LightByte: Communicating Wirelessly with an Underwater Robot

using Light

Robert Codd-Downey and Michael Jenkin

Department of Electrical Engineering and Computer Science, York University, Toronto, Ontario, Canada

Keywords:

Robot Communication, Underwater Robotics, LiFi.

Abstract:

Communication with and control of underwater autonomous vehicles is complicated by the nature of the water

medium which absorbs radio waves over short distances and which introduces severe limitations on the band-

width of sound-based technologies. Given the limitations of acoustic and radio frequency (RF) communication

underwater, light-based communication has also been used. Light-based communication is also emerging as

an effective strategy for terrestrial communication. Can the emerging Light Fidelity (Li-Fi) communication

standard be exploited underwater to enable devices in close proximity to communicate by light? This paper

describes the development of the LightByte Li-Fi model for underwater use and experimental evaluation of its

performance both terrestrially and underwater.

1 INTRODUCTION

Very few robots are designed to operate completely

autonomously. Rather their actions are controlled

(or at least inﬂuenced) by commands provided to

them by human operators. Leaving aside the com-

plexity of the development of a language for human-

robot communication, the actual problem of transmit-

ting electrical information in air is a relatively simple

task using modern technologies such as WIFI (IEEE,

2013), Bluetooth (IEEE, 2002), NFC(ISO, 2013) and

RFID(ISO, 2008). Unfortunately such approaches

ﬁnd limited application in the underwater environ-

ment because the electro-magnetic spectral bands as-

sociated with these technologies are highly attenu-

ated in the underwater domain rendering them unus-

able at any reasonable distance(Bogie, 1972). This is

not to say that RF-based communication is not pos-

sible underwater. Radio wave attenuation in water is

highly dependent upon frequency, and thus underwa-

ter communication technology is typically based on

very low frequency (VLF) radio waves in the 3-30kHz

range. Such signals can propagate long distances but

have particularly poor bandwidth. Given the short-

comings of RF transmission underwater, other tech-

niques are more popular. One effective approach is

to use acoustic communication. Sound propagates

more effectively underwater than in air. Underwa-

ter acoustic communication was developed for the US

Navy during the 1940’s(Quazi and Konrad, 1982) in

the form of the underwater telephone. Since then the

technology has matured with commercial off the shelf

(COTS) acoustic underwater acoustic modems being

readily available(EvoLogics, 2009). Such devices

are manufactured for different depth/distance appli-

cations, but performance in the 31.2kbit/s over a 1km

range are typical. Given the long ranges associated

with VLF and acoustic techniques, other approaches

are more appropriate over shorter distances. One in-

teresting approach here is to utilize visible spectrum

light communication. Such devices can be developed

with a range of different power/distance/bandwidth

tradeoffs and can be constructed using relatively inex-

pensive off the shelf components. Furthermore, given

the commercial interest in light-based communication

terrestrially, one can expect a substantive decrease in

component costs as the technology is deployed in the

terrestrial domain. Given this, here we consider the

questions: How effective can light ﬁdelity (Li-Fi)-

based technology be underwater, and is it practical to

develop ”Li-Fi modems” for robot-robot and robot-

human communication underwater?

Although Li-Fi can certainly be used to develop

a network-based communication infrastructure, here

we are particularly interested in bidirectional com-

munication between two underwater units; an au-

tonomous underwater device and an underwater de-

vice controlled by a human operator. Although the

human could certainly be underwater themselves,

we concentrate on the case in which a shore- or

Codd-Downey, R. and Jenkin, M.

LightByte: Communicating Wirelessly with an Underwater Robot using Light.

DOI: 10.5220/0006855202990306

In Proceedings of the 15th International Conference on Informatics in Control, Automation and Robotics (ICINCO 2018) - Volume 2, pages 299-306

ISBN: 978-989-758-321-6

299

Figure 1: Unmanned underwater vehicle control. Commu-

nicating with such a device wirelessly is complicated by

the nature of the aquatic medium. As a consequence wired

communication is common with its inherent problems re-

lated to cable management and fouling.

boat-based operator communicates via light with

a single thruster-based UUV such as the Milton

platform(Codd-Downey et al., 2017) shown in Fig. 1.

2 BACKGROUND

Underwater communication has a long history and

can be traced back at least to Leonardo da Vinci who

observed ”If you cause your ship to stop, and place

the head of a long tube in the water, and place the

other extremity to your ear you will hear ships at a

great distance from you”(da Vinci, 2010). Of course,

there have been substantive technological improve-

ments since then. Lack of space prohibits a full re-

view of advances in the use of sound, radio waves

and light energy for communication underwater here.

