## 1 Introduction  
Currently, there is an increasing interest in smart systems, e.g., smart metering or smart spaces, for which active sensing plays an important role. In such systems, a signal (acoustic, infrared, radio-frequency) is transmitted through a medium with the aim of measuring some particular parameters or characteristics from the received or reflected signal, which is captured by the corresponding transducers. Usually, spreading sequences are used to enhance the behavior of these systems, for instance to increase the signal-to-noise ratio (SNR) [1] or to contribute to the minimization of the Cramer-Rao lower bound [2]. At the low-level processing stage, three approaches can be used: optimizing the receiving filter [3], building the transmitted signals suitably [4],or jointly affording the optimization of both [5]. In a higher-level processing, some efforts have been done to extract task-relevant information from a sequence of observations and to launch the actions that lead to the most relevant observations [6]. If several emitters are using the same propagation medium, a multiple access technique, such as the Coded-Division Multiple Access (CDMA), may be required. Then, the system can deal with the simultaneous transmission of all the signals and the source separation at the receiver's processing stage [7]. Furthermore, using correlation techniques allows the receiver to determine the amount of energy received from each emitter, by quantifying the main correlation peaks. In the particular case of positioning systems, it is necessary to improve the positioning of people or devices in places where global navigation satellite systems (GNSS) have attenuation problems and multi-path effects that limit their performance. Throughout the years, different local positioning techniques have been developed to solve this problem, such as those based on optical, magnetic, acoustic, mechanical and radio-frequency (RF) signals [8]. In particular, the use of optical signals as a means of transmission has considerably spread due to the deployment of the light emitting diode (LED) technology. This type of technology has a low cost, long life and is present in the majority of the current infrastructure [9], characteristics that increase its interest. In local positioning systems (LPS) based on visible light or infrared, whereas the transmitter is typically a LED array, the receiver can be an imaging sensor or a photodetector. Those based on imaging sensors take advantage of the cameras available in commercial smartphones [10] or CMOS cameras [11], but they require image-processing algorithms that can slow down computation. If the receptor is a photodetector, it often uses an array of photodiodes [12], whose currents when the light impacts them can be used to estimate the position of the receiver. Typical receivers are position-sensitive detectors (PSD) with lenses [13] or quadrant photodiode angular diversity aperture (QADA) with apertures [14]. The majority of the systems are based on measurements of the angle-of-arrival (AoA), so they do not have to deal with the speed of light, which is required when using time-of-flight (ToF) measurements [15]. Additionally, they do not require an exhaustive analysis of the possible reflections of light or the multipath effect, which is the case when they use the received signal strength (RSS) measurements [16]. With the AoAs, the positioning algorithms are typically based on triangulation techniques, which result in a better accuracy than other approaches [9]. If the corresponding points of the emitters in the image plane are obtained and the calibration matrix of the camera-like receiver is available, perspective n-points (PnP) algorithms can be also used to obtain the pose of the receiver [17]. In the search for a centimeter-precise positioning system, it is important to consider the implemented medium access technique, often related to the modulation scheme used in the transmissions.