# A Novel Ranging Technique Based on Optical Camera Communications and Time Difference of Arrival
## 1 Introduction  
Positioning systems are gaining an essential role in indoor scenarios, with several applications on accessibility and marketing [1; 2]. In addition to classic Visible Light Positioning (VLP) indoor scenarios (commercial areas, hospitals, and visually impaired-people guidance [3]), applications of this technology can be also oriented to location on industrial environments, (e.g., robot positioning) where the presence of high electromagnetic (EM) noise levels induce huge errors when using traditional Radio Frequency (RF) systems [4]. Another scenario is underground mining, especially in the case of potentially flammable gases emission, as in coal extraction facilities (or even when coal is just present) or when there is a risk of the atmosphere becomes potentially explosive (e.g., oil and gas production or storage plants). Optical Camera Communication (OCC) and LED-based positioning is an alternative to avoid EM emissions [5]. Most of the techniques proposed until present are based on radio frequency signals, generated specifically for that purpose [6], or reused from other systems such as WiFi network devices [7]. Nevertheless, there are also some techniques based on optical signals, which make use of the Solid State Illumination (SSL) devices present in LED lamps [8; 9]. This alternative introduces a double purpose of lights in facilities: as light sources and as part of indoor location systems. Positioning techniques mainly consist of distance measurements among the target to be located, and several reference points to carry out a trilateration operation [1]. Several schemes for distanceestimation have been proposed such as Received Signal Strength (RSS), Time of Flight (ToF), Angle of Arrival (AoA) or Time Difference of Arrival (TDoA) [6]. In this work, a TDoA scheme based on a Visible Light Communication (VLC) measurement system, as previously proposed in [10], is presented. The main contribution of this work is the use of OCC techniques and image processing for measuring the signal's time of flight between the base station (BS) and the mobile station (MS). Some authors have proposed Optical Camera Communication for Visible Light Positioning (VLP). They are based on the extraction of different information from the camera's images and, in general, use them for estimating distances among reference points and mobile nodes [11; 12]. Additionally, the use of OCC to carry out node identification (reference and/or target) has also been studied [13; 14; 15; 16], achieving centimeter accuracy in small-room scenarios. Thus, the system can obtain both position and identification from the nodes. In this work, OCC technology is involved in the ranging process rather than in the trilateration one, and makes use of the images in a very different way, which is shown in this paper. The proposed method reduces the time resolution of the system, since it is limited by the camera scanning rate, but improves the received optical signal detection due to the camera's sensitivity. Furthermore, this scheme can be easily implemented over smartphones using the flash LED (optical emitter), the speaker (sound emitter), and the camera (OCC receiver). The remaining elements are allocated using an external module (optical and acoustic receivers, and optical emitter) unless they can also be integrated into another smartphone: embedded camera as the optical receiver, microphone as the sound receiver, and flash LED as the optical emitter. The presented work is focused on a new distance estimation technique and describes in depth the proposed distance-measurement scheme. The solutions implemented for adapting the signal transmission and processing, as well as the error performance are also discussed. Several of the presented modules are required for implementing a full positioning system, which are integrated following the general trilateration scheme used by other indoor location alternatives.