Bidirectional Underwater Visible Light Communication

Received Feb 13, 2018 Revised Jul 27, 2018 Accepted Aug 15, 2018 In this paper, a novel bidirectional underwater visible light communication (BiUVLC) is proposed. The VLC transmitter transmits an information signal using the one of RGB LED through the water tank that represents an underwater environment and then is received by VLC receiver via a color filter. The color LEDs and color filters are utilized in bidirectional systems. The single link is created by a LED on the transmitter and the color filter on the receiver with the same color which represents a single wavelength. The performance of the proposed BiUVLC system was evaluated via implementations. The experimental result shows that the transmitted signal undergoes attenuation over the underwater optical channel and the pair of the blue wavelength in link 1 and the green wavelength in link 2 have the best performance than the other wavelength pair. In the crosstalk measurement, the red wavelength color is the worst in the underwater environment. Keyword:

The use of Single Photon Avalanche Diode (SPAD) on the receiver improves the detection sensitivity in this study. Therefore, to our knowledge, there has been no the bidirectional VLC (BiVLC) research in the underwater environment.
In this paper, a novel bidirectional underwater visible light communication (BiUVLC) is proposed. The systems are implemented by the two transceivers in an experimental method. And the proposed system utilizes the one of Red Green Blue (RGB) LED and a color filter that represents a wavelength to build the bidirectional systems. The bidirectional condition in optical communication occurs while each link using a wavelength which is represented by a color LED and a color filter. The proposed system utilized the water tank that represents the underwater environment. The first link, the VLC transmitter with the single color LED transmits an information signal through the water tank that represents an underwater environment and then is received by VLC receiver via a color filter. The same condition occurs for the second link where the information signal is transmitted from VLC transmitter toward VLC receiver using same or different color LED. The crosstalk measurement is used. Therefore, this research is prospective to give the contribution to finding the best of the color pair for the bidirectional VLC system in underwater environment. The proposed systems can support the bidirectional communication between vehicles, divers, and devices in the underwater environment.

RESEARCH METHOD
In this chapter, we will discuss about the steps in conducting the research of bidirectional visible light communication in underwater environment.

System Design
In designing this system, the two transceiver are used which comprises of 2 (two) transmitters and 2 (two) receivers. The equipment is combined to get a single link transceiver. Then put the LED light on the transmitter and photodiode on the receiver. Figure 1 shows the block diagram of a visible light communication system, which consists of a transmit and receiving system. In the transmitter, the input section is given information in the form of an analog signal with the human frequency limit which is 400 -4000 Hz, generated by the audio generator. The signal processed on the transmitter circuit; then the information signal is transmitted through the light using the LED. The signal from transmitter undergoes a process of altering from an analog signal into the light through the Light Emitting Diode (LED). The light generated by the LED contains information from the transmitter. To proceed to the next section, the light contains information must be captured using components that can convert light into electricity. The part that can turn light into electrical are photosensors, and the one used in this design is a photodiode. The captured light by the photodiode contains information. Since the information received directly by the photodiode is too weak, the amplifier is used as a booster from the photodiode to forwarded to the receiver output.

VLC Transmitter
In the transmitter section, the process of information signal changes coming from the signal generator into the form of light, which will be transmitted through the color LED. At the time of sending information in the form of visible light waves, the LED will emit visible light according to the transmitter signal. The color of filters is to set the links to avoid wavelength interference. Figure 2 is a block diagram of the entire transmitter. Figure 2 shows the connection between components in the transmitter system, input from the audio generator connected to the power amplifier then to the transformer and to the LED to transmit the signal. The voltage input used is 12 volts, the cathode on the LED is connected to the secondary transformer winding, for the primary transformer winding is connected to the power amplifier. The connection of these components can be seen in Figure

Receiver VLC
In this receiver, the visible light that captured by the photodiode converted into an information signal. The workflow of this receiver section can see in Figure 4, the information transmitted captured by the photodiode proceeds to a circuit containing some resistor, capacitors, and bc547 transistors components, the component is a series of amplifiers. In the block diagram the captured light will be forwarded to the amplifier circuit to be amplified to a certain level, then the reinforced value will be displayed through the oscilloscope through the output of the amplifier. The circuit of receivers has specification: a. Source : Power Supply (9 V  This channel model implements the workings of VLC in underwater. The aquarium with dimensions 190x40x40 cm coated by scotlights on the inner side of the aquarium, it aims to avoid the incoming outer light from the side of the tank, then covered with black corriboard so that no light enters at the top. Next aquarium filled with tap water with 35 cm high. Figure 6 is the concept and realisation of channel model implementation.

RESULTS AND DISCUSSION
After the design process, the next step is to measurements. Measurements were made to obtain system performance data, then compared to determine the performance of a device that has been designed. The measurement results can be used as a reference in the overall analysis.

