# A Survey of Visible-Light-Communication-Based Indoor Positioning Systems
## 3 Technologies for Indoor Positioning Systems
### What are VLC-Based IPSs  
VLC harnesses the power of visible light, which falls within the wavelength spectrum of 380 nm to 780 nm, to enable seamless data transmission. This transformative technology, which can be traced back to the 1880s [30], has seen remarkable advances in recent decades. To promote its widespread adoption and further development, IEEE 802.15.7 [31] has meticulously defined the physical (PHY) and medium access control (MAC) layer standards for both broadcast and bi-directional communications. VLC is more than just a viable alternative to traditional RF communications, particularly Wi-Fi. With the ongoing transition from fluorescent lighting to energy-efficient light-emitting diodes (LEDs), VLC, often referred to as light fidelity (Li-Fi), holds great promise for revolutionizing indoor wireless connectivity [32]. These LED-based lighting infrastructures not only provide superior illumination but also have the potential to support high-speed data links. It is worth noting that the convergence of LEDs and lighting infrastructure presents exciting opportunities for the development of novel positioning and communication systems [33]. By leveraging the existing LED infrastructure, innovative positioning solutions can be developed, expanding the horizons of indoor localization.  
VLC-based IPSs benefit from breakthrough LED technology. For example, this article [34] describes how to improve the performance of visible light communication (VLC) systems by optimizing the superlattice interlayers (SLs) of Si-based LEDs. By receiving information such as the position of transmitters and detecting the direction and intensity of the received light, the position of the receiver can be estimated. The position of transmitters is usually broadcast using VLC. The driving force behind VLC-based IPSs is the use of existing and future LED infrastructure, as well as the integration of state-of-the-art smart mobile devices equipped with light sensors (see, e.g., [35]). By leveraging these resources and developing sophisticated algorithms, novel indoor positioning systems can be created; see Figure 2 in reference [14] for an illustrative example.  
In VLC-based IPSs, the primary choice for transmitters revolves around LEDs, as opposed to incandescent or fluorescent lamps/tubes, in line with the future trend of lighting systems. LEDs offer significant advantages, including energy efficiency and a long lifespan [36; 37; 38]. Furthermore, communications using LEDs enables high data rates due to their ability to support modulation with higher spectral efficiency.  
As for receivers, mobile phones have become one of the most prevalent and indispensable user devices in the 21 st century [39]. Leveraging the convenience and ubiquity of mobile phones, various design solutions can be explored. For example, the integration of a mobile phone's imaging camera, photodetectors (PDs), inertial sensors, or new light sensing functions can greatly enhance the receiver capabilities in VLC-based IPSs. Further technical details on these solutions are discussed in Section 4.