## 1 Introduction  
Transdermal optical wireless links that transmit information data via light waves through the skin have lately attracted particular research and commercial interest, mainly due to their effective high-speed operation within a very large unlicensed available bandwidth with low probability of intercept, low power consumption, ease of modulation and demodulation, compact and light equipment, flexibility for deployment and redeployment, and electromagnetic interference (EMI) immunity, [1, 2, 3, 4, 5, 6]. In view of these concrete advantages over their radio frequency (RF) and inductive-coupling counterparts, which are widely used in most transcutaneous communication systems with medical implants nowadays, TOW (transdermal optical wireless) technology affords a prime alternative, if not the only viable choice for establishing robust transdermal wireless communication links that can simultaneously reach the growing high capacity and low power requirements of different but equally important medical applications such as natural-feeling prosthetic control, neural cortical signal recording and communication with cochlear implants, as revealed in the findings of [7, 8, 9, 10, 11]. However, since skin is a very complex, multilayered and optically turbid channel that consists of many different structural components, the information-bearing light that propagates through the skin is reflected, scattered and absorbed by all these skin structural components, [12, 10, 13, 14]. In fact, even within the medical optical window; i.e., the desirable wavelength regime between 0.6 um and 1.3 um where photon absorption by tissue is minimized, skin highly attenuates the information-bearing light signal and limits its maximum depth of penetration into tissue by as much as several centimeters, [17, 8, 14, 15, 7, 16]. Apart from transdermal pathloss, TOW performance and availability is strongly degraded by the effect of pointing errors due to random misalignments between transmitter and receiver apertures that result, in turn, in significant random misalignment-induced irradiance fading of the propagating information-bearing light signal at the receiver side, [17, 18]. Indeed, despite the directional behavior of light sources and photodetectors, the relative motion between the transmitter and receiver in several biomedical applications is unavoidable due to biological functions [19]. Thus, in such cases the stochastic nature of pointing errors should not be neglected. More specifically, however, pointing errors consist of two components: boresight and jitter. The former represents the fixed displacement between the beam and the detector center, and the latter is the random offset of the beam center at the detector plane [12, 20, 21]. Hence, as is the case with the spatial-diversity technique, in which single-input multiple-output (SIMO) link configurations are employed, it could be more realistic to also consider boresight, i.e., NZB (non-zero boresight) pointing errors (PEs), since the transmitted light beam can be strictly aligned with only one photodetector due to the fixed spacing among the receiver apertures. It is to be recalled here that diversity is realized usually, in space, in time or in wavelength, and refers to the consideration of multiple copies of the propagated signals in an attempt to overcome a poor transmission media state [17, 20, 21, 22]. Consequently, through spatial diversity, the system can incorporate multiple receiver apertures [5]. At this point, we should clarify that there are two basic TOW link configurations: direct and MRR (modulated retroreflective) [3, 4, 23]. Through the former, the modulated light traverses the cutaneous channel in order to be collected at the receiver side, while through the latter, the in-body MRR modulates the transmitted interrogating light beam and reflects it back to the out-of-body receiver terminal. Consequently, since the light signal passes through the skin channel twice, the impact of the skin-induced attenuation effect becomes more significant in the latter case.