Analysis and Investigation of Dual-Polarized Color LED Based Visible Light Communication System
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photonics
Article
Analysis and Investigation of Dual-Polarized Color LED Based
Visible Light Communication System
Yun-Cheng Yang 1 , Chien-Hung Yeh 1, *, Shien-Kuei Liaw 2 , Chi-Wai Chow 3 , Wei-Hung Hsu 1 and Bo-Yin Wang 1
1 Department of Photonics, Feng Chia University, Taichung 40724, Taiwan; zxc706090@gmail.com (Y.-C.Y.);
whhsu@fcu.edu.tw (W.-H.H.); paul19961103@icloud.com (B.-Y.W.)
2 Department of Electronics and Computer Engineering, National Taiwan University of Science and Technology,
Taipei 106335, Taiwan; skliaw@mail.ntust.edu.tw
3 Department of Photonics, College of Electrical and Computer Engineering, National Chiao Tung University,
Hsinchu 30010, Taiwan; cwchow@faculty.nctu.edu.tw
* Correspondence: yehch@fcu.edu.tw
Abstract: To increase the data capacity of a light-emitting diode (LED) based visible light commu-
nication (VLC) transmission, a polarization-division-multiplexing (PMD) green (G)- and blue (B)-
light-based transmitter (Tx) module is demonstrated here. It was demonstrated that we can achieve
1200 and 1120 Mbps VLC capacities based on dual-polarized G- and a B-LED based light wave after 3
and 4 m free-space link lengths, respectively, at exceedingly low illuminance. Based on the presented
VLC system, paired G-LEDs or B-LEDs with dual-polarization can also be applied on the VLC-Tx
side for doubling and delivering VLC data. According to the obtained results, the largest polarization
offset angle of 50◦ between two polarizers (POLs) can be allowed experimentally to provide optimal
VLC traffic. Moreover, the relationships of polarization offset, the illuminance of LED and maximum
achieved VLC capacity are also performed and analyzed.
Keywords: visible light communication (VLC); light-emitting diode (LED); polarization-division-
Citation: Yang, Y.-C.; Yeh, C.-H.;
Liaw, S.-K.; Chow, C.-W.; Hsu, W.-H.;
multiplexing (PMD); dual-polarized; energy efficiency
Wang, B.-Y. Analysis and
Investigation of Dual-Polarized Color
LED Based Visible Light
Communication System. Photonics 1. Introduction
2021, 8, 210. https://doi.org/ Recently, mobile communication has progressed rapidly for high-speed and broad-
10.3390/photonics8060210 band wireless data access. However, the usable wireless spectra were continuously limited
and busy. Furthermore, the visible wavelength spectrum is more than 10,000 times the ex-
Received: 1 May 2021
isting wireless spectrum for future signal access applications [1]. Hence, the light-emitting
Accepted: 9 June 2021
diode (LED) based visible light communication (VLC) system has been presented and
Published: 10 June 2021
demonstrated to provide an alternative choice for wide capacity wireless access [2,3].
The LED-based VLC transmission could also provide beneficial benefits, such as cost-
Publisher’s Note: MDPI stays neutral
effectiveness, a large unlicensed spectrum, energy efficiency, a long lifetime, no electromag-
with regard to jurisdictional claims in
netic interference (EMI), high modulation bandwidth, and signal security [4–6]. Essentially,
published maps and institutional affil-
the commercial component of phosphor-LED could only provide the achievable bandwidth
iations.
of 1 to 10 MHz for VLC link, which would limit the data capacity [7,8]. To enhance the
obtainable VLC capacity, the analog equalization design [8–10], parallel VLC architec-
ture [11], optical color filter [12], optical multi-input multi-output (MIMO) method [13,14],
wavelength-division-multiplexing (WDM) [15,16] and polarization-division-multiplexing
Copyright: © 2021 by the authors.
(PDM) technologies [17–20] were studied and discussed.
Licensee MDPI, Basel, Switzerland.
Utilizing 2 × 2 PDM LED-based VLC methods was proposed for doubling data
This article is an open access article
rates in recent years. In 2014, Wang et al. presented 1 Gbit/s orthogonal-frequency-
distributed under the terms and
division-multiplexing (OFDM) modulation rate based on a 2 × 2 red-green-blue (RGB)
conditions of the Creative Commons
Attribution (CC BY) license (https://
LED-based PDM system [18]. However, they needed the avalanche photodiode (APD) to
creativecommons.org/licenses/by/
detect VLC signals through a free space transmission length of 80 cm. In 2018, Hsu et al.