Position based Measurement System
On the measurement, the system is tested in two conditions. First transmitter 1 and transmitter two also receiver 1 and receiver 2 are on the same level or can be called crossed. The second situation, the position of transmitter 1 and receiver 1 and transmitter 2 and receiver 2 are on the same level or can be called directional. The purpose of this measurement is to know the optimal position in transmitting the information between links. Figures 7 and 8 are diagrammatic of both previously conditions. Figure 9 show graph of measured values from the two previously mentioned conditions with frequency is 400 -4000 Hz and 5 Vp -p signal amplitude.  From the measurements on transmitter 1 and receiver 1 and transmitter 2 and receiver two positions in the same area display values that are not much different. In transmitter condition 1 and transmitter 2 and receiver 1 and receiver two are in the same area or can be called crossed shown that the values between links are much different. It can be assumed that directional position is the best condition on this system.

Bidirectional Measurement using RGB LEDs
The next measurement by using color LED on the transmitter side to compare. The color LED used as many as three pieces placed on the transmitter side and the receiver set a color LED similar to the color on the transmitter. Figure 10 is a block diagram in measurement using color LED. Measurements were made in water with a water depth of 35 cm with a distance between the links as far as 50 cm. Each color is tested based on a frequency range of 400 -4000 Hz with an amplitude of 5 Vp-p. Figure 11 shows the results of measurements using RGB LEDs.  Figure 10. Diagram block of bidirectional measurement using RGB LEDs In Figure 11, three graphs are showing the response of each link. The red line displays the response on link 2 and for the blue line represents the response on the link 1. The explanation for writing the title of each graph "Red -Red" the first color mentioned is a link one which contained a color LED on the transmitter and color filters on the receiver it aims to obtain one wavelength in one link. On the second color after the "-" sign is a link 2 containing the color LED on the transmitter and the color filter on the receiver it aims to obtain one wavelength in one link. In this measurement, the use of filters and color LEDs on link two are changed interchangeably. Table 1 shows the average results of each measured link.  Table 1 is the average value of the output voltage on the receiver side of link 1 and link 2, to determine whether a solid color combination is seen from a large number of values between links. In Table 4 the "Red -Blue" color combination has the highest output voltage value of 8.50 volts. In Figure 12, three graphs are showing the response of each link. The red line displays the response on link 2 and for the blue line represents the response on the link 1. The explanation for writing the title of each graph "Blue-Blue" on the first color mentioned is a link one that contains color LEDs on the transmitter and color filters on the receiver it aims to get one wavelength in one link. In the second color after the "-" sign is a link 2 containing color LEDs on the transmitter and color filters on the receiver it aims to get one wavelength in one link. Table 2 represents the average value of the output voltage on the receiver side of link 1 and link 2, to determine whether a solid color combination is seen from a large number of values between links. In Table  5 the "Blue-Green" color combination has the highest output voltage value of 8.90 volts. Figure 12 addresses the results of measurements using color LEDs.

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In Figure 13, three graphs are showing the response of each link. The red line represents the response on link 2 and for the blue line represents the response on the link 1. The explanation for writing the title of each "Green -Green" graph on the first color mentioned is a link one that contains color LEDs on the transmitter and color filters on the receiver it aims to get one wavelength in one link. In the second color after the "-" sign is a link 2 containing color LEDs on the transmitter and color filters on the receiver, it aims to get one wavelength in one link. Table 3 is the average value of the output voltage on the receiver side of link 1 and link 2, to determine whether a solid color combination is seen from a large number of values between links. In Table 6 the "Green -Blue" color combination has the highest output voltage value of 8.78 volts. Based on the nine pairs of tested colors and from the data obtained, proving that a good color pair is used as link1 and link two on bidirectional communication is a "Blue-Green" color LED pair with a value of 8.90 volts. This result is obtained from a significant amount of output voltage at the receiver, and this conclusion can be corroborated by the literature that states the best wavelength in the underwater environment is the blue-green wavelength of 400 -500 nm [2]. Based on the comparison in the use of filters and LEDs found that the color pair that produces a good response is on the color filter with a combination of blue and red with a value of 10 volt.

Crosstalk Measurement
To know the value of interference between the links, then the measurement of crosstalk by involving two links simultaneously but in the state of one of the links are not given signal information. In the measurement process, the information sent from link 1 has a distance of 50 cm, and the value of crosstalk obtained from the output link two connected to the oscilloscope. Figure 14 is a block diagram of crosstalk measurement.  Figure 14. Crosstalk measurement diagram block using RGB LEDs Figure 14 is diagram block of the previously mentioned conditions, subsequently for data retrieval, the transmitted frequency value of 1 Hz with the 5 Vp-p signal amplitude generated by the audio generator and for each link spacing of 50 cm. In this crosstalk measurement, the color used is similar to the previous test, red (~ 675 nm), blue (~470 nm), and green (~550 nm). Table 4 are the results of crosstalk measurements.