1

Localisation, Communication and Networking with
VLC: Challenges and Opportunities
Rong Zhang1

arXiv:1709.01899v1 [] 6 Sep 2017

1

School of Electronics and Computer Science, University of Southampton, UK

Abstract—The forthcoming Fifth Generation (5G) era raises
the expectation for ubiquitous wireless connectivity to enhance
human experiences in information and knowledge sharing as well
as in entertainment and social interactions. The promising Visible
Light Communications (VLC) lies in the intersection field of
optical and wireless communications, where substantial amount
of new knowledge has been generated by multi-faceted investigations ranging from the understanding of optical communications
and signal processing techniques to the development of disruptive
networking solutions and to the exploitation of joint localisation
and communications. Building on these new understandings
and exciting developments, this paper provides an overview
on the three inter-linked research strands of VLC, namely
localisation, communications and networking. Advanced recent
research activities are comprehensively reviewed and intriguing
future research directions are actively discussed, along with
the identifications of a range of challenges, both for enhancing
the established applications and for stimulating the emerging
applications.

I. I NTRODUCTION
We are at the dawn of an era in information and communications technology with unprecedented demand for digitalised
everything, for connected everything and for automated everything [1]. The next decade will witness a range of disruptive changes in wireless technologies, embracing all aspects
of cross-disciplinary innovations [2]. Fundamental challenges
arise when we have reached the limit of the conventional
Radio Frequency (RF) based wireless technologies, which are
increasingly less capable of meeting the escalating trafficdemands and of satisfying the emerging use-cases. Especially,
there have been substantial research efforts dedicated to the
high carrier frequencies, including the millimetre wave [3]
and the visible light spectrum [4] in the forthcoming Fifth
Generation (5G) wireless networks landscape. In this paper,
we provide an overview on the fast growing technology of
Visible Light Communications (VLC), which lies at the crosssection of optical and wireless communications, and focuses
on the human perceivable part of the electromagnetic spectrum, corresponding to wavelengths from 380nm to 780nm.
The use of the visible light spectrum for wireless communications has gained great interests. This is because the
visible light spectrum is licence-free, has a vast bandwidth
and does not interfere with the RF band. Historically, in
the late 19th-century, Alexander Graham Bell invented the
photo-phone by transmitting voice signals over modulated
sunlight [5]. Almost a century later, artificial light generated
by fluorescent lamps was also successfully demonstrated for

supporting low data-rate communications [6]. It is these exciting experiments that inspired the modern VLC using Light
Emitting Diodes (LEDs). In addition to facilitating communications, being a modern illumination technology as their main
function, LEDs have been increasingly dominating over the
traditional incandescent lamps and fluorescent lamps, owing
to their higher energy-efficiency, colour-rendering capability
and longevity [7]. Hence, the potential for VLC is further
supported by the anticipated presence of a ubiquitous and
efficient LED lighting infrastructure.
The pioneering implementation of VLC using LEDs for
the dual purpose of indoor illumination and communications
was carried out by the Nakagawa laboratory in the early
2000s [8]. Subsequently, tremendous research efforts have
been invested in improving the link-level performance between
a single LEDs array and a single receiver, focusing on both
the LED components and on the VLC transceivers, where
ambitious multi Gbps targets have been achieved. These exciting link-level achievements set the basis for broadening the
scope of VLC research beyond point-to-point applications [9],
with the focus on VLC aided networking, where various
promising protocols, architectures and cross-layer solutions
have been proposed. Furthermore, LEDs based localisation
has also attracted dedicated research interests, where recent
advances have demonstrated sub-centimetre accuracy and 3D
positioning capability [10]. In addition to the main thrust
research in localisation, communications and networking, VLC
can also provide innovative solutions for a number of usecases [11], including vehicular, Device-to-Device (D2D) and
underwater applications, just to name a few.
Along with the above technical advances, there have been
significantly increased activities in the VLC domain. Large
scale research programmes were launched, bringing together
research-led universities and industries, as exemplified by
the Visible Light Communications Consortium (VLCC), the
EU-FP7 project OMEGA, the consortium on ultra parallel
VLC. Most recently, VLC was also included in the scope
for networking research beyond 5G under the framework
of EU H2020. Dedicated research centres have been also
established, such as the Smart Lighting Engineering Research
Center in US, the Li-Fi Research and Development Centre
in UK and the Optical Wireless Communication and Network Centre in China, etc. Furthermore, in recent years,
ComSoc has published three special issues on VLC in the
Communications Magazine [12], [13] and in the Wireless
Communications Magazine [14]. Moreover, three consecutive

number of papers

2000
1500
1000

conference
journal
total

500
0
07~08

09~10

11~12

13~14

15~16

Fig. 1. Biannual statistics on the number of papers published in IEEE and
OSA in the topic of visible light communications and positioning. These data
are gathered by searching for the keywords (Visible Light Communications,
VLC, Visible light Positioning, VLP).

