This document discusses using semi-transparent solar cells as light-fidelity (LiFi) receivers. Two types of solar cells were tested - a copper indium gallium selenide (CIGS) module and an amorphous silicon (a-Si) module. Testing showed the CIGS module performed better for weak LiFi signals, while the a-Si module could receive high LiFi signals up to 270,000 lux. Additionally, the a-Si module's signal-to-noise ratio (SNR) and data rate increased with ambient lighting levels, an unexpected result that makes it suitable for combined indoor and outdoor LiFi applications.

1. NamYong Kim
UCS Lab
Email: nykim@seoultech.ac.kr
SeoulTech
Specific innovative semi-transparent solar
cell for
indoor and outdoor LiFi Applications

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1. Introduction
2. LIFI Experimental
3. LIFI Only
4. LIFI and Ambient lighting
5. Conclusion
6. In my opinion

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• light intensity emitted from LEDs can be modulated at
high speed, permitting optical data-transmission in
addition to classical illumination.
• An optical sensor such as a photodiode is used to
receive the data.
• LiFi has some specific advantages
– LiFi does not create electromagnetic interferences and so
secures some sensitive environments like hospitals, nuclear
powerplant, airplanes’ instruments without adding too many
cables.
– The visible bandwidth is totally free.
– visible wavelength does not harm the human skin and does not
cause any health issues
– Optical communication provides higher level of security than RF
communication because the signal does not pass through walls.
1. Introduction

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• good receiver should have the following characteristics
– should be thin, omnidirectional, energy sufficient, integrated on
all kind of surfaces.
– should have wide LiFi bandwidth, large surface, high sensitivity.
– should not deteriorate LiFi signals when it works in ambient
lighting environments.
– should generate a sufficient signal power that could directly be
used by LiFi modems without adding amplifiers.
• So, Photovoltaic module could appear as a good
candidate for a relevant solution
1. Introduction

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• we study the electro-optical
responses of two commercial
photovoltaic modules: the CIGS
(Copper Inidium Gallium Selenide)
DisaSolar and the WysipsrConnect
Crystal receiver from SunPartner
Technologies.
• In this paper, we compare LiFi
characteristics of these two
modules at different lighting
configurations.
1. Introduction

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• a-Si: solar cell that injects amorphous silicon between
substrates
– Advantages : Technical stability, easy process implementation,
various substrates available
– Disadvantages : Low conversion efficiency
• CIGS: Solar cell using Copper Indium Gallium Selenide
Compound
– Advantages : High conversion efficiency
– Disadvantages : Manufacturing process complexity
1. Introduction

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1. OFDM(Orthogonal Frequency Division Multiplexing)
– we choose to use an experimental setup which connects
software-based DCO (Direct Current Optical)-OFDM emitter and
receiver to hardware optical front-ends.
– This discrete OFDM signal is sent to an Arbitrary Function
Generator(AFG) which converts it into an analog signal with a
14-bit DAC resolution.
– Then, this OFDM signal goes through an amplifier to reach a
modulation depth between 30% and 50% of the current.
– A Bias-Tee adds this analog OFDM signal to constant current
coming from a power supply(CC power supply). The resulting
AC+DC unipolar signal is injected into the LED.
2. LIFI Experimental

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2. LIFI reception
– Their DC component is removed by the software-based DCO-
OFDM digital processing at reception. Receiver modules are in
front of the LED lamp and aligned to it.
– The output of the photodetector is then sampled by a digital
oscilloscope triggered by the AFG to simulate perfect
synchronization.
2. LIFI Experimental

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– the SNR on each sub-carrier is estimated. It can be noticed that
tested photovoltaic modules are directly connected to the
oscilloscope with no need of analog circuitry such as trans-
impedance amplifiers which are always used with photodiode
(PD) or avalanche photodiode (APD).
3. Data rate estimation
– The data rate is estimated based on the SNR and the spectral
efficiency.
– It is considered that a raw Bit-Error-Rate (BER) lower than 10-4
without channel coding could be reached if the SNR is larger
than 10dB.
– For such condition, a spectral efficiency of 2bit/s/Hz can be
fairly achieved by means of advanced digital signal processing
and this figure is considered to get an estimation of the
available data rate.
2. LIFI Experimental

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• Before analyzing the ambient lighting effects on the LiFi
signal, we need to analyze LiFi saturation effects on
photovoltaic modules.
1. Lighting level
– In these experiments, we consider that ambient DC lighting
under 80 Lux could be neglected.
– Channel attenuation relies on electrical signal level. When
channel attenuation increases, electrical signal level decreases.
3. LIFI ONLY

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– For each of the measurements at constant ambient lighting, VPP
and VDC decrease when channel attenuation increases.
– Lux ↑, VPP ↑, VDC ↑
3. LIFI ONLY

