## 2 Description of the Proposed System
### Reception System  
The reception system consists of a QADA circular photoreceptor QP50-6-18u-TO8 [25], a filtering system, a synchronism signal detector and an acquisition system STM32F469I Discovery [26]. The general operation of the receiver system is shown in Figure 3. Note that the acquisition system starts acquiring at the instant it receives the synchronism pulse from the synchronism detector. The signals  
Figure 2: Block diagram of the transmission system. to be acquired come from the signals measured by the QADA after being filtered. The acquired signals are sent through an USB port for further processing on a computer. Firstly, the signals transmitted by the beacons added in the transmission medium are received by the QADA, to which a square aperture has been added so that the incident signal goes through the center of the aperture and illuminates part of its surface (see Figure 4) [21]. The QADA is a photoreceptor divided into four quadrants, so that it generates three output signals according to the area illuminated in each of the quadrants, which depend on the angle of incidence from the received signal. These three signals are the global sum of the incident energy, \(v_{sum}\); the voltage difference on the X-axis (left-right), \(v_{LR}\); and the voltage difference on the Y-axis (bottom-top), \(v_{BT}\). Since the QADA has a radius of \(r\) = 3.9 mm and the aperture height and length are \(h_{ap}\) = 2.6 mm and \(l_{ap}\) = 7.8 mm, in order to have all the quadrants illuminated, the light must have an angle of incidence below 56.3\({}^{\circ}\). Therefore, if a transmitter is placed at a height of 3.4 m from the receiver, the QADA could be placed at a maximum distance of 5.1 m from the projection of the transmitter. Secondly, a filtering and amplification stage is required before starting the acquisition process, with the aim of maximizing the amplitude of the received signals, as well as to reduce possible interferences in the desired signal with the light coming from the windows and the room's own illumination. The diagram of the filtering and amplification stage for the signal \(v_{sum}\) can be observed in Figure 5. Similar circuits are used for the signals \(v_{LR}\) and \(v_{BT}\). The acquisition system is based on a microcontroller STM32F469 NIH6U, which manages an adjustable potentiometer [27] included in the filtering and amplification system. Depending on the received signal amplitude (_maxValue_), the circuit gain is adjusted with this digital potentiometer controlled by a SPI bus, increasing or decreasing the internal resistance (_StepValue_). As was shown in Figure 3, three 12-bit ADCs are used in the acquisition of the signals, which have a maximum dynamic range of 4096, reduced to 4000 for security and to avoid introducing higher voltages than the maximum allowed at the input of the acquisition system. Figure 4: Geometrical representation of the quadrature angular diversity aperture (QADA) receiver. Figure 3: Block diagram of the reception system. With this objective, three ranges are defined to increase the gain of the signal to be filtered: the first range covers signal amplitudes up to 3000, where _StepValue_ is increased by 20; the second range covers amplitudes from 3000 to 3500, where _StepValue_ is increased by 10; and, finally, the third range contains amplitudes between 3500 and 4000, where _StepValue_ is increased by 1. This process is independent for each channel, \(v_{sum}\), \(v_{\eta LR}\) and \(v_{\eta BT}\), and is implemented in order to maximize the dynamic range of the used acquisition module. Note that the acquisition does not start and, therefore, the programmable resistance does not vary until the sync pulse is detected. The flowchart explaining the interrupt service routine is presented in Figure 6.  
The complete reception system has a power consumption of 0.23 A, with a voltage supply of 5 V. It is worth mentioning that, when the acquired signals are transmitted via the USB, the value of the gain used in the capture of those signals is also transmitted.