## 3 Analytical Results
Figure 2 shows the ABER evolution of the examined MRR TOW system over a wide average electrical SNR range for both SISO and SIMO configurations with OC, assuming different levels of stochastic pointing mismatch. The skin thickness was fixed at 7 mm. In any case, the illustrated results indicated that the total ABER decreased for larger average electrical SNR values. Indeed, lower ABER values needed higher electrical SNR values \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value(s)** \\ \hline \(\theta_{m}\) & \(20^{\circ}\) \\ \(P_{s}\) & \(1\)\(\mu\)W\(/\)MHz\(-\)\(20\)\(\mu\)W\(/\)MHz \\ \(N_{0}\) & \(\left(1.3\,\mathrm{pA}/\sqrt{\mathrm{Hz}}\right)^{2}\) \\ \(\mu\) & \(30\)–\(60\) dB \\ \(\eta_{m}\) & \(0.8\) \\ \(M\) & \(\{1,\,2,\,3\}\) \\ \(\lambda_{m}\) & \(1.1\)\(\mu\)m \\ \(r_{m}\) & \(0.5\) mm \\ \(\delta_{m}\) & \(\{7\) mm, \(8\) mm\} \\ \(\mu_{x,m}/\,r_{m}\) & \(\{0,\,2\}\) \\ \(\mu_{y,m}/\,r_{m}\) & \(\{0,\,1\}\) \\ \(\sigma_{x,m}/\,r_{m}\) & \(\{4,\,4.5,\,5\}\) \\ \(\sigma_{y,m}/\,r_{m}\) & \(\{4,\,4.5,\,5\}\) \\ \(\xi_{1}\) & \(\{1.14,\,1.30\}\) \\ \(\xi_{2}\) & \(\{0.87,\,0.96,\,0.99,\,1.10\}\) \\ \(\xi_{3}\) & \(\{0.87,\,0.96,\,0.99,\,1.10\}\) \\ \hline \end{tabular}
\end{table} 
Table 2: Link parameter values. to be achieved, as expected. Additionally, the impact of pointing errors with nonzero boresight on the total ABER performance was highlighted. In fact, the depicted results revealed significant ABER degradations when the \(\xi_{2}\) parameter ranged from 0.87 to 0.96; i.e., as nonzero boresight pointing errors were getting weaker. It also became evident that the spatial-diversity technique could significantly alleviate the negative side effects of NZB misalignments. Indeed, for the same amount of pointing mismatch, we could observe significantly reduced corresponding ABER values as the diversity order was higher; e.g., as \(M\) becameequal to 3 instead of 2; i.e., by utilizing one more photodetector at the receiver's side. Nevertheless, the significant availability enhancements that stemmed from diversity-method employment were realized at the expense of a higher degree of system complexity, along with lower achievable data rates. Thus, it was not wise to further increase the diversity order of the proposed MRR TOW system. Figure 3 highlights the impact of skin thickness, which, according to Equation (6), is highly related to pointing-error strength, on the total ABER of SIMO MRR TOW links with OC. In this context, the only difference between the link configurations in Figures 2 and 3 was the transdermal link length, which was assumed to be extended by 1 mm for each link. Therefore, although we assumed for both figures the same values for the parameter set \(\left({{{\mu}_{x,m}}/{{r}_{m}}},\ {{\mu}_{y,m}}/{{r}_{m}},\ {{\sigma}_{x,m}}/{{r}_{m}},\ {{ \sigma}_{y,m}}/{{r}_{m}}\right)\), we eventually observed that the corresponding \(\xi_{1}\), \(\xi_{2}\) and \(\xi_{3}\) hadbeen increased due to Equation (6). The latter translated into a weaker amount of both zero and NZB pointing mismatch, and led to an enhanced ABER performance. For typical skin thicknesses, this behavior was consistent with findings in [11], until skin-induced attenuation became the dominant effect. It should be also noted that as is the case in Figure 2, the impact of NZB was stronger than the impact of zero-boresight pointing errors. This is because the laser could be strictly aligned with only one receiver aperture (in the case of zero boresight). The unavoidable geometric distance between receiver apertures  
Figure 2: Average BER evolution over a wide range of electrical average SNR for various link configurations of \(\delta_{m}=7\) mm with spatial diversity and OC along withvarying generalized PE. caused the additional boresight pointing-error component, which aggravated the total pointing errors' negative side effects.