4.0/).
demonstrated a 2 × 2 phosphor LED-based 1.4 Gbit/s OFDM VLC system with a Volterra
Photonics 2021, 8, 210. https://doi.org/10.3390/photonics8060210 https://www.mdpi.com/journal/photonics
Photonics 2021, 8, 210 2 of 12
nonlinear equalization (VNE) method by using PIN PD for detection at the illuminance of
1000 Lux [19]. Here, the larger illuminance and complex VNE algorithm were required to
decode PDM and WDM VLC signal, and the wireless VLC length was not indicated. As
mentioned above, they required OFDM modulation format for a shorter VLC connection
length (or at the greater illuminance) to achieve >1 Gbit/s data rate. In 2017, Lu et al.
presented a hybrid PDM and RGB-WDM VLC system to achieve a 2.16 Gbit/s OFDM rate
through 3 m link length within 100 MHz available bandwidth [20]. However, the related
locations of the VLC transmitter (Tx) and receiver (Rx) were both fixed to deliver and
decode the VLC signal for indoor application.
In this demonstration, we propose and investigate a PDM-based G- and B-LED VLC
transmission system with an orthogonal-polarization state to double the data traffic. Here,
to obtain the dual-polarized VLC signals, the two orthogonal polarizers (POLs) at the
receiver (Rx) side can be controlled for rotation automatically to match the dual VLC
signals from the transmitter (Tx) site. The presented dual-polarized VLC system can
achieve the total modulation rate of 1200 and 1120 Mbps (VLC(Green+Blue) ) passing through
3 and 4 m free-space transmission lengths, respectively when the detected corresponding
illuminances of G- and B-LEDs are 2.4 and 0.5 lux and 5.4 and 2.3 lux. To make the
presented LED VLC system more flexible, we also employed paired G-LEDs and B-LEDs
acting as the optical transmitter (Tx) to produce dual-polarized VLC(Green) and VLC(Blue)
signals for alternative indoor applications, respectively. Hence, the proposed 2 × 2 PDM
G- and B-LED VLC system does not need any color filter for decoding signals. When the
VLC client is moving randomly, the two polarizers (POLs) of the Rx side are controlled to
match the dual-polarized VLC signals and avoid missing received signals. In addition, we
also perform and discuss the influence of different polarization offset at a fixed VLC rate
through a wireless link length of 3 and 4 m, respectively. Based on the observed results,
the largest offset angle of two POLs should be less than 50◦ to maintain the optimal VLC
modulation rate below the forward error correction (FEC) level. The presented LED VLC
system is for fully static scenarios in the investigation.
The optical wireless signal indoors might also bring a possibility of occupying by
unauthorized stealer. To reach the signal security in the VLC connection, applying the
encryption algorithm in the upper layer is one of the methods [21,22]. Due to the two mixed
VLCs with various modulation formats, it would also generate signal mutual interference
to cause a security procedure in the physics layer [23]. To decode the dual-polarized
signals, two orthogonal POLs with precise direction are required at the Rx side. Moreover,
if there are many clients with only one Rx detector in the same room for receiving the
dual-polarized VLC signal, the polarization of POL at each client can be controlled properly
to share the total capacity. As a result, compared with the previous work [17–19], the
presented 2 × 2 PDM G-B LED VLC system can not only achieve >1120 Mbit/s through
4 m wireless link at very low illuminance status but can also provide signal security in the
physical layer when the same color LEDs are used.
2. Proposed VLC Architecture and Experiment
Generally, to obtain a higher VLC data rate via the LED-based device, wavelength-
division-multiplexing (WDM) LEDs were proposed and studied [15,16]. Hence, the red
(R), green (G) and blue (B) LEDs could be used to serve as the optical transmitter (Tx)
for WDM VLC transmission at the VLC-Tx side, respectively. Here, the three various
modulation signals could be applied to each LED to deliver optical wireless traffic. After a
VLC transmission length, the blended WDM RGB signals could be received and decoded
by the corresponding optical receiver (Rx), when the RGB filters were in front of each Rx
to avoid channel interference at the VLC-Rx side, respectively. However, the optical color
filters were needed to choose and match the output spectrum of RGB-LED. Moreover, the
achievable VLC rate of R-, G- and B-LEDs depended on their useful modulation bandwidth.
In our demonstration, to achieve a higher modulation capacity of LED-based VLC
traffic systems, the G- and B-LEDs with orthogonal polarization are applied at the VLC-Tx,
Photonics 2021, 8, x FOR PEER REVIEW 3 of 12
Photonics 2021, 8, 210 In our demonstration, to achieve a higher modulation capacity of LED-based3 of VLC12
traffic systems, the G- and B-LEDs with orthogonal polarization are applied at the VLC-
Tx, as illustrated in Figure 1a. Here, the G- and B-LEDs are utilized simultaneously serv-
ing
as as the wireless
illustrated VLC
in Figure 1a.signal.
Here, To thedeliver
G- andtwo B-LEDsvarious VLC modulations
are utilized simultaneouslyon theserving
G- andas B-
LEDs, respectively, two bias-tees (BTs) are used to connect
the wireless VLC signal. To deliver two various VLC modulations on the G- and B-LEDs, re-the individual LED and VLC
modulation
spectively, two format,
bias-teeswhen
(BTs) theareproper
used tobias currents
connect (Ibias) are applied
the individual LED andsimultaneously.
