## 2 Background  
In this section we present some recent research on the VLP topic related to this work, both from our research group and others. In previous works carried out by our research group, the influence of the multipath effect on IPSs based on optical signals has been analyzed. In [44] a light reflection model was developed to characterize how light will be reflected by different materials. Thanks to this reflection model, an algorithm was developed [43] which, given a certain environment, allows one to obtain the signal that would be received by the receivers of the positioning system. With this algorithm, in [42] it was demonstrated that the AoA measurement technique is more immune to the multipath effect than the PoA technique. AoA-based systems are affected by different electronic component tolerances and drifts. Therefore, an electrical calibration method was developed in [50] to compensate for these effects. In [51], it is shown how the frequency division multiple access (FDMA) technique is the most suitable for IPSs based on PSDs. FDMA allows one to discriminate between the different emitters of the system with almost no interference, unlike code division multiple access (CDMA), which due to multiple access interference (MAI), causes errors when obtaining the point of impact. In [52] is shown the implementation of an IPS based on PSDs [51] in a microcontroller unit (MCU)-based system-on-chip (SoC) system. In [52] it is demonstrated that it is possible to integrate into an MCU both the hardware and software requirement for signal acquisition and processing, for an IPS detector. Recent research by other groups is presented below. In [53], the actual communication characteristics of a VLP system that uses a QADA receiver are studied. The authors calculated the signal-to-noise ratio and bit-error-rates for a range of scenarios, proving that communication will not be a limiting factor when using QADA in VLP systems. In [54] is presented an autonomous method of collecting data for VLP, and a comprehensive investigation of VLP using a large set of experimental data is presented. RSS data were collected using a method that utilizes consumer grade virtual reality (VR) tracking for accurate ground truth recording. The quality and volume of the data allow for robust study of machine learning (ML)- and channel model-based positioning utilizing visible light. A sensor utilizing low-range infrared (IR) signals in the line-of-sight (LOS) context, providing high precision AoA estimation, is presented in [46]. The proposed sensor was used as a pragmatic solution to the localization problem that avoids NLOS propagation issues by exploiting the powerful concept of the wireless sensor network (WSN). The evaluation outcomes reached centimeter-level accuracy. In [55] a three-dimensional VLP algorithm using the Cayley-Menger determinant (CMD) with a cost function was proposed, and it was experimentally tested to track a drone for industrial applications. The proposed algorithm uses optical RSS for estimating the drone's 3D position without prior knowledge of its height. Ref. [56] proposed an artificial neural network (ANN)-based approach for accurate modeling and positioning with on-site data. Likewise, the proposed approach was proven applicable to accurate modeling of initial time delay distributions of LED chips in VLP systems based on phase differences of arrival (PDOA). To improve the robustness by mitigating the impact of intensity variations, they introduced a selection strategy utilizing both PDOA and RSS measurements. In [57], a fast and high-accuracy single-LED based VLP system was proposed. Firstly, an unbalanced single-LED VLP algorithm was proposed to increase the positioning accuracy and reduce the computational complexity. Secondly, a fast beacon searching algorithm was proposed to further reduce the processing time for each captured image. A new optical signal modulation technique suitable for intensity-modulation/direct-detection (IM/DD)-based VLP systems was developed in [58].