## 3 WDM-FSO Architecture  
Acronyms introduced in the manuscript and their definitions are listed in Table 1.  
Figure 1 shows a WDM-FSO architecture based on a multi-wavelength comb source, including DeMux and Mux stages, as well as broadband modulators. At the source, the UWB is created by integrating several optical wavelengths, each modulated by an input RF OFDM signal. An OFDM signal is produced as a sum of orthogonal subcarriers, which are modulated using different encoding techniques, for instance, quadrature amplitude modulation (QAM). For this work, the system was designed using 16-QAM modulation, which offers higher transmission rates and better spectral efficiency. Additionally, FSO systems benefit from the use of OFDM due to their resilience to multipath fading, frequency selective fading, and intersymbol interference (ISI), thereby increasing their transmission range. The information data are processed by a QAM sequence encoder to transform the data bits into symbols (4 bits per symbol for 16-QAM). The QAM symbols are then modulated into multiple orthogonal sub-carriers and the QAM-OFDM signal generated is used to modulate the optical WDM signal. A non-return to zero (NRZ) 223 - 1 pseudo random bit stream (PRBS) sequence with a data rate of 40 Gbps has been generated for each WDM channel. In the OFDM modulator, the number of OFDM subcarriers is 512 and the number of Fast Fourier Transform (FFT) points is 2048, so the generated bit rate is (512/2048) \(\times\) 40 = 10 Gbps. ISI between OFDM symbols is avoided by adding a cyclic prefix of 100 for each OFDM symbol after the Inverse FFT (IFFT). The proposed system has an \(N=32\) wavelengths WDM FSO link (32 \(\times\) 10 Gbps) that covers the frequency range of 193.1 to 194.65 THz (1552.52 to 1540.16 nm) and has a total transmission capacity of 320 Gbps. Cosine roll-off filters are employed to shape pulses before transmission to minimize ISI. A laser source and an MZM are used to up-convert the OFDM-modulated signal from the electrical to the optical domain. Then the resulting optical signal is multiplexed with other WDM signals, amplified using a gain-flattened Erbium-doped fiber amplifier (EDFA), and sent over the FSO channel. Next, the WDM signals are separated by a DeMux, where various wavelength signals are directed to the desired receiver. At the receiver, each WDM signal is converted to an electrical signal by a PIN photodetector (PD). An electrical amplifier then amplifies the received electrical signal. The amplified signal is then downconverted and recovered using a quadrature demodulator. After that, using an OFDM demodulator, the transmitted QAM symbols are successfully retrieved. Then the received symbols are converted into bits by a QAM sequence decoder based on the number of bits per symbol. \begin{table}
\begin{tabular}{c c} \hline \hline
**Acronym** & **Description** \\ \hline WDM & wavelength division multiplexing \\ FSO & free space optical communication \\ DCNs & data center networks \\
16-QAM & 16-quadrature amplitude modulation \\ VCSEL & vertical cavity surface emitting laser \\ PAM-4 & 4-level pulse amplitude modulation \\ OFDM & orthogonal frequency division multiplexing \\ EDFA & erbium-doped fiber amplifier \\ RSOA & reflective semiconductor optical amplifier \\ PONs & passive optical networks \\ OWC & optical wireless communication \\ RoF & radio over fiber \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of acronyms. A bidirectional WDM FSO system can be achieved by replicating the original topology for upstream transmission. However, a wavelength reuse scheme can be employed to reduce the cost since there is no optical source needed for upstream transmission. These systems maximize wavelength utilization by using the same wavelength for upstream and downstream channels. Downstream wavelength signals are remodulated with upstream data and then sent back as upstream signals [38; 39].