An Analysis of Distance Extension Method in Visible Light Communication (VLC) Performance

A Visible Light Communication (VLC) adalah teknologi yang menawarkan konsep inovatif karena VLC menerapkan cahaya tampak untuk mentransmisikan informasi dari satu titik ke titik lain. Tantangan utama dalam penerapan VLC adalah pelemahan sinyal cahaya tampak karena faktor jarak dari titik sumber ke titik tujuan. Penelitian ini berfokus pada metode untuk merancang dan menerapkan pemancar dan penerima VLC pada media udara. Dengan membandingkan berbagai macam tipe LED, pengukuran yang didapatkan menunjukkan bahwa pemancar dan penerima VLC dapat ditingkatkan kemampuannya sehingga mencapat jarak maksimum 8.5 meter dengan menggunakan LED HPL.


INTRODUCTION
Visible Light Communication (VLC) is an optical wireless communication system that has been an increasing interest in many fields such as in seashore ( . Therefore, our work focuses on designing the VLC transmitter and enhancing the reception of signal (Hecht, 1997), (Max Born, 1999). The enhancement of signal reception is obtained by applying a distance extension method. This paper is structured as follow: Section 1 describes the challenge in VLC system. Section 2 provides a prototype of both VLC transmitter and reception. Section 3 evaluates the design system and analyses the output based on the distance extension method. Finally the conclusion and future work are provided in the Section 4.

VLC TRANSCEIVER
The design of VLC transceiver as the prototype is divided to two subsections as follows:

The VLC Transmitter
The VLC transmitter prototype (Figure 1.b) is designed according to the VLC's transmitter circuit as shown in Figure 1.a. The realization of the transmitter prototype includes only one LED in each circuitry and assembles LED's numbers when required.

Figure 1. VLC's Transmitter (a) VLC Circuit, (b) Prototype
The transmitter sends the audio signal with the frequency of 3 kHz through LEDs. In order to suppress the noise, the transmitting signal is filtered by a Band Pass Filter (BPF). The transmitting signal is detected by an oscilloscope that shows the output of BPF in Figure 2. Using the prototype as mentioned, the signal with a frequency of 3 KHz is transmitted applying an equation as follows: Substituting Equation (1) and Equation (2)

The VLC Receiver
The receiver consists of photodiode and amplifier as shown in Figure 3. This circuit receives the visible light which is subsequently converted to electrical signal. The output of the receiver is amplified with the calculation of ,as follows: The similar method in designing the VLC transmitter, Figure 3 shows the realization of VLC receiver. As illustrated in Figure 3.b the Photodiode number increases in numbers when required based on the VLC receiver circuit in Figure 3.a.

EVALUATION AND ANALYSIS
This work is conducted in 10 times measurements using parameters as shown in Table 1.

Distance Extension by Applying Different LED Types
The evaluation of distance extension method by applying different LED types shows signal reception captured by the oscilloscope (Figure 4.)

Figure 4. VLC Transceiver Signal Measurement: (a). 1 meter 5mm LED, (b). 1 meter HPL LED, (c). 3 meters 5mm LED, (d). 3 meters HPL LED
The VLC transceiver's measurement that is shown in Figure 5, indicates the signal attenuation toward the distance extension on average. By applying 5mm LED type, the signal voltage remains on the 5.36 Vpp, while it drops on 1.72 Vpp by using the HPL LED both at distance 1 meter. When the distance reaches 3 meters, the signal voltage drops to under 1 Vpp by applying both LED types. Thus, 5mm LED is more suitable than HPL LED because HPL LED experiences a narrowing signal beam.

Distance Extension by Applying Reflector on LED
This experiment evaluates the applied reflector on LED which is shown in Figure 6. Based on those experiments, the measurement of applying reflector (Figure 8) shows the stable received voltage that is achieved by 52mm reflector with the diameter of 6 meters on average. The maximum distance that can be achieved by this 52mm reflector is 9.5 meters. However, the 32mm reflector has the maximum distance of 6 meters which is mainly caused by noise.

Distance Extension by Adding Photodiode Number
The aim of this third experiment is to evaluate the signal reception by adding the number of photodiodes that is put in particular position according to LED's position. Figure 9 shows the measurement of using two 3mm and 5mm photodiodes. By adding the number of photodiodes, the signal measurement as shown in Figure 10, represents the voltage attenuation using at the distance of 1.5 meters. It can be noticed that the attenuation shows the drastic decrease at the distance of 2.5 meters. However, the 2x2 5 m photodiodes performs better than other types of photodiodes since the VLC receiver captures light in the wider scopes.

Distance extension by applying solar cells
The last experiment is applying solar cells as the alternative of using photodiodes. The solar cell is arranged on the VLC receiver side as illustrated in Figure 11. The measurements which are viewed on the oscilloscope are shown in Figure 12.  Figure 13 shows the result of signal measurement using solar cell instead of photodiode. The output voltage (Voutput) shows inconstantly voltage rate at distance 10 cm to 5 m. In addition, the signal quality decreases as the distance increases from 5.5 m -9.5 m. The poor signal quality occurs because of the low light intensity due to the further distance between VLC transmitter and receiver. Moreover, the solar cell receiver is highly interfered by the surrounding environment we assumed as temperature when the device is located in the outdoor. The maximum distance of VLC transceiver using the solar cell is 9 m and it is indicated as the signal attenuation because the signal is decreased below 1V ( Figure 13).

CONCLUSION AND FUTURE WORK
This work implemented the VLC transceiver and evaluated the different distance extension methods. The transmitted signal worked on the 3kHz audio frequency. The evaluation showed different maximum distances of each extension method. However, the VLC transceiver reached the maximum distance of 9.5m which was obtained by applying 5mm photodiode. It is also concluded that the size of the reflector has the significant impact to the light beam since it gained the stable voltage.
In future, the VLC transceiver will be implemented in a real environment i.e., on a road side unit (RSU) infrastructure.