## 1 Introduction  
TOW links are rapidly gaining popularity as a very promising alternative to conventional transdermal radiofrequency (RF) wireless links, which are commonly used todayfor varying medical applications [1, 2, 3]. Transdermal RF has been shown to support data rates of some Mbps up to 24 Mbps, consuming at the same time at least 30 mW [4]. However, many emerging medical applications such as recording neural signals from in-body devices and actuating these devices from out-of-body signals to guide a prosthesis, require even higher capacities to emulate similar performance to that of human organs, such as the cochlea, along with lower power consumption [5, 6, 7, 8, 9, 10]. Another representative example of the need forhigher-speed bidirectional transdermal communication than RF is when neural signals are recorded, sampled, processed, and used to actuate artificial limbs for rehabilitation human affected by paralysis resulting from stroke, head injury, spinal cord injury, and other neurological disorders. This process can relieve pain, improve neurological disorder, or even regain functionality of damaged limbs [11]. Additionally, in comparison with the achievable data rates via transcutaneous RF, even higher-speed transdermal wireless links may be needed for such brain-machine interface applications [12, 13, 14, 15]. In fact, 50 Mbps would be required when 100 channels are simultaneously recorded [1]. As is the case with the wider free-air optical wireless communication systems [16, 17, 18], according to TOW modality, the data retransmitted via a very-high-frequency light carrier and, therefore, with an extremely high data rate which can meet the above growing capacity and energy consumption demands [19]. Nevertheless, owing to the different channel properties of human skin in comparison with atmospheric channels, the utilized wavelength window for TOW is between 600 nm and 1300 nm, while typical TOW link lengths are limited up to a few millimeters due to skin-induced attenuation and transdermal pathloss [20; 21; 22; 23]. Another but equally significant advantage of TOW versus RF transdermal modality is that the former ensures that there is no interference with existing RF networks or electronic equipment, while the latter suffers from electromagnetic interference (EMI) and multipath fading within a highly regulated spectrum [24; 25; 26]. In view of the above, TOW links have attracted particular research interest in the last few years. The feasibility of establishing TOW links has been experimentally validated by different research organizations around the world [2; 5; 12; 13; 27; 28; 29; 30; 31; 32; 33]. In short, the feasibility of establishing modulated retroreflective and direct TOW links was demonstrated in [5; 30], while, in [33], a bidirectional TOW link system for artificial hearts was employed. In [28], a 75 Mbps invitro TOW link was presented, while the potential of establishing in-vivo TOW links was verified in [29] by achieving data rates up to 100 Mbps with 2.1 mW of electrical power consumption. Moreover, the feasibility of establishing a bidirectional TOW link for brain-machine interface was validated in [12; 13]. Nevertheless, what all these papers have in common is that pointing errors, which refer to unavoidable, random misalignments between transmitter and receiver terminals and may result in significant TOW performance degradation, were either neglected or considered as a deterministic effect. The concept of the stochastic nature of pointing errors was first modeled and reported in [34] and thenin [9; 35]. Even more recently, the stochastic impact of pointing errors was reported in [3; 10; 19; 24; 36; 37; 38; 39; 40]. In these papers, considering the joint impact of transdermal pathloss and stochastic pointing errors, the TOW performance was evaluated in terms of outage probability, average signal-to-noise ratio (SNR), average ergodic capacity, average bit error rate (ABER), or average symbol error probability (ASEP) for various TOW system realizations.