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2. LiFi only performances
– CIGS Characteristics
• LiFi signal is higher than 140 000 Lux LED: when the LiFi signal increases, the channel
attenuation increases and the electrical signal VPP decreases. Nevertheless, the SNR
saturation effects begin when the LiFi signal is higher than 4200 Lux.
• Increasing the LiFi signal higher than 4 200 Lux LED does not improve the SNR level, thus
does not improve the data rate.
3. LIFI ONLY

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ￚ Specific semi-transparent a-Si characteristics
• On the contrary, the a-Si module does not suffer form LiFi signal saturation effects until
230 000 Lux. the a-Si SNR threshold is about 230 000 Lux.
– In these experiments:
• the CIGS module appears as the best candidate for weak LiFi signal level.
• the a-Si semi-transparent module is the best candidate for high LiFi signal level.
3. LIFI ONLY

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ￚ the experiment consists in adding a huge ambient light (in
Lux DC) coming from an external source to observe the
saturation due to high levels.
1. CIGS saturation effects
For the CIGS experiment, useful light signal coming from DCOOFDM LED
lamp is maintained around 330 Lux. The distance between the LED lamp
and the CIGS module is about 2m.
– Without adding ambient light, at 80 Lux DC, the bandwidth is 4.2 MHz.
• At 10 000 Lux DC, the bandwidth is 3.7 MHz.
• At 80 000 Lux DC, the bandwidth is 2.5 MHz.
• At 200 000 Lux DC, the bandwidth is 0.6 MHz.
4. LIFI AND AMBIANT LIGHTING

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ￚ there is a classical ambient lighting saturation effect for the channel
attenuation and the SNR: for fixed LiFi signal, when the ambient lighting
level increases, the channel attenuation increases and the SNR decreases.
ￚ These results appear as classical saturation effects of optical LiFi receiver :
when the ambient lighting level is too high, the data rate decreases
drastically and the communication stops.
4. LIFI AND AMBIANT LIGHTING

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2. Specific semi-transparent a-Si SNR
– SNR improvements
• The specific innovative semi-transparent a-Si solar cell presents a classical channel
attenuation : when the ambient lighting level increases, the channel attenuation
increases and the electrical signal level decreases.
• Nevertheless, there is an unexpected result about this a-Si module: when the
ambient lighting level increases, the SNR and so the data rate increases.
4. LIFI AND AMBIANT LIGHTING

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• In this case, the ambiant lighting allows the improvement of photo-electric effects
in reducing noise.
• We tested six other semi-transparent a-Si solar cells developped by SunPartner
with different parameters, we always obtained the same results: adding ambiant
lighting increases the SNR level.
– Experimental on mobile phone
• In the experiment, we used a mobile phone (Alcatel one Touch) equiped with the
WysipsrConnect Crystal Screen module (based on the technology described above)
and a geolocalisation software.
• A LiFi Tag is sent to the mobile phone through a LED source. This tag is composed
by a short Manchester sequence.
• The mobile phone triggers geolocalisation contents until 25 cm (around 23 000 Lux
LiFi).
• We increase the distance of geolocalisation up to 57 cm. The LiFi signal is about
5300 Lux. This implies that this kind of specific modules could be used in very low
LiFi signal level if it is accompanied by a DC lighting source.
4. LIFI AND AMBIANT LIGHTING

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– we analyzed the LiFi responses of two different kinds of photovoltaic
modules: a CIGS module of DisaSolar and an a-Si module of
SunPartner Technologies.
– These two kinds of photo receivers could achieve a data rate of
around 8 Mbit/s with appropriate digital processing at the receiver
part.
A. Classical lighting saturation effects:
• When LiFi signal level is too high, channel attenuation and SNR have deteriorated
LiFi performances. In this study, the CIGS module appears as the best candidate for
weak LiFi signal level and the a-Si semi-transparent module as the best candidate
for high LiFi level (until 270 000 Lux).
• When the ambient lighting level is too high, CIGS SNR level and CIGS data rate
drastically decrease.
B. Specific ambient lighting effects:
• Adding ambient lighting level increases the a-Si semitransparent SNR level.
• To the best of our knowledge, WysipsrConnect Crystal receiver is the only one to
have this specific behavior: SNR and data rate are always better in the presence of
ambient lighting.
5. Conclusion

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• Specific semi-transparent a-Si SNR allows for faster speeds.
• Using Specific semi-transparent a-Si SNR to combine Cloud
and SDN / NFV, We can have both the advantages of Leafy
and the advantages of cloud-based.
• We need to continue researching the efficiency of using the
geolocalization software phone (Alcatel one Touch).
• When using LIFI, it would be nice to include energous wattup
technology to solve battery problems such as computer or
mobile.
6. In my opinion

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• EMILIE BIALIC, LUC MARET, DIMITRI KTÉNAS, Specific
innovative semi-transparent solar cell for indoor and
outdoor LiFi Applications, Applied Optics, 2015
참고문헌

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Q & A