## 2 System Description of OFDM-OCDMA RoF System
\begin{table}
\begin{tabular}{c c} \hline \hline
**Modulation Technique** & **4-QAM** \\ \hline Data Length & 32,768 bits \\ Data Transmission Rate & 10 G bits/s \\ IFFT Size & 1024 \\ Upscale Frequency (\(f_{h}\)) & 7.5 GHz \\ Electronic Filter Bandwidth (\(B\)) & 6 GHz \\ Input power & 10 dBm \\ Number of Optical Carriers & 4 \\ Laser Beam Width & 5 MHz \\ Optical Carriers Spacing & 100 GHz \\ AWG Bandwidth & 15 GHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The Simulation Parameters. of 32,768 bits and a transmission rate of 10 G (bits/s). The signals were then transmitted to the OFDM modulator to produce OFDM signals. In the OFDM modulator, the input digital signals were processed using four-quadrature amplitude modulation (4-QAM). The modulated signals were transmitted to the OFDM block to produce OFDM signals with an inverse fast Fourier transform (IFFT) size of 1024 bits. In order to maintain the wave pattern of the transformed signals consistent with that of the original signals, only the middle 512 carriers were used to load signals. The remaining carriers were loaded with zero signals. The carriers with zero signals imposed no influence on the transformed signals. Then, a low-pass filter was used to de-noise the signals of I-channel and Q-channel produced by the OFDM block. The two baseband signals were separately multiplied by \(\cos(2\pi f_{u}t)\) and \(-\sin(2\pi f_{u}t)\) to upscale them to 7.5 G. The spectrums of the two baseband signals are illustrated in Figure 4a,b. The overall bandwidth of the signals is 15 G. However, the primary signal range was between 5 and 10 G, and the preceding and succeeding sections (5 G) were the guard intervals. Figure 3: An architectural diagram of the base station (BS) in a simulation environment: (**a**) the base station; (**b**) the OFDM modulator; and (**c**) the OCDMA encoder. A distributed feedback laser (DFB) laser array was used to produce four light sources with a linewidth of 5 MHz and central frequencies of 193.1, 193.2, 193.3, and 193.4 THz to serve as the optical carriers (Figure 4c). The optical carriers were polarized into two different SOPs (vertical and horizontal) using a polarization splitter. In addition, an ideal Mach-Zehnder modulator (MZM) was employed to modulate the upscaled signals of I-channel and Q-channel onto the two types of polarized optical carriers. All optical carriers and I-channel and Q-channel signals possess a 5 G guard interval, as illustrated in Figure 4d,e. Figure 4f illustrates a spectral diagram of the combined vertically and horizontally polarized signals using the polarization combiner. Finally, the combiner was used to collect the signals from Output Terminals #1 and #3 of the AWG based on codeword (1010) and the signals were transmitted to the receivers. Figure 4g illustrates a spectral diagram of the encoded signals. Figure 4: _Cont._  
Figure 5 illustrates an architectural diagram of a receiver in a simulation environment. First, a polarization splitter was used to split the received signals into vertically and horizontally polarized signals. Then, the two polarized signals were transmitted to the decoder for decoding. Figure 6a,b illustrate the spectral diagrams of the two polarized signals. In the decoder (using the decoder for the vertical SOP as an example), an AWG was used to filter the optical carrier and the attached OSSB signals. The signals were transmitted to the photodetectors to convert optical data into electrical data. Then, the signals were balanced using the BS codeword. Combiner #1 collected the digital signals from Photodetector #1 and Photodetector #3 (\(C_{1}\)), and Combiner #2 collected the digital signals from Photodetector #2 and Photodetector #4 (\(\overline{C}_{1}\)). The signals of the two combiners were subtracted from one another to eliminate MAI, as illustrated in Figure 5b. The operations of the decoder for the horizontal SOP were similar to those of the decoder for the vertical SOP.