## 1 Introduction
Nevertheless, NZB pointing errors had never been examined in TOW literature, just as no method had been proposed in order to enhance the TOW outage performance and availability. Only, very recently, authors demonstrated for the first time, in [39], that a diversity method--i.e., in space, in time, or in wavelength--is an effective technique to enhance the outage performance of a typical TOW in terms of outage probability metric, by considering the joint impact of both attenuation-induced fading and misalignment-induced fading with NZB, modeled by the accurate Beckmann distribution [3; 41; 42; 43; 44; 45; 46]. In basic terms, diversity refers to the consideration of multiple copies of the propagated signals in an attempt to overcome a poor transmission media state so as to reinforce the communication link's availability and robustness. Diversity can be realized either in space, in time or in wavelength [3; 47; 48; 49; 50; 51]. Considering spatial diversity [47; 50], an FSO system employs multiple transmitters and/or receivers at different places that transmit and receive copies from the same information-bearing signal, resulting in an enhanced error performance. Through time diversity [49; 52; 53], an FSO system employs a single transmitter-receiver pair, but the signal is retransmitted at different time slots. Finally, assuming wavelength diversity [49; 54], an FSO system uses a composite transmitter, and thus, copies of the same signal are transmitted at the same time at different wavelengths towards a number of receivers [51]. It becomes clear that via diversity an initial single input-single output (SISO) FSO link is transformed into a more performance effective, by means of error rate and availability, single input-multiple output (SIMO), multiple input-single output (MISO), or multiple input-multiple output (MIMO) FSO system. Additionally, to the best of the authors' knowledge, in previous works related to TOW communication, only the simplest, but not always efficient enough, intensity modulation/direct detection (IM/DD) with on-off keying (OOK) modulation formats have been investigated. Although OOK is the most widely used modulation scheme in FSO area mainly due to its simplicity, when it is operating under fading conditions the detector threshold must be estimated and adopted by appropriate means, according to the variable channel states. This drawback can be circumvented via Pulse Position Modulation (PPM), which outperforms OOK in power efficiency [3; 55; 56; 57]. The latter happens at the expense of higher complexity and lower spectral efficiency, which is however, of minor importance considering the huge, in any case, available optical modulation bandwidth. Moreover, to the best of our knowledge, TOW links have been evaluated only in terms of instantaneous BER, when the reality is that the average bit error rate (ABER) metric is recommended in order to be able to draw meaningful conclusions for their reliability. Motivated by these facts, a key novelty in this work is the estimation of the ABER performance of a SIMO TOW system that may employ apart from OOK, \(L\)-PPM modulation format alike, under the joint impact of skin-induced attenuation and pointing errors with non-zero boresight. It should be highlighted here that in previous published works concerning FSO, receiver's diversity in terms of SIMO configurations have been used in order to enhance the performance of a terrestrial, outdoor FSO communication link, and thus, random atmospheric channel's impairments have been addressed, such as atmospheric turbulence-induced signal's fading [2; 4; 46], while in [2]\(L\)-PPM signaling technique has been proposed in order to achieve further such performance enhancements. However, for a TOW link which is investigated in this work, there is an essential difference concerning the nature of the transmission medium. In this work, the transmission medium is biological tissue instead of atmosphere.