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Abstract—In the past few years, the demand for high data rate
services has increased dramatically. The congestion in the radio
frequency (RF) spectrum (3 kHz ~ 300 GHz) is expected to limit
the growth of future wireless systems unless new parts of the
spectrum are opened. Even with the use of advanced engineering,
such as signal processing and advanced modulation schemes, it will
be very challenging to meet the demands of the users in the next
decades using the existing carrier frequencies. On the other hand,
there is a potential band of the spectrum available that can provide
tens of Gbps to Tbps for users in the near future. Optical wireless
communication (OWC) systems are among the promising
solutions to the bandwidth limitation problem faced by radio
systems. In this paper, we give a tutorial survey of the most
significant issues in OWC systems that operate at short ranges
such as indoor systems. We consider the challenging issues facing
these systems such as (i) link design and system requirements, (ii)
transmitter structures, (iii) receiver structures, (iv) challenges and
possible techniques to mitigate the impairments in these systems,
(v) the main applications and (vi) open research issues. In indoor
OWC systems we describe channel modelling, mobility and
dispersion mitigation techniques. Infrared communication (IRC)
and visible light communication (VLC) are presented as potential
implementation approaches for OWC systems and are
comprehensively discussed. Moreover, open research issues in
OWC systems are discussed.
Index Terms— Optical wireless communication (OWC) system,
indoor optical wireless systems, IRC , VLC.
I. INTRODUCTION
RADITIONAL radio communication systems suffer from
limited channel capacity and transmission rate due to the
limited radio spectrum available, while the data rates requested
by the users continue to increase exponentially. Different
techniques have been proposed to use the radio spectrum in
more efficient ways including advanced modulation, coding,
smart antennas and multiple input and multiple output (MIMO)
systems [1], [2].
Achieving very high data rates (beyond 10 Gbps and into the
Tbps regime) using the bandwidth of radio systems is
challenging. According to Cisco, mobile Internet traffic over
this half of the decade (2016-2021) is expected to increase by
27 times [3]. Given this expectation of dramatically growing
demand for data rates, the quest is already underway for
alternative spectrum bands beyond radio waves. The latter are
bands currently used and planned for near future systems, such
as 5G cellular systems.
Different technology candidates have entered the race to
provide ultra-fast wireless communication systems for users,
such as optical wireless communication (OWC) systems for
indoor usage [4]–[6].
OWC systems are candidates for high data rates in the last
mile of the access network. They have been investigated for
more than three decades as an alternative solution to support
high speed data instead of radio frequency (RF) systems [4].
However, this technology was not deployed until the 1990s
when the transmitter and the receiver components became
available at low cost [7]. Their rate of adoption is expected to
grow now in view of the use of semiconductor sources in
lighting applications indoor and in view of the significant
growth witnessed in data rates. In OWC systems, a modulated
beam of infrared, visible or ultra violet light is transmitted,
typically through the atmosphere, by an OW transmitter. OWC
systems use light emitting diodes (LEDs) or laser diodes (LDs)
to transmit light similar to what is done in fibre optic systems.
Indoor OWC systems include systems such as those
standardized by the infrared data association (IrDA), and
systems developed to support infrared optical wireless
communication (IRC) and visible light communication (VLC)
[8].
Fig. 1: The Electromagnetic spectrum.
OWC systems have been shown in many studies to be able
to transmit video, data and voice, at data rates up to 25 Gbps in
indoor systems [9], [10], [11], [12], [13].
OWC systems offer many favourable features when
compared to RF systems, such as rapid deployment, low start-
up and operational costs and higher bandwidth, similar to fibre
optics. The available OW spectral regions offer virtually
unlimited bandwidth, such as 1 mm – 750 nm for infrared, 780
nm – 380 nm for visible light and 10 nm – 400 nm for UV as
shown in Fig. 1. However, there are certain wavelengths that
are suitable for communication in OW spectrum such as the
range between 780 – 950 nm which is the optimal range
considered in IRC and the ultraviolet-C band, which is the best
range for wide field of view (FOV) communication [14].
Optical Wireless Communication Systems, A
Survey
Osama Alsulami1, Ahmed Taha Hussein1, Mohammed T. Alresheedi2 and Jaafar M. H. Elmirghani1
1School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT, United Kingdom 2Department of Electrical Engineering, King Saud University, Riyadh, Kingdom of Saudi Arabia
[email protected], [email protected], [email protected], [email protected]
T
2
Since, the OW spectrum is unregulated and is licence free,
the cost of the OW systems can be reduced compared to RF
systems.
