Adaptive Underwater Optical Wireless Sensor Network Using LED-

Based Visible Light Communications

Xin Lin

Nakagawa Laboratories, Inc. and NAKATEN, 2-9-8 Ryogoku, Sumida-ku, Tokyo, Japan

Keywords: underwater optical wireless sensor network, visible light communication, wavelength adaptation control,

LED.

Abstract: An adaptive underwater optical wireless sensor network (AUOWSN) is proposed for marine environment

development and seafloor resources observation. The method of LED-based visible light communication

(VLC) is employed for creating the wireless network among the sensor nodes, and each sensor node is a

VLC transceiver. The LED is incoherent source, it can neither damage the marine creatures like as a

coherent laser nor lose the inherent high speed of light yet. The wavelength-adaptation-control technique is

used for seawater turbidity and marine environment which have the spatiotemporal change. The

effectiveness of proposed AUOWSN is demonstrated in the experiments of underwater one-to-one image

transmissions.

1 INTRODUCTION

A wireless sensor network (WSN) consists of

multiple spatially distributed wireless terminals.

Each wireless terminal is an autonomous sensor

node. And each sensor node equipped with a

wireless transceiver, a microprocessor, and a battery

or energy harvesting device. The sensor nodes of a

WSN are scattered in a specific physical space to

collect information from these wireless sensors

(Dargie and Poellabauer, 2010). Figure 1 shows a

mesh-type WSN. Because the WSN is a multipoint

simultaneous measurement system, hence it is

effective for grasping distribution changes of

environmental situations and physical phenomena.

And it is also one of core technologies used in

current Internet-of-Things (IoT) system (Ootsuka

and Kazama, 2014).

Figure 1: Mesh-type WSN.

On the other hand, the expansion of human

activities in marine environments such as the

monitoring and exploration of the ocean, offshore oil

field exploration and so on make the needs for

underwater WSN (UWSN) is increasing (Detweiller

et al., 2007). Like this that the developments of

marine environment and resources have many tasks

for the observations and analysis of many

phenomena including marine physics, marine

chemistry, marine biology, and so on. And data

among these different fields are related to each

other. Therefore, how to construct a smart UWSN is

also interesting and important in the aquatic world

with multi-data, so, data between different detectors

can mutually use and reference. Figure 2 shows a

detector net in a seafloor observatory.

Figure 2: A detector net in a seafloor observatory.

sensor node

gateway

sensor node

sensor 1

for biology

sensor 2 for

physics

sensor 3 for

chemistry

seafloor observatory

data in different fields

222

Lin, X.

Adaptive Underwater Optical Wireless Sensor Network Using LED-Based Visible Light Communications.

DOI: 10.5220/0008187902220227

In The Second International Conference on Materials Chemistry and Environmental Protection (MEEP 2018), pages 222-227

ISBN: 978-989-758-360-5

However, the techniques of conventional

terrestrial WSN to the marine environments have

intrinsic difficulties. In fact, the major obstacle in

using radio for underwater communication is the

severe attenuation due to the conducting nature of

seawater. In particular, the attenuation is very high

for high-frequency radio waves and, since the

current terrestrial technology for wireless

communication is often based on high frequency in

the order of Gbps, so it is practically impossible to

use terrestrial techniques in underwater application.

In this paper, an underwater optical WSN

(UOWSN) is proposed by using the visible light

communication (VLC) technique. And the method

of wavelength-adaptation control is used for

seawater turbidity and marine environment which

have the spatio-temporal change.

2 UNDERWATER OPTICAL WSN

An alternative to radio wave communication is using

visible light wave. Seawater exhibits a window of

reduced absorption in the visible spectrum, as shown

in Figure3 (Nakao, 1987). Particularly between 400-

650nm, where water is relatively transparent to light

and absorption takes its minimum value. Also,

because the inherent high bandwidths and space

divisionality of the light wave, make the techniques

of underwater optical wireless communication

(UOWC) the most important approach to construct

the UWSN (Ghelardoni et al., 2012).

Figure 3: Absorption coefficients of electromagnetic wave

in the water.

2.1 Underwater Optical Channel

Study of underwater optical channel is important for

creating a UWSN. Figure 4 shows the physical

model of visible light propagation in seawater.

Figure 4: Physical model of light propagation in seawater.

Natural seawater is a translucent substance with

a lot of micro particle and water molecule. The

performance of the UOWSN relies on how well light

propagates though this “translucent substance”.