international workshops on VLC have also been sponsored by
ComSoc along with ICC’15, ICC’16 and ICC’17. Meanwhile,
GlobeCom has continuously offered annual workshops on
optical wireless since 2010. These investments and efforts have
all contributed to the growing success of the subject.
Hence, we see a strong momentum on the research of VLC,
as evidenced by the publication statistics in Fig 1, that motivates this special issue and our paper is organised as follows.
We will first introduce some basics in Section II-A. We will
then review the achievements of three main research strands
of VLC, namely localisation in Section II-B, communications
in Section II-C and networking in Section II-D. After that, we
will discuss about the challenges of VLC in Section III-A and
paint our vision on the future of VLC in Section III-B. Finally,
we will conclude our discourse in Section IV.
II. M AIN R ESEARCH S TRANDS
A. Basics
The modern VLC relies on LEDs for the dual purpose of
illumination and communications. LEDs are solid-state semiconductor devices [7], [15], which are capable of converting
electrical energy to incoherent light through the process of
electro-luminescence. When using LEDs for VLC, data can
be readily modulated by electronically flickering the intensity
of light at a rate fast enough to be imperceivable by human
eyes. At the receiver, data can be directly detected using inexpensive photo-detectors or imaging sensors. This transceiver
structure may be referred to as the Intensity Modulation (IM)
and Direct Detection (DD), which is much simpler than the
conventional coherent RF receivers relying on complicated
and energy-consuming heterodyne structure. Although there
exist implementations of Laser Diodes (LDs) for VLC [16],
[17] that can provide an even higher data-rate, our following
discussions will be based on the more popular LEDs.
The primary function of LEDs is for illumination, hence
the performance of VLC is naturally dependent on the specific characteristics of LEDs [18]. White light is the most
widely used color for many applications, where there are three
different methods to produce while light LEDs. The most
common method is the use of blue light LEDs with yellow
phosphor coating layer [19]. This is a low-cost method, but
the coating layer limits the modulation bandwidth to only
tens of MHz. Early experiments have demonstrated tens of
Mbps data-rate [20], while hundreds of Mbps data-rate may

also be achieved with advanced processing [21] and adaptive
receiver [22]. The second method to generate white light is to
carefully mix the color of Red, Green and Blue (RGB) emitted
from the RGB LEDs [23]. This method is more expensive, but
it offers much wider modulation bandwidth upto hundreds of
MHz, supporting multi Gbps data-rate [24]. The third method
is resulted from the recent advances on micro-LED array [25],
which produces white light through wavelength conversion.
This method is capable of facilitating parallel communications
with even higher data-rate [26].
When the receiving side is considered, there are generally
two types of detectors that are often employed, namely the
photo-detectors and the imaging sensors [27]. In particular,
the Positive-Intrinsic-Negative (PIN) photo-detectors are most
widely considered in current studies, owing to its low cost
to implement and high sensitivity to strong incident light.
By contrast, in the scenario of weak incident light, avalanche
photo-detectors are often more preferred. On the other hand,
imaging sensors are constituted by an array of photo-detectors,
which inherently support the use of pixelated reception owing
to its desirable capability in better separating the optical
channels [28]. Another advantage of imaging sensors is that
they can be readily integrated with camera lens in smart handheld devices [29]. Since the frame-rate of the commonly used
camera lens is low, this type of receiver is particularly suitable
for applications requiring low date-rate [30], such as the
project of CeilingCast [31]. Sometimes, focusing lens may also
be used on top of photo-detectors [32] or imaging sensors [33]
to enhance the reception by collecting more light energy from
diffuse links with increased Field of View (FoV). In addition
to the above receivers, other interesting developments include
prism-array receiver [34], solar-cell receiver [35], rotational
receiver [36] and aperture-based receiver [37], etc.
Since the surface diameter of typical photo-detectors is
several thousand times of the light wavelength, the fast fading
effects are averaged out at the receiver. Hence, when the
indoor propagation is considered, the Line of Sight (LoS)
path-loss effect is typically observed, plus the reflections from
diffuse links [38]. In most of the studies, (dispersive) channel
modelling is carried out by assuming Lambertian type emitters
and for simplicity, we adopt the well-established Infra-Red
(IR) channel modelling [39], with the aid of the convolutionbased simulation method to properly characterize the Power
Delay Profile (PDP) of the propagation channels [40]. This
method comes at the expense of complexity, hence more
efficient computational methods were proposed [41]. Furthermore, there are three typical sources of noise, namely the
ambient noise, short noise and thermal noise. In fact, they are
mutually dependent, hence it is important to carry out system
design by taking into account this property [42].
B. Localisation
The widely used Global Navigation Satellite System
(GNSS) technology provides a coarse localisation capability
that works just fine for the majority of outdoor applications [43]. However, its use for indoor localisation fails, since
the satellite signal has a poor indoor penetration and the multipath reflections are highly complex in indoor environment.