The main features and drawbacks of OWC systems are
presented here. The nature of light gives OWC systems
immunity against interference caused by adjacent channels in
neighbouring rooms, with the possibility of frequency reuse in
different parts of the same building, which means increased
capacity. OWC systems also offer better security at the physical
layer. This is due to the fact that light does not penetrate opaque
barriers, which means the potential for eavesdropping is
reduced unlike in radio systems, and this reduces the need for
data encryption [5], [15]. In addition, the detector (photodiode)
has a very large area, typically tens of thousands of
wavelengths, and this leads to efficient spatial diversity at the
receiver [15]. These combined favourable features make OWC
systems suitable replacements for conventional radio systems.
However, OWC systems have several drawbacks. In indoor
systems, optical wireless access points, in different rooms, have
to be connected via a wired backbone as light cannot penetrate
walls. Also, because of multipath propagation, reflections and
signal spread, optical signals suffer from attenuation and
dispersion leading to inter symbol interference (ISI).
Furthermore, the signal at the receiver can include shot noise
induced by intense ambient light sources (sunlight,
incandescent lighting and fluorescent light), and this leads to
signal corruption by this background noise [16], [17].
Moreover, the transmitter power is limited by eye and skin
safety regulations [18], [19].
OWC systems require a receiver with a photo sensitive
detector with large area to collect the maximum optical signal
possible, and this leads to an increase in the capacitance of the
photodetector, consequently the available receiver bandwidth
decreases.
Table 1 compares Radio and OW systems. To place OWC in
the communications scene, Fig. 2 summarizes the coverage area
and data rates of the available systems in traditional RF and
OWC systems. Table 1: Comparison of radio and OW systems
Property Radio System OW System
Bandwidth Regulated YES NO
Security Low High
RF Electromagnetic
interference YES NO
Passes through walls YES NO
Technology Cost High Low
Beam Directionality Low Medium
Available bandwidth Low Very high
Transmitted Power Restricted
(Interference) Restricted (Eye safety
and interference)
Noise Sources Other Users and
Systems
Sun light and Ambient
Light
Power consumption Medium Relatively low
Multipath Fading YES NO
OWC systems can provide high data rates beyond 10 Gbps
[9], [11], [12], [13], however, there are many challenges to
implement OW systems. For instance, fog, rain and dust reduce
data rates and coverage area of free space optics (FSO) outdoor
systems, while multipath propagation, receiver noise and
interference tend to limit the capacity of indoor OWC systems.
Fig. 2: Data rates and coverage area of different wireless technologies [9],
[11], [12], [13], [20].
The rest of the paper is organized as follows: Section II
presents a discussion of link design, modulation techniques and
the channel characteristics of OWC systems. Section III
discusses the available transmitter architectures. Section IV
summarizes the OW receiver structures and hence the design of
OWC systems. In Section V, the main challenges faced in OWC
systems are described. Due to the importance of VLC systems,
we provide a comprehensive review of VLC systems in a
separate Section VI. Section VII summarizes the main
applications of OWC (IRC and VLC) systems. Section VIII
gives an overview of the open research issues in IRC and VLC
systems. Finally, we conclude the paper in Section IX.
II. LINK DESIGN, CHANNEL CHARACTERISTICS AND
MODULATION TECHNIQUES IN OW SYSTEMS
In this section, we discuss various link designs that can be
employed in OWC systems. A brief description of the
modulation techniques and channel characteristics of indoor
OWC system is also provided.
A. Link design
There are two criteria to classify OWC links: (a) the level of
directionality of the receiver and the transmitter and (b) if there
is a direct path between the receiver and the transmitter. These
classifications are based on the radiation pattern of the
transmitter and field of view (FOV) of the receiver. Link
classification schemes are illustrated in Fig. 3. The two most
common link configurations for indoor systems are line of sight
(LOS) and non-LOS (NLOS) [5], [9], [21], [22]. LOS links
provide a direct path between the transmitter and the receiver
which minimizes multipath dispersion and enhances the power
efficiency of the communication system. However, LOS links
suffer from shadowing. On the other hand, NLOS links rely on
reflections from the walls, ceiling and other objects. They offer
robust links and protection against shadowing but are severely
affected by multipath dispersion, which results in ISI and pulse
spread [22].
Both LOS and NLOS schemes can be classified into directed,
hybrid and non-directed links [22]. The NLOS non-directed
3
scenario is considered to be the most desirable method for
indoor systems from the point of view of mobility, however it
suffers dispersion which can limit the achievable data rate and
is also limited by the low power collected. A very popular
diffuse system is the conventional diffuse system (CDS) [22].
CDS employs a wide FOV receiver and a transmitter with wide
radiation pattern. The transmitter and receiver point towards the
ceiling and the received signal reaches the receiver through
multiple reflections. The CDS impairments can be reduced by
using specific receivers, such as the triangular pyramidal fly-
eye diversity receiver which uses an optimum value of the FOV
to mitigate ISI in indoor OWC systems [21]. However, non-
directed links do not have good power efficiency. Thus,
employing different degrees of directionality at the transmitter
and receiver (hybrid link) can give good performance [22].
