## 2 System Description of OFDM-OCDMA RoF System  
Figure 1 presents a block diagram of the proposed OFDM-OCDMA network. In a system architecture, the transmitter largely consists of \(N_{c}-1\) base stations (BSs). The optical communication simulation software Optisystem 10 which was made by Optiwave Systems Inc., Ottawa, ON, Canada and was used to simulate the proposed system architecture. Figure 2 illustrates a simulation of the OFDM-OCDMA system with three BSs and three receivers in the CS. The BSs loaded the signals of two OFDM channels onto the optical carriers in two SOPs. A polarization combiner was used to combine the polarized signals and transmit the combined signals to an encoder. The encoder coded the signals based on a codeword assigned to each of the BSs. Subsequently, a combiner was used to collect the coded signals, and the signals were transmitted to the CS. Figure 1: Integrated configuration of orthogonal frequency-division multiplexing-optical code-division multiple-access (OFDM-OCDMA) radio-over-fiber (RoF) system. Figure 2: An architectural diagram of an OFDM-OCDMA system with three BSs. Signals transmitted from the transmitter were the cumulative signals of all active BSs. These signals were transmitted via optical fiber to the receiver. The receiving signals illustrated in Figure 1 of star coupler are the cumulative signals of all the BSs, which can be expressed using the following equation:  
\[r(t)=r_{v}(t)+r_{h}(t)=\sum_{k=1}^{K}\left[l_{k}(t)+Q_{k}(t)\right], \tag{1}\]  
where \(t\) is time parameter; \(r_{v}(t)\) and \(r_{h}(t)\) represent the two receiving signals in the SOPs; \(l_{k}(t)\) and \(Q_{k}(t)\), respectively, represent the encoded signals of I-channel and Q-channel that were modulated by BSs onto the optical carriers; and \(K\) represents the number of BSs. However, the present study used the Hadamard code. Therefore, \(K=N_{c}-1\), where \(N_{c}\) represents the length of the code. The CS consisted of an equal number of receivers as the number of BSs. Thus, each receiver decoded the signals based on the codeword of one BS. Finally, an OFDM demodulator was used to restore the electrical signals to digital signals. The output signals in the vertical and horizontal SOPs for Receiver #\(l\) can be expressed using Equations (2) and (3). The results of the two equations indicate that the proposed method can completely eliminate interference between BSs in ideal conditions. \[r_{v}(t)\cdot C_{l}-r_{v}(t)\cdot\overline{C}_{l}=\left\{ \begin{array}{cc} \dfrac{N_{c}}{2}\cdot I_{k}(t),&k=l\\ 0,&k\
eq l\end{array} \right., \tag{2}\]  
\[r_{h}(t)\cdot C_{l}-r_{h}(t)\cdot\overline{C}_{l}=\left\{ \begin{array}{cc} \dfrac{N_{c}}{2}\cdot Q_{k}(t),&k=l\\ 0,&k\
eq l\end{array} \right., \tag{3}\]  
Finally, the balanced electrical signals in the two SOPs were combined to form complete OFDM signals, which were sent to the OFDM demodulator for demodulation, restoring the electrical signals to digital signals, as expressed in Equation (4). \[\left[r_{v}(t)\cdot C_{l}-r_{v}(t)\cdot\overline{C}\right]+\left[r_{h}(t)\cdot C _{l}-r_{h}(t)\cdot\overline{C}_{l}\right]=\dfrac{N_{c}}{2}\left[l_{k}(t)+Q_{k} (t)\right], \tag{4}\]  
In the system simulations, the present study assumed that the system adopted a back-to-back method to transmit signals. Only the attenuation of the fiber channel was investigated in this section. The effects of CD are discussed in Sections 3 and 4. Moreover, the OFDM signals transmitted by the wireless transmitter were set at ideal conditions because only the non-ideal effects of the fiber channel were examined in this study. The simulation parameters are tabulated in Table 1.  
Figure 3 is an architectural diagram of the OFDM modulator and optical encoder in a simulation environment. In Figure 3a; a PRBS generator was used to produce random digital signals with a length