When incident light I

passes through the seawater,

it become an attenuation light I

after a propagation

distance L due to absorption and scattering of micro

particle and water molecule. The mathematical mode

for this attenuation process can be written as





















 











 



(1)

















 







(2)

where



is wavelength of the incident light, K() is

total attenuation coefficient of seawater, a() and

b() is the absorption and the scattering coefficient,

respectively. The experimental data of K() in

horizontal light propagation direction are given by

Hulbert et al in 1945. Using these data, spectrum-

intensity attenuations for different seawater types

that correspond to Eq. (1) are obtained and plotted in

Figure5

Figure 5: Spectrum-intensity attenuation for different

seawater types in horizontal direction.

visible light

-2

-4

absorption coefficient

(dB/m)

X ray

seawater

microwave

radio wave

ultraviolet

infrared

distilled water

frequencies (Hz)

light intensities (W)

distances (m)

bay (high turbidity)

red (650nm)

green (550nm)

blue (450nm)

pure seawater

red (650nm)

green (550nm)

blue (450nm)

seawater

(translucent substance)

scattering

incident light (I

)

micro particle

attenuation light (I

)

water molecule

absorption

converted energy

Adaptive Underwater Optical Wireless Sensor Network Using LED-Based Visible Light Communications

223

For pure seawater type on upside graph of Figure

5, the absorption is dominated almost by the

attenuation of seawater molecule, the region of ideal

wavelength with lowest attenuation for visible-light

propagation is within the blue-green band between

400nm and 500nm. In case of high-turbidity bay on

bottom of Figure5, the total absorption in visible-

light band is dominated by the combination of

organic and inorganic particles, the ideal

transmission wavelength is shifted from blue-green

wave band towards green-yellow band around 550-

600nm.

On the other hand, the spatiotemporal change of

seawater color (Figure 6(a)) is also an intrinsic noise

which can affect light propagation. And the marine

snow (Figure 6(b)) in deep sea is an external noise

which able disrupts the optical link because it is a

visually observable enough large particulate organic

materials.

(a) (b)

Figure 6: Noises in underwater optical channel: (a)

spatiotemporal change of seawater color, it is an intrinsic

noise (picture by Johnson, The Univ. of Warwick) and (b)

marine snow in Sagami bay of Japan (picture by Kitamura,

2006).

2.2 Underwater Link Configuration

Underwater link configuration between sensor nodes,

and what is the difference between underwater links

and terrestrial links in a VLC system also should be

considered. Typical underwater diffuseness link

configurations between a transmitter and a receiver

are tabulated in Table 1 (Johnson et al., 2014).

Table 1: Typical diffuseness links for UOWC.

LOS

diffuseness

non-LOS

diffuseness

LOS diffuseness

retroreflector

Line of sight (LOS) refers to the ability to see the

transmitter from the receiver. The LOS link is the

simplest type which a direct link between the

transmitter (Tx) and receiver (Rx). Non-LOS link

uses reflections from the sea surface to overcome

underwater obstacles. LOS retroreflector link is

useful when bidirectional communication is

required, but the receiver is too low power to

support a full transceiver for underwater sensor

nodes. Theoretically, all these configurations have

enough good bit error rate (BER) and are viable for

short-range of under 15m underwater data

transmission (Arnon, 2010).

For turbid nature seawater, the attenuation of

light propagation is dominated by scattering, a wide

field-of-view (FOV) angle of transmitter can

compensate for the increased attenuation. Incoherent

visible-light LED (Light Emitting Diode) is most

suitable for this diffuseness-link application.

2.3 Constructing an Adaptive UOWSN

Such as a WSN in terrestrial environment, a WSN in

marine environment also has different types to favor

different applications. Figure 7 shows link

topologies of different UWSN types.

Figure 7: The link topologies of different UWSN type.

In order to construct a UOWSN which is viable

to work in a seafloor observatory (see Figure 2), the

mesh-type WSN topology is adopted to multipoint

simultaneously measure and transmit data from

different detectors and share these data each other

without human operations. The wavelength-

adaptation control technique (Lin, 2017) is used to

help overcome the “spectrum-intensity attenuation”

which shown in Figure 5 in different seawater types.

The links between sensor nodes are “LOS

diffuseness” type configuration because it is easier

to implement and most energy efficient for UOWC.

