# Characterization of Light-To-Frequency Converter for Visible Light Communication Systems
## 1 Introduction  
In optical wireless communication (OWC), it is common to find optical receivers conformed by positive intrinsic negative (PIN) photodiodes or avalanche photodiode (APD) and analog-to-digital converters (ADC) [1; 2; 3; 4; 5; 6], as proposed by the IEEE 802.15.7 standard [7] for visible light communication (VLC) systems. Among these, the PIN photodiodes (PD) are presented more often in VLC systems, because they have better immunity to noise and low parasitic capacitance, and can be used to speed up the transmission (ultrafast PIN-PD), which is a milestone in VLC works [2; 6]. This is a result of the intrinsic material between the p-n junction, which leads to a reduction of the time constant, and thus better bandwidth [2; 8]. In fact, the PIN-type photodiodes adapted to the red-green-blue (RGB) sensors have been studied for applications in VLC systems [9], the authors characterized these sensors and determined its frequency response. Furthermore, PIN photodiodes are being adapted to other systems, to perceive intensity levels of light and turn over periodic electrical signals with frequencies that correspond to the incident power in the photodetector [10]. These devices are known as light-to-frequency (LTF) converters, and are internally comprised of PIN-type photodiodes and a module that transforms the photocurrent to frequency. The mentioned module employs a voltage-controlled oscillator (VCO), which reduces and simplifies the conditioning circuit ofthe photodiode, allowing its adaptation with embedded low-cost systems that do not require ADC [10; 11; 12; 13; 14; 15; 16]. Therefore, the LTF converters are exposed as an attractive solution for the detection of communication signals in the visible range of the electromagnetic spectrum because of the reduction of the system complexity [10]. Recently, LTF converters have been investigated, for example, in optical communication systems for the design of portable transceivers [11] and in health applications to detect levels of oxygen in the blood [12], among others.  
In this paper, we propose the use of a light-to-frequency converter as an alternative to design the receiver of a VLC system. In this scheme, the characterization and performance evaluation of LTF converter as a receiver in a VLC system, based on on-off keying (OOK) modulation, is presented. The main contribution of this article is summarized as follows: initially, the characterization of an LTF converter is presented, and an equation is derived based on both the incident optical power and the frequency generated in the LTF, in order to estimate the system's SNR value. The second contribution is the evaluation of the LTF by using a periodic optical signal, which reveals the advantages and disadvantages regarding its use as a receiver in a VLC system.  
The rest of the paper is organized as follows: Section 2 shows the model of the VLC system and the LTF converter. The characterization of the LTF converter for a VLC system is presented in Section 3. The results and discussions are presented in Section 4. Finally, we summarize the main conclusions.