VLC modulation In
addition, two optical polarizers (POLs, 400700 nm) are placed
format, when the proper bias currents (Ibias ) are applied simultaneously. In addition, two in front of the G- and B-
LEDs to
optical produce (POLs,
polarizers orthogonal
400−x- 700andnm)y-polarization
are placed instatus
front of andtheavoid
G- and theB-LEDs
interference of two
to produce
VLC signals, respectively, as shown in Figure 1a. Through
orthogonal x- and y-polarization status and avoid the interference of two VLC signals, a length of optical wireless
transmission,
respectively, astwo
shown wireless
in Figuregreen 1a.and blue VLC
Through signals
a length (VLC(Green)
of optical andtransmission,
wireless VLC(Blue)) with or-
two
thogonal polarization can be received by the two 400 MHz avalanche
wireless green and blue VLC signals (VLC(Green) and VLC(Blue) ) with orthogonal polariza- photodiodes (APDs)
at the
tion can VLC-Rx side, respectively.
be received by the two 400 To MHz
enhance two detected
avalanche VLC power
photodiodes (APDs)andatcorresponding
the VLC-Rx
VLCrespectively.
side, signals, the two focusingtwo
To enhance lenses and POLs
detected VLCare also exploited
power for VLC demodulation,
and corresponding VLC signals,
respectively.
the two focusing To lenses
improve andthe POLsdetected
are alsopower sensitivity
exploited for VLC of demodulation,
the VLC signal,respectively.
the APD is
applied
To improve forthesignal demodulation.
detected Commonly,
power sensitivity of thetheVLCindoor
signal,illuminance
the APD isisapplied
between for300 and
signal
750 lux, sometimes the illuminance will be lower. So, to detect
demodulation. Commonly, the indoor illuminance is between 300 and 750 lux, sometimes and decode the VLC signal
effectively
the illuminance under a longer
will be lower.wireless
So, tolength
detectatanda lower
decodeor extra-low
the VLC signalilluminance indoors,
effectively under the
aAPD-PD can be applied
longer wireless length at in aan LED or
lower VLC system illuminance
extra-low as an alternative. Although
indoors, APD iscan
the APD-PD expen-
be
sive, it is
applied inanother
an LED option
VLC systemin theas VLC system. As aAlthough
an alternative. result, the APD proposed G anditB-LED
is expensive, VLC
is another
transmission
option in the VLC system could
system. Asdouble
a result,thethetraffic
proposedrate G byand
utilizing
B-LEDdual-polarized
VLC transmission VLC chan-
system
nels. However,
could double thethe tworate
traffic POLs by at the VLC-Rx
utilizing must match
dual-polarized VLC thechannels.
x- and y-polarization
However, thedirec- two
tions at
POLs of the
the VLC-Rx
VLC(Green)must
and match
VLC(Blue) thesignals. Commonly, thedirections
x- and y-polarization offset of x- of and y-polarizations
the VLC (Green) and
VLC
of POL
(Blue) signals. Commonly, the offset of x- and y-polarizations
x and POLy would result in different insertion losses when the of POL and POL
x VLC-Rxy module would
result
movesinarbitrarily.
different insertion
In previouslossesstudies
when the VLC-Rx
[17–20], module
there was no moves arbitrarily.
mention of VLC In previous
problems
studies
caused [17–20], thereoffset
by different was no mention
angles. of VLC in
Therefore, problems causedVLC-Rx
the proposed by different offsettoangles.
module, detect
Therefore, in the proposed
the two orthogonal VLC-Rxfrom
VLC signals module, to detectcompletely,
the VLC-Tx the two orthogonal VLC signals
two rotatable from
dual-polar-
the
izedVLC-Tx
POLs can completely, two rotatable
be controlled dual-polarized
automatically based onPOLs can be controlled
the maximum received automatically
illuminance
based on the maximum
for decoding VLC data.received
Moreover, illuminance
to examine for the
decoding
bit errorVLC data.
rate Moreover,
(BER) performanceto examine
of the
the bit error rate (BER) performance of the proposed
proposed LED VLC system, a BER tester is applied for signal measurement. LED VLC system, a BER tester is
applied for signal measurement.
Figure 1. (a) Proposed dual-polarized G- and B-LED VLC transmission system. (b) Observed output
spectra of G- and B-LEDs, when the bias currents are both operated at 10 mA.
Photonics 2021, 8, 210 4 of 12
3. Result and Discussion
In the experiment, the bias currents of the G- and B-LEDs are both operated at 10 mA.
The measured output spectra of the G- and B-LEDs are also shown in Figure 1b. We
observed that the central wavelengths of G- and B-LEDs are 457 and 521 nm, respectively.
To reach a broader modulation bandwidth of LED, an analog front end (AFE) circuit with
pre-equalization was also designed to carry various on-off keying (OOK) modulation
signals for delivering VLC traffic at the VLC-Tx [6,8]. In addition, to enhance the detected
VLC illuminance, two focusing lenses with 5 cm diameter were also in front of the two
APDs, as exhibited in Figure 1a. Due to the smaller Ibias on each LED, the demonstrated
LED VLC system also can achieve better energy efficiency.
