# Signal Quality Assessment for Transdermal Optical Wireless Communications under Pointing Errors +
## 1 Introduction  
Medical implants (MIs) have received significant attention from both academia and industry because of the improvements that they offer to an increasing amount of patients. Conventional MIs use the radio frequency (RF) band, which is bandwidth (BW) saturated [1], and transmission power in the order of some 10 s of mW [2]. Consequently, when operating with reasonable transmission power, they are unable to achieve the high data rates required for the emulation of human organs [3, 4]. Another disadvantage of conventional MI appliances is the interference from other sources operating in the same band, which can significantly degrade their performance. As a result, RF MIs are considered a mediocre solution [5, 6]. To overcome these barriers, it is necessary to examine the feasibility and effectiveness of transdermal wireless links that operate in non-standardized frequency bands and are characterized by a large BW availability. In this context and due to the partial transparency of skin at infrared (IR) wavelengths as well as the extremely high immunity to external interference, the use of optical wireless communications (OWCs) has been reported as an innovative alternative (see, for example, [7, 8] and references therein). Consequently, a great amount of research effort was put into identifying the particularities of the transdermal optical wireless links (TOLs) [9, 10, 11] and experimentally validating its feasibility [5, 12, 13, 14, 15, 16, 17]. In more detail, in [9], in vitro measurements of the optical properties of human skin are presented in the range of visible and infrared wavelengths, while, in [10, 11], the authorsanalyzed the optical properties of skin and subcutaneous tissues measured in a wide wavelength range. In the aforementioned works, the channel models used are based on the numerical results to accommodate the impact of the transdermal propagation loss. Consequently, it is difficult to evaluate the TOL's performance as well as to provide the appropriate design guidelines and tools. Furthermore, in [12], TOLs have been used to establish transdermal high data rate communications between the internal and external unit of the medical system, whereas, in [15], the authors presented experimental results of direct and retroreflection transdermal link configurations. Additionally, in [13], the optical transdermal link's fundamental design parameters and their interaction have been quantified, while, in [14], the authors demonstrated the tradeoffs revolving around the design of OWC medical systems. Finally, the authors of [16] evaluated the system model of a transdermal optical link with regard to tissue thickness, data rate and transmitted power. Most of the previously reported published works have neglected the effect of pointing errors caused due to the light emitting diodes' (LEDs) and photo-detectors' (PDs) directional behavior. On the contrary, in [5; 15; 17], the authors examined its impact on the TOL's error performance. Particularly, in [5], a low power, high data rate TOL was evaluated in terms of energy consumption and bit error rate (BER), for a given pointing errors tolerance. Finally, in [17], the in vivo feasibility of the TOL is verified, proving that high data rate can be delivered with BER of \(2\times 10^{-7}\), in the presence of pointing errors. The findings verify the feasibility of high-reliable low-power consumption transdermal optical wireless links. However, in all of these contributions, deterministic models have been employed to accommodate the impact of pointing errors. Thus, the stochastic nature of the pointing errors due to the relative motion between the transmitter (TX) and the receiver (RX) is not taken into account. Of note, this relative motion is unavoidable in several biomedical applications, due to the biological functions. Motivated by this, in this paper, we quantify the impact of pointing errors in terms of average SNR.