Various alternatives were proposed for indoor localisation
services [44], mainly based on RF techniques such as Wireless
Fidelity (Wi-Fi) or Ultra-Wide Band (UWB). However, the
associated cost is often too high to reach the desired accuracy.
Hence, LEDs based localisation becomes an interesting option
to meet the above demand [45]. This is because the LEDs can
be readily integrated in the existed lightening infrastructure
and several nearby LEDs may provide joint localisation to
achieve a very high level of accuracy. Furthermore, the visible
light spectrum can be used in RF sensitive environment and
the imaging sensors built-in the smart hand-held devices
constitute a convenient receiver for LEDs based localisation, or
inexpensive PDs may be purposely installed. In the future 5G
systems, both manufacturing and service robots will become
dominant Machine Type Communication (MTC) devices. In
homes, factories and warehouses, light is useful for accurate
position control of robots [46], again thanks to its linear propagation property and short wavelength. Explicitly, there exist
several approaches for LEDs based localisation, including the
proximity based approach, the triangulation based approach
and the fingerprinting based approach, where each of them
will be elaborated in detail as follows.
The simplest way to realise LEDs based localisation is the
proximity based approach, where the position of the object is
roughly classified to its nearest LED transmitter. This procedure can be conveniently integrated in the cell search stage,
where no complicated algorithms are imposed [47]. Hence,
the achievable localisation accuracy is very coarse, in terms of
decimetres. When further exploiting the geometric properties
of multiple LEDs, the triangulation based approach is capable
of determining the absolute position of the object and reaching
a localisation accuracy in terms of centimetres. This approach
exploits the measured distance (angles) between the multiple
LEDs and the localisation object, where the measurement
could be Time of Arrival (ToA) [48], Time Difference of
Arrival (TDoA) [49], Received Signal Strength (RSS) [50]
and Angle of Arrival (AoA) [51]. There are several challenges
associated with these measurements that may deteriorate the
localisation accuracy. For example, both ToA and TDoA
require strict timing synchronisation, RSS requires detailed
knowledge of radiation pattern and AoA requires careful
calibration of light directionality. Hence, localisation based
on a combination of different measurements is a good practice [52], [53], particularly for 3D localisation [54]. Finally, the
fingerprinting based approach determines the position of the
object by comparing on-line measured data with pre-stored
off-line measured data [55]. Depending on the richness of
the pre-stored data set, this approach may provide a higher
level of localisation accuracy, but at the cost of an increased
complexity. Nevertheless, this approach has a poor scalability
to use owing to its scene-dependence.
Unique approach exists for LEDs based localisation using
camera sensors available in smart hand-held devices [56].
Thanks to the high density of pixels, camera sensors can
extract detailed spatial information in order to determine the
position of the object with high localisation accuracy by using
vision analysis and image processing. In addition to only
use camera sensors, accelerometer build-in the smart hand-

held devices can also be exploited together to achieve 3D
localisation [57], [58]. To conclude, despite the existence of
various LEDs based localisation approaches and their achievements, LoS blockage constitutes the single dominant problem,
which is lacking of considerations in the current research.
Hence, the recent study on the effect of multi-path reflections
on positioning accuracy becomes highly practical [59]. More
importantly, when localisation is considered, it should be
included in the entire design chain for achieving a better
integrity [60]. We believe that with further scientific advances,
powerful and robust solutions for LEDs based localisation will
be developed to meet diverse requirements.
C. Communications
As discussed above, VLC is capable of realising the dual
purpose of illumination and communications. Amongst the
cascaded physical layer components of VLC, we elaborate
on those that require contrived design and dedicated discussions, namely optical domain modulation and Multiple
Input Multiple Output (MIMO). Generally, there are two types
of modulation techniques that are commonly employed for
VLC, namely the single-carrier modulation and multi-carrier
modulation. To elaborate on the former, On-Off Keying (OOK)
constitutes the simplest technique for VLC modulation. Hence,
it is the most commonly studied and experimented technique
together with various different types of LEDs. Despite its simplicity, multi Gbps data-rate has been reported in recent experiment [61]. Developed from the plain OOK, there is a family
of pulse-based modulations, such as Pulse Width Modulation
(PWM) [62] and Pulse Position Modulation (PPM) [63]. These
modulations belong to the M-ary orthogonal signalling, which
is particularly suitable for IM. Unique to VLC, Color Shift
Keying (CSK) has been introduced in the VLC standardisation 802.15.7 for simultaneously achieving a higher data-rate
and a better dimming support [64], [65]. Specifically, CSK
modulates the signal based on the intensity of the RGB color,
relying on the employment of RGB LEDs. Other interesting
schemes exploiting the color property of VLC may be found
in [66], [67]. It is worth noting that as a modulation technique,
CSK is different from the concept of Wavelength Division
Multiplexing (WDM) [68]. This is because in WDM system,
additional modulation may be designed together with each of
the multiplexing layers.
When considering the multi-carrier modulation family, the
celebrated Optical Orthogonal Frequency Division Multiplexing (OOFDM) schemes are often employed. This is because
OOFDM scheme allows parallel data transmissions, and it is
also capable of combating the detrimental channel dispersion
without complex time domain equalisations. Different from the
conventional RF based OFDM schemes, transmitted signals
of OOFDM need to be real-valued and positive in order to
facilitate IM. There are a family of OOFDM schemes, typically including the Asymmetrically Clipped OOFDM (ACOOFDM) scheme and the DC-biased OOFDM (DCO-OFDM)
scheme [69]. In both schemes, the property of Hermitian
symmetry is exploited for rendering the complex-valued signal
to be real-valued, at the cost of halving the bandwidth efficiency. In order to maintain positivity of the transmitted signal,

