# Channel Modeling for In-Body Optical Wireless Communications +
Footnote †: This paper is an extended version of our paper published in part at the 10 th International Conference on Modern Circuits and Systems Technologies (MOCAST), Thessaloniki, Greece, 5–7 July 2021.  
skin were studied in the visible and near-infrared spectral range. From the aforementioned works, it is observed that the majority of published works have focused on quantifying the optical characteristics of specific tissues at certain wavelengths [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. Despite the significance of these results, they cannot always be exploited from future researchers due to the fact that they may not include all the necessary wavelengths, while even if the desired wavelength is available, the constitution of a tissue is different enough between distinct individuals that the results cannot be regarded as confident.  
A method that estimates the optical properties of any generic tissue based on its constitution is required in order to aid the development of novel biomedical applications that utilise the optical spectrum for communication. As a result, specific formulas have been reported for the pathloss evaluation of a generic tissue that take into account the variable amounts of its constituents (i.e., blood, water, fat, melanin) but require their optical properties at the exact transmission wavelength, which hinders the use of these formulas [19, 20]. The development of such a method will open the road towards not only the theoretical analysis of in-to-out-of body OWC links but also the design of novel transmission and reception schemes, as well as scheduling and routing techniques for next generation networks. Motivated by this, this work derives a novel mathematical model, which requires no experimental measurements for the calculation of the pathloss for in-body OWCs. In more detail, the technical contribution of this work is summarized as follows:  
* The utilization of an ML-enabled mathematical framework for the extraction of analytical expressions for the absorption coefficients of the main constituents of tissues, namely oxygenated and de-oxygenated blood, water, fat, and melanin.
* Based on these expressions, we drew a general model that enabled the estimation of the absorption coefficient of any generic tissue (i.e., skin, muscle, etc.) based only on its constitution.
* The usability of this model was extended by incorporating the phenomenon of scattering in the analysis and, therefore, increasing the estimation accuracy of the attenuation due to the existence of generic tissues. This model is expected to have a great impact in the design and optimization of future medical devices that require the transmission of optical radiation inside the human body.
* We validated the extracted expression twofold. On the one hand, we fed experimental data into the ML algorithm to extract the mathematical expressions of the aforementioned coefficients and we drew the numerical results to visualize their performance. On the other hand, we compared the extracted numerical results against experimental data taken from different published papers and provided proof that they coincide. This twofold comparison illustrates the validity of the presented channel model.
* We provided the design with insightful discussions based on the pathloss variations, with regard to variable transmission wavelength, complex tissue types, and tissue thickness.  
The organisation of this paper and a list of variables alongside their names are presented in Figure 1 and Table 1, respectively. In more detail, Section 2 is devoted to presenting the pathloss model based on the absorption and scattering properties of the constituents of any generic tissue. Section 3 presents respective numerical results that verify the mathematical framework and insightful discussions, which highlight design guidelines for communication protocols. Finally, future research directions are highlighted in Section 4, while closing remarks are summarized in Section 5.