LOS communication using directed links makes use of a
narrow radiation pattern between the transmitter and the
receiver (FOV), see Fig. 3 [22]. This scenario reduces the
impact of ambient noise and minimizes the path loss, while also
improving the power efficiency.
Directed links are however not suitable for mobile
applications especially for indoor OWC usage. In contrast, non-
directed links use a wide transmitter radiation pattern and wide
FOV at the receiver. Therefore this configuration can provide
mobility for users (see Fig. 3).
Fig. 3: Optical wireless link classification
B. Channel characteristics and modulation techniques
This section discusses the indoor OW channel characteristics
and the popular modulation schemes used.
1. Channel characteristics
Channel characterization in OWC is vital to better
understand the system performance. The channel impulse
response ℎ(𝑡), which can fully describe the multipath
propagation in the OWC system, is given by [23]:
𝑦(𝑡, 𝐴𝑧, 𝐸𝑙) = ∑ 𝑅𝑥(𝑡) ⊗ ℎ𝑘(𝐾
𝑘=1𝑡, 𝐴𝑧, 𝐸𝑙)
+ 𝑅 ∑ 𝑁𝑘𝐾𝑘=1 (𝑡, 𝐴𝑧, 𝐸𝑙) (2)
where 𝐾 is the total number of reflecting elements in the
room, 𝐴𝑧 and 𝐸𝑙 are the directions of arrival in the azimuth and
elevation angles, respectively. By estimating the impulse
response, many parameters can be determined, such as the root
mean square, delay spread, 3 dB channel bandwidth, spatial
power distribution and signal to noise ratio (SNR), [23].
To give examples of the OW channel impulse response, we
show the impulse responses of CDS and of line strip multi beam
system (LSMS). The latter is a NLOS hybrid system where the
transmitter faces up and produces a number of spots on the
ceiling [23]. In addition, the impulse response when using
different types of receiver configurations in VLC systems are
shown. A ray-tracing algorithm simulation package was
developed using the same room configuration and parameters
used in [21], [23], [24], [25] and the simulation scheme was
based on [26]. The room has length × width × height of
8𝑚 × 4𝑚 × 3𝑚. The devices used for communication are
placed on a “communication floor”, ie typical desk surfaces that
are one meter above the floor. In the IRC system, the transmitter
and receiver were located at the center of the room on the
communication floor, and the receiver specifications for the
IRC example were based on [27]. In the VLC system 8 light
units were used as transmitters and each light unit has 9 LEDs
where the light units are located in different positions on the
ceiling and the receiver is located at the room center on the
communication floor. The locations of the light units, room
dimensions (same as above) and receiver specifications of the
VLC system are based on [24], [25]. Fig. 4A illustrates the
impulse responses of the most well-known indoor IRC systems
(CDS, LSMS), Fig. 4B shows the impulse response of a VLC
system with a single wide field of view (WFOV) receiver, Fig.
4C shows the impulse response of a VLC system with three
branch angle diversity receiver (ADR) and Fig. 4D shows the
impulse response of the VLC system using a 50 pixel imaging
receiver.
Fig. 4: (A) Impulse responses of CDS and LSMS systems with single
element wide FOV receiver (𝐹𝑂𝑉 = 90°) and LSMS systems with three
branches ADR; transmitter and receiver are located at the room center, (B) Impulse responses of VLC system with single wide FOV receiver, (C)
Impulse responses of VLC system with three branch ADR, (D) Impulse
responses of VLC system with 50 pixels imaging receiver.
Fig. 5 (A) shows that the CDS has the highest delay spread
and hence the smallest channel bandwidth compared to the
other systems. The LSMS system results in a lower delay spread
due to the presence of the diffusing spots which act as
secondary transmitters close to the receiver. Also as Fig. 5 (A)
shows, reducing the receiver FOV, results in the collection of a
smaller range of rays and hence a larger channel bandwidth.
Table 2 provides a comparison of the OW channel
bandwidths achievable using different OW systems[24]. The
results are shown at x=2m and along the y axis. The minimum
channel bandwidth is observed in the case of the CDS with wide
FOV receiver, and is 41 MHz in the 8𝑚 × 4𝑚 × 3𝑚 room
using the parameters in [24]. The use of an imaging receiver in
4
conjunction with the CDS increases the minimum channel
bandwidth to 160 MHz due to the use of narrow FOV pixels in
the imaging receiver which reduce the range of rays collected.
The imaging receiver however provides wide coverage through
its many pixels which also result in increased collected power.