ACO-OFDM scheme resorts to use only the odd sub-carriers,
while DCO-OFDM scheme resorts to apply sufficient DC bias.
Hence, the former approach is more power efficient, while the
latter approach is more bandwidth efficient [70]. Moreover,
there exist many other interesting realisations of OOFDM,
such as the flip OOFDM [71], unipolar OOFDM [72], multilayer OOFDM [73], hybrid OOFDM [74], and DC-informative
OOFDM [75]. In general, OOFDM schemes suffer from
the classic problem of high Peak to Average Power Ratio
(PAPR) [76] and their performance is also limited by the
clipping distortion [77] and the LEDs’ non-linearity [78],
where many countermeasures have thus been proposed [79],
[80], [81]. Despite all these challenges, OOFDM schemes
have attracted great attentions owing to their high data-rate
potential, robustness to channel dispersion and flexibility in
resource allocation [82].
It is important that the above modulation techniques should
be co-designed with LEDs’ illumination requirements to avoid
flickering and support dimming [83]. Flickering refers to the
undesirable effect of human perceivable brightness fluctuation,
or in other words light intensity changes. This is relatively
easy to cope with by using Run Length Limited (RLL) coding
in order to balance the resultant zeros and ones [84], [85].
On the other hand, modern LEDs have now been capable of
supporting arbitrary levels of dimming for energy saving and
environmental-friendly illumination. Hence, a more intriguing
aspect is to jointly design modulation and dimming [86]. In
general, there are two approaches to support dimming with
modulation, namely to control either the light intensity or the
signal duty cycle, where the former is easier to implement
and the latter achieves higher precision [87]. As an example,
in OOK, its on or off levels can be defined to support dimming
or one may introduce compensation period along with the
signalling period without resorting to intensity modification.
Amongst others, pulse-based modulations show great flexibility in support dimming, such as the variable PPM scheme
proposed in the 802.15.7 standard [88]. On the other hand,
dimming support in multi-carrier modulation requires further
investigations, where recent research has been dedicated to
this direction [89], [90]. In addition to modulation, channel
coding schemes can also be co-designed with dimming support
in mind, as demonstrated in various research efforts, including
turbo codes [91], Reed-Muller codes [92], adaptive codes [93]
and concatenated codes [94].
Similar to the conventional RF based wireless communications systems, MIMO in VLC is also capable of providing
promising throughput enhancements [95]. It is known that the
full potential of MIMO can only be achieved in a fully scattered environment. However, in VLC, the particular challenge
is that the optical MIMO channels are often highly correlated
and the resultant MIMO channel matrix appears to be rank
deficient [96]. To create full rank MIMO channel matrix, it is
crucial to maintain clear separation of the transmitters at the
(non)-imaging receiver by careful calibration. In addition to
support full rank MIMO channel matrix, robustness to receiver
tilts and blockage is also desirable, where it has been shown
that specifically designed receivers to harness angle diversity
constitutes a good design practice [97], [98]. Most recently,

