isible light communications (VLC) is an emerging field of optical communications that focuses on the part of the electromagnetic spectrum that humans can see.Much existing work in optical communications exists, mainly optimized for capacity and transmission performance in fiber and free-space with biases toward spectrum that minimizes attenuation in the medium. However, the use of the visible spectrum has gained interest due to its availability and the ease at which it can be modulated using light emitting diodes (LEDs). Recent demand factors due to the burgeoning mobile industry and the rapid evolution of LED-based light-ing are also driving this interest. Here, and in this special issue, we outline the context of VLC, its unique benefits, and describe the state of the art research contributions of the assembled papers.
D EMAND F
ACTORS
The potential for VLC is being driven by the increasing adop-tion of mobile electronic devices. The demand for wireless capacity as predicted most recently by Cisco (Cisco VNI, Feb 2015) indicates a 10x growth in mobile traffic over the next five years. But over the same time period, mobile carrier speed is predicted to grow by only 9 percent. Because more than 70 percent of all mobile traffic occurs indoors, and much occurs at fixed locations, there is a huge opportunity to offload traffic to localized access points as WiFi or small cells.Key to this technology is the placement of access points where
mobile users are active but also to reconcile contention caused by these populous indoor spaces. The problems with contention are where VLC can provide significant benefits over RF technologies. VLC can provide very high data rates when used directly as a directional medium. When combined with a lighting mission — providing light — VLC is ideally suited for where humans exist in these indoor spaces because they consume light at the same time that they consume data.
B ASICS OF VL
C AN
D B ENEFITS
In its basic form, VLC uses light that is intensity modulated to transmit data. LEDs are inexpensive, fast, and are widely adopted in lighting, and hence VLC is and ideal match in lighting systems using LEDs. Because of their efficiency as compared to other lighting sources, LEDs appear in many
applications in lighting and display including traffic lights, flat panel displays, and instrumentation. In this sense, any LED-based applications fall into the category of “green” technolo-gies.
In terms of communications, LEDs are “fast” in that they can be intensity modulated very quickly (order of MHz),much faster than conventional lighting. At a receiver, the sig-nals are sensed as intensities via “direct detection” using a photo detector that can be very inexpensive. Modulation for-mats in VLC vary extensively, and their optimization under different application scenarios is an active area of research.This work is unique because unlike RF modulation, optical signals are unipolar due to the nature of the intensity-modu-lated signal.
VLC has some interesting characteristics that are unique to optical communications systems. Light-based systems are confined by opaque walls and thus improve security and enhance reused of the channel in densely packed cells (e.g.adjacent rooms in an office). However, this does not mean that VLC is a strict line-of-sight technology; it has been shown that VLC also works when the light in a room is severely obstructed. Nor does VLC interfere with RF; VLC can be added to an existing network without introducing new inter-ference. Moreover, in cases when RF signals are perceived as a hazard, for example, in hospitals, airplanes, mines, or as RF “pollution,” VLC can be applied as a practical alternative.Due to its directionality and containment properties, VLC is also a good candidate for near field communications (NFC).VLC is also a contender for providing “indoor” GPS. Light-based positioning and localization is being explored by a vari-ety of researchers as potentially more accurate and more easily deployed than RF or acoustic techniques.
F UTURE AND S UMMARY
What does the future hold for VLC? While traditionally VLC had been conceived as a point-to-point, cable replacement technique, there are many works that highlight that VLC has the potential to augment cellular communications by provid-ing a means to decrease the cell sizes in cellular communica-tions even further without incurring significant installation cost by piggy-backing on existing lighting infrastructure. This is especially important as current research into 5G suggests that typical cell sizes will be around 50 m, which will pose
V ISIBLE L IGHT C OMMUNICATIONS : D EMAND F ACTORS , B ENEFITS AND O PPORTUNITIES
V
N AN C HI H ARALD H AAS M OHSEN K AVEHRAD T HOMAS DC L ITTLE X IN -L IN H UANG
severe challenges on backhaul and infrastructure deployment. As LEDs increasingly displace incandescent lighting over the next few years, general applications of VLC technology are expected to include Internet-of-Things, wireless Internet access, vehicle-to-vehicle communications, broadcast from LED signage, machine-to-machine communications, position-ing systems, and navigation. The long lifetime of LED lights means infrequent replacement of lights, resulting in the need for new business models in the lighting industry, and we see light-as-a-service (LAAS) being introduced, especially as it relates to the adoption of new integration of lighting as con-
trollable devices, each with its own Internet address.
A RTICLES
However, there are still many unsolved issues. The objective
of this Feature Topic is to present a collection of articles
focusing on the state of the art in visible light communica-
tions. Our Call for Papers attracted many submissions world-
wide. After a rigorous review process, 13 papers were selected
that best fit the theme of this Feature Topic. These cover a
broad spectrum of research topics including network protocol
and network architecture for VLC, practical applications with
VLC systems, new modulation methods for VLC systems, and
new materials, components, and devices for VLC.