The use of an LSMS transmitter results in multiple diffusing
spots cast on the ceiling, which act as secondary emitters close
to wherever the receiver is located. This increases the channel
bandwidth. The use of power adaptation, where the spot closest
to the receiver is allocated more power, reduces the delay
spread further, resulting in a minimum channel bandwidth of
7.5 GHz as Table 2 shows.
Table 2: Comparison of the 3dB channel bandwidth of
different OW systems [28]
Figs. 5 (B) and (C) show the delay spread of VLC systems
with wide FOV receiver, angle diversity receiver (ADR) and
imaging receiver. Both the ADR and imaging receiver employ
multiple receivers with smaller FOV which reduces the delay
spread. Full coverage of the room is achieved through the use
of the multiple receivers in ADR and the many pixels in the
imaging receiver. The smaller FOV of each receiver element in
the imaging receiver used, results in reduced delay spread
compared to ADR, as Figs 5 (B) and (C) show.
The VLC system with an imaging receiver provides low
delay spread as shown in Fig. 5. It also provides good SNR at
high data rates compared to the other systems as shown in Fig.
6. The oscillation in delay spread is due to the proximity (or
otherwise) from the transmitters (lighting sources in VLC) as
the receiver moves along the y axis. The oscillations in the SNR
are due to similar effects and can be more significant in infrared
systems where the room lighting can act as a noise source. The
use of optimum receiver element selection or receiver element
signal combining techniques in ADR [24] and imaging
receivers [24], [25] can result in more uniform SNR in the room
where the best receiver element that avoids noise and/or
interference for example is selected.
(A)
(B)
(C)
Fig. 5: (A) Delay Spread of CDS and LSMS systems with single element
wide FOV receiver (90°) and LSMS system with three branch ADR;
transmitter and receiver are located at the room center, (B) Delay Spread of
VLC system with single wide FOV receiver and three branch ADR, (C) Delay
Spread of VLC system with single wide FOV receiver and 50 pixels imaging
receiver.
(A)
(B)
(C)
(D)
Fig. 6: (A) SNR of CDS and LSMS systems with single element wide FOV
receiver (90°) and LSMS system with three branch ADR; transmitter and
receiver are located at the room center, (B) SNR of VLC system with single wide FOV receiver and three branch ADR, (C) SNR of VLC system with
single wide FOV receiver and 50 pixels imaging receiver at low data rate, 30
Mbps, (D) SNR of 50 pixels imaging receiver at high data rate, 5 Gbps.
2. Modulation techniques
OWC channels are different from traditional RF channels.
This has resulted in different methods of modulation being
used. Modulation schemes that fit well in RF channels do not
necessarily perform well in the optical domain. There are four
criteria that guide the choice of a specific modulation technique
for OWC systems. Of particular importance as a criterion to be
applied, is the average power used (power efficiency) of a given
modulation format. This is important in view of eye hazards and
power consumption in mobile terminals. The second criterion
is the available channel bandwidth and receiver bandwidth
requirements. The third factor is the complexity of the
modulation format (and power consumption in portable
devices). The last set of factors relate to the physical limitations
in the transmitter (i.e. LD or LED) which the modulation format
may have to take into account. Modulation in OWC systems
consists of two steps: in the first step the information is coded
as a waveform(s) and then (second step) these waveforms are
modulated onto the instantaneous power of the carrier [22].
5
Intensity modulation (IM) and direct detection (DD) is the
preferred transmission technique in OWC systems [9], [22]. IM
is achieved by varying the bias current of the LD or the LED.
In OWC systems, the transmitted signal must always be
positive in intensity. Direct detection is the simplest method
that can be used to detect an intensity modulated signal. The
photodetector generates a current that is proportional to the
incident optical power intensity. A simple description for the
IM/DD channel is given as [22]:
𝑦(𝑡) = 𝑅𝑥(𝑡) ⊗ ℎ(𝑡) + 𝑅𝑛(𝑡) (1)
where 𝑅 is the photodetector responsivity, 𝑦(𝑡) is the
instantaneous photo current received, 𝑡 is the absolute time, ⊗
denotes convolution, ℎ(𝑡) is the channel impulse response, 𝑥(𝑡)
is the instantaneous transmitted power and 𝑛(𝑡) is the
background noise (BN), which is modelled as white Gaussian
noise, and is independent of the received signal.
Three broad types of modulation schemes can be applied in
OWC (a) baseband modulation, (b) multicarrier modulation and
(c) multicolor modulation [14], [29].
a. Baseband modulation
The main baseband modulation techniques considered in
OWC include (i) pulse amplitude modulation (PAM), (ii) pulse
position modulation (PPM), (iii) pulse interval (PIM)
modulation and (iv) carrierless amplitude phase (CAP)
modulation.
i. PAM
PAM is a simple modulation technique which is widely
considered in OWC systems. On-off keying (OOK) modulation
is a type of PAM with only two levels. OOK is a suitable OWC
modulation approach and is easy to implement in such systems
due to the ability of LDs and LEDs to switch on and off quickly.