the research on MIMO in VLC under diffuse channel is also
emerging, leading to substantial practical insights [99]. As
far as the MIMO functions are considered, MIMO in VLC
can achieve diversity gain by using space time coding [100],
multiplexing gain by using parallel transmission [101] and
beamforming gain by using electronic beam steering [102].
Multiple transmit luminaries can also be used to improve
the security of VLC transmission by making the VLC signal
difficult to intercept by an eavesdropper. Several linear beamforming schemes for active and passive eavesdroppers have
recently been presented in [103], [104], [105]. Being an
important generalisation, Multiple Input Single Output (MISO)
transmission and in particular, Multi-User MISO (MU-MISO)
transmission have attracted substantial research interests [106].
This is because the MU-MISO scheme provides beneficial
multi-user diversity gain without incurring rank deficient
MIMO channel matrix. However, in this multi-user scenario,
challenges arise when performing inter-user interference cancellation, where (non)-linear transmit pre-coding scheme is
required.
D. Networking
The above mentioned advances in physical layer research
of VLC have lead to the development of VLC aided networking [107]. Being next to the physical layer, there are
several different candidates for the Medium Access Control
(MAC) layer, including both the multiple access and random
access schemes. Let us now elaborate on the multiple access
scheme first, the most straightforward arrangement is the
Time Division Multiple Access (TDMA) scheme [108], [109],
where users simply access the network in different time slots.
When multi-carrier modulation is employed, the Orthogonal
Frequency Division Multiple Access (OFDMA) scheme allows
users to be allocated different Time and Frequency (TF)
resource blocks [110]. When compared to TDMA scheme,
OFDMA scheme provides a higher flexibility in terms of resource allocation and user scheduling, at a modestly increased
complexity. In addition to the above orthogonal multiple
access schemes, in Non-orthogonal Multiple Access (NOMA)
scheme [111], [112], two or more users may be multiplexed in
power domain in addition to the conventional orthogonal TF
domain. At the receiver side, onion-stripping type interference
cancellation is required to separate the users from powerdomain non-orthogonal multiplexing [113]. Other than relying
on the power-domain, spatial domain could also be exploited
at the transmitter to realise NOMA [114]. Differently, (multicarrier) Optical Code Division Multiple Access (OCDMA)
scheme relies on assigning each user a unique and specific
optical code [115], [116]. Finally, when random access is
considered, the classic Carrier Sense Multiple Access (CSMA)
scheme remains highly attractive. Importantly, early implementations have already shown its successful usage [117] and
in slotted random access, both contention access periods and
free periods are included, where the latter ensures guaranteed
time slots for resource limited applications.
The most straightforward way of constructing an indoor
VLC cell is to simply consider each Access Point (AP)

function as an individual cell and to adopt the unity frequency
reuse across all cells. This construction would result in the
highest spatial reuse but it tends to suffer from the typical
problem of Inter-Cell Interference (ICI) amongst neighbouring
cells. Following the traditional cellular design principle [118],
different (fractional) frequency reuse patterns could be used to
mitigate ICI, at the cost of a reduced bandwidth efficiency. An
effective method of improving the efficiency, whilst mitigating
the detrimental ICI is to employ cell merging [119], where a
group of neighbouring APs jointly form an enlarged VLC cluster. In this way, the previously ICI-contaminated area becomes
the cluster-centre of the newly formed cluster. Multiple users
can be served simultaneously by using sophisticated Vectored
Transmission (VT) techniques. The underlying principle is to
totally eliminate the ICI at the multiple transmitters side by
using transmit pre-coding, so that the multiple users receive
mutually interference-free signals [120]. However, this technique requires that both Channel State Information (CSI) and
the users’ data have to be shared amongst multiple APs. All of
the above cell formations follow the conventional cell-centric
design approach, which is based on defining a cell constituted
by a fixed set of one or more APs and then associating the
users with it. By contrast, the newly proposed user-centric
design approach relies on the dynamic association between
APs and users [121]. More explicitly, by taking into account
the users’ geo-locations, the new user-centric design flow is
based on grouping the users together and then associating the
APs with them, leading to amorphous cells [122], which is
capable of supporting video service on the move [123]. Finally,
an intriguing piece of research emerges to consider the optimal
placement of LEDs [124], resulting into rich implications on
throughput, delay and mobility.
Holistically, owing to the existence of lightening infrastructure, Power-Line Communications (PLC) constitute a convenient back-haul for VLC as indoor access technology [125],
[126]. PLC can reach luminaries that serve as VLC transmitters to supply the data streams as well as to coordinate
transmission between multiple VLC transmitters to support
multi-user broadcasting [127]. The VLC transceivers can be
considered as relays that can operate in a full-duplex model
and different relaying paradigms such as amplify-and-forward
and decode-and-forward are possible [125]. From a networking perspective, VLC can be considered as a new member in
the small-cell family of the Heterogeneous Networks (HetNet)
landscape for complementing the over-loaded Radio Access
Technology (RAT). Indeed, the interplay between VLC and
RAT system has been an active area of research [128], [129],
where there are two different interplay scenarios that may be
envisioned, namely single-homing and multi-homing. In the
single-homing scenario, only one access system is allowed
to maintain its association at any instant. In this scenario,
dynamic Load Balancing (LB) will prevent traffic congestion
caused by blockage or mobility through diverting the traffic
flow appropriately [130]. To better exploit the access system’s
diversity potentials, in the multi-homing scenario, each user
maintains multiple associations at the same time by using the
Multipath Transport Control Protocol (TCP) to connect multiple interfaces [131]. In either scenario, robust vertical han-

