## 1 Introduction
Hence, the connections between nodes are optical in nature and they rely on non-coherent transmission and detection techniques, i.e., Intensity Modulation and Direct Detection (IM/DD). Additionally, depending on the Tx directivity and Rx field-of-view (FOV), these optical connections can broadly be classified into LoS and non-LoS (NLoS). While directed LoS links depend on accurate pointing between transceivers, they do so at the cost of mobility, whereas the NLoS (diffuse) links support mobility and do not require strict alignment [5]. Using these broad link configurations, two types of connections can be established in OBANs for UL and DL communication, i.e. : (i) between coordinator and AP (tier- II) and (ii) amongst on-body sensors (tier-I). These two cases correspond to MBAN channels identified as S4 to S7 according to IEEE ratified 802.15.6 standard [4]. Even though the two-way data transmission and reception in OBANs are critical at all the three tiers of communication, UL and DL connections at tier-II are of utmost importance because they not only carry patients' sensitive information, but also establish a link between tier- I and tier- III. Furthermore, the QoS of the bi-directional UL and DL wireless transmissions encompasses robustness and reliability with the bit error rate (BER) expected to be \(10^{-9}\) or less [4; 6; 7], for instance, in applicationslike ECG, EEG, drug delivery, and glucose level monitoring [7]. Thus, any negligence at this level could be life-threatening and, so, QoS remains a fundamental design issue at tier-II. Accordingly, it is imperative that the data collected by the coordinator from the SNs at tier- II must be transmitted/ received reliably to/from the AP and vice versa to avoid any mishaps. While reliability in OBANs depends on several factors, such as blocking effects [6], ambient light interference [8], and mobility [9], broadly, it primarily relies on the received \(SNR\) levels at the photodetector (PD) which in turn determine the system performance. For a typical OBAN employing IM/DD with OOK-NRZ modulation format, the required theoretical \(SNR\) is 15.6 dB to maintain a minimum (threshold) BER of \(10^{-9}\)[6; 10]. This implies that a minimum \(SNR\) of 15.6 dB must be made available throughout the room to ensure desired communication performance for OBANs. Hence, the ultimate challenge for an indoor OBAN designer is not only to maximize the value of \(SNR\) at the receiver, but also to maintain nearly uniform \(SNR\) at all points in the room, regardless of the Tx or Rx positions for UL and DL, respectively. The \(SNR\) is principally determined by the channel's direct current (DC) gain. While, in an OBAN link, the channel characteristics are fixed for a given position of transmitters, receivers, and intervening reflecting objects, they tend to change when these components are moved by distances in the order of centimeters [10]. Therefore, changing the transmitter positions will significantly impact the overall communication performance. Additionally, transmitter placement also plays a crucial role in obtaining the uniform distribution of illuminance, which determines the received power (\(P_{r}\)) and, subsequently, the \(SNR\). On the contrary, the non-uniform distribution may not only be displeasing aesthetically, but it also leads to a high degree of fluctuations in the received \(SNR\). These fluctuations restrict the system's ability to provide equal communication quality to users, especially in the  
Figure 1: Indoor OBAN communication architecture. corners of the room. Hence, it leads to poor communication quality and an unreliable link. Currently, the widely adopted placement technique [2; 11] in the literature fails to provide seamless user experience when it comes to reducing the \(SNR\) fluctuations. As the ultimate aim in an OBAN is to establish reliable bi-directional communication, it is critical to analyze and optimize the transmitter locations to improve \(SNR\) and illuminance uniformities of the DL connection.