Implementation of a Visible Light Communication Link: Li-Fi with

Smartphone Detection

David Andrade

, João Pedro Gomes

and Paulo S. André

Department of Electrical and Computer Engineering and Instituto de Telecomunicações, Instituto Superior Técnico,

University of Lisbon, 1049-001 Lisbon, Portugal

Department of Electrical and Computer Engineering and ISR, Instituto Superior Técnico, University of Lisbon,

1049-001 Lisbon, Portugal

Keywords: Optical Communications, Visible Light Communications, Light Emitting Diodes, Colour-Shift Keying, Li-

Fi.

Abstract: The average bandwidth per user increases daily, being necessary to present solutions that can satisfy this

growth. One possible solution is to explore the light emitting diodes (LED) features. Visible Light

Communications (VLC) is an emerging technology that presents several advantages comparing to other

alternatives that currently exist on the market. This solution can be use simultaneously for illumination and

data transmission, being an economical alternative to wi-fi. In this work we proposed the use of a mobile

phone camera combined with an application to attain a bitrate up to 1 kbit/s with BER lower that 10

-3

. The

concept of colour shift multiplexing/modulation was explored showing the capacity to successfully increase

the aggregate bitrate in a 3 time fold.

1 INTRODUCTION

In the last decade, the world has seen a huge growth

in the data traffic transmitted by the

telecommunication networks. The increase of devices

accessing mobile networks and the high demand for

internet services that require high transmission rates

(social networks, video calls, cloud-based services,

mobile applications, etc.) has increase the research

and development of new technologies with high

transmission rates (Ghassemlooy, 2017). According

to CISCO, in 2021 the mobile traffic will be around

seven times higher, when compared to 2016 (Cisco,

2015).

Radiofrequency based communication systems

suffer from multipath propagation effects in dense

urban environments. This situation can reduce the

performance and number of available connections.

The limited bandwidth of these systems combined

with spectrum congestion implies only a few high

definition channels can be use. Therefore, to increase

the system capacity and have a greater bandwidth a

new spectral region must be utilized or the spectral

efficiency must be improved. However, these two

solutions imposes very high costs and higher

complexity in the design and management of the

emitters and the receivers (Ghassemlooy, 2017).

.Depending on the frequency used, radio-based

communications may also result in safety problems,

in human health effects and can cause interference in

different systems, as aircrafts communication and

navigation system (Khan, 2017).

One possible solution is based in the use of the

visible band in the 380 nm and 750 nm spectral range,

named as Visible Light Communications (VLC).

Since this communication type has an extremely high

frequency can provide a high degree of spatial

confinement, making reuse of frequency almost

unlimited and providing high modulation bandwidth.

This characteristic reduces licensing costs and

increases security in data transmission

(Ghassemlooy, 2017).

The optical signal can be generated by using light-

emitting diodes or laser diodes (Ball, 2012), allowing

simultaneously the illumination and data

transmission. This application is usually called as Li-

fi, that is a sustainable and ecological technology of

VLC, allowing the transmission of information by

modulating the light. In the future, it is expected that

Li-Fi become complement to the Wi-Fi.

Andrade, D., Gomes, J. and André, P.

Implementation of a Visible Light Communication Link: Li-Fi with Smartphone Detection.

DOI: 10.5220/0007691303150319

In Proceedings of the 7th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2019), pages 315-319

ISBN: 978-989-758-364-3

315

2 STATE OF THE ART

The state-of-the-art in Visible Light Communications,

can be divided into two main categories: indoor and

outdoor communications. The studies related with Li-

Fi, the current experiences are mainly base on the

indoor environments.

In (Haas, 2015) is presented an alternative to Wi-

Fi, based on the Color Shift Keying (CSK), that has

the advantage of ensuring a constant illumination

flow. In the emitter, they used a chip developed for

ultra-parallel visible light communication design and

the receiver was based on an avalanche photodiode.

By combining the LED light with wireless data

networks, it was possible to achieve a considerable

reduction in the size of the cells and consequently an

increase in the transmission rate, in the number of

users served and in the total traffic. Thus, the authors

showed that is possible to achieve transmission rates

in the order of 1 Gb/s. In this study, a comparison is

made between Wi-Fi and Li-Fi, concluding that the

performance is higher when both techniques are used

simultaneously, in a balanced way.

In 2017 PureLiFi (PureLiFi, 2018) launched the

Li-Fi-XC, a device that allows wireless

communications at very high transmission rates, in a

safely way using LEDs. The Li-Fi-XC is a certified

USB plug and play device. Because of its small size,

it can be integrated into computers, tablets or smart

devices. Allows transmission up to 43 Mbps from

each LED, enabling two-way communication in Full

Duplex mode. This system also allows the user to walk

between different LEDs, maintaining the connection.

