## 2 SNR Analysis of the VLC System with Light Interference
### Novel Receiver Design  
In VLC systems, the following modulation techniques, namely OOK, pulse width modulation, variable pulse position modulation, and multiple pulse position modulation use DC signals with a rectangular waveform. For the demodulations of those methods, a fixed and stable signal is required. According to the Fourier series expansion method, a rectangular wave is the resultant of superposition of an infinite number of sinusoidal waves. Figure 2 shows the conventional receiver with a HPF. When we use the HPF at the receiver, low-frequency signals are removed. It causes the end of each half cycle of the square wave to drop or tilt back towards zero because of the phase shift in the harmonic components. Furthermore, the relaxation time effect leads to poor performance. Thus, it is necessary to design a novel circuit to solve these problems.  
Figure 3 illustrates the principle of our novel receiver circuit design. It has six stages such as photodiode converting optical signals into electrical signals, a transimpedance amplifier (TIA), AVT for detecting the average value of the converted voltage signal, a voltage subtractor (VS) to remove the bias signal, a voltage amplifier (VA), and a voltage comparator (VC).  
To detect the photodiode signal, voltage or current could be used. Current measurements have a better linearity, offset, and high-speed response. Thus, by using the TIA, we can convert the generated photocurrent into the voltage signal to process. There are two methods to design a TIA: photoconductive (PC) (reverse biased) and photovoltaic (unbiased) (PV). The PC mode reduces the response time because of an increase in the depletion region width and a consequent decrease in the junction capacitance. However, applying a reverse voltage makes the dark and noise currents increase. The PV mode uses zero bias and operates based on the photovoltaic effect, which is the basis for solar cells. The PV mode is suggested when a photodiode is utilized for low-frequency or ultralow-light intensity applications. The selection of the operation mode depends on the specific requirements of the application considered. In this work, we used the PC mode, because it could help to improve the bandwidth, and the high dark current is acceptable because of the ambient light conditions.  
After the TIA stage, the raw signal received from the LED and other sources is amplified to around 1-2 V. The AVT stage is designed as an active low-pass filter with a 100 Hz frequency cutoff to obtain a 50-60 Hz signal and the harmonic frequency signals from the lamps and below 10 Hz from others (sunlight or light devices... ). Through the VS stage, the raw signal subtracts the filtered signal to minimize the effect of noise. Then, only the signal received from the LED goes to the next stage. After these stages, the amplitude of the voltage signal ranges from \(-50\) mV to 50 mV.  
Before comparing with a reference voltage, the original data is amplified to the range from \(-1\) to \(1\) V in order to avoid electrical circuit noise. Moreover, we adapt a hysteresis method to offer a high  
Figure 3: Proposed VLC receiver diagram.  
Figure 2: Conventional visible light communication (VLC) receiver diagram.  
noise immunity and a stability comparator. The hysteresis comparator uses two different threshold voltages to prevent multiple output state transitions.  
The OOK output signal from our receiver is demodulated in a microcontroller (STM32F4, STMicroelectronics, Geneva, Switzerland) to calculate the BER.