The interested reader is directed to(Kaushal and Kad-

doum, 2016) for a recent review of the RF, acoustic

and optical communication strategies underwater and

their limitations.

Although RF and acoustic communication strate-

gies have their advantages, we are particularly inter-

ested in short range, reasonably-high bandwidth com-

munication for which optical approaches seem partic-

ularly useful. A number of such systems have been

developed and deployed underwater using both laser

and LED-based systems. Although high-power sys-

tems are necessary for long distance communications,

for shorter distance (<10m) communication, a wide

range of technologies exist that can be applied to the

problem. Terrestrially, digital communication over

short ranges has also received substantive attention

and standard communication technologies have been

developed using RF technologies including Blue-

tooth, WIFI and other technologies. Light-Fidelity

communication (Li-Fi) (Condliffe, 2011) aims to use

visible light as the communication medium for digital

Figure 2: Manchester Encoding.

communication (See (Haas et al., 2016) for a review

of the technology.) Although still in its infancy, Li-Fi

has shown substantive promise, however, there have

been few large-ﬁeld tests of the technology.

Beyond the terrestrial domain there have also been

a number of efforts to deploy Li-Fi technology under-

water. For example, (Medhekar et al., 2016) looks at

the transmission properties of different light sources

for Li-Fi underwater and observes that LED-based

communication has advantages when line of sight

cannot be guaranteed. (Wang et al., 2016) demon-

strates a long distance (100m) light-based communi-

cation system that utilizes optics to concentrate the

emitter and a single photon avalanche diode to en-

hance detection.

The IEEE 802.15.7 standard for visible light com-

munication (VLC)(Rajagopal et al., 2012) utilizes on-

off keying (OOK) to encode the format of the data

stream from the transmitter to the receiver. The basic

idea here being that by turning a light on and off at

the transmitter the receiver can decode this sequence

into the transmitted message. A popular approach

for this OOK process is Manchester encoding (Tanen-

baum and Wetherall, 2011) (Fig. 2), which is a recom-

mended OOK approach in the IEEE VLC standard.

Essentially this approach modulates the data stream

using a clock signal. One downside of this mecha-

nism is its relatively high overhead in terms of the

communication signal consuming 100% more band-

width than a raw encoding scheme.

3 UNDERWATER LI-FI MODEM

Evaluating the potential for the use of VLC for UUV

control involves developing waterproof modems that

modulate an electronic signal onto the light signal

and then back again to an electronic signal. Rather

than developing a system that simulates this process

above ground (e.g., by placing a water tank between

emitter and receiver), or developing a small tank that

can be suspended from the surface, we are particu-

larly interested in testing performance under realistic

conditions where human-robot interaction will hap-

pen. That is, at depths greater than 10m and under re-

alistic turbidity and external illumination conditions.

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Table 1: A list of the primary components that make up

each transceiver. For the purpose of brevity resistors and

transistors are excluded from this list.

Primary Components

Teensy 3.2 32bit ARM Cortex-M4 arduino-

compatible micro-controller.

The micro-controller has 256kb

program storage space, 64kb of

dynamic memory

TSL12S-LF Eight light-to-voltage convert-

ers, combines a photodiode and

transimpedance ampliﬁer.

C503D-WAN Twenty-four cool white LEDs

with high luminous intensity

(64600 mcd).

SSD1306 An I2C OLED graphical dis-

play.

The goal here is to not only identify issues related to

Li-Fi generally, but also to investigate potential inter-

actions between underwater housing construction and

illumination conditions found at depth. This decision

drives the basic physical design of the LightByte sys-

tem shown in Fig. 3. The LightByte ”modems” are es-

sentially small cylindrical housings with port glands

for power and data (USB). Light emission and sens-

ing is performed using emitters and sensors mounted

in an octagonal 3D printed structure that is mounted

inside the housing and isolates the emitters from the

receivers to reduce crosstalk and also enables both

emitters and receivers to cover a full 360

◦

horizontal

ﬁeld. Operationally, these devices can be mounted as

small external pucks on the external shell of a UUV.

Each LightByte transceiver communicates using

ﬁve sets of emitters each composed of three LEDs,

ﬁve light sensors that act as receivers and a small

amount of electronics to drive and monitor the emit-

ters and receiver. A small display is also included

within each transceiver to enable monitoring of the

state of the device when it is operating. The cost of

each unit is under $200 US including the cost of the

housing. Table 1 lists the components housed within

each transceiver node.

