## 3 Experiment and Results
### Proposed Receiver Performance  
Figure 6 shows our proposed circuit in great detail. First, we chose Photodiode S5107 (Hamamatsu, Hamamatsu City, Shizuoka Pref., Japan) as the photodetector in our system. For the TIA, VS, and VA stages, the selected op-amps must have a wide bandwidth and a high slew rate to process high-data-rate OOK signals. Therefore, AD8056 (Analog Devices, Norwood, MA, USA) and TLV3501 (Texas Instruments Incorporated, Dallas, TX, USA) were chosen as the high-speed op-amps for these stages and the high-speed comparator, respectively.  
The OOK signal was employed to modulate the light signal through the LED. Because of the relationship between the semi angle and the SNR [19], several experiments have been conducted with different semi angles and the same receiver height of 1.25 m to validate the robustness of the performance of our proposed systems with the SNR.  
Figure 5: Signal-to-noise ratio (SNR) distribution: (**a**) Different distances from the light-emitting diode to the receiver; (**b**) Different semi angles (no tilting) at 1.25 m.  
The experimental scenario involves sending the sensor data between two microcontrollers via a universal asynchronous receiver/transmitter (UART). To perform the frequency response test, the microcontroller in the transmitter was programmed to generate a UART signal with the baud rate from 115.2 Kbps to 1 Mbps. At the receiver, the TIA bandwidth was designed for 10 MHz. The experiment was conducted in a room with windows and fluorescent light. The signal output after each stage was observed with the DSO7054A oscilloscope (Keysight, Santa Rosa, CA, USA).  
As shown in Figure 7a, the yellow signal consists of a rectangular wave signal and the DC signal is affected by noise. It is shown that the VA output signal has eliminated the average voltage of the signal (DC offset \(\approx\) 0). The final output of the VC stage is the OOK signal that can be retrieved by using a UART. To verify the adaptive feature of the average-voltage detection circuit, the experiment is repeated with the LED under lower illumination condition.  
Figure 6: Proposed VLC receiver circuit.  
Figure 7: Output waveforms of the VLC receiver: (**a**) 115.2 Kbps band rate with the SNR of \(-\)0.8 dB; (**b**) 1 Mbps baud rate with the SNR of \(-\)2.7 dB.  
The result in Figure 7b shows that the peak-to-peak amplitude of the signal after the TIA stage is much lower than before, but the DC voltage component remains at the same amplitude because of the same 400-lux indoor condition, dark current, and electrical noise. At a high frequency, we can clearly see the impact of the electrical noise; therefore, we must employ a hysteresis comparator. The better output signal is still obtained. As we mentioned in the introduction section, based on coding for the balance of the number of \"0\" and \"1\" bits, the system can operate with \(10^{-6}\) BER in 2.5 dB SNR condition [3] as the lowest SNR for the conventional receiver. Comparing with \(-2.7\) dB SNR with error-free in our results, the use of simple elements such as LED, PD, TIA, AVT and hysteresis comparators demonstrate a good system for OOK modulated error free communications to high data rates and low S/N ratios.