# A Photolysis-Assist Molecular Communication for Tumor Biosensing
###### Abstract  
Molecular communication (MC) is a promising bioinspired paradigm for exchanging molecule information among nanomachines. In this paper, we propose a synchronization-assist photolysis MC system that aims to transmit the biosensing signal of the tumor microenvironment, facilitated by mitigating redundant molecules for improved bit error rate (BER) performance. Benefits from biocompatible MC, biosensors could transmit biosensing signals of the tumor in vivo instead of converting them to electrical signals. Due to diffusion motion's slow and stochastic nature, intersymbol interference (ISI), resulting from previous symbols' residual information molecules, inevitably occurs in diffusion-based MC. ISI is one of the challenges in diffusion-based MC, which significantly impacts signal detection. Inspired by on-off keying (OOK) modulation, the proposed modulation implements a switch of molecules and light alternatively. The light emitted is triggered by a synchronization signal, and the photolysis reactions could reduce the redundant molecules. An expression for the relevant channel impulse response (CIR) is derived from a hybrid channel model of diffusion and photolysis reaction. In this paper, we implement the maximum posterior estimation scheme to find the optimal decision threshold and analysis the BER performance in terms of different time intervals of the system. Numerical simulations demonstrate that the proposed method can improve the channel capacity and BER performance. We believe that our work may pave the way for MC application in biosensing. biosensing; intersymbol interference; molecular communication; synchronization 202  
of a tumor and corresponding tumor biosensing [5]; however, up to now, little attention has been paid to transmit this biosensing information in MC. In this paper, we aim to develop a synchronization-assist photolysis MC system that transmits biosensing signals of the tumor microenvironment metastasis (DMEM), such as pH conditions. Diffusion, molecules in a fluid propagating via Brownian motion, is the basic model for MC. Brownian motion is the stochastic motion of molecules in a fluid medium, which leads to the information transmitted previously arriving at the receiver after the current arrival molecules, causing the intersymbol interference (ISI), limiting the transmission rate of MC. Indeed, in avoiding the ISI, we could send a date over long time intervals and lower the transmission rate. However, the real-time biosensing in TMEM requires a high transmission rate, as detailed discussed in Section 2. Thus, it is fundamental to mitigate the ISI in MC for real-time tumor sensing. There is a growing body of literature that recognizes the importance of mitigating ISI in MC [6; 7]. The above methods can be divided into two types, passive strategies, not eliminating the surplus molecules directly instead shaping the channel impulse response (CIR) symbols [8; 9], while active strategies, in which the redundant information molecules are degraded by the enzymes [10] or light [7] so that the receiver can not recognize them. [PERSON] et al. proposed Concentration Shift Keying (CSK) and Molecule Shift Keying (MoSK) modulation in [8]. The CSK modulation implements the received molecules' concentration as the amplitude of signal; for example, high concentration represents the signal '1', and low concentration indicates signal '0'. In comparison, MoSK modulation uses different types of molecules to represent the signal. The authors argue that compared with CSK, MoSK considerably reduces the ISI since a different molecule type represents each bit. [PERSON] et al. [11] proposed a derivative-based scheme to detect the symbol by the derivative of CIR, which has a much shorter peak time than CIR itself. The tail, therefore, vanishes much faster than that of CIR, resulting in reducing ISI. In [9], [PERSON] et al. proposed a nonlinear mapping method in MC, which reduces the delay profile of the sampled data to mitigate ISI at the receiver.