However, OOK is not efficient when compared with other
modulation formats [14].
ii. PPM
PPM is another type of modulations that is widely considered
in OWC systems [17]. Differential PPM (DPPM) was evaluated
in OWC system to increase the achievable data rates. It
consumes less power on average when compared to PPM.
However, the distortion in the DPPM signal is greater than in
the PPM signal [30].
iii. PIM
PIM encodes the information by inserting empty slots between
two pulses. Its design relies on accurate synchronization which
can increase its complexity compared to PPM. Digital pulse
interval modulation (DPIM) is a digital version of PIM. The
performance is improved in DPIM compared to PPM by
removing the redundant frame space in PPM, however errors
can propagate from a frame to the next [31].
iv. CAP
CAP modulation is a promising modulation format that can
be used in OWC systems to achieve higher data rates. It is a
bandwidth-efficient two-dimensional passband transmission
scheme [14]. In CAP modulation, two orthogonal filters are
chosen to modulate two different data streams.
OOK and PPM are the most popular modulation schemes
applied in OWC systems [32].
b. Multicarrier modulation
More advanced techniques can be used in OWC systems to
transmit multiple carriers, such as subcarrier modulation
(SCM). This can provide multiple access for simultaneous users
and a high data rate. However, SCM is not power efficient like
single carrier schemes [33]. For instance, each quadrature phase
shift keying (QPSK) or binary PSK subcarrier requires about
1.5 dB more power than OOK. In [34] a high data rate was
achieved while reducing the average power requirements in
SCM modulation. In [35], orthogonal frequency division
multiplexing (OFDM) was applied in indoor OWC systems to
achieve high data rates over a noisy channel and to reduce ISI.
The system, however, did not achieve a high SNR. The main
disadvantages of OFDM are the sensitivity to frequency offset
and phase noise as well as the high peak to average power ratio
(PAR) [36]. In general, the use of complex modulation leads to
an improvement in the performance of the OWC systems, such
as mitigating the ISI effect and increasing the data rates.
However, these modulation techniques require a complex
transceiver.
c. Multicolor modulations
Multicolor modulation has recently been considered to
provide high data rates or multiple access for users [11], [14].
White light can be generated from red, green and blue (RGB)
type LEDs which means data can be transferred through each
color or wavelength. Wavelength division multiplexing
(WDM) is the multiplexing used here. In addition, [37]
achieved white light from four color LDs which provide a better
result compered to three colors LEDs in term of multiple access
and higher data rates due to the improved modulation
capabilities of LDs.
III. TRANSMITTER ARCHITECTURES IN OWC SYSTEMS
This section presents a review of the available transmitter
architectures in OWC systems. The CDS system, the most
commonly investigated transmitter - receiver configuration, is
widely considered [4], [9], [21], [23]. A number of spot
diffusing transmitter architectures were proposed and
evaluated. A uniform multi-spot diffusing transmitter system
was first proposed in [38]. While a diamond multi-spot
diffusing system configuration can be found in [39]. In addition,
there are two attractive methods for multi spot diffusing
configurations: the LSMS and the beam clustering method
(BCM) proposed in [23], [39], [40]. The last two methods have
also been widely investigated by other researchers [41] – [47].
The results of these investigations are very promising. Different
transmitter structures for IRC are shown in Fig. 7. Table 3
summaries the main advantages and disadvantages of the
different transmitter structures in OWC indoor systems.
6
Fig. 7: Different spot diffusing transmitter structures in IRC systems; (A)
Spot diffusing transmitter [4], (B) uniform multi-spot diffusing transmitter
[47], (C) diamond multi-spot diffusing transmitter [38], (D) line strip multi-
beam transmitter [36], (E) multi-beam clustering transmitter [39].
Table 3: Comparison of different transmitter structures in IRC indoor
systems
Type of
Transmitter Advantages Disadvantages
Spot Diffusing
transmitter for
CDS system [4].
Provides link
robustness against
shadowing and blockage.
LOS links do not exist.
(Signal is received due to
reflections from walls and ceiling).
High path loss.
High delay spread.
ISI.
Uniform multi-
spot diffusing
[38].
Enhanced received
power, due to existence
of LOS link between
spots and receiver.
Good coverage area.
High delay spread due to
large numbers of beams.
ISI.
Diamond multi-
spot
transmitter
[39].
Provides good
coverage of the sides
and corners of the room.
Signal power received is
very low in the room
center.
High delay spread.
Line strip
multi-beam
transmitter
(LSMS) [23].
