### Prototype Conceptualization
#### 3.1.1 Purpose Statement
These transceivers employ visible light for receiving data from the indoor lighting system and Infrared (IR) links for uploading information requests. The schematic of the prototype's optical wireless communications component is shown in Figure 2, emphasizing the infrastructure-integrated transmitter and one of the smart backpack transceivers. This bidirectional connection allows the user to request information and, in future developments, can potentially enable the VLC lighting infrastructure to determine the user's location using Time-of-Flight (ToF) measurement properties combined with Received Signal Strength protocols [9]. Currently, user requests primarily focus on emergency assistance, and are limited to several specific points of interest such as the location of restroom, elevator, door, stairs, campus restaurant, or a few other objects. Nevertheless, as the concept is further developed, additional points of interest could be defined. Figure 1: Schematic representation of the VLC-based smart backpack for blind users’ assistance. The VLC transmitter has been integrated into the indoor lighting infrastructure from one of our research laboratories. Specifically, the actual setup is based on a luminaire positioned at a height of 3.5 m above the floor. The luminaire uses four 60 cm off-the-shelf LED tubes, each having a power of 9 W. Controlled through a digital driver by an ARM Cortex M7 processor operating at 600 MHz, the LED tubes-based luminaire becomes an information broadcasting device. For VLC data transmission, the system uses an adapted form of Variable Pulse Position Modulation (VPPM) [37], with asynchronous communication, capable of providing a data rate of up to 100 kb/s. VPPM is a modulation technique suitable for VLC applications, which combines Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM), enabling precise light dimming without compromising data transmission. The design provides optical intensity of 101 lux at the workspace level (2.7 m from the ceiling), when a 50% duty cycle is used. However, the light intensity can be adjusted from virtually the off state (1% duty cycle) to the nearly maximum intensity of the lighting device (obtained at a 99% duty cycle). Thus, it is important to emphasize that the system is designed to be compatible with light dimming, making it suitable for energy-saving applications while maintaining its blind assistance functions. For situations that imply high optical noise, the concept is also compatible with Binary Frequency-Shift Keying (BFSK) modulation [38], solution that further enhances the system's resilience to noise. In its turn, the VLC receiver represents a vital component of the smart backpack. It consists of an optical collecting system, a signal processing segment, and a data processing unit. The front-end employs an optical filter used to eliminate unwanted spectral components and a PIN photodiode-based optical detector having a Field-of-View (FoV) of \(\pm 53^{\circ}\), which is able to convert the incident light into a proportional electrical signal. Then, the signal processing module handles tasks such as signal band-pass filtering, signal amplification,  
Figure 2: Schematic of the smart backpack visible light communications component. and signal reconstruction. More precisely, the signal passes through a 1 kHz-500 kHz 4 th order band-pass Bessel filter, several preamplification stages, an adaptive gain control circuit that stabilizes the signal amplitude, and a Schmitt trigger circuit which provides the digital output containing the binary data. Finally, the data processing unit is responsible for real-time data decoding, data analysis, and Bit Error Rate (BER) measurement. To accomplish these tasks, an ARM M7 Cortex processor running at 1008 MHz is used. Table 1 summarizes the parameters of the prototype's VLC component. The following block is the _environment perception block_. This unit consists of sensors that have the purpose of analyzing the users' movement and to detect potential obstacles.