# A Novel Ranging Technique Based on Optical Camera Communications and Time Difference of Arrival
## 2 Brief Literature Review
### Photodiode-Based Visible Light Positioning  
Most contributions found in the literature regarding VLP address applications of the technology and the use of different techniques. [PERSON] et al. reported a fully integrated transmitter using 180 nm BIPOLAR-CMOS-DMOS (BCD) integrated circuits technology [17]. For VLC and VLP applications to get into the mass market, integrated circuit based System on Chip (SoC) and System in a Package (SiP) is the only solution in the real world. The proposed transmitter is based on a Manchester-encoded 12 MHz signal. Although the authors claimed the feasibility of their implementation for VLP, they offered no application results. An Indoor Positioning System (IPS) using LED identification technology was proposed [18] by [PERSON] et al. LED sources transmit unique ID codes using 200 kbps OOK and the positioning is cell-based, achieving poor accuracies around 1.3 m.  
[PERSON] et al. evaluated a TDoA-based technique applied to VLP [19]. They demonstrated that it is possible to perform the position estimation without the need for lamp identification but using a pure pilot signal instead. Nevertheless, the pilot signal time requirements (10 ns pulses in a periodic 50 ns signal) makes this scheme unfeasible using conventional lighting LED lamps. VLP has also been tested on vehicles. An example was presented by [PERSON] et al. [20], who proposed a phase-difference-of-arrival scheme for estimating the distance between the leading and following vehicles. The system employs the leading car tail lights as emitters of amplitude modulated ranging tones, and photodiodes in the next car headlight for detecting those tones. Although the authors presented their work as VLP, this system is not a positioning system but a ranging scheme to obtain the relative position between vehicles. An example of an actual VLP system for vehicles was presented [21] by [PERSON] et al. They proposed a positioning technique based on TDoA measurements of signals transmitted by traffic lights. Methods for VLP with one traffic light and two traffic lights are proposed. This work and the previous one present the problem of detecting differences in the received optical signal by two photodiodes separated by a short distance (the vehicle's headlights distance), because of the high propagation speed of light. This problem requires the use of more complex and faster electronic devices, as well as synchronized sampling. [PERSON] et al. described the demodulation characteristics of an optical receiver using a microphone jack [22]. They found that the communication is feasible and that the smartphone can decode the lamp ID. An Indoor Positioning System (IPS) based on this VLC system is proposed and demonstrated. Nevertheless, no accuracy data are provided, but it could be tens of centimeters since this method only provides relative positioning based on the beacon data frame that the LED lamp send. [PERSON] and [PERSON] explored VLP with z-axis restriction to provide xy-coordinates [23]. The system retrieves both ID and relative emitter coordinates using OCC under on off keying (OOK), and then applied closed-form equations to provide the xy estimate. The experiments were carried out at 1 m height with an accuracy of around 3 cm. Generally, IPS based on visible light assume that receivers are pointing upwards. Some authors have explored the inclusion of the receiver's attitude estimation into the positioning system with relative success. The estimated position may present a significant error when the receiver is tilted. In the work presented by [PERSON] et al. [24], a positioning estimation compensation technique with known tilting angle is presented. Feasibility is verified both theoretically and experimentally, achieving accuracies around 1.5 cm at 100 cm. In [25], the authors evaluated the performance of a multi-photodiode receiver in a Received Signal Strength Indicator (RSSI) positioning system. They used the relative position of each photodiode in the receiver and the coordinates of the reference light sources.