In the first article, “Visible Light Communications in Het-
erogeneous Networks: Paving the Way for User-Centric
Design” by Rong Zhang et al., the authors introduce the user-
centric design of VLC for heterogeneous networks (HetNet),
with special emphasize on three key aspects, namely its signal
coverage quality, system control, and service provision. Both
the traditional and the user-centric VLC cell formation are
discussed. Additionally, the user-centric design of VLC in the
holistic HetNet environment and a range of open challenges
are discussed.
In the second article, “A Practical In-home Illumination
Consideration to Reduce Data Rate Fluctuation in Visible
Light Communication” by C. W. Chow et al., the authors
investigate joint illumination and communication systems to
provide an in-home lighting and VLC system. The illumina-
tion constraints set by different lighting conditions are ana-
lyzed considering different kinds of commercially available
luminaries, beam angles, LED lamp arrangements and reflec-
tions by different materials and colors. Besides, a white-light
phosphor-based LED adaptive data rate VLC system is pro-
posed and experimentally demonstrated. Based on the experi-
mental results, an environment with reduced VLC data rate
fluctuation can be achieved.
In the third article, “Grouped Modulation Scheme for
LED Array Module in a Visible Light Communication Sys-
tem” by Aiying Yang et al., the authors propose grouped mod-
ulation to generate multiple-level optical signals and enhance
the data rate for LED array module based indoor visible light
communication systems. The concept of grouped modulation
is dividing the LEDs in an array module into smaller size
groups and driving each group with a separate circuit. The
results of the experiments on 2-grouped modulation demon-
strate that the data rate can be enhanced and the perfor-
mance of a VLC system can be improved. Furthermore, more
groups and other modulation formats such as pulse position
modulation (PPM) and overlapping pulse position modulation
(OPPM) can be considered.
Although VLC is capable of concurrently providing com-
munication as well as illumination, to make commercial
implementation of VLC feasible, it is necessary to incorporate
dimming schemes. The fourth article, “Dimming Schemes for
Visible Light Communication (VLC): The State of Research”
by Fahad Zafar et al., presents the latest concepts and method-
ologies involved in dimming control of a VLC system. It gives
a detailed overview of trending dimming schemes based on
modulation and coding techniques. Adaptive techniques are
described that can be implemented to enhance communica-
tion capacity together with the concepts involved in develop-
ing the driver circuitry that would facilitate the practical
implementation of such schemes. Finally, future prospects
along with the key areas requiring attention and improvement
are characterized.
Many physical devices used in VLC systems exhibit nonlin-
ear effects which can significantly degrade overall system per-
formance. The fifth article, “Nonlinear Distortion Mitigation
in Visible Light Communications” by Kai Ying et al., summa-
rizes topics related to the LED nonlinearity distortion mitiga-
tion in VLC systems. The authors present the modeling of
LED nonlinearities followed by two major approaches for their
mitigation: a waveform-specific mitigation and a waveform-
agnostic mitigation. For the latter, optimal nonlinear mapping
is considered for dynamic-range-limited nonlinearities together
with the linearization approaches. Finally, challenges of nonlin-
ear distortion mitigation in VLC are highlighted.
Advanced modulation formats are becoming increasingly
important in VLC systems. In the sixth article, “Multi-band
Carrier-less Amplitude and Phase Modulation for Band-limit-
ed Visible Light Communications Systems” by Paul Anthony
Haigh et al., the authors introduce a new modulation scheme
into the VLC domain: multi-band carrier-less amplitude and
phase modulation (m-CAP) and describe in detail its perfor-
mance within the context of band-limited systems where m-
CAP’s considerable potential in achieving higher spectral
efficiency is demonstrated.
In the seventh article, “DC-Informative Modulation for
Visible Light Communications Under Lighting Constraints”
by Qian Gao et al., DC-informative modulation schemes are
introduced as viable alternatives to conventional none DC-
informative counterparts for multi-carrier visible light commu-
nication systems for the purpose of boosting the system
energy efficiency. Optimal constellations are constructed
either in time-domain or frequency-domain by compact
sphere packing. Then several DC informative system architec-
tures are proposed and compared. Finally, challenges in tech-
nical implementation are also discussed.
In the eighth article, “A High Performance Blue Filter for
White LED Based Visible Light Communication System” by
Shao-Wei Wang et al., the authors introduce a high perfor-
mance blue filter, which has a very wide stopband (500–1050
nm), high transmittance passband (average 97.5 percent in the
blue signal range of 430–485nm), and sharp and precise cut-
off edge. Employing this blue filter, BER performance of
white LED VLC systems is shown to improve by two orders of
magnitude compared with the no filter case.