Each LightByte node provides a USB connection

for data transfer. For low power operations the Li-Fi

LED’s can be powered from the USB bus. For higher

power operations, power can be provided externally

or a battery can be included inside the transceiver

node.

(a)

(b)

(c)

Figure 3: LightByte Transceiver node. (a) 3D printed

mounting rig. (b) A LightByte transceiver node in its under-

water housing. (c) The electrical components of the node.

4 COMMUNICATION

PROTOCOL

The IEEE 802.15.7 standard for VLC provides stan-

dards at both the data modulation and physical lay-

ers. The IEEE standard speaks to the use of VLC

for local communication networks. As the goal here

is for point-to-point communication we deviate from

the IEEE standard at the physical layer, introducing a

simple frame-based communication structure (based

on UDP with components borrowed from TCP). The

LightByte: Communicating Wirelessly with an Underwater Robot using Light

301

two transducers are identical in structure with identi-

cal communication protocols in both directions. Fol-

lowing the IEEE 802.15.7 standard frames are en-

coded using a Manchester line code. In addition to

being an effective encoding scheme, this scheme has

the beneﬁt of using a constant amount of power irre-

spective of the data being sent and thus the ﬂicker of

the emitting LEDs cannot be perceived. This makes

the emitter appear as a dim stable illuminator to any

operator in the vicinity.

4.1 Physical Layer

As signals are emitted they are encoded using the

Manchester coding scheme. This requires the emit-

ter to maintain a stable clock to drive the encoding

process. All of the output LED’s are driven to the

same level (100% of their maximum possible out-

put (20 mA)). The complete set of LEDs use a total

of 480mA in addition to the 45mA consumed by the

micro-controller.

4.2 Data Link Layer

The data link layer utilizes the capabilities of the

physical layer to construct a protocol for data trans-

mission that deals with errors and regulates the ﬂow

of data in an effective manor. This is accomplished

by encapsulating data into 128 byte ﬁxed sized pack-

ets/frames with 8 byte headers that can be used to vali-

date, sequence and identify each piece of data. Data is

packed into a frame/packet as shown in Table 2. The

ESC, PREAMBLE, SYNC, STX and ETX symbols

are reserved. In order to avoid becoming confused by

such tokens appearing in the signal itself, these sym-

bols are reserved in the data packet and must be re-

coded there. Like UDP packets there is no guarantee

of delivery, however unlike UDP this protocol does

guarantee packet ordering. The physical layer also

provides duplicate packet protection because data is

only transmitted once and not relayed by subsequent

nodes. Data frames are of ﬁxed sized.

4.3 Error Detection

The Data Link Layer provides the ability to per-

form a certain amount of error correction. The two-

byte Fletcher-16 (Fletcher, 1982) checksum facilitates

the identiﬁcation of transmission errors in the data

stream. Frames that fail the checksum test are dis-

carded, invalidating the current data stream. Consid-

ering that frames are sent sequentially the ID, DID

and FID framing bytes can be used to identify non

sequential frames and intrusive frames from another

Table 2: Packets are 128 bytes long. The Preamble, STX,

SYNC and ETX characters are reserved in the data packet

and are escaped in the data transmission using the ESC char-

acter 0x1B.

Packet/Frame Structure

PREAMBLE (0xAA) used to compute a signal

average

SYNC (0xD5) allows the receiver to

align itself, signiﬁes the

start of a frame

ID (1-byte) unique identiﬁer of the

sending node

DID (1-byte) unique identiﬁer for

the current sequence of

frames

FID (1-byte) numerical identiﬁer for

the frame in sequence

CHKSUM (2-bytes) two-byte checksum of

the data in the frame

(Fletcher-16)

STX (0x02) signiﬁes the start of data

transmission

DATA (119-bytes) user data

ETX (0x01) signiﬁes the end of data

transmission

sender. Error detection helps to prevent invalid data

from being propagated further up the communication

stack. Thus, only validated transmitted and received

data is transferred to the application layer.

4.4 Data Communication

Given the ability to send bidirectional packets with

checksums a variety of different lossless bidirectional

communication strategies are possible. For ease

of implementation a modiﬁed version of the classic

ZMODEM(Forsberg, 1988) protocol is used to trans-

mit binary data between the two transceivers.

5 EVALUATION

The bottleneck of this VLC device is the response

time of the TSL12S-LF light sensor which has a re-

sponse time of approximately 10µs. This equates

to absolute maximum bit rate of approximately 6.25

kilobytes per second. Each data frame has an eight

byte overhead, which translates to a theoretical maxi-

mum of approximately 5.8 kilobytes per second. This

level of throughput is capable passing controller in-

formation akin to a wireless joystick and other low

bandwidth sensors.

