# A Novel and Adaptive Angle Diversity-Based Receiver for 6G Underground Mining VLC Systems
Among other problems identified in the literature in UM-VLC schemes are the low light coverage that exists in tunnels due to the low density of optical transmitters and the effect produced when the optical receivers are in the overlapping area of two adjacent optical cells [6]. These problems decrease the Signal-to-Interference-plus-Noise-Ratio (SINR) of the receivers considerably due to poor signal propagation and the existence of strong interference produced by the phenomenon of Inter-Cell Interference (ICI), which severely degrades the performance of the systems. Furthermore, specifically in UM environments, there are some SINR fluctuations, which are caused by Non-Line of Sight (non-LoS) components, which are due to reflections of the optical signal on irregular tunnel walls. This problem also reduces the system efficiency [7]. In the literature, there are several solutions to efficiently mitigate optical signal propagation issues. Recently, a particular technique, based on the Angle Diversity Receiver (ADR), has been introduced in indoor VLC environments [8]. An ADR consists of multiple narrow Field-of-View (FoV) Photo-Diodes (PDs) which, when combined, result in a large overall FoV and coverage as a single PD. Each of the narrow-FoV PDs can be selected or combined using signal combining schemes, thereby mitigating the effect of poor signal propagation and reducing the interference effect of ICI-producing signals. Another feature of these PD arrangements is to assume that the PDs orientation is fixed. However, limiting the PDs orientation also limits the flexibility of the applications and does not allow achieving better signal reception. Consequently, due to the hostility of the UM environments toward the VLC system, it is necessary to optimize the orientation and positioning of the PDs in the ADRs to improve the received signal and increase the system efficiency. According to the orientation mechanisms reviewed in the literature, the orientation of PDs is commonly estimated by proximity, fingerprints [9], Time of Arrival (ToA) [10] and Received Signal Strength (RSS) mechanisms [11; 12]. In this sense, algorithms based on RSS are the most used due to their high precision and low synchronization requirement. Specifically, one of these is the Received Signal Strength Ratio (RSSR) algorithm, which estimates the orientation of the target (the PD, in this case) depending on the received optical power. This parameter is affected by the relative distance and angles between the LED and the PD. However, the inherent challenge is to determine the influence of the relative angle between the LED and the PD in the RSSR method, since the PD orientation is commonly random and unknown in practice. Since the study of VLC systems applied to underground mines is considered a new area of application, these solutions have not been tested in these scenarios, either. In the context of these challenges, in the next subsection, we will present and discuss the most relevant optical receiver orientation methods and algorithms in the state of the art and related works. Works Related to Receiver Orientation Algorithms and Methods  
In general, receiving orientation algorithms are also applied for the location of receivers in a VLC scenario. For example, in [13], a hybrid RSS+Angle of Arrival (AoA) indoor positioning algorithm implemented in VLC is discussed. The interesting feature about this study is that ICI exists in the scenario, so the authors mitigate this problem using a unique frequency address. On the other hand, the authors in [14] present a mathematical model for a 3D positioning system in VLC. For this, they first derive the channel gain as a function of the source and receiver location in Cartesian coordinates. Then, they develop a cost function based on channel gain. In the experimental scenario, they consider receiver tilt, which is similar to UM-VLC environments. The results show that the proposed 3D method is more accurate compared to the 2D method.