The ninth article, “An Analog Modulator for 460 Mbit/s
Visible Light Data Transmission based on OOK-NRZ Modu-
lation” by Honglei Li et al., describes an analog modulator
that consists of modulation, metal oxide semiconductor field-
effect transistor (MOSFET) drive, and pre-emphasis circuits
for high-speed VLC application systems. The analog modula-
tor successfully combines pre-emphasis and AC-coupled mod-
ulation technologies together. With the combination of
blue-filtering, the proposed analog modulator extends the 3-
dB bandwidth of VLC system from 3 MHz to 175 MHz, which
allows on-off-keying non-return-to-zero (OOK-NRZ) data
transmission up to 460 Mbit/s with bit-error-ratio (BER) below 10–9 under a 1 m radial distance.
In the tenth article, “Size- and Current-Density-Controlled Tunable Wavelength in GaN-Based LEDs for Potential Dense Wavelength-Division Multiplexing Application” by Dongdong Teng et al., the authors design and fabricate an array of mLEDs with hybrid pixel sizes from 30mm~60mm. Wave-length shift accompanying the size and current density varia-tions are investigated. A wavelength tunable range of 20 nm is obtained, which endows the device potential as a tunable light source to enable dense wavelength division multiplex (DWDM) technology for visible light communication (VLC) application. In addition, the full width at half maximum (FWHM) of the complex spectrum broadens from 16 nm to 45 nm which can increase lighting quality.
In the eleventh article, “Ubiquitous 3-D Positioning Sys-tems by LED-based Visible Light Communications” by
Jaechan Lim, the author presents a brief overview of position-
ing approaches in VLC systems, and shows enhancements in
positioning performance by employing an iterative maximum
likelihood approach using a least-squares solution as an initial
guess. This work shows the possibility of obtaining MSE per-
formance of the iterative ML approach that is similar to the
Cramer-Rao bound.
In the twelfth article, “Efficient Coding Modulation and
Seamless Rate Adaptation for Visible Light Communica-
tions,” by Min Wang et al., an analog rateless code (ARC)
modulation scheme is proposed for VLC in a hybrid VLC-RF
system, which can simultaneously improve spectral efficiency
and enhance robustness in dynamic lighting scenarios. The
author analyzes how to select a good weight set and design a
mapping matrix. A new weight set suitable for VLC systems is
given. Finally, simulation and experiment results show the
proposed weight set and mapping matrix achieve obvious gain,
and the proposed adaptive scheme works well in practical
VLC systems.
In the thirteenth article, “An Open-Source Research Plat-
form for Embedded Visible Light Networking” by Qing Wang
et al., an open-source research platform called OpenVLC is
introduced based on software-defined implementation. It
offers a basic physical layer, a set of essential medium access
primitives, as well as interoperability with Internet protocols.
OpenVLC is designed to demystify VLC and lower the barri-
ers to entry to VLC research for embedded system
researchers.
B IOGRAPHIES
N AN C HI(nanchi@https://www.sodocs.net/doc/251649223.html,) received the BS degree and Ph.D. degree in elec-
trical engineering from Beijing University of Posts and Telecommunications,
Beijing, China in 1996 and 2001, respectively. From July 2001 to December 2004,
she was an assistant professor at the Research Center COM, Technical Uni-
versity of Denmark. From January 2005 to April 2006 she was a research
associate at the University of Bristol, United Kingdom. In June 2006 she
joined Wuhan National Laboratory for Optoelectronics, Huazhong University
of Science and Technology, where she worked as a full professor. She joined
Fudan University in June 2008, in the School of Information Science and
Engineering. She is the author or co-author of more than 200 papers. She
has been the chair of the APOC 2007 OSRT workshop and ACP 2010. She has
served as the technical program committee member of many conferences
such as APOC 08, ICAIT09, ACP 2011, WOCC 2012, ACP 2013, and IWOO
2014. She has been awarded the New Century Excellent Talents Awards from
the Education Ministry of China, Shanghai Shu Guang scholarship, Japanese
OKAWA intelligence Fund Award, Pujiang talent of Shanghai City, and Ten
Outstanding IT Young Persons awards of Shanghai City. Her research inter-
ests are in the area of coherent optical transmission, visible light communi-
cation, and optical packet/burst switching.
H ARALD H AAS received the Ph.D. degree from the University of Edinburgh in
2001. He currently holds the Chair of Mobile Communications at the Univer-
sity of Edinburgh. His main research interests are in optical wireless commu-
nications, hybrid optical wireless and RF communications, spatial modula-
tion, and interference coordination in wireless networks. He first introduced spa-
tial modulation. He was an invited speaker at TED Global 2011, where he
coined the term ‘Li-Fi.’ His talk has been watched online more than 1.5 mil-
lion times. He is co-founder and chief scientific officer (CSO) of pureLiFi Ltd.
