# A Photolysis-Assist Molecular Communication for Tumor Biosensing
###### Abstract
However, adopting the above passive strategies, the redundant information molecules causing ISI are not eliminated directly, which limits the transmission rate [10]. Meanwhile, the computational complexity required is high, which is a considerable burden on the limited capability of nanomachines. Thus in [10], [PERSON] et al. implemented enzymes to catalyse the degradation of molecules in the medium. Enzymes selectively transform the molecules so that they become unrecognisable by the receiver, which mitigates ISI, but the enzymes presented around the receiver could reduce the strength of the signal. In [7], [PERSON] et al. introduced photolysis reactions, in which the emitted molecules are transformed by light instantly. The light is required to be emitted at the optimal time. However, because light emissions time is strictly limited at the peak time of each slot, it is necessary to introduce the synchronization mechanism to ensure that the light is emitted at the right time. Due to the low-speed and stochastic propagation characteristics, precise synchronization in transmitters and receivers is particularly challenging in diffusion-based MC [12]. In response to this issue, some synchronization detection schemes have been proposed and divided into two types using two different types of molecules [13; 14] and using signal processing method instead [12; 15]. [PERSON] et al. [13] assumed a synchronization frame that utilizes two types of molecules, type A for information molecules and type B for synchronization molecules, facilitating simultaneous symbol and synchronization detection; however, the exact types of specific molecules are not mentioned in [13]. While in [14], [PERSON] et al. introduced a block synchronization technique by utilizing two types of molecules with different diffusion coefficients for synchronization and information transmission. As different from [13; 14], [PERSON] et al. [15] introduced a simple asynchronous detection scheme without requiring multiple molecule types and detectors, which is based on peak concentration detection. Following the idea of [15], [PERSON] et al. [12] derived an approximate maximum-likelihood delay estimator and a decision feedback equalizer to mitigate sensitivity to synchronization errors; however, it requires a more complex scheme in the receiver. It is important to note that synchronization detection occurs at each release time for all the aforementioned works. In this paper, we propose a synchronization-assist MC for biosensing that hybrids photolysis and molecules diffusion. The information molecules diffuse to the receiver, then degraded by photolysis when reaching around the receiver. The synchronization molecules are employed to trigger the receiver to emit light impulses during symbol \"0\" transmission. Then the photolysis reaction degrades the redundant information molecules to mitigate ISI. Because the emitting light time starts almost at the beginning of the symbol \"0\" slot, instead of calculating the peak time of the received signal, it can be adjusted with the change of the symbol period. The synchronization mechanism makes sure that it is easier to achieve precisely. In two scenarios, diffusion without photolysis and diffusion with photolysis, expressions for CIR are derived. Then we present the relationship between the channel capacity and the peak time by simulation and obtain the optimal symbol duration to achieve the maximum channel capacity. The rest of this paper is organized as follows. In Section 2, we introduce our system model, including tumor biosensing, molecules propagation, photolysis reaction, and diffusion with photolysis. We evaluate the system's performance and derive the bit error rate (BER) and channel capacity in Section 3. In Section 4, we present and discuss numerical and simulation results. Conclusions and the ongoing direction of our research are described in Section 5.