# A Novel Ranging Technique Based on Optical Camera Communications and Time Difference of Arrival
## 3 System Description  
As mentioned above, the proposed system is a variation of the scheme presented in [10]. Fundamentally, that system uses an optical signal instead of the RF one included in Cricket devices [49], and introduces an additional optical receiver and emitter to provide distance-measurement capability on both devices. The measurement procedure is triggered by the Base Station (BS), which sends both optical and ultrasound signals simultaneously. When the Mobile Station (MS) detects those signals, it carries out the distance estimation taking into account the time delay between signals. Besides, at the moment of receiving the ultrasound signal, the MS transmits an optical pulse. It is used as a reference for the distance estimation at the BS side. Distance is then calculated from the time lapse between the BS optical signal emission and the reception of the MS optical signal. Figure 1 depicts the block diagram of the system. Figure 1: Hybrid Visible Light Communication-ultrasound system for distance estimation. Where MCU is a microcontroller unit and FPGA is a Field Programmable Gate Array. Modifications proposed in this work mainly consist of substituting the BS optical receiver (photodiode) by a Complementary Metal Oxide Semiconductor (CMOS) camera, which implements a Rolling Shutter (RS) scanning method [50]. Photodiode-based receivers integrate all the incoming light (restricted by its attached optics FOV), generating an electrical signal that depends on the received power and its responsivity. This signal is typically conditioned using amplifiers and finally is analog-to-digital converted at a given sample rate, producing a continuous data stream. This type of receivers is characterized by its minimum detectable signal or sensitivity, and its bandwidth. When used in ranging applications, these parameters determine the maximum measurable range and the maximum distance resolution. Optical cameras can be considered as photodiode arrays attached to imaging optics and optical filters, in which each row is enabled sequentially. The scanning process limits the maximum received signal frequency and, hence, the maximum achievable time resolution. The maximum measurable range will depend on the minimum allowed image size for the performed ranging method. All these characteristics are studied in depth in the following sections, where Table 1 shows the maximum range and minimum resolution for different camera types. In this new scheme, the time interval measurement is performed by an image processing algorithm instead of the classical pulse arrival detection used in TDoA systems. Furthermore, if audible elements replaced the ultrasound ones, the system's BS and MS would be suitable for a COTS devices-based implementation. The location process is thought to be performed, fully or partially, through commercial smartphones or webcams, microphones and speakers. That assumption makes cost implementation similar to a smartphone-based APP. If needed, some specific electronics can also be used in the Mobile Station, but no specific, ad-hoc hardware circuits are needed. Positioning systems require n + 1 reference points on an n-dimensional space (e.g., three points to find position over a surface) to perform the trilateration scheme. In this case, three MS can be used as reference stations, reducing the cost. MS design leads to a very simple and cheap implementation, as only an optical and acoustic receiver, an optical emitter, and some basic control electronics are required, without further computation and communications elements. The new block scheme is shown in Figure 2.  
In addition to the hardware modifications, some changes must be introduced in the signals generated in the measurement process. Chronograms for both systems are represented in Figure 3.  
The ranging procedure attaining the MS-side distance estimation remains unchanged from the one in [10]. Nevertheless, when the MS receives the optical signal, it starts its optical emission and does not finish until the reception of the ultrasound signal.