The UOWSN with mesh type that is proposed in

this work is shown in Figure 8. It consists of one

gateway sensor node (i.e. main node) and multiple

sensor nodes (i.e. sub nodes). Each node is

connected to other nodes to make a mesh; such,

gateway node

end-device node

router node

star type (1 to n)

tree type

linear type

mesh type

one to one type

MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection

224

multiple communication paths can be generated,

which cooperatively pass their data through the

mesh network to the land-base station. The distances

between the sensor nodes are about 3-10m. Data

from each sensor node are long-distance transmitted

to a land-base station via the gateway node. The link

between gateway node and land-base station is

optical fiber.

Figure 8: The mesh-type UOWSN.

Each sensor node is a wavelength-adaptation

LED-based VLC transceiver, and its control

principle as shown in Figure 9. Each VLC

transceiver as a sensor node is installed into a

seafloor detector for bidirectional data transmission.

The links between these sensor nodes are optical

seawater channels. The space division and visibility

of visible light can ensure each sensor node is

independent and identifiable both in space and time.

Figure 9: LED-based VLC transceiver with wavelength

adaptation.

In order to reach the wavelength-adaptation

control, a multi-chip white LED is used as the light

source of the transceiver. Figure 10 is a white LED

module with three chips of R(red), G(green), and

B(blue) which is used in this system. Each chip has

an independent wavelength peak, which can as a

separate channel for different seawater-types

adaptation control. The LED is incoherent source, it

can neither damage the marine creatures like as a

coherent laser nor lose the inherent high speed of

light yet. Also, it can as a lighting equipment for

underwater lighting.

Figure 10: Three-chips white LED module.

To detect three-colors light from LED source

with high accuracy simultaneous, at receiving side

of the transceiver, a photodiode (PD) with three-

primary-colors sensor chips is used, as shown in

Figure 11. Three sensor chips are arranged in mosaic

shape on the light detection surface of PD.

Figure 11: Three-sensors color photodiode.

The baseband intensity modulation is main

carrier techniques employed in optical wireless

communication. The on-off keying with no return to

zero (OOK-NRZ) and the pulse position modulation

with L levels (L-PPM) are two most common

schemes of the baseband intensity modulation

(Manea et al., 2011). Figure 12 shows the coding

methods of OOK-NRZ and 4-PPM (L=4). The

OOK-NRZ transmits data in a bit unit of binary “0”

and “1”, and the 4-PPM transmits data in a symbol

unit which is a string of 4 bits. Although OOK-NRZ

scheme has higher data transmission rate, L-PPM

yields an average power requirement that decreases

steadily with increasing L. This decreased average

power requirement makes L-PPM more suitable to

land-base

station

bidirectional communication

optical channel

optical fiber

gateway

sensor node

(main node)

space division

sensor node

(sub node)

3～10m

300 400 500 600 700 800

wavelength (nm)

spectrum

red

green

blue

wavelength adaptation

data

LED

Transmitter

modulation

control

Receiver

de-

modulation

control

color PD

data

optical

seawater

channel

illumination

&.signal

light-emitting principle

white light

LED chips

R: 620nm

G: 520nm

B: 470nm

spectrum

B-PD

G-PD

R-PD

photo-detection surface

Adaptive Underwater Optical Wireless Sensor Network Using LED-Based Visible Light Communications

225

underwater data transmissions. Especially, for deep-

sea environment, more low power consumption is

required.

data

0 0

0 1

1 0

1 1

OOK-NRZ

data

0 0

0 1

1 0

1 1

4-PPM

Figure 12: The coding methods of OOK-NRZ and 4-PPM.

The underwater digital optical wireless

communication is employing binary encoding to

accomplish optical data transmission and using BER

parameter to describe the optical seawater channel

performance. BER is a conditional probability; it

must be averaged over the probability-density

function of random digital signals to determine the

unconditional BER. The BER of L-PPM given by





























(3)

















 















(4)

where SNR is the mean signal-to-noise ratio in the

seawater channel and electronic circuit of

communication system. By using Eq. (3) and (4), the

SNR requirement of L-PPM scheme to reach a

certain BER can be calculated and the BER

performances for different L values are plotted in

Figure 13. The SNR requirement is decreased with

increasing L, for example, to reach BER of 10

-8

, the

SNRs of L=2, 4, 8, 16 are about 15, 9, 4.5, and 0.5,

respectively. The larger L can obtain better BER

performance. However, an increase in the L causes a

decrease in the communication speed. L=4 is a

preferred value employed in common underwater

optical wireless communications due to 4-PPM

scheme has appropriate efficiency both in BER and

speed.

