# Characterization of Light-To-Frequency Converter for Visible Light Communication Systems
## 5 Conclusions  
This paper proposed a novel receiver for visible light communication system using a LTF converter as the detector. The LTF can convert the modulated light into periodic square signal with a frequency proportional to light intensity. Under this scheme, the frequency estimation can be performed with the period measurement technique, without the need of ADC modules in the receiver side. In this system, the characterization of LTF converter was presented in terms of SNR, its input optical power, and the frequency of its output squares signal. The main objective of this characterization is to identify the parameters that determine the performance of the LTF converter as a receiver in a VLC system. Among the measured parameters of the LTF detector, responsivity \(R_{e}\) was big enough to make the device too sensitive to light intensity variations. However, such a characteristic could affect the overall SNR of the communication, as the detector is also sensitive to power related to outside optical sources. Experimental results showed that the LTF output signal has a maximum output frequency of 600 kHz at a distance of 6.2 cm. In this setup, measured SNR reached 18.75 dB, while the lowest obtained SNR with 1.1 m length was roughly \(-\)35.1 dB. Therefore, we conclude that it is possible to use the LTF converter as a receiver in a VLC system, considering a minimum SNR value that guarantees the \(f_{o}\geq 4f_{OOK}\) inequality. Given the condition that \(f_{o}\geq 4f_{OOK}\), a theoretical bit rate can be achieved and would be less than or equal to 150 kbps; however, we must be aware that no bit error rate measurement was made, because the electronic elements used are noisy and under the mentioned conditions, we obtained an SNR of 18.75 dB. This could have an impact on the speed of the system, but we consider that it could be a functional VLC system, because the main application is in low data rate sceneries, so the bit rate is not critical. The \(f_{o}\geq 4f_{OOK}\) is necessary for the identification of the LTF output frequency with the measurement system used in the receiver. Additionally, it must be ensured that the application scenario has a reduced number of non-transmitting lighting sources, in order to mitigate the negative effects of noise on the VLC receiver, and then the frequency shift in the LTF output.  
The main advantage of using the LTF converter as a detector in a VLC system lies in the low complexity to convert light intensity to an electrical signal, which can be directly processed in a digital  
Figure 7: LTF frequency estimation for the different \(f_{OOK}\) value and variation link distance.  
device such as a microcontroller, a field-programmer gate array (FPGA), or an embedded system equipped with a fast reference clock [14; 15].  
This way of detection is suggested for low data rate sceneries (indoor location, sensor networks, VLC-ID systems) because of limited bandwidth of the LTF, which is less than 800 kHz. Hence, higher data speed can be improved with an increased order of the CSK modulation format, at the cost of a penalty in transmission reach [23].  
Future works include evaluation of multilevel modulation techniques like CSK [22; 23] and pulse-width modulation (PWM) [24; 25], which allows one to maximize the low bandwidth of the LTF converter.  
All authors contributed equally to writing the paper. [PERSON] and [PERSON] proposed the research project and designed the experiments; [PERSON] wrote the paper; [PERSON] contributed to the formal analysis; [PERSON] and [PERSON] derived the analytic model and revised the paper.  
This research received no external funding.  
This work was supported by the Instituto Tecnologico Metropolitano (ITM) of Colombia and Universidad Autonoma de San Luis Potosi (UASLP) of Mexico.  
The authors declare no conflict of interest.