## 4 Experimental Evaluation
### Results and Performance Analysis
The relationship between the BER and the roll-off factor in the experiment is shown in Figure 8. In the experimental results of the NNWSC-based receiver, the BER trends of S1 and S3 differ from that of S2 as the roll-off factor increases. This divergence can be attributed to the greater influence of IBI on S2. These experimental results are also in good agreement with the simulation results. Next, we investigated the issue of STO caused by path differences when signals arrive at the receiver in the two-transmitter one-receiver system. To address this issue, we first calculated the time delay difference between signals arriving from different paths while receiving a known training set. Subsequently, timing compensation was applied at the transmitter to reduce timing errors arising during signal transmission. The primary objective of the experiment was to verify the extent to which the NNWSC-based receiver can overcome STO after transmitter-side timing compensation. In the experimental setup, the roll-off factor was set to 0.4, and the sub-band spacing factor was set to 0.2. The results, shown in Figure 9, illustrate the relationship between BER and STO for each sub-band. The unit Ts represents the symbol period. The experimental results indicate that the NNWSC-based receiver maintains favorable BER performance at small STO values, but its BER increases significantly once the absolute value of STO exceeds 10%. This is because the NNWSC processes combined signals  
Figure 8: (**a**) The relationship between the BER and roll-off factor per sub-band at a sub-band spacing factor of 0.2; (**b**) constellation diagrams at roll-off factors of 0.6. Figure 9: The relationship between the BER and STO per sub-band. directly without a separation step, which suggests that, although the NNWSC performs well under no or minimal STO, its ability to handle larger STOs is limited. In contrast, the conventional receiver, which separates the two signals from different transmitters during matched filtering, remains unaffected by changes in STO. While it does not achieve as low of an absolute BER as the NNWSC, it shows stronger robustness against larger STO values. Overall, for sub-bands S1 and S3, where IBI is minimal, the impact of STO on the BER of the NNWSC-based receiver is particularly pronounced. On S2, where IBI is more severe, the NNWSC shows both an advantage in handling IBI and a disadvantage in managing STO, resulting in relatively stable BER performance. This experiment further verifies the NNWSC-based receiver's performance after timing compensation. While the NNWSC effectively compensates for STO-induced interference within a certain range, its compensation capability is limited beyond a specific threshold. Next, we investigated the impact of ROP attenuation on BER. The experimental conditions were set with a roll-off factor of 0.4 and a sub-band spacing factor of 0.2. We evaluated the performance of both the conventional receiver and the NNWSC-based receiver under varying ROP conditions in the two-transmitter one-receiver system. As shown in Figure 10, due to the inherent insertion loss of the variable optical attenuator (VOA), the BER performance at the 0.5 dB and 1 dB attenuation levels could not be measured in the experiment. First, for each receiver, when the ROP is high (i.e., power attenuation is low), the BER for each sub-band remains low, indicating a good SNR for the received signal, which contributes to lower BER. As ROP gradually decreases, the BER increases correspondingly, with a significant rise observed at low ROP levels. This trend aligns with theoretical expectations: as the ROP decreases, noise becomes relatively more significant, leading to a degradation in system performance and, consequently, an increase in BER. In the two-transmitter one-receiver system, all three sub-band signals are transmitted simultaneously. There is a notable difference in performance between the conventional receiver and the NNWSC-based receiver under the multi-sub-band interference environment.