In the demonstration, Figure 2a is the effective bandwidth measured by G-LED and
B-LED itself by using the 400 MHz bandwidth APD, respectively. Here, the corresponding
detected bandwidths of G- and B-LEDs are nearly 415 and 380 MHz within a 10 dB
bandwidth range, respectively. This is due to the limitation of the original 400 MHz APD.
According to the achieved results of Figure 2a, the modulation rate of the G- and B-LEDs
should be able to reach 400 Mbit/s initially. The valuable bandwidth of the G-LED is better
than that of the B-LED in the observation. Furthermore, the original illuminances of the
G- and B-LEDs are also detected under various VLC transmission lengths before entering
the POL or color filter, respectively, as schemed in Figure 2b. The detected illuminances
of the G- and B-LEDs are between 7 and 136.1 lux and 39.1 and 255.1 lux, respectively,
under the free space connection lengths of 1 to 4 m. First, we executed the bit error
rate (BER) performances of WDM G- and B-LEDs through 3 and 4 m VLC connections,
respectively, when the POLs are not used and the corresponding color filters are applied
in the measurement. Here, the greatest OOK modulation rates of G- and B-LEDs was
670 and 580 Mbit/s and 600 and 560 Mbit/s at the forward error correction (FEC) limit
(BER ≤ 3.8 × 10−3 ) through 3 and 4 m VLC link lengths, respectively, when the pre- and
post-equalizations are applied on the Tx- and Rx-VLC sides, as shown in Figure 3a,b. We
only used 400 MHz APD with an equalization circuit in the presented VLC system to
decode 670 Mbit/s OOK signal. These have two benefits, the first one can reduce costs,
and the other one can increase the decoding bandwidth in the demonstration.
Second, we performed the proposed dual-polarized VLC system through different
free-space transmission lengths. Hence, the corresponding illuminances of the G- and
B-LEDs after passing through two POLs with parallel polarization before launching into the
focusing lens were measured first. Figure 4 displays the detected illuminance of the G- and
B-LEDs after a free-space link length of 1 to 4 m, respectively. The achievable illuminances of
the G- and B-LEDs are obtained between 0.5 and 22.7 lux and 2.3 and 31.9 lux, respectively,
as seen in Figure 4. Here, to establish the long indoor VLC transmission length, we selected
3 and 4 m links for the VLC(Green) and VLC(Blue) connections, respectively. Figure 5a,b
exhibit the obtained bit error rate (BER) performances of the VLC(Green) and VLC(Blue)
traffics versus various OOK modulation rates after an optical wireless transmission length
of 3 and 4 m, respectively. In the measurement, the largest VLC(Green) data rates of 640
and 580 Mbps were achieved below the forward error correction (FEC) threshold (BER of
3.8 × 10−3 ), respectively, through a wireless link of 3 and 4 m, when the corresponding
illuminances were 2.4 and 0.5 lux (before entering the lens), as shown in Figure 5a. The
insets of (i) and (ii) of Figure 5a are the corresponding eye diagrams at the 640 and 580 Mbps,
respectively. Identically, the OOK VLC(Blue) traffics could also reach 560 and 540 Mbps
after 3 and 4 m free-space transmissions to satisfy the FEC target under the corresponding
illuminances of 5.4 and 2.3 lux, respectively, as illustrated Figure 5b. The insets of (iii) and
(iv) of Figure 5b are the measured eye diagrams at the 560 and 540 Mbps VLC(Blue) rates,
respectively. As displayed in Figure 5a,b, the modulation rate of the G-LED seems better
than that of B-LED due to the characteristics of the component itself. According to the
presented dual-polarized G- and B-LED VLC system, the largest VLC traffics of 1200 Gbps
(640(Green) + 560(Blue) Mbps) and 1120 Mbps (580(Green) + 540(Blue) Mbps) were obtainedPhotonics 2021, 8, x FOR PEER REVIEW 5 of 12
Photonics 2021, 8, 210 5 of 12
seems better than that of B-LED due to the characteristics of the component itself. Accord-
ing to the presented dual-polarized G- and B-LED VLC system, the largest VLC traffics of
1200 Gbps (640(Green) + 560(Blue) Mbps) and 1120 Mbps (580(Green) + 540(Blue) Mbps) were ob-
tained under 3 and 4 m free-space link lengths, respectively, when the equalization
under 3 and 4 m free-space link lengths, respectively, when the equalization method was
method was applied on the AFE side under the very low illuminance for decoding data.
applied on the AFE side under the very low illuminance for decoding data.
5
Normalized Response [dB]
(a)
-5
-15
-25
B-LED
-35 G-LED
-45
0 100 200 300 400 500 600
Frequency [MHz]
300
(b)
250
Illuminance [lux]
200 G-LED
B-LED
150
100
50
without POL
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
VLC Length [m]
Figure2.2.(a)
Figure (a)Measured
Measured achievable
achievable bandwidths
bandwidths of
ofthe
theG
G−and
andBLEDs.