In 2018 Philips launched two models of LED

luminaires ready to illuminate and transmit

information simultaneously, the LuxSpace PoE

(Philips, 2018) and the PowerBalance gen2 (Philips,

2018b). Both have a Power-over-Ethernet (PoE)

technology that allows to transmit electric energy and

data through a single standard Ethernet cable. These

devices allow a transmission rate up to 30 Mb/s in a

connection that can be bidirectional. Depending on the

chosen model, for an input power varying from 9.2 W

and 16.2 W, a luminous flux of 1200 lm and 2200 lm

can be obtained, allowing a reduction up to 80% in the

electric consumption (Lux, 2018).

MyLifi was introduced in 2018 and is another

example of LED lighting prepared for Li-Fi use

(Oledcomm, 2018), release by Oledcomm, it can

reach transmission rates up to 23 Mbps in download

and 10 Mbps upload, being used simultaneously for

illumination. This device is also considered more

efficient, since the lamp with 800 lumens requires 13.5

W, less than the 20 W of conventional Wi-Fi router

(Takahashi, 2018). In a Li-Fi system, it is necessary

for the receiver to capture the emitted light. In this way

Oledcomm also as a USB device that allows any

device with this interface to stablish the connection

(Oledcomm, 2018).

3 SYSTEM DESIGN

The proposed VLC system is composed by a

transmitter based in white emitting LED + RGB

LEDs, with an individual electrical power

consumption of  1W (Cree XLamp MC-E Colour (Cree,

2018)), a smartphone camera is used as receiver, as

illustrated in Figure 1. A DAC board, with a sampling

rate of 100 kHZ, (Adalm100) and a LED driver

(model T-Cube LEDD18 ThorLabs) allows to drive

the LED current accordingly with the digital signal

synthetized in Matlab. The capture in the mobile is

done using an application (Luximeter) that measure

the camera light intensity with a maximum

acquisition rate of 10 samples per second.

Figure 1: Circuit diagram for the implemented Li-Fi

system.

The considered modulation used was On - Off Keying

and the detection was made with a minimum distance

rule, in this situation considering an equal bit

probability, the decision level is in the middle

distance between the ‘1’ and ‘0’ intensity levels.

This solution allows to use the smartphone as a Li-

Fi receiver, without requiring additional hardware.

On the other hand, the camera used has some

limitations, like a typical frame rate of 60 fps, making

the transmission rate lower. As the Li-Fi system can

be used for illumination and data transmission. Is

essential that intensity variations ascribed to the

information cannot be perceptible by the human eye,

therefore, the extinction ratio must be reduced.

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

316

Using the white LED the detection is made with

the absolute intensity level. By exploring the RGB

orthogonal colour space is possible to implement a

colour multiplexing system. In this case the detection

if made by the intensity on those 3 axes of the colour

space.

However, by analysing independently the signals

ascribed with blue, green and red colours, the

presence of cross interference effects are observed.

This is due to the mismatch between the colour

primaries and the LED emission wavelengths. The

RGB components can be described by:





 







 







 







(1)





 







 







 







(2)





 







 







 







(3)

Using those equations it is possible to construct a

matrix model (matrix S) that allows to correct the

cross-interference effects. In these equations, b

, r

and g

are the percentage of blue, red and green colour

present in each of the individually studied LEDs. The

parameters 



, 



and 



are the intensity measured in

each channel.

To obtain the parameters of matrix S, assigning to

each LED the same intensity value and measuring the

intensity for the three colours is possible obtain the

matrix coefficients, show in table 1.

Table 1: Normalized coefficients for the S matrix.

LED

Blue

0.98

0.01

Red

0.09

0.82

0.09

Green

0.14

0.12

0.74

It must be referred that these values are only valid for

a specific combination of emitter-receiver.

4 EXPERIMENTAL RESULTS

Using the application Luximetro (developed by

Crunchy ByteBox, available for Android devices on

Google Play) and the front camera was possible to

implement a direct detection receiver. A sample

function of the signal transmitted is presented in

Figure 2 top, corresponding to s test 9-bit NRZ

sequence 100100101. Each bit has a duration T

= 2s

with 10

samples per second and an extinction ratio

of 1.1. Due to the limitation imposed by the

smartphone camera, the capture is limited to 100

samples per second. The samples received over the

time, for a transmission distance of 22 cm, are

presented in Figure 2 bottom. This value is in the

range of the typical distance between a desk lamp and

the table.

