## 4 Simulation Results
This is because of the reduced likelihood of signal diffusion and loss over shorter distances, which results in more molecules successfully arriving at the RN and thus a higher data rate. Furthermore, for all distances, the ADR for \(M=2\) is higher than that for \(M=4\). This suggests that the simpler modulation scheme (lower \(M\)) is more robust against the increased path loss and potential interference that come with longer distances or more complex modulation schemes. As \(M\) increases, more information can be transmitted per symbol, leading to an increase in data throughput. However, despite the theoretical increase in the amount of data that can be transmitted at \(M=4\), the relatively high SER compared to \(M=2\) makes efficient data transmission more challenging in practice. The comparison between modulation schemes with \(M=2\) and \(M=4\) also shows an expanding ADR gap as \(r\) increases. Figure 6: SER versus number of interfering users (\(N-1\)) for different TN\(-\)RN distances. Figure 8 demonstrates that the SER decreases as the number of hops, \(N_{h}\), increases for distances of 1 cm and 2 cm for a fixed number of interfering users at five. This is because the increase in \(N_{h}\) reduces the interference at the RN. However, it notably increases slightly for a distance of 3 cm. This indicates that increasing \(N_{h}\) at shorter distances would decrease the interference in the system. However, at larger distances, the effect of increasing \(N_{h}\) is negligible. This pattern can be explained by the MC channel behavior observed in Figure 5. From the channel model (Equation (2)), it can be seen that concentration \(C(r,t)\) depends on the distance \(r\) in an inverse exponential manner. Specifically, as \(r\) increases, \(C(r,t)\) decreases. As shown in Figure 5, for \(r=1\) cm and 2 cm, the peak of the CQR occurs at times \(t=1\) and 2 s, respectively. This indicates that the molecules quickly reach the receiver, maintaining a high concentration. In addition, the dispersion of the molecules in the channel is less, and the SER decreases with increasing \(N_{h}\). However, at \(r=3\) cm, the peak of the CQR is notably delayed at \(t=4\) s, with a significant reduction in amplitude. This implies that fewer molecules reach the receiver, and they do so after a long delay. Moreover, the molecules disperse over a wider area as they travel a longer distance, resulting in a diluted concentration by the time they reach the receiver. Therefore, regardless of the increase in \(N_{h}\), we did not see any improvement in the SER. Therefore, we can say that for \(r=3\) cm and above, reducing the interference at the receiver does not improve the performance of the SER, making the SER nearly flat. Figure 9 depicts a closer look at the SER versus \(N_{h}\) plot for the distance of 3 cm. We can clearly observe that the SER is indeed showing a slowly increasing trend, suggesting the use of lower-order modulation for greater distances. The slowly increasing SER pattern at \(r=3\) cm with increasing \(N_{h}\) in Figure 9 can be explained by the time resources consumed by additional hops. More hops increase the separation between transmitted symbols, leading to further dispersion in the channel. While increasing \(N_{h}\) reduces interference, it also increases the dispersion of transmitted molecules, particularly pronounced at greater distances. This behavior is also observable at \(r=2\) cm, where the SER slope is less steep compared to that at \(r=1\) cm. Overall, we found that for \(r\geq 3\) cm, the precision in detecting the molecule concentration at the RN drops due to channel bias, making it more challenging to accurately discern the signal location. Therefore, at large distances, where channel bias has a significant impact, using fewer hops may reduce errors. In addition, we found that with an increase in \(N_{h}\), the received signals do not overlap, indicating a reduction in ISI.