IEEE Wireless Communications • April 2015
8
1536-1284/15/$25.00 © 2015 IEEE
Rong Zhang and Lajos
Hanzo are with the
University of
Southampton.
Jiaheng Wang and
Chunming Zhao are with
Southeast University.
Zhaocheng Wang is with
Tsinghua University.
Zhengyuan Xu is with
USTC.
The financial support of
the EPSRC under the
India-UK Advanced
Technology Centre (IU-
ATC), that of the EU
under the Concerto pro-
ject as well as that of the
European Research
Council’s (ERC)
Advanced Fellow Grant
is gratefully acknowl-
edged. This work is also
supported in part by the
National Key Basic
Research Program of
China (Grant No.
2013CB329201).
VISIBLE LIGHT COMMUNICATIONS
INTRODUCTION
BACKGROUND AND MOTIVATION
At the time of this writing, visible light commu-
nications (VLC) constitutes a rapidly developing
research area, drawing significant attention both
from academia and industry. Historically, the
first experiment of VLC can be traced back to
the old English beacon system relying on bon-
fires lit on top of regularly-spaced beacon-hills.
Then in the late 19th-century, Alexander Gra-
ham Bell invented the photophone by transmit-
ting speech over modulated sunlight. This
pioneering experiment inspired the recent imple-
mentation of data transmission using light emit-
ting diodes (LEDs) by the Nakagawa laboratory [1]
in 2004 and raised a significant amount of invest-
ments, as exemplified by the EU-FP7 project
OMEGA (http://www.ict-omega.eu/). Moreover,
several collaborative projects were launched
across the globe, such as the VLC standardiza-
tion process (http://www.ieee802.org/15/pub/
TG7.html), the LiFi Consortium (http://www.lifi-
consortium.org/), the VLC Consortium (http://
www.vlcc.net/), etc. Following from these
advances, link-level data rates attaining hun-
dreds of Mega-bits/sec have been reported using
state-of-the-art LEDs and photo-detectors [2],
while a variety of low-rate indoor positioning
applications have also been commercialized
(http://www.bytelight.com/).
The development of VLC solutions essential-
ly relies on the maturing wireless communica-
tions techniques and on the gradually improving
performance as well as on the growing populari-
ty of LEDs. It is envisioned that LEDs will dom-
inate the general illumination market owing to
their energy-efficiency, color-rendering capability
and longevity. Thus, LEDs constitute pervasive
luminaries providing an attractive modulation
potential in relation to the current technology.
By modulating data on the visible light produced
by the LEDs way above the human eye’s fusion
rate, the dual goal of communication and illumi-
nation can be realized simultaneously. To elabo-
rate a little further, in their most basic form,
VLC systems generally exploit intensity modula-
tion at the LED transmitters, which can be read-
ily detected by the photo-diode (PD) receivers.
The attainable link-level data rates can be
improved from using simple on-off keying
(OOK) and pulse position modulation (PPM), to
employing more sophisticated modulation
schemes, such as the Optical Orthogonal Fre-
quency Division Modulation (OOFDM) [3, 4].
More ambitiously, Giga-bits/sec transmissions
have also been achieved with the aid of multiple
input multiple output (MIMO) techniques [5].
Even higher link-level data rates are achievable
by using the RGB-LED with Wavelength Divi-
sion Multiplexing (WDM) [6].
The advantages of VLC are multi-fold,
including using the unlicensed spectrum, avail-
ability of vast bandwidths, the presence of a
ubiquitous lightening infrastructure, energy effi-
ciency, etc. Naturally, disadvantages also exist,
such as a confined coverage, sensitivity to line-
of-sight (LoS) blocking as well as sun-light, and
the lack of up-link (UL) support, etc. Nonethe-
less, there have been tremendous efforts invest-
ed in increasing the link-level data-rate of VLC
RONG ZHANG, JIAHENG WANG, ZHAOCHENG WANG, ZHENGYUAN XU, CHUNMING ZHAO,
AND LAJOS HANZO
ABSTRACT
At the time of this writing, there is substan-
tial research interest in the subject of visible
light communications (VLC) owing to its ability
to offer significant traffic offloading potential in
highly crowded radio frequency (RF) scenarios.
We introduce the user-centric design of VLC for
heterogeneous networks (HetNet), where three
key aspects are identified and elaborated on: sig-
nal coverage quality, system control, and service
provision aspects. More explicitly, the concepts
of amorphous cell formation as well as separated
up-link (UL) and down-link (DL), decoupled
data and control, dynamic load balancing (LB),
etc., all demand radically new thinking. The
advocated user-centric VLC design is of key sig-
nificance in the small-cell scenarios of the emerg-
ing 5G design philosophy.
VISIBLE LIGHT COMMUNICATIONS IN
HETEROGENEOUS NETWORKS:
P
AVING THE WAY FOR USER-CENTRIC DESIGN
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