dover has to be properly designed to mitigate any ping-pong
effect, where the load-ware mobility management appears to
be a promising solution [132]. Different from using higher
layer converging approaches, seamless rate adaptation between
VLC and RAT systems may also be achieved by network
coding and rate-less coding [133]. To sum up, in the light of
the information and communications technology convergence,
the above mentioned network layer functions should be software defined to maximise its full potential [134], [135].
III. C HALLENGES AND O PPORTUNITIES
A. Challenges
1) Channel Modelling: Most of the current channel modelling in VLC was directly adapted from the IR communications. However, it would be ideal to develop specific VLC
channel modelling, corresponding to different types of LEDs
for the ease of cross-calibration. In particular, with regards
to shadowing, there is a lack of both empirical and statistical
modelling. In most of the studies, the shadowing effect was
often assumed to follow the over-simplified Bernoulli distribution. However, given the sensitivity of VLC over shadowing
in all aspects of localisation, communications and networking,
a well-calibrated model is indeed of critical importance. Also
importantly, VLC channel modelling for vehicular applications
is still in its infancy, which requires dedicated efforts.
2) Interference Mitigation and Management: VLC relies
on visible light spectrum which overlaps with solar radiation
and indoor/outdoor lighting and display. A VLC system is
inevitably interfered by those sources, from day and night
time. Therefore, effective techniques are needed to mitigate
not only inter-system interference from neighbouring LEDs,
but also external light interference. Considering those interference sources typically emit a large dynamic range of light
intensities, a robust and sensitive detector needs to respond
reliably to transmitted signals while suppressing interference
to certain extent. It is necessary but very challenging to
develop advanced methods and algorithms to recover useful
signals, weak or strong, from noisy signals overwhelmed by
interference, weak or strong. For example, normal operation of
a vehicular receiver or an indoor receiver under direct sunlight
through a window at noon is extremely difficult.
3) Channel Feedback: Several contributions make a massive use of channel state information. While this aspect is
not critical in approaches that move all the processing toward
the receiver since estimation techniques are not so difficult
to implement both with training-based or (semi)-blind, when
the communications requires a feedback link, as for preequalization, bit loading, retransmission schemes, the feedback
channel performance becomes an issue. The problem of how
a feedback channel is used is a very challenging task either if
it is an optical one and if it is based on RF. In fact, important
aspects for example quantisation and error control must be
properly taken into account, in addition to delay and jitter.
4) Access Techniques: A high percentage of access techniques proposed in the literature and described above start
from the tentative of adapting access mechanisms and procedures already used in the RF for implementing them into the

VLC context. However, mechanisms that take into account
the fact that VLC receivers usually have one or more photodetectors that are not omnidirectional as well as for emerging
VLC applications such as vehicular and camera communications, should be investigated. Access techniques become more
complicated, when a user is covered by some LEDs in indoor
environment, due to the tilt of the device, the signal can be
received from another set of LEDs. Hence, access techniques
are strictly related also to the localisation techniques.
5) Mobility Management: In many VLC applications, VLC
terminals or transceivers are mobile, such as hand-held devices, vehicles, and robots. Maintaining a quality communications link in a point-to-point case and network connectivity in
a multiple-user case are important to avoid communications
losses. Protocols for mobile optical wireless communication
networks are in urgent need, for dynamic system resource
adaptation to communications environments, easy node access
and drop-off, smooth handover from one AP to another and
from one network to another. Meanwhile, mobility prediction
and network topology modelling are under-explored, and these
topics open new room for further investigation.
6) Integrated Smart Lighting: Since VLC is not a paradigm
separated from illumination, the ultimate future challenge can
be a full integration that takes into account not only communications performance related to the single link under illumination constraints but also a holistic optimisation regarding both
the networking (system performance and seamless handover)
and the perceived illumination (a good level of light all over
the room). This is an issue especially in large rooms such
as museums, where multimedia content may be based on the
position of the user in the information centric system. Hence,
the aim is to integrate lighting, communications, networking
and positioning in a single homogeneous framework.
B. Opportunities
1) 5G-Home: The concept of 5G-Home is based on the
unified heterogeneous access-home network with wired and
(optical) wireless connectivity, which is capable of creating
tens of millions of additional 5G-Home sites. Technically, to
succeed in a timely and affordable manner, both the existing
copper or cable based fixed access and in-home wiring technique need to be exploited in addition to fibre [136], [137].
Deep in the home, the number of wireless APs and terminals
will be going up corresponding to the increasing bandwidth,
delay and coverage requirements. At least one AP per room
may become the norm given the higher carrier frequencies that
will be used for 5G, including Wi-Fi standards 802.11ac/ad
and VLC aided networking [138], [139]. Hence, we envision
beneficial convergence between fixed access network infrastructure and in-home (optical) wireless networks, in order to
eliminate the boundary between these two domains [140].
2) 5G-Vehicle: Future 5G systems will embody a number
of new applications which span across vast areas beyond enhanced mobile broadband services, such as media distribution,
Smart Cities, and Internet of Things (IoT). In Smart Cities,
light-enabled vehicular communications networks utilize a
large number of densely distributed street lamp poles as

