## 1 Introduction
It means that any source of light can be tuned to be used as an access point (AP) to provide both illumination and now, data transmission. In this sense, each of these optical APs can be integrated in the heterogeneous cellular networks as an attocell [18; 19] in order to extend the coverage and increase the achievable throughput of the overall system. On the receiver side, any device with photo-sensing capabilities may be used, such as photodiodes (PDs) [20], solar cells [21], image sensors, i.e., cameras [22], or even LEDs [23]. It is worth noticing that VLC also hold the belief that they are healthier than RF transmission because light does not affect the human body [24] as long as it complies with the illumination regulation. Since light does not go through solid elements, it also confers an inherent security umbrella upon VLC [25], which is a very appreciated property nowadays, where security and safety awareness are growing up in our society. However, the obstructions in the direct link between transmitter and receiver for VLC are critical for communication and it is one of its main disadvantages, which is known as the line-of-sight (LoS) link blockage or shadowing [26]. Links are often obstructed in normal scenarios due to either human body, furniture or other elements. If the system must provide a high data rate and reliable transmission network, this issue must be carefully addressed [27]. Providing proper lighting within an area according to the illumination standards entails necessarily the intrinsic overlapping of light beams in order to avoid non-illuminated regions. Although these overlapped areas suffer from high interference, it can be intelligently handled to provide a reliable communication without outages. For illustrative purposes, let us consider a 20 m \(\times\) 20 m \(\times\) 7 m scenario, which may correspond to an industrial building. In this scenario, a set of luminaries (Specifically, we have considered the LEDs Citizen CLU058-3618C4 [28], which are mainly intended for illumination. A discussion about the LEDs employed for high data rate VLC is provided afterwards.) providing about 24,000 lm each has been deployed on the ceiling. The obtained illuminance assuming a concentrator lens at each LED that generates a beam angle of 20\({}^{\circ}\) and 50\({}^{\circ}\) is shown in Figures 1 and 2, respectively, assuming a receiving plane 1.5 m above the floor. For comparison purposes, both figures consider the same range for the illuminance index. Notice that the first trade-off is related to providing a constant and satisfactory illuminance. Narrow beams obtain peaks of illuminance above 500 lux, while shadowing below 100 lux may appear in some areas. However, as seen in Figure 2, using wider beams involves reducing the average illuminance to about 250 lux. On the other hand, increasing the number of LED lights also increases complexity, cost and interference. Figure 1: Illuminance in an indoor scenario. Beam angle of 20\({}^{\circ}\). Moving to VLC, the second trade-off is related to the interference. In this sense, it is obvious that avoiding non-illuminated areas by overlapping the illuminance footprint of the luminaries involves increasing the interference. The achievable spectral efficiency (The receiver parameters are the same as in [29]. Furthermore, the noise is considered negligible so that the spectral efficiency does not depend on the modulation bandwidth.) for the considered configuration is shown in Figures 3 and 4, where each LED light works as an independent AP, i.e., without any cooperation among them. As occurs in previous figures, the same range for the spectral efficiency index is considered for comparison purposes. It is shown that using narrow beams reduces the interference among optical APs, and therefore, great spectral efficiency is achieved around each optical AP. However, very low spectral efficiency is achieved in the overlapped areas.