Significant improvement in the
received power
compared to the techniques above.
Reduces ISI when combined with angle
diversity receivers.
LSMS suffers from delay spread when a wide FOV
receiver is employed (see
Fig. 4A).
Power received in the
sides and corners of the room is very low.
Does not provide full mobility in the room.
Multi-beam
clustering
transmitter
(BCM) [39].
Provides full mobility
for the optical receiver.
Increased power
received at the sides and the corners of the
room (good coverage
of the surroundings and room center).
The delay spread is still
high when using wide FOV receiver
(see Fig. 4A.).
Adaptive Line
strip multi-
beam
transmitter
(ALSMS)[28],
[48], [43]
Improved allocation of
power to spots
according to receiver location
Increased complexity to
achieve optimum
transmitter power allocation
Reduced delay spread and increased SNR
Single line of spots, hence poorer coverage in some
areas
Multi beam
transmitter
with Power and
Angle
Adaptation
System
(MBPAAS) [44]
Improved coverage
through beam steering and optimum allocation
of power to spots
Further reduction in
delay spread and
increase in SNR
Increased complexity as
the optimum beam direction has to be
determined and the
optimum power allocation has to used
Beam Delay
and Power
Adaptation
LSMS
transmitter
(BDAPA-
LSMS) [41]
Adapting the
differential delay
between the beams leads to transmitter pre-
distortion and hence
improved data rates
Complex technique, but
simple implementation
algorithms given
Beam steering
transmitters
[12]
Improved LOS
component, hence
reduced delay spread
and improved SNR
User mobility is difficult
IV. RECEIVER STRUCTURES USED IN OWC SYSTEMS
There are many types of receiver configurations for use in
OWC systems. The single element receiver with a wide FOV
(90o) is the most basic configuration. The use of a wide FOV
receiver enables improved collection of the optical signal. On
the other hand, huge amounts of undesirable ambient light are
also received with the desired optical signal and this reduces the
SNR. An angle diversity receiver (ADR) that employs narrow
FOV elements aimed in different directions is an attractive
receiver architecture. Such ADRs were proposed in [49], [50]
however the same FOV was used for all branches in the ADR.
The authors in [21], [23] developed and optimized ADR
structures using an optimum FOV in each receiver branch. They
also considered several diversity schemes, such as selection
combining (SC), maximum ratio combining (MRC) and equal
gain combining (EGC). The MRC approach can achieve better
performance compared to the other methods [39]. An ADR has
several advantages: reduced effect of ambient light noise,
reduced signal spread and improved SNR. These advantages are
realized as MRC does not ignore the unselected branches (as
SC does) where these branches can make a useful contribution.
It also does not simply give an equal weight to all receiver
branches (EGC case) with no regard to their SNR. Its main
drawbacks are design complexity, high cost (separate
concentrator used for each detector) and bulky size.
Significant performance improvements can be achieved
through an imaging receiver, which is one of the most attractive
receivers in OWC systems due to its reduced complexity
compared to ADR (only one image concentrator (eg. lens) is
shared among all pixels), reduced transmitter power needed,
reduced signal spread hence reduced ISI and therefore support
for high data rates. It also allows space division multiple access
(SDMA) to be used as the image formed resolves the signals
received from different directions. An imaging receiver was
originally proposed in [51]. In [51] the imaging receiver
improved the SNR by 20 dB. Development and modification of
the imaging receiver can be found in [41], [28], [43]–[45],
[52],[10]. where the impact of the number of pixels was
considered together with change in the shape of pixels, use of
multiple imaging receivers mounted in different directions and
7
power adaptation of transmitter power to fit the imaging
receiver. These changes enhanced the performance of the
overall system. Fig. 8 illustrates the main types of receivers. An
adaptive receiver was presented in [53] for reducing ISI.
Fig. 8: Types of receivers used in IRC systems; (A) single element wide
FOV (900) receiver, (B) angle diversity receiver employing narrow FOV
elements and (C) imaging receiver using single imaging concentrator.
V. MAIN IMPAIRMENTS IN OW SYSTEMS
OWC systems including IRC and VLC face many
challenges. In this section we will cover the main impairments
in IRC systems. While, the VLC system impairments will be
covered in Section VI.
The major design limitations encountered in IRC systems
include: background noise, multipath dispersion, transmission
power restrictions and the large capacitance associated with
large area photodetectors.