B−LEDs.(b)(b)
Detected corresponding
Detected corresponding
illuminances of the G and BLEDs without using POL versus various VLC lengths before launch-
illuminances of the G− and B−LEDs without using POL versus various VLC lengths before launching
ing into the focusing lens.
into the focusing lens.
In the demonstration, we could not only apply two various color LEDs in the presented
dual-polarized VLC system but also we could utilize two same color LEDs. We could also
utilize a pair of polarization-multiplexing G-LEDs (or B-LEDs) at the VLC-Tx for delivering
wireless VLC traffics. Therefore, the maximum dual-polarized G-LED VLC(Green) and
B-LED VLC(Blue) rates of 2 × 640 and 2 × 580 Mbps; and 2 × 560 and 2 × 540 Mbps were
achieved after 3 and 4 m optical wireless connections, respectively. This means that we
can arbitrarily use the combination of two LEDs for dual-polarized VLC transmission.
Moreover, the observed BER measurements of dual-polarized VLC(Green) and VLC(Blue)
under different modulated rates were similar to Figure 5a,b, therefore we do not exhibit
the BER measurements here. As mentioned above, this means that we can arbitrarily use
the combination of two LEDs for dual-polarized VLC transmission.Photonics 2021, 8, 210 6 of 12
According to the results of Figures 4 and 5, the achieved largest VLC rate depends on
the detected illuminance. The obtained VLC power would affect the signal performance.
As the VLC transmission length increases, the available signal capacity will be relatively
smaller, due to the weaker corresponding detected power. Therefore, a >1 Gbit/s OOK rate
through a 4 m wireless connection can be achieved in the proposed dual-polarized LED
VLC system.
However, while the polarization of two orthogonal POLs is offset at the VLC-Rx, the
obtained VLC signal would be influenced and interfered with due to two mixed VLC
traffics. To avoid this problem, the two POLs can be controlled for rotation automatically
based on the maximum detected illuminance. In the investigation, we demonstrate and
discuss the VLC performance of the G- and B-LEDs under the different offset angles. For
example, according to the measured results of Figure 5a, we select the 620 and 440 Mbps
OOK modulations under the free-space link of 3 and 4 m for VLC(Green) BER demonstration
when the various rotation angle of POL is changed gradually. Figure 6a,b present the
observed BER performances of G-LED versus the various rotation angles of 0◦ to 90◦ with
the corresponding illuminances of 0 to 2.4 lux and 0 to 0.5 lux, respectively. To meet with
the VLC(Green) traffic below the FEC threshold, the offset angles must be small 6than
Photonics 2021, 8, x FOR PEER REVIEW of 12 50
◦
◦
and 45 , respectively, after 3 (illuminance = 0.9 lux) and 4 m (illuminance = 0.3 lux) optical
wireless transmissions, as illustrated in Figure 6a,b.
10-1
10-2 (a) G-LED
10-3
Bit Error Rate
10-4
10-5
10-6
10-7
3m
10-8 4m
10-9 FEC
10-10
0 100 200 300 400 500 600 700 800
VLC Rate [Mbit/s]
10-1
10-2 (b) B-LED
10-3
Bit Error Rate
10-4
10-5
10-6
10-7
3m
10-8 4m
10-9 FEC
10-10
0 100 200 300 400 500 600 700 800
VLC Rate [Mbit/s]
Figure 3.
Figure 3. Different
Different measured
measuredVLC
VLCrates
ratesofof(a)
(a)G
G−and
and(b)(b)
BLEDs versus
B−LEDs the the
versus corresponding BERBER
corresponding
after 3 and 4 m freespace transmissions under WDM VLC connection, respectively.
after 3 and 4 m free−space transmissions under WDM VLC connection, respectively.
35
30 G-LED
B-LED
[lux]
2510-10
0 100 200 300 400 500 600 700 800
VLC Rate [Mbit/s]
Photonics 2021, 8, 210 7 of 12
Figure 3. Different measured VLC rates of (a) G and (b) BLEDs versus the corresponding BER
after 3 and 4 m freespace transmissions under WDM VLC connection, respectively.
35
30 G-LED
B-LED
Illuminance [lux]
25
20
15
10
5
with POL
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
VLC Length [m]
Photonics 2021, 8, x FOR PEER REVIEW 7 of 12
Figure 4. Measured corresponding
corresponding illuminances
illuminances of
ofthe
theGG− and −LEDs after passing through two
and BBLEDs
POLs with parallel polarization before launching into the focusing
focusing lens.
lens.
Figure5.5. Measured
Figure Measured different
differentVLC
VLCrates
ratesof of
(a)(a)
GGand (b) BLEDs
− and versusversus
(b) B−LEDs the corresponding BER
the corresponding BER
after33and
after and 44m
m free-space
free-spacetransmissions. Insets
transmissions. are the
Insets are corresponding
the eye diagrams
corresponding eye at the maximum
diagrams at the maximum
VLC rate below the FEC limit of BER = 3.8 10−3.