Figure 13: BER versus SNR for L-PPM with different L

values.

3 EXPERIMENTS AND RESUTLS

Using the developed LED-based transceiver which

shown in Figure 14, the experiment for underwater

bidirectional image (640×480 pixels) transmissions

is implemented between a one-to-one link. The

principles of this transceiver have been described in

Figure 9-11. The 4-PPM method is used for

baseband light intensity modulation.

Figure 14: The prototype of LED-based VLC transceiver.

The results of image transmission with different

wavelength light are shown in Figure 15. In the

experiment, the transceiver is placed in a transparent

waterproof container which has high visible-light

transmissivity of about 93%, and then, the

waterproof container is placed into a water tank

which has 600mm width. Turbidity of water at the

tank is adjusted by using white-sand particles. A

water-flow equipment is used to generate dynamic

flow water which similar to natural water. The blue-

green light at 490nm wavelength and green-yellow

light at 590nm are used to demonstrate the data

transmission effectiveness of the proposed

wavelength-adaptation control method in lower-

and higher-turbidity water. Stable data transfer rate

is 5Mbps and BER less than 10

-8

1E-12

1E-10

1E-08

1E-06

1E-04

1E-02

0 2 4 6 8 10 12 14 16

SNR (dB)

BER

2-PPM

4-PPM

8-PPM

16-PPM

1 symbol

1 bit

LED source

detector

1 bit

MEEP 2018 - The Second International Conference on Materials Chemistry and Environmental Protection

226

Figure 15: The results of underwater one-to-one image

transmissions with different light wavelength.

4 CONCLUSIONS

The purpose of this study is to effectively develop

marine environments and resources, and the

incoherent LED is an attractive light source because

it not only can transmit data and never damage the

marine creatures, but also can as a lighting

equipment for underwater lighting. So, by using the

techniques of LED-based VLC, a mesh-type

UOWSN with wavelength-adaptation-control

function has been constructed for data transmission

in aquatic environment. A prototype of LED-based

VLC transceiver has been developed to perform the

one-to-one image data transmission experiments

with different light wavelength, and good BER and

data rate have been obtained both in lower-turbidity

and higher-turbidity water. Further studies include

that:

 underwater optical experiments for one-to-

many and many-to-one data transmissions;

 underwater link methods with low power

consumption;

 vertical direction characteristics of optical

seawater channel.

ACKNOWLEDGEMENTS

This work was supported by Ministry of Land,

Infrastructure, Transport and Tourism, Japan.

REFERENCES

Arnon, S., 2010. Underwater optical wireless

communication network. Optical Engineering, vol. 49,

pp. 1-6.

Dargie, W., Poellabauer, C., 2010. The book,

Fundamentals of wireless sensor networks: theory and

practice. Wiley.

Detweiller, C., Vasilescu, I., Rus, D., 2007. An underwater

sensor networks with dual commucations, sensing, and

mobility. OCEANS 2007, pp. 1-6.

Ghelardoni, L., Ghio, A., Anguita, D., 2012. Smart

underwater wireless sensor networks. IEEE 27

Convention of Electrical and Electronics Engineers in

Israel, pp.1-5.

Johnson, L. J., Jasman, F., Green, R. J., Leeson, M. S.,

2014, Recent advances in underwater optical wireless

communications. Underwater Technology, vol. 32, no.

3, pp.167–175.

Lin, X., 2017. Underwater wireless communication system

of adaptation wavelength using visible light. IEICE

Trans. Fundamentals, vol. E100-A, no. 1, pp. 185-

193.

Manea, V., Dragomir, R., Puscoc, S., 2011. OOK and

PPM modulations effects on bit error rate in terrestrial

laser transmissions. Telecomunicatii Anul LIV, nr.2,

pp. 55-61.

Nakao, S., 1987. Attenuation of electromagnetic waves in

seawater. Defense Technology, pp. 22-30.

Ootsuka, H., Kazama, H., 2014. A study of system

integration related to advanced IT networks in IoT.

IEICE technical report, vol. 114, no. 166, pp. 169-176.

blue-green light (490nm)

Tx image Rx image

transceiver

green-yellow light (590nm)

600mm

water-flow equipment

white sand

Tx image Rx image

Adaptive Underwater Optical Wireless Sensor Network Using LED-Based Visible Light Communications

227