• Background noise
OWC receivers detect the desired signal as well as
background noise due to ambient light. The main sources of
ambient light in OWC systems are sun light, fluorescent and
incandescent bulbs [42]. The substantial amount of power
emitted by these sources within these wavelength ranges is
detected in the OWC system. This can introduce shot noise at
the receiver as well as saturate the photodetector when their
intensity is high [16]. The impact of ambient light can however
be reduced by using optical and electronic filters. Inexpensive
daylight filters can eliminate the components of the sunlight
below 760 nm [54]. Fig. 9 shows the spectrum of the different
sources of ambient light and the responsivity and transmission
of a typical silicon p-i-n photodiode that employs a daylight
filter. The characteristics and effects of ambient noise on the
OWC system transmission have been investigated widely in
[16], [17], [53] – [56]. The work has found that the background
noise contribution from fluorescent light is negligible compared
to incandescent and sunlight light (see Fig. 9A). Fluorescent
lights produce most of their optical power outside the pass band
of commonly used filters [16], [55].
Various techniques have been proposed and investigated to
eliminate the effects of directional ambient light noise. For
instance directional ambient noise can be reduced through the
use of the fly eye receiver proposed in [38], which can optimally
select a reception direction where ambient noise (for example
from a window or spot light) is minimum [7], [58]. An imaging
receiver can introduce better spatial resolution of signals and
directional noise [41], [28], [43]–[45], [52],[10]. The authors in
[21] proposed a pyramidal fly-eye diversity antenna to reduce
the impact of background noise. However, even with its great
advantages, this design still needs more optimization in terms
of azimuth and elevation angles, triangular base dimensions as
well as number of sides. In [59], an ADR was used to mitigate
the background noise. The main problem with this receiver is
the size of the optical elements. In addition, the imaging
receiver was investigated widely, and one of its main
characteristics is reduced background noise [41], [43], [51],
[60].
(A)
(B)
Fig. 9: (A) Power spectra of the main sources of ambient light, (B) responsivity and transmission of a silicon p-i-n photodiode employing
daylight filter [22].
• Multipath dispersion
The signal received through multipath channels may lead to
ISI. ISI is normally characterized through the delay spread of
the channel. Multipath dispersion and ISI have a huge impact
on the performance of the OWC system. In [21], the optimum
value of the FOV (60o) was determined for an ADR in the
presence of ISI and background noise. A considerable decrease
8
in multipath induced dispersion can be achieved through careful
design of the detecting element’s (orientation and their)
directionality. However, the main drawbacks of such a receiver
(i.e. ADR) are their bulky size and high cost of fabrication. An
imaging receiver is a suitable alternative and can be effective in
combating multipath dispersion and ISI [41]. Beam angle and
delay adaptation techniques were proposed by the authors in
[42], [61] to improve the link performance and reduce the
mobility induced impairments.
• Transmission power restriction due to eye safety
Transmitted IR beams, if operated incorrectly, can cause
injury to eyes and/or skin. However, the damage to the eye is
more significant due to the ability of the eye to concentrate and
focus optical energy. The eye can focus light to the retina within
the wavelengths 400-1400 nm [62]. The cornea (front part of
eye) absorbs other wavelengths before the energy is focused.
There are a number of factors that determine the level of the
radiation hazard, for example, beam characteristics, operating
wavelength, exposure level, exposure time and distance from
the eye [18]. The optical power generated by LDs and high-
radiance LEDs within the wavelength band 700-1550 nm may
cause eye damage if absorbed by the retina. The maximum
permissible exposure (MPE) levels are very small in this band
because the incident light can be focused by the human eye onto
the retina by a factor of 100,000 or more [63]. According to the
international electro technical commission (IEC) standards
[64], an IRC system that employs a laser source must not
transmit power in excess of 0.5 mW (class A) [64]. Different
transmitter technologies are used to meet the IEC standard
when using LDs with high transmit power. Safety regulations
have been revised downward by a factor of 100 when using a
hologram [65], (typically computed by computer driven
algorithms and known as a computer generated hologram
(CGH)). In addition, using an optimum holographic diffuser
leads to reduced path loss for the diffused signal [66]. An
alternate solution can be found in [67] where an integrating-
sphere diffuser for an IRC system, was designed. This diffuser
is eye safe up to 0.84 W at 900 nm.
Skin safety should be considered in IRC system design. Short
term effects, such as skin heating, are considered in eye safety
regulations, due to the power level regulations for eyes allowing
lower power levels than the power levels allowed for skin [68].
• Photodetector capacitance
Many OWC systems employ intensity modulation and direct
detection. The SNR of the DD receiver is proportional to the
square of the received optical power. Furthermore, the
transmitted power is limited by eye safety regulations and
power consumption. These constraints mean that a
photodetector with a large photosensitive area should be used
to gather maximum power, to improve the SNR. Unfortunately,
a large photosensitive area leads to high capacitance (as the
photodetector capacitance is proportional to the area of the
photodetector). Large detector capacitance limits the available
receiver bandwidth due to the capacitance at the input of an
amplifier acting as a low pass filter. The authors in [69]
proposed the use of an array of photodetectors instead of a
single photodetector to mitigate the effects of large capacitance
and maximize the collected power at the same time. The
photodetector’s effective area can be enhanced by using a
hemispherical lens as suggested in [22]. Bootstrapping was
proposed by authors in [27] to minimize the effective
capacitance of the large area photodetector.