VLC rate below the FEC limit of BER = 3.8 × 10−3 .
In the demonstration, we could not only apply two various color LEDs in the pre-
sented dual-polarized VLC system but also we could utilize two same color LEDs. We
could also utilize a pair of polarization-multiplexing G-LEDs (or B-LEDs) at the VLC-Tx
for delivering wireless VLC traffics. Therefore, the maximum dual-polarized G-LED
VLC(Green) and B-LED VLC(Blue) rates of 2 640 and 2 580 Mbps; and 2 560 and 2 540
Mbps were achieved after 3 and 4 m optical wireless connections, respectively. This meansOOK modulations under the free-space link of 3 and 4 m for VLC(Green) BER demonstration
when the various rotation angle of POL is changed gradually. Figure 6a,b present the ob-
served BER performances of G-LED versus the various rotation angles of 0 to 90 with
the corresponding illuminances of 0 to 2.4 lux and 0 to 0.5 lux, respectively. To meet with
the VLC(Green) traffic below the FEC threshold, the offset angles must be small than 50 and
Photonics 2021, 8, 210 8 of 12
45, respectively, after 3 (illuminance = 0.9 lux) and 4 m (illuminance = 0.3 lux) optical
wireless transmissions, as illustrated in Figure 6a,b.
10-1 3
(a) 3m VLC
Illuminance [Lux]
10-2
Bit Error Rate
2
10-3
1
10-4
10-5 0
10-1 0.6
(b) 4m VLC BER
illuminance 0.5
Illuminance [Lux]
FEC
Bit Error Rate
10-2 0.4
0.3
10-3 0.2
0.1
10-4 0.0
0 15 30 45 60 75 90
Rotoaion Angle [degree]
Figure 6. VLC(Green) BER measurements of 620 and 440 Mbps after the freespace link of (a) 3 and (b)
Figure 6. VLC(Green) BER measurements of 620 and 440 Mbps after the free−space link of (a) 3 and
4 m under different corresponding illuminance.
(b) 4 m under different corresponding illuminance.
Next, to
Next, to understand
understand thethe effect
effectof
ofoffset
offsetangle
angleforforthe
theB-LED-based
B-LED-basedVLC VLCtraffic,
traffic,wewe
applied 520 and 500 Mbps OOK VLC signals for the BER measurements
applied 520 and 500 Mbps OOK VLC signals for the BER measurements under various under various
offsetangles
offset anglesaccording
accordingtotothe
theobtained
obtainedresults
resultsof ofFigure
Figure5b.5b.Therefore,
Therefore,Figure
Figure7a,b
7a,bexhibit
exhibit
theobserved
the observedBER BERpresentations
presentationsof ofB-LED
B-LEDversus
versusthethevarious
variousrotation
rotationangles
anglesofof00toto90
◦ 90
◦
underthe
under thecorresponding
correspondingilluminances
illuminances ofof 0 to
0 to 5.45.4
luxlux
andand
0 to0 2.4
to 2.4
lux,lux, respectively.
respectively. To fitTo fit
the
the VLC signal below the FEC threshold, the offset angles must be
VLC(Blue) signal below the FEC threshold, the offset angles must be small than 45 and 35◦
(Blue) small than◦ 45 and
after 3 (illuminance = 1.6 lux) and 4 m (illuminance = 2.9 lux) free-space connections, as
demonstrated in Figure 7a,b.
The influence of the offset angle on the maximum VLC modulation rate is also a critical
issue. As mentioned above, the maximum VLC modulation rates are 580 and 540 Mbps
for the G- and B-LED VLC transmissions under a 4 m free-space link, respectively. In the
measurement, when the offset angle of POL is increased gradually from 0◦ to 50◦ , the
corresponding largest modulation rates of G- and B-LED VLC traffics can be obtained
within the FEC target, as shown in Figure 8. In this measurement, the maximum G-LED
VLC of 500, 440 and 300 Mbps are achieved under the offset angle of 30◦ , 40◦ and 50◦ ,
respectively. The 500, 450 and 400 Mbps traffics of B-LED VLC link are also observed under
the offset angle of 30◦ , 40◦ and 50◦ , respectively. This is because the illuminance of the
G-LED itself is smaller than that of the B-LED. Once the angle is over 50◦ , the VLC(Green)
and VLC(Blue) traffics would be not obtained due to the larger insertion loss. Therefore, the
results show that the corresponding illuminances of the G- and B-LEDs must be greater
than 0.2 and 1 lux to obtain optimal VLC performance through a 4 m free-space connection.
As exhibited in Figure 8, we can obtain the same data rate of 500 Mbps for the VLC(Green)
and VLC(Blue) connections, when the offset angle is rotated at 30◦ . In addition, with aPhotonics 2021, 8, x FOR PEER REVIEW 9 of 12
Photonics 2021, 8, 210 9 of 12
35 after 3 (illuminance = 1.6 lux) and 4 m (illuminance = 2.9 lux) free-space connections,
gradual increase ininthe
as demonstrated offset7a,b.
