## 1 Introduction
Indeed, the only available RF bands that could be used for high capacity wireless links are increasingly crowded, because of commonly used protocols and applications such as 802.11, Bluetooth, and others. Moreover, RF transdermal links are highly vulnerable to EMI, and thus, susceptible to interference from other RF emitters and devices. It becomes now evident that exploiting the above-mentioned distinct advantages of FSO communication, TOW technology is a very promising alternative to all these conventional methods. Indeed, considering mainly the high data rates they provide (above 20 Mbps rate which is the threshold to satisfy anticipated needs [24], especially with regard to neural recording [25; 26; 27]), their extremely high immunity to external interference along with their operation with low power consumption (of the 10 mW target [24]), TOW transmissions can meet easily the essential requirements of transdermal high-data-rate communication, including wireless operation, i.e., no wires traversing skin that could increase the possibility of infection and discomfort, high noise immunity to ensure operational independence of the ambient environment and operation with low-power consumption for the internal device to extend the longevity of the implant [14], which sometimes is strongly associated with patient's longevity. However, the widespread deployment of TOW technology is significantly hampered by several performance-degrading factors that mainly arise from the variable characteristics and the nature of the skin as a transmission medium. Skin is a complex, multilayer, highly anisotropic biological entity--composed mainly of dermis, epidermis, and hypodermis--which entail many and different structural components such as cells, corpuscles, blood vessels, nerve endings, glands, muscles, and hair follicles. Consequently, light incident on skin is reflected, scattered, and absorbed by all its structural constituents that depend strongly on variable conditions and characteristics such as skin's thickness, tissues' heterogeneities, as well as different topology, region, race, gender, and race [28]. This leads to an important skin-induced loss in optical signal power, and hence, to a spatially broadened power distribution at the receiver, alike [29]. Specifically, photon absorption arise primarily from water content or hemoglobin and melanin, and can be minimized within the so-called skin's 'optical window', i.e., with radiation at wavelengths between 600 nm and 1300 nm, while scattering, which is usually the most dominant process, happens mostly into the direction of photon travel, and thus, a highly forward-directed scattering occurs [30]. Due to the strong scattering induced by thick tissue, photons arrive in a very anisotropic pattern at the receiver and the power distribution is widely broadened, and as a result, a large photodiode should be used to collect the scattered photons [29]. However, larger size photodiodes not only lead to a limited bandwidth, and thus, to a lower link speed but also generate a greater input-referred noise [9; 29; 31]. Additionally, many commercially available emitters and photodiodes are optimized for operation in skin's 'optical window' range, as is the same wavelength range used for many fiber optics or free-air applications [32]. Nevertheless, this skin-induced attenuation described above remains a major concern in TOW communication since even for the highest tissue transmittance, i.e., for wavelengths between 800 nm and 1300 nm, only 10% to 30% of incident optical power is transmitted through skin of typical thickness, i.e., 2 mm to 6 mm, when the emitter and the detector are aligned [33; 34; 35]. Considering also the pointing errors due to unavoidable misalignments between transmitter and receiver terminals that is also an effect of primary concern in TOW communication, as well as in the wider FSO communication, the performance and availability of TOW transmissions is further degraded [2; 4; 7; 36; 37; 38; 39].