Professor Haas holds 31 patents and has more than 30 pending patent
applications. He has published 300 conference and journal papers including
a paper in Science. He was co-recipient of a best paper award at the IEEE
Vehicular Technology Conference in Las Vegas in 2013. In 2012, he received
the prestigious Established Career Fellowship from the EPSRC (Engineering
and Physical Sciences Research Council) within Information and Communica-
tions Technology in the UK. He is the recipient of the Tam Dalyell Prize 2013
awarded by the University of Edinburgh for excellence in engaging the pub-
lic with science. In 2014 he was selected by EPSRC as one of ten RISE (Recognising
Inspirational Scientists and Engineers) Leaders.
M OHSEN K AVEHRAD completed his Ph.D. degree at New York University Polytechnic
(formerly Brooklyn Polytechnic Institute), Brooklyn, New York in electrical
engineering in November 1977. Between January 1978 and March 1989 he
worked on telecommunications and networking problems for Fairchild
Industries, GTE (Satellite and L abs.) and AT&T Bell L aboratories. In 1989 he joined
the University of Ottawa Electrical Engineering Department as a full profes-
sor. Since January 1997 he has been with the Pennsylvania State University
Electrical Engineering Department as the W.L. Weiss Chair Professor and
founding Director of the Center for Information and Communications Tech-
nology Research. He is a Fellow of the IEEE for his contributions to wireless
communications and optical networking. He received three Bell Labs awards
for his contributions to wireless communications, the 1990 TRIO feedback award
for a patent on an optical interconnect, the 2001 IEEE VTS Neal Shepherd
best paper award, three IEEE L asers and Electro-Optics Society best paper awards
between 1991 and 1995, and a Canada NSERC Ph.D.-thesis award in 1995
with his graduate students for contributions to wireless systems and optical
networks. He also received the 2009 DesignCon Paper Award in the High-
Speed and RF Design Category, and the Paper of the Year Award from ETRI
Journal in December of 2009. He has close to 400 published papers, several
book chapters, books, and patents in these areas. His research interests are
in the areas of communications networked systems of all types. He is a for-
mer technical editor for IEEE Transactions on Communications, IEEE Commu-
nications Magazine, and the IEEE Magazine of Lightwave Telecommunications
Systems. He served as the General Chair of leading IEEE conferences and
workshops. He has chaired, organized, and been on the advisory committee
for several international conferences and workshops.
T HOMAS DC L ITTLE received his BS degree in biomedical engineering from RPI
in 1983, and his MS degree in electrical engineering and Ph.D. degree in
computer engineering from Syracuse University in 1989 and 1991, respec-
tively. Currently a professor in the Department of Electrical and Computer
Engineering at Boston University, he is also Associate Dean for Educational
Initiatives for the college, and serves as associate director of the National Sci-
ence Foundation Smart Lighting Engineering Research Center, a collabora-
tion of Rensselaer Polytechnic Institute, the University of New Mexico, and Boston
University. His recent efforts address research in pervasive computing using wire-
less technologies. This includes video streaming, optical communications
with the visible spectrum, and applications related to ecological sensing,
vehicular networks, and wireless healthcare. He is a successful entrepreneur
and most recently nurtured the spinoff of Bytelight, a company focused on indoor
positioning with lighting. He is a Senior Member of the IEEE, a member of
the IEEE Computer and Communications Societies, and a member of the
Association for Computing Machinery.
X IN-L IN H UANG[S’09, M’12] received the M.E. and Ph.D. degrees in the
Department of Information and Communication Engineering from Harbin
Institute of Technology (HIT), Harbin, P. R. China, in 2008 and 2011, respec-
tively. He is currently an associate professor in the Department of Informa-
tion and Communication Engineering, Tongji University, Shanghai, P. R.
China. His research focuses on cognitive radio, cognitive networks, machine
learning, VANET, OFDM technology, and massive MIMO feedback. He has
published over 40 research papers, two patents, and three book chapters in these
fields. He was a recipient of Chinese Government Award for Outstanding
Ph.D. Students in 2010, Best Ph.D. Dissertation Award from HIT in 2013,
Shanghai High-level Overseas Talent Program in 2013, and Shanghai “Chen-
guang” Scholar Program in 2014. From August 2010 to September 2011 he
was supported by the China Scholarship Council to do research in the
Department of Electrical and Computer Engineering, University of Alabama (USA),
as a visiting scholar. He was invited to serve as session chair for IEEE
ICC2014. He also serves as IG leader for the IEEE ComSoc-MMTC. He is a
paper reviewer for IEEE Transactions on Wireless Communications, IEEE
Transactions on Signal Processing, IEEE Transactions on Vehicular Technolo-
gy, IEEE System Journal, IEEE Communications Letters, Computer Communi-
cations, Wireless Personal Communications, and the International Journal of
Communication Systems.