Figure angle, the measured modulation signal of the B-LED would
also be larger than that of the G-LED.
10-1 7
(a) 3m VLC
6
Illuminance [Lux]
Bit Error Rate 10-2 5
4
10-3
3
10-4 2
1
10-5 0
10-1 3
(b) 4m VLC BER
illuminance
Illuminance [Lux]
FEC
Bit Error Rate
2
10-2
1
10-3 0
0 15 30 45 60 75 90
Rotoaion Angle [degree]
Figure7.7.VLC
VLC(Blus) BER measurements of 620 and 440 Mbps after the freespace link of (a) 3 and (b)
Figure (Blus) BER measurements of 620 and 440 Mbps after the free−space link of (a) 3 and
4 m under different corresponding illuminance.
(b) 4 m under different corresponding illuminance.
InThe
theinfluence of the offset
demonstration, owing angle on blended
to two the maximum VLC modulation
orthogonal VLC signals,rate the is alsotraffic
VLC a crit-
ical can
also issue. As mentioned
provide above, the
security operation maximum
in the VLC based
physical layer modulation ratesinterference
on channel are 580 and 540
if the
Mbps for the
polarization ofG-
theand
POLsB-LED VLC
are not transmissions
matching. In our under
previousa 4 study
m free-space
[23], thelink,
two respectively.
mixed VLC
In the measurement,
signals with similar power when the offset
would causeangle
very of POL data
serious is increased gradually
interference. from the
To decode 0 to 50,
dual-
the corresponding largest modulation rates of G- and B-LED VLC traffics
polarized VLC signals, two orthogonal POLs with precise direction controls are required at can be obtained
within
the VLC-Rx the FEC
side, target,
when theas client
shownisin in Figure 8. arbitrary
a state of In this measurement,
movement. The thecontrol
maximum of theG-LED
POL
isVLC of 500, 440
dependent on theandmaximum
300 Mbpsdetected
are achieved under the
illuminance offset angle
for proper of 30,
rotation. The40 and 50,
schematic
respectively.
diagram of theThe 500, 450
control and is
design 400 Mbps traffics
illustrated of B-LED
in Figure 9 forVLC
proof link
of are also observed
concept. Hence, the un-
der the offset angle
corresponding of 30, can
VLC signal 40 be
anddetected
50, respectively. This isrotation.
through proper because Additionally,
the illuminance of the
if there
G-LED
are manyitself is smaller
clients with only than
onethat of the B-LED.
Rx detector Onceroom
in the same the angle is over 50,
for receiving the VLC(Green)
a dual-polarized
VLC signal, the polarization of the POL for each client can be adjusted
and VLC(Blue) traffics would be not obtained due to the larger insertion loss. Therefore, properly to sharethe
and adapt
results showthethat
totalthe
capacity. Therefore,
corresponding the proposed
illuminances dual-polarized
of the G- and B-LEDs LED VLC
must besystem
greater
can
thannot0.2only
anddouble
1 lux to the data rate
obtain but can
optimal VLC also provide a security
performance throughfunction accordingconnec-
a 4 m free-space to the
polarization-based
tion. As exhibitedfilter. in Figure 8, we can obtain the same data rate of 500 Mbps for the
VLC(Green) and VLC(Blue) connections, when the offset angle is rotated at 30. In addition,
with a gradual increase in the offset angle, the measured modulation signal of the B-LED
would also be larger than that of the G-LED.0
250 300 350 400 450 500 550 600
Photonics2021,
Photonics 2021,8,8,210
x FOR PEER REVIEW 1010ofof1212
VLC Rate [Mbit/s]
Figure 8. Obtained corresponding modulation rates of VLC(Green) and VLC(Green) within the FEC level
under the different rotation angles from 0 to 50.
60
Rotation Angle [degree]
In the demonstration, 50 owing to two blended orthogonal VLC signals, the VLC traffic
also can provide security operation in the physical layer based on channel interference if
the polarization of the POLs40 are not matching. In our previous study [23], the two mixed
VLC signals with similar power would cause very serious data interference. To decode
the dual-polarized VLC 30 signals, two orthogonal POLs with precise direction controls are
required at the VLC-Rx side,20 when the client is in a state of arbitrary movement. The con-
trol of the POL is dependent on the maximum Green detected illuminance for proper rotation.
The schematic diagram of 10the control Bluedesign is illustrated in Figure 9 for proof of concept.
Hence, the corresponding VLC signal can be detected through proper rotation. Addition-
0 with only one Rx detector in the same room for receiving a
ally, if there are many clients
dual-polarized VLC signal,250 300
the polarization 350 of the400POL450 for each 500client550 can be600 adjusted
properly to share and adapt the total capacity. VLC Therefore,
Rate [Mbit/s] the proposed dual-polarized
LED VLC system can not only double the data rate but can also provide a security function
Figure8.8.Obtained
Figure Obtained corresponding
corresponding modulation
modulation rates
rates of
ofVLC
VLC(Green)
(Green)and VLC
and (Green) within
VLC the FEC
(Green) within level
the FEC
according to the polarization-based filter.angles from 0 to 50.
under the different rotation ◦ ◦
level under the different rotation angles from 0 to 50 .
