# Wheel-Based MDM-PON System Incorporating OCDMA for Secure Network Resiliency
## 3 Results and Discussion
The higher order modes (other than fundamental mode) are more susceptible to fiber bending and other variants of impact on the fiber as compared to fundamental mode (mode 0) due to the mode field cross distributions character in fiber. Additionally, the  
Figure 6: Measured BER vs. received optical power at 10/2.5 Gbit/s data rate over 1 km range for both User 1 and User 2 operating at modes 0 and 1 in downstream and upstream transmission. Insets: corresponding eye patterns over \(-\)20 dBm received power for User 1 operating at mode 0.  
major differential mode delay (DMD) between mode 0 and mode 1 transmission cause distributions in different operating higher-order modes. Thus, the modal dispersion caused by DMD would result in intense signal degradation at the receiver, and it restricts both the transmission distance and data rate at mode 1 than mode 0 in MMF [38; 39]. Figure 8a,b show the measured BER performance of the 10/2.5 Gbit/s hybrid MDM-OCDMA-PON system over 1 km fiber distance for downstream and upstream transmission, respectively, at modes 0 and 1 for both users 1 and 2. It is depicted that the system operating at mode 0 offers better performance than mode 1 for both downlink and uplink transmission. For the downstream transmission operating at mode 0, it is analyzed that system using MWZCC, ZCC, RD and MD code, the minimum optical power needed to achieve the BER threshold is \(-30\), \(-28\), \(-22\) and \(-20\) dBm, respectively, as shown in Figure 8a. Moreover, in the downstream transmission operating at mode 1, when the MWZCC, ZCC, RD and MD codes are used in the system, the minimum received power is \(-20\), \(-14\), \(-13\) and \(-8\) dBm, respectively, under the BER threshold. Further, Figure 8b shows that for the upstream transmission operating at mode 0, the minimum received power for MWZCC, ZCC, RD and MD code is \(>-30\), \(-30\), \(-29\) and \(-26\) dBm, respectively, under the BER limit. Likewise, the power of \(-24\), \(-20\), \(-16\) and \(-22\) dBm was received for MWZCC, ZCC, RD and MD code, respectively, in the system for the upstream transmission operating at mode 1. Figure 7: Measured BER vs. fiber length at 10/2.5 Gbit/s data rate for both User 1 and User 2 using modes 0 and 1 in downstream and upstream transmission. Insets: corresponding eye patterns over 1 km fiber length for User 1 mode 0. Figure 9a,b show the BER versus the number of active ONUs for different OCDMA codes employed in the system at 10/2.5 Gbit/s data rate over 1 km range for modes 0 and 1 for different codes in downstream and upstream transmission. It is noted that with the increasing number of ONUs, the system performance degrades along with an increase in respective BER as the received power is restricted by the power splitters for both downstream and upstream coded signals. The proposed fiber link using the MWZCC code provides superior performance with respect to the other OCDMA codes in both downlink and uplink transmission. The maximum number of ONUs obtained for mode 0 and mode 1 for MWZCC, ZCC, RD and MD code is 58, 32; 50, 24; 42, 16 and 34, 15 simultaneous ONUs, respectively, at the BER threshold in the downstream transmission as shown in Figure 9a. Additionally, Figure 9b illustrates that the maximum number of uplink ONUs handled by MWZCC, ZCC, RD and MD code is >58, 58, 50 and 44, respectively, operating at mode 0. However, in the downlink, for MWZCC, ZCC, RD and MD codes, the highest number of supported ONUs are 42, 38, 34 and 28, respectively. The performance of the proposed system for a large number of ONUs decreases because of the presence of inter-symbol interference and four-wave mixing effects, which again increases the fiber noise, non-linearities and dispersion.