## 4 Simulation Results  
This section evaluates the symbol error rate (SER) and achievable data rate (ADR) of the proposed TH-CPSK scheme. Our multiuser MC environment considers an MC channel characterized by free diffusion in air with the coefficients presented in Table 1. This implies that the molecules emitted by transmitter nanomachines undertake a random walk, diffusing toward the receiver through Brownian motion. Table 1 also outlines the default parameters used for simulations, applicable unless specified otherwise. The chosen default parameters allow us to maintain consistency with prior research [26], ensuring our results are both comparable and reliable. User 1 is the desired user that we want to receive, and perfect knowledge of its time-hopping pattern \(\{c_{i}^{1}\}\), together with ideal synchronization between the TN and RN, is assumed. Figure 6 illustrates the SER according to the number of users, \(N\), in the system. The figure compares the SER for two modulation schemes, \(M=2\) and \(M=4\), at three different TN-RN distances \((r)\): 1 cm, 2 cm, and 3 cm. First, we can observe that the SER escalates with an increase in the number of users for all distances and both modulation orders. This is because of the increase in multiuser interference at the RN, where the signals from multiple TNs overlap, making it difficult for the RN to decode the signal from User 1. With more users, the probability of signal overlap increases, which can lead to more frequent and severe multiuser interference. The impact of this interference is more noticeable as the user count grows. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Values** \\ \hline Symbol duration \((T_{s})\) & 5 s \\ Modulation order \((M)\) & 2, 4 \\ Number of hops \((N_{h})\) & 5 \\ TH code length \((N_{p})\) & 3 \\ Number of users \((N)\) & 6 \\ Diffusion coefficient \((D)\) & 0.43 cm\({}^{2}\) s\({}^{-1}\) \\ Range of TNs-RN distance \((r)\) & 1 cm to 10 cm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Default simulation parameters. Second, the SER is directly proportional to the distance between the TN and RN. For both \(M=2\) and \(M=4\), the error rate is highest at \(3\,\mathrm{cm}\) and lowest at \(1\,\mathrm{cm}\). This trend suggests that at a distance of \(3\,\mathrm{cm}\), the MC channel characteristics become more pronounced, potentially due to a critical point where the channel effects, such as bias, noise, and interference, have a substantial impact on the signal. The distinct rise in errors at \(3\,\mathrm{cm}\), particularly for the \(M=4\) scheme, suggests that the channel at this distance is causing significant distortion or loss of information. This could be due to a combination of effects, including diffusion dynamics, molecule degradation, and the time it takes for molecules to reach the RN. Lastly, the system with modulation order \(M=2\) exhibits a lower SER compared to \(M=4\). The increased SER of the \(M=4\) scheme, which represents more bits per symbol, contributes to a higher probability of error, especially as the number of users grows. To more accurately compare the performance of TH-CPSK, we also evaluated the ADR performance. Figure 7 examines the ADR across varying distances with an increasing number of interfering users. The data exhibit a decline in the ADR as the user count rises, aligning with the anticipated inverse relationship between the ADR and SER. This is indicative of multiuser interference--as more users try to communicate over the same channel, the chances of signal collision increase, leading to a loss of molecules and a decrease in the ADR. Moreover, the greater the distance the molecules have to travel, the more likely they are to be dispersed or lost in the channel, resulting in fewer molecules reaching the RN. For instance, at a closer range (\(r=1\,\mathrm{cm}\)), the channel influence diminishes, resulting in a higher ADR.