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Figure 4: Aquatic Test. Both the transmitting and receiving

nodes during an aquatic range test are shown. The emitter

appears as a constant (but dim) light. Here the receiver is

not transmitting.

The experiments that follow are all conducted in a

similar manner, each consisting of a one-way commu-

nication between transmitter node and a receiver node

as shown in Fig. 4. The transmitter is conﬁgured to

send out a thousand numbered “test string” messages.

The receiver is connected to a laptop where incoming

packets are recorded and then analyzed to recover the

packet drop percentage.

5.1 Range Test

Unlike most Li-Fi systems which operate in con-

trolled environments where ambient light is a minimal

competitor, the Li-Fi modems operate in direct com-

petition with light from the sun and other intensive

artiﬁcial sources and ambient light in the surround-

ing environment. Competition becomes a problem

when the attenuation of light through the environmen-

tal medium (water or air) causes the brightness of the

transmitter to be indistinguishable from the ambient

light. In an environment with no ambient light the

maximum range of transmission is based on the at-

tenuation of light through the environmental medium

(water or air) and the light sensitivity of the receiver.

The range test experiment evaluates the opera-

tional range of the device under normal and optimal

ambient conditions in both terrestrial and aquatic en-

vironments. Both devices are setup in the same ver-

tical plane with a single opposing transmitter and re-

ceiver manually aligned at 10cm, 50cm, 1m, and 5m

apart. Results are given in Fig. 5 and Table 3. Rates

are for 1000 packets. Note that performance in air un-

der strong ambient illumination shows very high drop

off rates at 5m but that performance in the water in the

2-3m range is quite good even under strong illumina-

tion.

Figure 5: Range packet drop-off rates. The terrestrial dark

signal is occluded by the aquatic dark signal in the above

graph. Each test involved transmitting 1000 packets from

the sender to the receiver. Numerical values are given in

Table 4.

Table 3: Packet/Frame drop percentages in different envi-

ronments and lighting conditions at various distances. Rates

were estimated over 1000 packets.

Terrestrial Aquatic

Offset Light Dark Light Dark

10 cm 0.01% 0.01% 0.25% 0.00%

50 cm 0.00% 0.00% 0.00% 0.00%

1 m 0.07% 0.00% 3.70% 0.00%

5 m 91.3% 32.7% 29.40% 32.65%

5.2 Radial Offset Test

Each LightByte modem consists of eight separate

units arranged radially around the modem. A set of

three vertically aligned transmitters and one receiver

is positioned on each face of the octagonal layout.

Transmitter LEDs have a viewing angle of 15

◦

with a

sharp falloff in brightness after 22.5

◦

. Receivers have

a nominal viewing angle of ±30

◦

. These speciﬁca-

tions suggest that at certain radial misalignments of

the transmitter and receiver the communication sig-

nal will be attenuated due to the optics of the receiver

and emission properties of the transmitting LED. Due

to the octagonal layout of the sensor, realignment

should occur every 45

◦

. However, since the trans-

mitter/receiver pair order is inverted every other emit-

ter/transmitter pair, realignment with a similar trans-

mitter/receiver pair conﬁguration occurs every 90

◦

This experiment evaluates packet/frame drop per-

centage as a function of the orientation alignment of

the transmitter and receiver. Both devices are set up in

the same vertical plane with a single opposing trans-

mitter and receiver manually aligned 1m apart. One

sensor is then rotated from 0

◦

to 90

◦

in increments of

◦

. Results are shown in Fig. 6. As the receiver is ro-

LightByte: Communicating Wirelessly with an Underwater Robot using Light

303

Figure 6: Radial offset packet drop-off rates. Terrestrial

light test (red), terrestrial dark test (fuchsia), aquatic light

test (cyan), and aquatic dark test (blue). Data was captured

from 0

◦

(horizontal) to 90

◦

(horizontal) offsets of 5

◦

incre-

ments.

Figure 7: Vertical offset packet drop-off rates.

tated relative to the transmitter in the light throughput

drops at the angles at which the sensor-receivers are

misaligned. Performance in the dark is excellent at all

orientations.

