## 1 Introduction
Similar to PPM, concentration-position shift keying (CPSK) leverages both the concentration and the spatial position of the molecular signal to reduce the ISI and increase the data rate. However, CPSK still struggles because the signal can shift even without noise, especially as the distance increases, due to the way MC channels work [24]. To tackle this, recent studies have explored the application of machine learning techniques to improve the performance of MC [24; 25]. By utilizing machine learning, the complex characteristics of MC channels can be trained, allowing the development of communication strategies that can predict and adapt to changes in the channel, thereby enhancing the performance of CEMC [24]. To achieve a fully functional nanonetwork, multiple nanomachines should be able to communicate simultaneously within a shared medium--a concept known as multiple access (cf., Figure 1). Traditional EM communication systems utilize various multiple access techniques [26], but the unique properties of MC demand novel approaches. CEMC offers a promising pathway by utilizing the concentration of molecules as a means to encode information. However, as the number of devices within an MC system increases, the potential for signal interference and degradation also rises. In this context, multiple access techniques tailored for CEMC can significantly enhance system capacity and reliability. In this paper, we propose a method to realize multiple access within the domain of CEMC by applying the time-hopping concentration-position shift keying (TH-CPSK) scheme to CEMC systems. By leveraging time hopping in conjunction with CPSK, our approach significantly reduces the likelihood of interference and enables higher throughput in densely populated MC networks. In particular, TH-CPSK utilizes multiple spatial positions for signal transmission, thereby mitigating the impact of channel biases or ISI that can dominate a single location. This strategy not only lowers the error probability at each position but also bolsters the system's resilience to channel variations. In other words, in scenarios where strong interference or channel fading occurs at one hop, TH-CPSK compensates by leveraging other hopping positions. The main contributions of this study are as follows. First, we propose the TH-CPSK scheme for CEMC systems in a multiuser MC environment. The TH-CPSK technique operates by modulating the temporal position of the transmitting molecules, which effectively minimizes interference among multiple users by transmitting each user's signal in an irregular time pattern. Second, we present in detail the mathematical model to implement the TH-CPSK technique. In addition, a free-diffusion MC channel [14; 27] is considered. Finally, we evaluate the performance of the TH-CPSK scheme under various communication distances and environmental conditions. This paper is organized as follows. Section 2 lays the groundwork by detailing the system model for the conventional CPSK scheme. Section 3 introduces our proposed TH-CPSK scheme, including the system model and signal model, and discusses the characteristics of the free-diffusion MC channel. In Section 4, we present the simulation results for the proposed TH-CPSK scheme, comparing its efficacy under various system design parameters. The paper concludes with Section 5.