## 3 Downlink and Uplink Transmission Operations
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**RNX**} & \multirow{2}{*}{**Frequencies per RN**} & \multicolumn{2}{c}{**Frequency Ranges**} \\  & & **(20 GHz Frequency Spacing)** \\ \hline RN1 & F\({}_{01}\) to F \({}_{10}\) & 192.52 THz\(\rightarrow\)192.7 THz \\ \hline RN2 & F\({}_{11}\) to F\({}_{20}\) & 192.72 THz\(\rightarrow\)192.9 THz \\ \hline RN3 & F\({}_{21}\) to F\({}_{30}\) & 192.92 THz\(\rightarrow\)193.1 THz \\ \hline RN4 & F\({}_{31}\) to F\({}_{40}\) & 193.12 THz\(\rightarrow\)193.3 THz \\ \hline RN5 & F\({}_{41}\) to F\({}_{50}\) & 193.32 THz\(\rightarrow\)193.5 THz \\ \hline RN6 & F\({}_{51}\) to F\({}_{60}\) & 193.52 THz\(\rightarrow\)193.7 THz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the frequency distribution. Figure 4: Schematic diagram of the RN + ONU. Upon receiving the signal at \(OS_{w1-RNj}\), as shown in Figure 4, which is split after the identification via an optical filter, part of it is reused for uplink transmission. The signal is identified using a Gaussian optical filter and fed into an intensity modulation on-off keying (IM-OOK) for data modulation. For uplink transmission, the data rate of the simulation \begin{table}
\begin{tabular}{|c|c|c|c|} \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \multicolumn{3}{|c|}{Global Parameters} & \multicolumn{2}{|c|}{Optical Fiber} \\ \hline Bit rate & \(20\times 10^{9}\) bit/s & Reference wavelength & 1552 nm \\ \hline Sequence length & 256 bits & Length & 5 km \\ \hline Samples per bit & 128 & Attenuation & 0.2 dB/km \\ \hline Symbol rate & \(10\times 10^{9}\) & Dispersion & 16.75 ps/nm/km \\ \hline \multicolumn{3}{|c|}{CW Laser} & Dispersion slope & 0.075 ps/nm\({}^{2}\)/km \\ \hline Frequency & 193.1 THz & Differential group delay & 0.2 ps/km \\ \hline Power & 5 dBm & Effective area & 80 um\({}^{2}\) \\ \hline Linewidth & 0.1 MHz & Fract. Raman contribution & 0.18 \\ \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters. Figure 5: (**a**) The multiplexed signal received at each RN: (**b**) bit error rate (BER) vs. received signal power. environment is reduced to 10 GHz. The use of a duobinary modulation scheme in the downlink transmission proved to be useful to re-modulate the downstream signal into the OOK upstream signal at the RN [35]. A symmetrical operation at 10 Gbps over a 25 km reach has already been proposed and demonstrated experimentally [36]. Even a symmetric 10 Gbps WDM-PON has been proven to reach distances of 80 km without using any remote amplification [37]. After modulation, the signals of both ONUs are combined and sent back to the optical fiber. Before injecting the signal into the fiber, \(OS_{uw\_RNj}\) is used to direct the signal to the working fiber or protection fiber. The OSw1-co and OSw2-co are utilized at the central office to identify the signal mode. If the signal is received from the working fiber, the \(OS_{w1\_co}\) directs the signal to the \(OC_{oupl\_co}\), which is then forwarded to the \(OC_{ircl\_co}\). If the signal is received from the protection fiber, the \(OS_{w2\_co}\) directs the signal to the \(OC_{oupl\_co}\), which is then forwarded to the \(OC_{ircl\_co}\). The output of both circulators is combined using an ideal Optical Combiner, and the output is fed into a 1 \(\times\) 10 WDM De-MUX. A Gaussian optical filter is utilized at the De-MUX to identify each wavelength. After wavelength identification, a direct detection optical receiver (DDOR) receives the signal. The DDOR is based on a PIN photodiode and a low-pass Bessel filter, and the results are calculated using BER spectrum analyzers. The received channels promise a good performance, and the results can be seen in Figure 6a, where the multiplexed signal shows a continuous downfall in the received optical power of around 1 dBm. The same VOA is used to test the performance of the scheme.