## 1 Introduction
Moreover, since the transmission of patient's data via UL is more crucial than the DL, therefore, \(SNR\) uniformity must also be retained for UL, irrespective of the patient's posture and his/ her position inside the room. In our previous work [12], we analyzed the effects of transmitter layout on illuminance uniformity for which we studied three different scenarios, i.e. : (a) single transmitter, (b) the generalized centrally lit quadrants (CLQ) with four transmitters, and (c) the proposed scheme of relocating all four transmitters towards each corner of the room referred to as corner illuminated quadrants (CoIQ). The single transmitter scenario offers a significant variation in \(SNR\) and reliable communication is not feasible. The proposed cornered LED lamp placements provided better results than the conventional four transmitter quadrant lit scenarios. Moreover, in both cases, there remained an area with low illuminance, known as the blind spot, which significantly degraded the illuminance uniformity. To eradicate this issue, a fifth transmitter was added to the system in [13]. Currently, there are several different lamp placement designs available in the literature, for instance, with five [13], six [14], and sixteen [15] luminaires. Even though the addition of transmitters may improve \(SNR\) and illuminance uniformities, it comes at the expense of cost, increased root mean square (RMS) delay spread, and reduced bit rates [16]. Accordingly, this work aims to increase the DL \(SNR\) and illuminance uniformities by heuristically optimizing the transmitter locations with the minimum number of transmitters. Subsequently, we use these optimum positions to achieve reduced \(SNR\) fluctuations; hence, a reliable link with a minimum BER of \(10^{-9}\) at a data rate of 20 Mbps for OOK-NRZ modulation format. Besides, the application and validation of the principle of reciprocity (POR) is presented. It is assumed that, while using POR [17], this DL analysis will also ensure the UL communication quality in terms of improved \(SNR\) uniformity throughout the indoor coverage area. To the best of the authors' knowledge, the characterization of a UL connection based on optimized DL transmitter locations has not been reported in the literature. The main contributions of this study are listed below:  
* OBAN DL connection is characterized by analyzing the impact of transmitter locations to improve SNR and illuminance uniformities and to enable QoS support. * A simple and heuristic placement scheme is proposed for generalized square room geometry that guarantees better performance when compared to the traditional approach. * An algorithm is developed that, given the room dimensions, yields optimized coordinates with minimum fluctuations in SNR over a resolution of 0.25 m.
* A comparative study on placement schemes and validation of the proposed heuristic design formulae is also presented by analyzing twelve different placement scenarios. * Using the principle of reciprocity and based on the proposed DL transmitter and receiver locations, UL \(SNR\) uniformity is estimated and validated. The rest of the paper is organized, as follows: Section 2 covers the related work followed by a comprehensive system description in Section 3. A presentation of results and their elaboration is available in Section 4. Finally, Section 5 concludes this paper.