In the demonstration, owing to two blended orthogonal VLC signals, the VLC traffic
POLx POL y
POL POL POL
also can provide security xoperation iny the physical layer POLy interference if
x on channel
based
the polarization of the POLs are not matching. In our previous study [23], the two mixed
VLC signals with similar power would cause very serious data interference. To decode
the dual-polarized VLC signals, two orthogonal POLs with precise direction controls are
illuminance
illuminance
required at the VLC-Rx side, when the client is in a state of arbitrary movement. The con-
trol of the POL is dependent on the maximum detected illuminance for proper rotation.
The schematic diagram of the control design is illustrated in Figure 9 for proof of concept.
Hence, the corresponding VLC signal can be detected through proper rotation. Addition-
ally, if there are many clients with only one Rx detector in the same room for receiving a
dual-polarized VLC signal, the polarization of the POL for each client can be adjusted
VLC Signal properly to share andVLC adaptSignal
VLC Signal
the total capacity. Therefore, the proposed dual-polarized
(a) Original Power
LED VLC system (b)canPOL Offset
not only double the data rate(c) Proper
but can Rotation
also provide a security function
according to the polarization-based filter.
Figure 9. Schematic
Figure diagram
9. Schematic of the
diagram POL’s
of the POL’srotation control
rotation control forfor decoding
decoding dual-polarized
dual-polarized VLC traffics.
VLC traffics.
POLx POLy POLx POLy POLx POLy
4. Conclusions 4. Conclusions
We proposed and investigated 2 × 2 PDM G- and B-LED VLC transmission systems
We proposed andwith investigated 2 2 PDM
orthogonal-polarization G- and
to double the B-LED VLC
data traffic. transmission
Here, to obtain the systems
dual-polarized
illuminance
illuminance
with orthogonal-polarization
VLC signals,tothe
double the dataPOLs
two orthogonal traffic. Here,
at the to obtain
VLC-Rx the dual-polarized
side could be controlled for rota-
VLC signals, the two orthogonal
tion POLs
automatically at thethe
to match VLC-Rx side could
dual-polarized VLC be controlled
signals from thefor rotation
VLC-Tx site. The
presented dual-polarized VLC system could accomplish the total modulation rate of 1200
and 1120 Mbps (VLC(Green+Blue) ) passing after 3 and 4 m free-space transmission lengths,
respectively,
VLCwhen
Signalthe corresponding VLC illuminances
Signal of the G- and B-LEDs were 2.4 and
VLC Signal
0.5 lux and 5.4 and 2.3 lux. To produce the more flexible proposed dual-polarized VLC
(a) Original Power (b) POL Offset (c) Proper Rotation
system, we also can employ paired G-LEDs and B-LEDs acting as the optical Tx to deliver
dual-polarized VLC(Green) and VLC(Blue) signals for alternative indoor applications. Hence,
Figure 9. Schematic diagram of the POL’s rotation control for decoding dual-polarized VLC traffics.
the proposed 2 × 2 PDM G- and B-LED VLC system did not need to use a color filter for
decoding signals. In addition, we also executed and analyzed the influence of different
4. Conclusions
polarization offsets at a fixed VLC rate through a wireless link length of 3 and 4 m, respec-
tively.We proposed
Based and investigated
on the observed 2 largest
results, the 2 PDMoffset
G- and B-LED
angle VLC
of the twotransmission
POLs shouldsystems
be less
with orthogonal-polarization
◦ to double the data traffic. Here,
than 50 to maintain the optimal VLC modulation rate below the forward to obtain theerror
dual-polarized
correction
VLC signals,
(FEC) the two orthogonal
level. Furthermore, POLsmany
if there were at theclients
VLC-Rx side
with could
only onebeRxcontrolled
detector infor rotation
the samePhotonics 2021, 8, 210 11 of 12
room for receiving dual-polarized VLC traffic, the polarization of the POL for each client
could be controlled properly to share and adapt the total capacity. Hence, the proposed
dual-polarized LED VLC system not only doubled the data rate but also provided signal
security according to the PDM method.
Author Contributions: Conceptualization, C.-H.Y.; methodology, S.-K.L.; validation, Y.-C.Y., W.-H.H.
and B.-Y.W.; resources, C.-H.Y. and C.-W.C.; data curation, Y.-C.Y., W.-H.H. and B.-Y.W.; writing—
original draft preparation, C.-H.Y. and Y.-C.Y.; writing—review and editing, C.-H.Y.; supervision,
C.-H.Y. and S.-K.L. All authors have read and agreed to the published version of the manuscript.
Funding: This paper was supported by the Ministry of Science and Technology, Taiwan, under Grant
MOST-109-2221-E-035-071.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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