a high-speed transport network, that is, the FSO fronthaul interface, which can support data rates up to several gigabits. This significantly improves the economy of scaling of the fronthaul networks' design. In fact, this approach becomes particularly interesting for FSO technology when transmission of an optical signal can be realized from a fiber termination point over the free-space channel without the need for any additional complexity. Afterwards, at the receiver side, the signal is directly coupled into an optical fiber. Thus, the need to convert the signal from electrical-to-optical or vice versa for transmitting or receiving through the free-space channel is removed. The mobile fronthaul accommodates a pool of BBU at the CO premises, whereas the RU at the tower site becomes simpler (in design, complexity, and lower cost) and can be implemented based on a straightforward radio frequency (RF) front-end antenna [109]. This centralized architecture eventually grants additional flexibility and reduces overall power consumption of the system [110]. The adoption of centralized fronthaul avoids the need for modulation/demodulation stages at the RU site, which contributes to energy consumption reduction. Accordingly, the optical carrier is modulated with RF data signals at the CO premises to be transmitted over the FSO channel to many antennas. The modulation is carried out by means of a graphene-based modulator as part of the optoelectronic integrated BBU capability [2,111]. This modulation happens in intensity, as supported by the proposed graphene modulator, which will be addressed in the next section. At the transmitter side, a single-wavelength laser source is modulated by an ON-OFF keying technique using a novel graphene modulator to generate bits. Sequentially, this optical signal is coded by an OCDMA encoder [49]. The spread spectrum technique is used to increase the security of data transmissions. This process is repeated for a given number of antennas or RU. The spread spectrum technique extends the original data bandwidth over a broader bandwidth via a \"spreading the code\" sequence. In this way, an exclusive code sequence with weight \(W\) and length \(L\) is assigned to each signal granting data codification at the physical layer. OCDMA coding techniques offer a certain level of security and built-in privacy, since the OCDMA receiver can only retrieve the transmitted data with knowledge of the employed code. Since the original signal is spread using a pseudo-random code, that is, the signal appears random-like, it becomes more resilient against eavesdropping. The total number of simultaneous users, thus transmitted signals, in the network is \(N\); the code's chip duration for all users is \(T_{c}=T_{b}/L\), where \(T_{b}\) is the bit period. After users'  
Figure 1: Architecture of the mobile fronthaul network based on FSO, graphene modulator, and spread spectrum codes. FSO: free-space optics. OCDMA: optical code-division multiple-access.