## 1 Introduction  
The fifth-generation (5G) telecommunication standards have set the new platform for mobile wireless networks, rather than just extending the transmission capacity and reliability of the 4G network [1]. With the three main focuses on capacity enhancement, massive connectivity, and ultra-high reliability (i.e., low latency), 5G development stems largely from the increasing number of users and smart devices within the context of Internet of things (IoT)-based smart environments being connected to the cellular networks [2]. In future smart environments, such as homes, offices, cities, etc., there will be a growing need for the communications networks that can facilitate connectivities between a large number of devices or sensors and the end-users [3]. This will mean access to substantial transmission resources (i.e., bandwidth), and thus a paradigm shift in the way the wireless transmission resources are utilized effectively. To address this paradigm, a number of technologies have been proposed, including millimeter-wave [4], massive multiple-input multiple-output (MIMO) [5], small cell [6],and optical wireless communications (OWC) [7; 8], in order to meet the requirements of 5G-IoT. The popularity of the latter is due mainly to its inherent advantages of safety, security, low cost, and large transmission bandwidth, which are essential in IoT applications, particularly in indoor environments [8]. In indoor environments, the OWC technologies of visible light communications (VLC) and optical camera communications (OCC) have been widely considered and investigated [8; 9]. While VLC can support major features of 5G in terms of the data rates, OCC can offer a promising solution. The VLC technology, which uses light-emitting diode (LED)-based lights as the transmitter (Tx), has a lower initial deployment cost. The cameras in smartphones (six billion front and rear cameras), and pre-existing camera-based infrastructures, such as traffic lights, security, surveillance, and vehicles can be adopted as the OCC receiver (Rx) effectively [10]. Reflecting on the OCC potential and its availability, the IEEE 802.15.7r1 task group has been established to develop a standard for OCC within OWCs [9]. Various OCC issues and considerations, such as image sensor architecture, synchronization, data rate, perspective distortion, flickering and dimming, MIMO, and diversity are studied in [11]. The use of high-rate Tx's, such as LEDs, and low-rate Tx's, such as liquid crystal displays and digital signage, in OCC schemes is also provided in [11]. Although OCC does not support high capacity link connectivities, due to the speed limit of cameras, it can be employed in numerous low data rate applications, such as indoor positioning, mobile robot navigation, vehicular communications, small identification information, and advertisements [8; 9; 10; 11]. Despite lower data rates, OCC is simple to implement, with low path loss, additional imaging functionality, and spatial modulation capability, as compared with high rate VLC links [12]. However, in OCC, the data rate is limited by the frame-rate of the image sensors (ISs). The data rate can be increased by using higher frame rate cameras, which are very costly, and increasing the camera capture speed, which is defined as the physical parameter of the sensor (electronics) and the graphics processor speed in the hardware domain. Therefore, OCC is further extended to offer massive MIMO capabilities in order to increase the data rate, using LED and photodetector (PD) arrays in the form of multiple pixels in ISs for IoT applications, in both indoor and outdoor environments [13]. In addition, hybrid modulations schemes based on the intensity, color, spatial, phase, and frequency are also suggested in [12] as a solution to improve the transmission data rates in OCC. The IEEE 802.15.7-2018 standard [14] defines new clauses for the physical layer (PHY) types V and VI for OCC links.