## 2 System Design  
### Transmitter  
Figure 1 shows a schematic of the transmitter system, which consists primarily of three parts: (1) an LED bulb, (2) a metal-oxide-semiconductor field-effect transistor (MOSFET) chip, and (3) a microcontroller (MCU) chip. A circular-shaped white LED with diameter and power of 15 cm and 15 W, respectively, was used as a transmitter LED. To control the current of the LED bulb, we constructed a driver circuit, which comprised a high-speed switching device MOSFET with two parallel resistances, R1 and R2. The resistance R1 was connected to the data pin of the MCU, and R2 was connected to pin number three of the  
MOSFET. An ATmega328p MCU was used to encode the data to the LED lighting. Table 1 show the list of configurations of the transmitter parameters for transmitting data. Phase-shift keying (PSK) is mainly used to apply LAN, Bluetooth, and RFID communication. The problem with PSK is that the receiver cannot know the exact phase of the transmitted signal to determine whether it is in a mark or space condition. In contrast, the case of frequency shift-keying (FSK) uses a pair of discrete frequencies to transmit binary (0 s and 1 s), and it is used for caller ID and remote metering [24]. In addition, the frequency shift on-off keying (FSOOK) was used, as it offers advantages for the separation of dark and bright strips width from the LED light through the transmission with its constant frequency. The camera field-of-view (FOV) is the main factor that increased the number of captured dark and bright strips from the LED. The captured dark and bright strips are inversely proportional to the distance at the different communication links. This means that the amount of covered dark and bright strips is smaller at a longer communication distance. However, with the distance variation, the loss of dark and bright strips increases for OOK modulation due to the separation problem of strip width to the captured FOV of the image sensor. No technique can solve this properly because out of the protocol control rule. This is a significant limitation of OOK modulation when establishing communication between LEDs and the camera image sensor. Hence, we proposed the FSOOK modulation technique to represent the modulated bit by multiple cycles, where the distance variation between the transmitter and receiver does not affect the decoding process. With the base of the frequency subcarrier, the binary data bit arrangement function appears at the group of dark and bright stripes at the receiver image and were determined in the FSOOK. If the  
Figure 1: (**a**) Simple transmitter circuit including its parameter; (**b**) the electrical schematic of transmitter circuit with its connection design. width of the strips of the modulated frequency is constant, then the subcarrier frequency remains same. The FSOOK modulation has multiple high frequency shifts for a high binary input and low frequency shifts for a low binary input of the LED light. However, the generated binary modulated bits of 1 s and 0 s of the FSOOK modulation are known as Mark and Space frequencies, respectively, as shown in Figure 2a. The transmitter LED flickers are controlled according to the modulation frequency during the transmission process. However, below the frequency of 200 Hz, the human eyes can observe the flicker [25]. Because the human eyes can observe it, the visual flicker is undesirable and modulation frequency should be higher than 200 Hz; thus, the frequency for modulation is generally between 200 Hz to 8 kHz. The FSOOK modulation scheme uses different flicker-free frequencies of the square wave to represent the different symbols of the binary data signal. The number of bits in one symbol depends on the number of frequency shifts. The receive image by the image sensor can be demodulated by those data signals if it can calculate the dark and bright strip widths of the received image. In our system, we used 8-FSOOK and received three-bits in every frequency. The packet format of modulation light is shown in Figure 2b.