APs, while vehicular lights, roadside street lights, pedestrian
signage and traffic lights can all act as ubiquitous transmitters to meet the need of massive connectivity and high
throughput [141], [142]. Equipped with image sensor or other
types of receivers, these nodes in the immediate vicinity
of vehicles provide ultra-reliable and low latency (mission
critical) communications and control links [143], [144], which
will serve well intelligent transportation, collision avoidance,
autonomous driving and telematics. However, challenges will
arise when experiencing strong ambient noise, while uneven
speed and diverting route may be also difficult to handle [145],
[146]. Nevertheless, vehicles become mobile offices and entertaining homes, enjoying amount of exterior resources.
3) Underwater Communications: In addition to groundbased indoor and outdoor applications, VLC also paves the
way for outreach applications deep into water. Currently,
underwater acoustics is the major wireless technology for
underwater object detection and communications. Due to the
very low data rate allowed by acoustic communications in
underwater environment and also the poor performance from
the propagation point of view in the RF bands, light sources
offer a unique opportunity for short-range high speed communications potentially at low cost and low power [147], [148],
[149], [150]. Data rate of hundreds to thousands of Mbps is
possible, and the communications distance is extendible to
hundreds of meters in clear water. The VLC technology will
undoubtedly play a critical role in marine resource exploration,
water sensing, monitoring and tracking of marine organism,
and saving endangered species.
4) Emerging Applications: LEDs are natural cheap communications transmitters, and CMOS image sensors are equipped
in pervasive consumer electronics as detectors. New applications emerge with these tiny optical sensors. Image Sensor
Communications (ISC) and positioning is easily realized based
on CMOS sensors built upon smart phones. Millions of pixels
can be maximally explored for communications [151] and
accurate positioning [152]. The LED screen-to-camera communications constitute another interesting application, which
may facilitate unimaginable potentials in entertainment, education, broadcasting and near-field communications, etc. Several
early experiments have been already carried out with exciting findings, including projects of SoftLight [153], Uber-inlight [154] and SBVLC [155] etc. In addition to traditional
LEDs, emerging Organic LEDs (OLEDs) are attractive in
flexibility, easy integration and fabrication, convenient color
selectivity, and wide viewing angle. Thus they serve as wearable and portable VLC transmitters as well as fixed-location
large screen communication transmitters [156], [157], [158].
Application domains encompass those of LEDs as well as new
body area networks and sensor networks. Last but not least,
LED based lighting infrastructures are becoming ubiquitous
and have already been dubbed as the ‘eyes and ears of the
IoT’ in the context of smart lighting systems. Hence, VLC is a
promising solution for indoor communications to a broad class
of smart objects in particular in scenarios with high density
of IoT devices. Steps along this direction have been presented
in [159], [160], [161].

5) Commercialisation: To create a larger economic and
societal impact, the above mentioned academic research has
to be in collaboration with industries. More importantly, the
future success of VLC urges the joint efforts from both information industry and illumination industry, driven by demands
from verticals, such as health, automotive, manufacturing,
entertainment etc. The inclusive of mobile phones within
the ecosystem would be highly desirable to the wide public
acceptance. One promising area for real-world VLC to grow
is the future pervasive IoT in consumer electronics, retailers,
warehouses, offices and hospitals, etc. For example, Philips
has recently commercialised VLC aided indoor localisation for
the hypermarket in France 1 . They have also teamed up with
Cisco on an exciting IoT project called ‘Digital Ceiling’ 2 ,
which connects all of a building’s services in a single and
converged network, where empowered LEDs can be used to
collect, send and analyse data.

