## 1 Introduction
In view of the above improving communication with IMDs still remains a critical research issue due to the limitations including power loss, misalignment-induced fading, and the simultaneous need for very high data rates [40]. Thus, several works have been published by different research organizations around the world concerning optical transdermal transmission of data. In short, the feasibility of establishing TOW links has been experimentally verified in [14; 28; 29; 31]. Specifically, in [28], it has been shown that FSO is an attractive solution in medical applications, in terms of TOW communication, because of its low power consumption that is of order of \(\mu\)W or below 10 mw for very high data rates contrary to that of some tens of mW that their current RF counterparts require. Next, in [14] the concept of a \"Medical Spectral Window\", commonly known as skin's'optical window' mentioned above, has been experimentally proven. Within this wavelength region, optical radiation is least attenuated traversing human skin, and therefore, the propagating light experiences its maximum depth of penetration, penetrating into tissue as much as several centimeters [14; 30; 32; 33; 35]. Additionally, by using a vertical cavity surface emitting laser (VCSEL) with 850 nm wavelength very effective TOW experimental links have been reported in [19; 29; 35]. More precisely, in [35], data rates up to 16 Mbps have been achieved through a skin thickness of 4 mm with a bit error rate (BER) of order of \(10^{-9}\) and misalignment between transmitter and receiver terminals of 2 mm, while consuming a power of 10 mW. In [19], a TOW link capable of transmitting data at 50 Mbps through sufficiently thick tissue, i.e., 2-6 mm, with a BER below \(1\times 10^{-5}\) and power consumption as low as 1.1 mW to 6.4 mW respectively, considering 2 mm misalignment between terminals, while in [29] in vivo tests performed on an anesthetized sheep demonstrated transmitting data at 100 Mbps with a BER below \(2\times 10^{-7}\), consuming only 2.1 mW of electrical power. With regard to pointing errors, in the majority of works concerning TOW communications, this effect has been either considered as a negligible one or it has been described by deterministic models, which are less suitable for the description of such random misalignment-induced irradiance fades [14; 19; 29; 31]. Recently, in [36; 37; 38], the authors considered the stochastic nature of a TOW link and derived closed-form expressions for the average signal-to-noise-ratio (SNR) and its outage performance,in terms of outage probability metric. In basic terms, they demonstrated that pointing errors can degrade the TOW performance by more than 10%. However, in the latter works, the stochastic nature of misalignment due to the relative motion between the transmitter and the receiver terminals is modeled by the commonly used model that has been proposed in [7] for outdoor FSO communication links, which takes into account the jitter but ignores the non-zero boresight displacement. Indeed, pointing errors consist of two components, i.e., boresight and jitter. The former is the fixed displacement between the beam and the detector center, while the latter is the random offset of the beam center at detector plane [41]. Thus, we should use a more generalized and practical pointing error model, commonly known as nonzero boresight (NZB) pointing error model, where apart from the effect of beam width, detector size, and different jitters for the elevation and horizontal displacement, the effects of NZB errors are also taken into consideration [42]. For the sake of clarity, this boresight displacement must be necessarily always considered at the receiver side when more than one detector aperture is employed, since each laser can be strictly aligned with only one receive aperture. Hence, there is an inherent boresight displacement that depends on the spacing among receiver detectors, due to the different distances from the specific detector aperture that determines the alignment point [41].