## 1 Introduction
However, the latter configuration has the distinct advantage of placing both transmitter and receiver terminals on the outside of body, which makes this configuration much more flexible for deployment and redeployment [18, 23]. Additionally, it makes possible to conserve the energy of the battery of the implanted device in a more effective and flexible way [3, 18]. It is noteworthy that the feasibility of both direct and MRR TOW link modalities have been experimentally validated in [1, 7, 8, 10, 13, 15] and in [1, 3], respectively. Nevertheless, in all these papers, the effect of pointing errors has been either neglected or described by deterministic models. The stochastic nature of zero-boresight pointing errors for direct TOW links has been only recently first modeled and quantified by the authors in [11, 19, 24, 25]. Next, the authors in [12, 17] extended this concept by modeling the more generalized NZB pointing errors impact on direct TOW typical links with diversity in space, in time or in wavelength. More recently, the authors in [4, 18] introduced the presence of stochastic zero-boresight pointing errors for MRR TOW links with diversity. In particular, the total outage probability and the total average bit error rate (ABER) have been evaluated in [18] and in [4], respectively. Thus, to the best of our knowledge, the influence of NZB pointing errors has not yet been estimated for MRR TOW links with diversity. Moreover, contrary to the research that has been conducted for direct TOW links, there are still only a few papers that reported on MRR TOW links. Motivated by these facts, in this work, we introduce the presence of stochastic NZB pointing errors for MRR TOW links with spatial diversity, along with the OC (optimal combining) technique for signal reception. The total error performance of the investigated system is evaluated through the critical ABER performance metric. Specifically, an ABER analysis is performed that includes the most crucial link parameters, such as the skin thickness, the transdermal pathloss and the intensity of PE with NZB. In this context, the joint impact of spatial diversity, along with NZB stochastic pointing errors, is assessed and quantified. Indeed, novel ABER expressions are derived that mainly incorporate the number of the utilized receiver apertures, the total transdermal distance and the NZB pointing errors' strength. Additionally, representative analytical ABER results are presented over a wide average electrical SNR (signal-to-noise ratio) range and are further validated by simulations. Furthermore, in order to verify the feasibility of achieving such average SNR values, i.e., without exceeding the maximum power levels permitted for transdermal links with IMDs (implanted medical devices), we estimate the average electrical SNR obtained for appropriate power spectral (PSD) density values, taking into account the total transdermal pathloss together with the NZB pointing errors' strength for each MMR TOW link. In short, the main contributions of this paper can be summarized as follows:  
* A novel system and channel model for MRR TOW links is developed that incorporates the basic characteristics of this type of link. * The impact of the more realistic stochastic pointing errors with nonzero boresight is introduced and estimated in the MRR TOW area for the first time. * A spatial-diversity technique with optimal combining is considered by employing multiple out-of-body receiver apertures, and thus, MRR TOW data transmissions of enhanced reliability are achieved. * An outage bit error rate analysis is performed that evaluates the effectiveness of the spatial-diversity technique versus the joint impact of transdermal pathloss and generalized pointing errors with nonzero boresight. * An average SNR estimation is performed that reveals the feasibility of the proposed MRR TOW system architecture. * Novel analytical mathematical expressions are derived along with proper corresponding illustrated results that can be utilized in the design of MRR TOW links.