## 3 Downlink and Uplink Transmission Operations  
During the regular mode operation, the downlink and uplink data are transmitted in the feeder fibers (working fibers) from the CO to RNs and vice versa. The CO contains an OMC source as an OLT that generates X number of frequencies, utilized for downlink transmission, which can be received by Y number of receivers (X = Y) back into the CO after an uplink transmission. The X number of transmitted wavelengths is received by the M number of RNs, where each RN passes Z number of frequencies. A DeMUX can separate the desired tones for the dedicated number of ONUs. As demonstrated in Figure 2a, port 1 of the OSw1-co is connected to the working fiber. The schematic of RN and ONU is shown in Figure 4, where OSw1-RNx followed by a 1\(\times\)2o-fork receives the signal. The 1\(\times\)2o-fork performs the following two tasks: (i) it extends the traffic towards the end-users through 1 xNo-fork, or by using DeMUX to separate the frequency tones, where N represents the number of users connected to the fork/DeMUX, and (ii) it extends the signal from output port 2 towards the second switch (OSw2-RNj), which extends the fiber in the clockwise direction towards the next RN (OSw-RNj1), where the same procedure is repeated and a ring network is formed. Consequently, if \"M\" RNs are connected in the ring, and at each RN, 1:No-forks are connected across the ring, the total number of subscribers in the network can be given by M \(\times\) N. The end-face of each 1:No-fork or the DeMUX terminates in the subscriber premises through a distribution fiber (DF). The DF is connected to a bandpass filter that works as a DeMUX arrangement and recovers the desired carrier frequency for the intended subscriber, which is split in two by a 1 \(\times\) 2 OSp; half of it is used for demodulating the signal. In contrast, half of the signal's power is used for colorless uplink transmission. In the proposed setup, the multiplexed signal that is transmitted from the CO is received by six RNs. Therefore, the overall spectrum of 1.2 THz is divided into six groups of frequencies with a range from 192.50 THz to 193.70 THz. The frequency range dedicated to the RN1 is from 192.52 to 192.70 THz. Similarly, RN2 is from 192.72 to 192.90 THz, and so on. At each RN, ten tones are dedicated to the end-users. The summary of the frequency distributions among the different RNs is given in Table 1.  
For downlink transmission among the six RNs, we simulated five RNs, and for each RN, two frequencies were selected for carrying the 20 Gbps ODBM-based modulated data. As mentioned in Table 1, 60 subscribers can be facilitated at six different RNs, where each RN is at the span of 5 Km feeder fiber, while each user is at a distance of 2 km from the RN. In this huge network, we have provided the simulation results for two users per RN for downlink and uplink transmission. The multiplexed signal received at 5 km (at RN1), 10 km (at RN2), and up to 25 km (at RN5), each with a fiber span of 5 km, is demonstrated in Figure 5a, which shows the drop/loss of power from one ONU to the other. The power loss across each RN can be observed clearly in the figure. To test the applicability of the scheme, a variable optical attenuator (VOA) is used at the output of the OS\({}_{\text{w1-co}}\) and OS\({}_{\text{w2-co}}\). The VOA attenuates the signal's power level and distorts the signal, and we can calculate the desired BER values. The value of the VOA is set in an iterative manner, where the min value is 0, and the maximum is restricted to 9. Each iteration advances the value by an addition of 1. The simulation parameters are listed in Table 2.  
The achieved low bit error rate of \(10^{-13}\) to \(10^{-3}\) is shown in Figure 5b, which shows a min. BER of approximately \(10^{-13}\) for 20 Gbps downlink transmission. Hence, the scheme guarantees efficient performance for 20 Gbps downlink transmission. The graph shows the power loss across the channel and other passive components.