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[208] F. Jin, R. Zhang, and L. Hanzo, “Frequency-swapping aided femtocells
in twin-layer cellular networks relying on fractional frequency reuse,”
in 2012 IEEE Wireless Communications and Networking Conference,
April 2012, pp. 3097–3101.
[209] J. Zhang, R. Zhang, X. Xu, G. Li, and L. Hanzo, “Effects of practical
impairments on cooperative distributed antennas combined with fractional frequency reuse,” in 2012 IEEE Wireless Communications and
Networking Conference, April 2012, pp. 1088–1092.
[210] X. Xu, R. Zhang, and L. Hanzo, “Digital RoF aided cooperative
distributed antennas with FFR in multicell multiuser networks,” in 2011
IEEE Vehicular Technology Conference Fall, Sept 2011, pp. 1–5.
[211] J. Feng, R. Zhang, and L. Hanzo, “Auction-style cooperative medium
access control,” in 2011 IEEE Vehicular Technology Conference Fall,
Sept 2011, pp. 1–5.
[212] J. Feng, R. Zhang, S. X. Ng, and L. Hanzo, “Relay selection for energyefficient cooperative media access control,” in 2011 IEEE Wireless
Communications and Networking Conference, March 2011, pp. 287–
292.
[213] R. Zhang and L. Hanzo, “Variable-rate network coding for multi-source
cooperation,” in 2011 IEEE Wireless Communications and Networking
Conference, March 2011, pp. 1517–1522.
[214] R. Zhang, K. Giridhar, and L. Hanzo, “Distributed downlink multi-cell
processing requiring reduced-rate back-haul data exchange,” in 2011
IEEE Wireless Communications and Networking Conference, March
2011, pp. 1277–1281.
[215] R. Zhang, X. Xu, and L. Hanzo, “Co-channel interference mitigation
capability of fixed relays connected by optical fibre,” in 2010 IEEE
Vehicular Technology Conference Fall, Sept 2010, pp. 1–5.
[216] X. Xu, R. Zhang, and L. Hanzo, “Imperfect radio over fibre aided
distributed antennas with fractional frequency reuse,” in 2010 IEEE
Vehicular Technology Conference Fall, Sept 2010, pp. 1–5.
[217] R. Zhang and L. Hanzo, “Joint and distributed linear precoding for
centralised and decentralised multicell processing,” in 2010 IEEE
Vehicular Technology Conference Fall, Sept 2010, pp. 1–5.
[218] ——, “Harmony search aided iterative channel estimation, multiuser
detection and channel decoding for DS-CDMA,” in 2010 IEEE Vehicular Technology Conference Fall, Sept 2010, pp. 1–5.
[219] ——, “Multiplexed hybrid ARQ for energy efficient transmissions
under delay constraints,” in 2010 IEEE International Conference on
Communications, May 2010, pp. 1–5.
[220] M. F. U. Butt, R. Zhang, S. X. Ng, and L. Hanzo, “Superposition coding
aided bi-directional relay transmission employing iteratively decoded
self-concatenated convolutional codes,” in 2010 IEEE Vehicular Technology Conference Spring, May 2010, pp. 1–5.
[221] R. Zhang and L. Hanzo, “Superposition-coding aided multiplexed
hybrid ARQ scheme for improved link-layer transmission efficiency,” in
2009 IEEE International Conference on Communications, June 2009,
pp. 1–5.
[222] N. Bonello, R. Zhang, S. Chen, and L. Hanzo, “Reconfigurable rateless
codes,” in 2009 IEEE Vehicular Technology Conference Spring, April
2009, pp. 1–5.
[223] R. Zhang and L. Hanzo, “Physical-layer algebraic network coding and
superposition coding for the multi-source cooperation aided uplink,” in

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1–5.
——, “High-throughput non-orthogonal interleaved random space-time
coding for multi-source cooperation,” in 2008 IEEE Global Telecommunications Conference, Nov 2008, pp. 1–5.
N. Bonello, R. Zhang, S. Chen, and L. Hanzo, “Channel code division
multiple access and its multilevel structured LDPC based instantiation,”
in 2008 IEEE Vehicular Technology Conference Fall, Sept 2008, pp.
1–5.
R. Zhang, L. Xu, S. Chen, and L. Hanzo, “Repeat accumulate code
division multiple access and its hybrid detection,” in 2008 IEEE
International Conference on Communications, May 2008, pp. 4790–
4794.
L. Xu, R. Zhang, S. Chen, and L. Hanzo, “EXIT-chart aided hybrid
multiuser detector design for frequency-domain-spread chip-interleaved
MC-CDMA,” in 2008 IEEE Vehicular Technology Conference Spring,
May 2008, pp. 1816–1820.
R. Zhang and L. Hanzo, “Interleave division multiplexing aided spacetime coding for high-throughput uplink cooperative communications,”
in 2008 IEEE Wireless Communications and Networking Conference,
March 2008, pp. 465–469.
——, “EXIT chart based joint code-rate and spreading-factor optimisation of single-carrier interleave division multiple access,” in 2007
IEEE Wireless Communications and Networking Conference, March
2007, pp. 735–739.

