# A Review of Wireless Positioning Techniques and Technologies: From Smart Sensors to 6G
## 3 Wireless Technologies
### Visible Light Communication (VLC)  
The VLC is defined as the unguided optical transmission by using light emitting diodes (LEDs) or fluorescent lamps. It is a solid-state lighting technology in which data can be encoded in the emitting light to realise communication [26]. LEDs can be easily modulated [111], with higher Lumina's efficiency, low power consumption, low-cost, long lifetime, and fast speed response. Their fast switching, which remains imperceptible by human eyes, changes the light intensity and enables data encoding [15, 112]. Moreover, the high data transfer and enhanced security makes the VLC a complementary solution for the ubiquitous RF-based wireless communication [15]. It is, in fact, a good alternative solution for RF-sensitive places, such as hospitals and aircraft, with a short transmission range communication capable of distinguishing different enclosed areas in indoor settings. Its narrow bandwidth provides the opportunity for AoA measurements to be used for localization at the receiver's side. While it is less affected by multipath interference, when compared to RF technologies, it is affected by shadowing and body reflections, especially in the case of brightly coloured clothing [15]. The fingerprinting can also be utilized, since the light strength is stable at different times of the day. The VLC localization system, shown in Figure 16, typically consists of the LED transmitting nodes, the receiving node (e.g., smartphone) whose position needs to be estimated and the channel. Specifically, modified LEDs are deployed on the ceiling and transmit their information related to their physical locations. The commercial LEDs commonly follow the Lambertian radiation pattern [44, 113], which is the angular distribution of the escaping light from certain diffuse sources. Moreover, the receiving node must support light detection for analysing the light beacon message. Light detectors include light sensors, photodiodes  
Figure 16: VLC localization system, reproduced with permission from [PERSON], _Asynchronous indoor positioning system based on visible light communications_, published by SPIE Digital Library, Optical Engineering, 2014. (PDs), photodetectors, and the camera's image sensor. The PDs detect and convert the light to photo current, which is proportional to the received optical power on the detection area [114], and provide ToA, RSS, and TDoA measurements. On the other hand, the light sensors require multiplexing design for separating the light signals from the different light sources. Nevertheless, the image sensors are widely deployed on smart devices and can provide AoA measurements. AoA measurements can be used in combination with the RSS for localization and result to small mean localization error, i.e., 0.50 cm [111]. Moreover, a calibrated smartphone rear camera can be used as an optical sensor. For instance, in [108], four uniformly distributed LEDs were deployed on the ceiling and their IDs were decoded from the pixels transmitted from a 10 s real-time video stream. This approach reported mean positioning errors of 4.81 cm and 6.58 cm, at 50 cm and 80 cm heights, respectively. Image sensors can also be used in combination with IMUs, i.e., in [44], the image sensor was used together with a mobile device accelerometer. The accelerator sensor was used for sensing the image sensor tilt, motion, shock, and vibration of the smart device, while the light sensor received the light signals and demodulated the LEDs coordinates. Furthermore, various combinations can be used, such as the TDoA measurements with a single LED transmitter and multiple PDs or the combination of the TDoA and frequency division multiplexing (FDM) [112]. Multiple not time synchronized transmitting nodes can also be used with the basic framed slotted additive link on-line Hawaii system (BFSA-ALOHA) protocol [113]. The on-off-keying (OOK) [26], can also be used as a modulation technique.