## 1 Introduction  
Radio frequency (RF) based wireless technologies, such as Bluetooth, Zigbee, and Wi-Fi, have been in use for decades in various short- and long-range communication applications which employ frequencies in the unregulated Industrial, Scientific, and Medical (ISM) band. Recently, its widespread use can be attributed to the emergence of internet-of-things (IoT), particularly with its application in healthcare. Medical Body Area Network (MBAN) is one such example, which is a network of sensor nodes (SNs) placed in-, on-, or around a patient's body to transmit and receive information wirelessly. The Federal Communications Commission (FCC) has already allocated a spectrum of 2360-2400 MHzband for monitoring patient's data while using the healthcare capability of the MBAN devices [1]. However, the spectrum-hungry demands of healthcare-based IoT applications will soon find that these RF carriers are inadequate to meet the escalating needs. Currently, visible light communication (VLC) is considered to be an alternate technology to complement the overcrowded RF spectrum for many applications, including MBANs [2; 3]. Accordingly, these networks are termed as Optical Body Area Networks (OBANs). With its large unregulated bandwidth, which is almost 10000 times greater than the entire RF spectrum, optical technology offers higher data rates and lower costs. Additionally, the optical links do not incur electromagnetic interference and multipath fading. Nevertheless, when these links are established in an indoor environment, they experience multipath dispersion, leading to inter-symbol interference (ISI) at high data rates. However, even the non-directed line-of-sight (LoS) optical links support data rates up to few hundred Mbps (esp. 10 kbps-10 Mbps that are required for patient monitoring [4]); hence, multipath effects are not significant in OBANs [3; 5]. As illustrated in Figure 1, an OBAN generally uses the three-tier communication architecture of MBANs [4], which comprises of tier-I, tier-II, and tier-III communications as described below:  
_Tier-I or intra-BAN Communication:_ is the type of communication that occurs between in- or on-body SNs and the sink (also called hub or coordinator). The former collects physiological parameters from the body and transmits them to the latter via a wired or wireless medium. While it is also possible for the SNs to communicate with each other, ultimately it is the coordinator that receives information from the SNs, processes it, and then sends it to the next level, i.e., tier-II. _Tier-II or inter-BAN Communication:_ is responsible for establishing a bi-directional connection between the hub and an external/ off-body device, e.g., an access point (AP), so that the data collected at tier-I can be forwarded to the remote station at tier-III (see Figure 1). Here, UL communication occurs when the on-body coordinator sends data collected from tier-I to an AP located in the patient's vicinity, whereas DL communication happens when the coordinator receives information from the AP. Generally, in OBANs, infrared (IR) is used for UL whereas visible light (VL) is used for DL communication. _Tier-III or Beyond-BAN Communication:_ uses Internet or cellular (peer) networks to transfer patient-related information from AP/ gateway to the medical server (MS), as illustrated in Figure 1. Moreover, this component is responsible for processing, analyzing, and storing the information in a database server to maintain an electronic medical history of the patients as well as for decision making, e.g., to trigger alarms or prompt the emergency services in case any anomaly is observed in the data; for instance, when a parameter under observation increases above a threshold level. Although the communication hierarchy in OBANs is similar to that of MBANs, yet, instead of the conventional RF-based devices, it employs optical transmitters (Tx) and receivers (Rx) to create a communication network.