5.3 Planar Offset Test

Each set of three LED transmitters in the Li-Fi mo-

dem are vertically aligned along the eight walls of the

3d mount. The top and bottom LEDs are tilted 10

◦

in opposing directions. This provides a 25

◦

vertical

viewing angle with a sharp falloff in brightness at ±

◦

This experiment evaluates packet/frame drop per-

centage as a function of the vertical misalignment of

Table 4: Packet drop percentages under different environ-

ments and lighting conditions at different vertical offset dis-

tances at a range of one meter.

Terrestrial Aquatic

Offset Light Dark Light Dark

0 cm 0.01% 0.01% 0.25% 0.00%

10 cm 56.56% 0.01% 62.8% 0.00%

20 cm 69.78% 0.01% 62.66% 0.00%

30 cm 71.11% 0.50% 65.90% 0.00%

40 cm 88.23% 34.80% 81.5% 33.90%

50 cm 100.0% 93.10% 100.0% 89.00%

the transmitter and receiver. The devices were set

up 1m apart with different vertical offsets of 0cm to

50cm in increments of 10cm. Results are shown in

Fig. 7 and Table 4. Performance in the light falls off

quite quickly with vertical misalignment while perfor-

mance in the dark is good up to approximately 30cm

at 1m.

6 DISCUSSION

Li-Fi-based technology can be an effective mecha-

nism for underwater communication between a robot

and a human. For reasonably short (<5m) distances

and well aligned emitter-receiver pairs (±16

◦

) good

performance is obtained, even under illuminated con-

ditions. However, as revealed by the experimen-

tal evaluation outlined above, The current LightByte

hardware is not an ideal solution to the underwater

wireless communication problem. As currently con-

structed vertical misalignment along the axis of the

device can lead to substantive signal loss. That being

said these experiments do outline certain application

for which LightByte is well suited and other areas that

can be improved upon in future versions of the hard-

ware. For example, it would be straightforward to ex-

tend the vertical range of the device so that accurate

off-axis alignment is not required.

The performance gap between high and low am-

bient light conditions is not surprising. Performance

underwater in the light is acceptable, but performance

in lower levels of ambient illumination is very good.

As ambient illumination declines with depth this sug-

gests that transmission reliability will increase along-

side operational depth. Performance is also very good

radially save for a few problem zones. Again this is

likely to be easily addressed by increasing the density

of light detectors around the device.

Even with the current prototype implementation

transmission rates allow for the ability to transmit

joystick-like commands from a command/control de-

vice to a nearby UUV (<5m). Excess bandwidth can

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304

be used to transmit other relevant sensor data to/from

the robot. Considering the range/alignment limita-

tions identiﬁed in the evaluation above this would re-

quire the operator and robot to remain in the same

plane.

As far the writers are aware this is the ﬁrst LIFI ca-

pable system designed for real world applications in

the underwater domain. As such it is a prototype that

demonstrates the technologies readiness to compete.

Already its has demonstrated bandwidth comparable

to that of commercially available acoustic modems.

The range of this device is a severe limitation in com-

parison to acoustic modems. This limitation is ex-

plored below.

7 FUTURE WORK

The current version of both the hardware and com-

munication protocol have a number of known limi-

tations. The Cree C503D0-WAN LEDs have a rela-

tively high luminous intensity in comparison to their

power consumption. Using LEDs that output more

lumens while sacriﬁcing power would allow for com-

munication at a much longer distance and would also

preform much more desirably in well lit environ-

ments.

The Manchester coding scheme is very inefﬁ-

cient, requiring twice as much bandwidth in its en-

coded form than the raw data. IBM’s 8b/10b cod-

ing scheme(Franaszek and Widmer, 1984) is another

DC-balanced line code that has the beneﬁt of provid-

ing additional control symbols (no decoding) that can

be used to construct a frame. 8b/10b is much more

complex that Manchester but only uses 25% overhead

bandwidth. Switching from Manchester to 8b/10b en-

coding would increase overall bandwidth by 60%.

An obviously extension to this work would be

construct an VLC device that has better out of plane

performance and with more uniform radial sensitiv-

ity. This would allow any two devices to communi-

cate with each other regardless of their relative ori-

entations, provided their bodies do not occlude the

light emitted from the device. We are in the process

of mounting the LightByte sensor on Milton with the

goal of driving the UUV underwater wirelessly from

a diver in close proximity.

ACKNOWLEDGEMENTS

This work was supported by the Natural Sciences and

Engineering Research Council (NSERC) through the

NSERC Canadian Field Robotics Network (NCFRN).

This work was also supported by the Natural Sci-

ences and Engineering Research Council (NSERC)

through the York University Vision Science to Appli-

cation (VISTA) program.

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