# A Novel Ranging Technique Based on Optical Camera Communications and Time Difference of Arrival
## 2 Brief Literature Review
### Photodiode-Based Visible Light Positioning
A centimeter accuracy ia reported for realistic scenarios. RSSI-based positioning was theoretically studied by [PERSON] et al. in [26], concluding that its performance depends on the layout of the LED lamps, AoA, type of light source, and receiver characteristics. In [27], [PERSON] et al. proposed a fusion positioning system based on extended Kalman filters, which fuse the VLC position and the inertial navigation data. The VLC-based positioning data are acquired in an LED lighting environment with a trilateral RSS-based algorithm. The accuracy of the combined positioning system is better than the VLC-based positioning or inertial navigation alone, as expected. The fusion algorithm provides an improvement around 57% respect to VLC positioning, achieving centimeter accuracy. Some authors have addressed the problem of medium access both theoretically and experimentally. A typical solution is to assign a different Radio Frequency (RF) subcarrier to each anchor node (Frequency Division Multiple Access (FDMA)), but other access techniques have been proposed(Time Division Multiple Access (TDMA), Ortogonal Frequency Division Multiple Access (OFDMA), Code Division Multiple Access (CDMA), etc.). [PERSON] et al. implemented a Real Time IPS using RF carrier allocation in VLC [28]. In this work, the authors implemented and tested an RSS-based system in which each LED lamp sends different RF carriers, ranging from 0.1 to 2.1 kHz. The relative positioning error is approximately 7%. The same authors analyzed through simulation the impact of moving speed and receiver's orientation in OOK-based VLP [29], comparing the received optical power with pre-calculated Line of Sight (LOS) channel gains. As could be expected, moving speed and tilting dramatically affect performance. In [30], the authors analyzed the effect of different multiplexing methods in the performance of VLP systems. FDMA scheme provides the best results in positioning but lowers luminous efficiency. On the other hand, random access procedure shows similarly good behavior but also excellent luminous efficiency. However, the identification sequences transmitted by lights must be short relative to the average delay between two transmissions in order to ensure low collision probability. A two-phase positioning algorithm was proposed by [PERSON] and [PERSON] [31]. During the coarse phase, a positional estimate is returned even if only one anchor node is within the receiver's Field Of View (FOV). The fine phase requires several emitters and its accuracy increases with the number of viewed luminaires. The authors evaluated through simulation the positioning error using different modulations/encodings, medium access strategies and parameter estimation technique (AoA, RSS, ToF). The results suggest that Time Division Multiplexing in combination with Time of Flight measurements provides the best error performance (8 cm). Another interesting but sophisticated approach to IPS is fingerprinting. In this technique, offline captured data are associated with each position and nonlinear regression machines are trained to fit the data. This approach needs calibration but eases the positioning algorithm from the mobile node's point of view. In [32], [PERSON] et al. evaluated a Visible Light Positioning system based on RSS. They generated an offline fingerprint database in a 3.3 \(\times\) 2.1 m floor space illuminated by four luminaires, each one emitting a different square-wave frequency. The fingerprints comprise the RSS of each luminaire, and a weighed K-nearest-neighbors regression is carried out with interesting results. Different metrics are used and the best-achieved accuracy is 2.2 cm. Despite the promising results, the proposed methodology requires prior calibration. [PERSON] et al. carried out data fusion from RF and VLP systems [33]. In the proposed scheme, the authors used a photodiode-based approach for the VLP subsystem, consisting of four different frequencies ranging from 2 kHz to 4 kHz.