To analyse in detail the implemented solution and

overcome the limitations imposed by the smartphone

camera, was used a photodiode (ThorLabs PbS

PDA30G) as reference receiver. The optical power

was measure with an optical power meter (IF-PM200)

with a detection area of 1 cm

Figure 2: Sample function of the signal. Top) transmitted;

Bottom) captured samples using the camera.

The bit error rate evolution as function of the

transmission rate is shown in Figure 3. For

transmission rates up to 1200 bit/s it is possible to

obtain a BER of less than <10

-3

, that corresponds to

the FECs limit.

The BER evolution with the received optical power,

for a transmission rate of 2 kbit/s is displayed in figure

Figure 3: BER evolution as function of the transmission

rate.

Implementation of a Visible Light Communication Link: Li-Fi with Smartphone Detection

317

Figure 4: BER as function of the received optical power.

The minimum optical power required to operation

with BER lower than 10

-3

is 0.6 mW. To qualitatively

evaluate the quality of the signal received at 10 kbit/s

the eye diagram was obtained and illustrated in Figure

Figure 5: Eye diagram for a 10 kbit/s transmission rate,

obtained for a distance of 7 cm.

The result shows low noise and non-significant

distortion, verifying the quality of the received signal.

To increase the aggregate transmission rate, it was

considered a RGB emitter. The electrical current of

the three LEDs here adjusted to obtain a maximum

optical power of 7.2 mW, also the influence of the

external light sources in this system was considered.

In figure 6 is displayed the raw data for the

received signal, with high background intensity

levels, due to the presence of environmental light.

Figure 6: Signal received for the RGB system, considering

a natural environment background light.

Combining these values with the matrix S, it is

possible to retrieve the original signal, as show in

Figure 7.

Figure 7: transmitted signals reconstructed from the RGB

channel raw data and the matrix S.

5 CONCLUSIONS

VLC systems are an alternative to the technologies

that we currently use for transmission. Li-Fi is

considered an emerging technology that will help to

complement the existing systems such as Wi-Fi.

We demonstrate the possibility to transmit in

distance in the order of dozens of centimetres, with a

transmission rate up to 1200 b/s, with bit error rate

lower than 10

-3

, the received optical power needs to

be higher than 0.6 mW.

By using colour shift modulation /multiplexing is

possible to stablish a link with a mobile phone,

enabling a Li-Fi solution without extra hardware.

ACKNOWLEDGEMENTS

This work was developed within the scope of the

project Instituto de Telecomunicações (FCT Ref.

UID/EEA/50008/2013), financed by the FCT/MEC

and Portugal 2020 through European Regional

Development Fund.

REFERENCES

Ball, C., Tien, K., 2012, Design and Development of a

Visible Light Communications Link, Cooper Union

Adv. Sci. Art, pp. 1–8.

Cisco, 2015, The Zettabyte Era: Trends and Analysis.

Cisco, San José.

tempo

Amplitude

Amplitude (V)

time (s)

PHOTOPTICS 2019 - 7th International Conference on Photonics, Optics and Laser Technology

318

Cree, 2018, Cree® XLamp® MC-E LED, Online:

http://www.cree.com/led-

components/media/documents/XLampMCE.pdf.

Haas, H., Yin, L., Wang, Y., Chen, C., 2015, What is LiFi ?,

J. Light. Technol., vol. 34, no. 6, pp. 1533–1544.

Ghassemlooy, M., Alves, L., Zvánovec S., 2017, Visible

light communications Theory and Applications. CRC

Press - Taylor & Francis Group, Boca Raton.

Khan, L. U. Khan., 2017, Visible light communication:

Applications, architecture, standardization and

research challenges. Digit. Commun. Networks, vol. 3,

no. 2, pp. 78–88.

Lux, 2018, Philips: We’ll take Li-Fi mainstream. Online:

http://luxreview.com/article/2018/03/philips-we-ll-

take-li-fi-mainstream-.

Oledcomm, 2018, MyLifi. Online:

http://www.oledcomm.com/solution/mylifi.

Philips, 2018, LuxSpace. Online:

http://www.lighting.philips.pt/prof/luminarias-de-

interior/downlights/luxspace/luxspace-poe#.

Philips, 2018b, PowerBalance gen2. Online:

http://www.lighting.philips.com/main/prof/indoor-

luminaires/recessed/powerbalance-gen2#p-image-4.

PureLiFi, 2018, LiFi-XC. Online: https//purelifi.com/lifi-

products/.

Takahashi, D., 2018, MyLiFi is a smart lamp that beams

broadband to your laptop. Online:

https://venturebeat.com/2018/01/07/mylifi-is-a-smart-

lamp-that-beams-broadband-to-your-laptop/.

Implementation of a Visible Light Communication Link: Li-Fi with Smartphone Detection

319