## 1 Introduction
In indoor scenarios, VLC can potentially combine lighting and communication, as it can reuse the LED lighting systems, which already represent about 40-45% of the total lighting systems [16]. Other scenarios can also be considered. Underground mine sites are known for having issues with RF communications [17]. Already existent lighting system in mines could communicate with underground sensors/actuators. Furthermore, VLC may offer improved usage security in RF hazardous environments such as aircrafts [18; 19] and highly flammable environments [20]. Due to lighting systems' typical configuration, the light fixtures being in the ceiling illuminating downwards, the star network topology is naturally implemented in VLC systems: while node devices are spatially spread across a lit area, the ceiling light fixtures act as gateways, communicating both with the nodes and the Internet. Thus, a single ceiling gateway could simultaneously cover multiple users [21; 22; 23], with an increased probability of having a line-of-sight link. IoT-targeted VLC devices and systems are currently very limited, and they are in an early development state. Some scenario examples that can be found in the literature include Intelligent Transportation Systems (ITSs) [24; 25], Visible Light Positioning (VLP) [13], VLC smart labels [26; 27], and initialization of IoT devices for security improvement [28], amongst others, such as indoor healthcare data transmission, infotainment services, drone-to-drone communications, and augmented/virtual reality, relying on both photodiode- and camera-based communications [29; 30]. Although most proposed devices require significant processing resources, an effort to reduce their complexity and cost has also been pursued by some research teams, namely Corner Cube Retroreflectors (CCRs) (including battery-less devices) [31; 32; 33] and VLC backscattering architectures [34; 35; 36]. In [37], the authors proposed augmented spatial modulation to be used in VLC-based MIMO IoT scenarios,using modified Spatial Modulation (SM) to overcome the spatial identification problem for low complexity systems. Nevertheless, current developments in VLC systems are not targeting the IoT scenarios, where multiple users need to share the channel resources, along with the cost, power, and size of the IoT requirements, since they commonly consider a single user, or in the case of multiple-user architectures, they make use of complex modulations, which require powerful processing devices. IoT systems, in particular WSNs, usually have multiple devices in a network, simultaneous requiring the transmission of independent data. This requirement implies the usage of a system architecture capable of multi-user communications. Different approaches have been proposed in the literature such as Time Division Multiple Access (TDMA), Optical Code Division Multiple Access (OCDMA), Wavelength Division Multiple Access (WDMA), and Space Division Multiple Access (SDMA) [38]. Depending on the application, such schemes may present some drawbacks: the TDMA scheme decreases the spectral efficiency and requires complex medium-access management, including time synchronism between network nodes; WDMA allows simultaneous medium access; however, it requires different wavelengths to be generated, which is not compatible with current LED lighting systems; SDMA requires the creation of several narrow-beam VLC channels, as well as the usage of sophisticated protocols for handling the handover; OCDMA offers the best compromise for IoT systems, since it does not require additional medium access control; however, additional demodulation processing needs to be taken into account for low-cost and low-power devices. Another common scheme used in multi-user VLC is Orthogonal Frequency Division Multiple Access (OFDMA) modulation, where a set of orthogonal bands is assigned to different users.