VI. VLC SYSTEMS
In the past few years, the green agenda has made a huge
impact on the field of communications [70]–[79]. This has
pushed many researchers to produce and investigate reliable
and energy efficient communication systems [80]–[91]. VLC
systems are energy efficient communication systems as
communications is a useful by-product of the lighting /
illumination system. The main factors that fueled the growth in
VLC systems are the recent developments in solid state lighting
(SSL), which already provides commercial high brightness
LEDs of about 100 lx - 200 lx [92], [93], with a longer lifetime
(about six years) in comparison to conventional artificial light
sources, such as incandescent light bulbs (whose lifetime is
about four months, continuous). In addition, SSL has fast
response time (modulation rate), smaller size, low power
consumption and no known health hazards. Fig. 10 shows the
practical luminous efficacy of different available light sources
[92].
The dual functionality of the VLC system, i.e. illumination
and communication, makes it a very attractive technology for
many applications, such as car-to-car communication via LEDs,
lighting infrastructures in buildings for high speed data
communication, traffic light management in trains and high
data rate communication in airplane cabins. The first proposed
VLC system was at Keio University in cooperation with
Nakagawa laboratory in Japan [70] – [72].
Fig. 10: Comparison of different light sources in terms of luminous
efficacy [92].
A. VLC Standards
In 2003, VLC system standardization was started in Japan
[97]. The Japan Electronics and Information Technology
Industries Association (JEITA) proposed two standards based
on visible light communication in 2007. The first one is the
visible light communications system standard (CP-1221) while
the second is the visible light ID system standard (CP-1222)
[98]. Six years later, a new Japanese standard based on CP-1222
was proposed, the visible light beacon system standard (CP-
1223).
In 2007, the Smart Lighting Engineering Research Centre
(ERC) was established in the USA funded by the National
9
Science Foundation (NSF) for developing solid state lighting
systems that have an advanced functionality [99].
In 2011, the IEEE published the physical and medium access
control (MAC) layers as a standard for VLC systems, called
IEEE 802.15.7 [100]. This standard can provide a data rate of
up to 96 Mbps for transmitting video and audio services. The
standard also considers the mobility of visible light
communication links as well as noise and other interference
sources. The IEEE 802.15.7 standard utilizes only the visible
light spectrum and considers optical receiver specifications. In
the same year, a technology called light fidelity (Li-Fi), which
has features similar to the Wi-Fi technology but uses light as
transmission medium instead of RF, was launched [101]. Three
years later, the IEEE 802.15.7 standard group revised the
standard by releasing an update, the IEEE 802.15.7m. The
updated standard utilizes infrared, visible light and near
ultraviolet wavelengths as medium, also, considering optical
camera communications (OCC) and imaging receivers as
detectors [102]. In 2017, the IEEE 802.15.13 standard was
published for providing data rates up to 10 Gbps with range up
to around 200 m [103]. The standard is designed to support
point-to-point and point-to-multipoint communications that can
be in both coordinated and non-coordinated topologies. One
year later, in 2018, the IEEE announced a new light
communications task group for devolving a global standard in
wireless local area networks based on visible light
communication. The standard, named IEEE 802.11bb, intends
to engage with manufacturers, operators and end users to build
the system [102].
B. VLC sources
The VLC system structure is similar to that of IRC systems.
However, the main essential difference between VLC and IRC
systems is the function of the transmitter. The transmitter in
VLC systems is used as an illumination source as well as a
communications transmitter for data users. Therefore, the VLC
network designers must consider the following two
requirements. Firstly, the visible light transmitter (LEDs or
LDs) should work as sources of illumination. The optimum
lighting configuration and LED angles have been studied in
previous work [104], [105]. According to the international
standards organization (ISO) and European standards, the
illumination should be 300 lx -1500 lx for an indoor office
[106]. Secondly, the data should be transmitted through LEDs
without affecting the illumination.
An essential advantage of VLC systems is that they can
provide communication and illumination at the same time.
White light can be generated by using LEDs or LDs. The
following sections describe each source.
1. LEDs
At present, there are two different approaches generally used
to generate white light from LEDs. In the first method white
light is generated using a phosphor layer (emitting yellow light)
that is coated on blue LEDs. The light emitted by blue LEDs
(λ≈470 nm) is absorbed by the phosphor layer, where the blue
wavelength excites the phosphor causing it to glow white, then
the phosphor emits light at longer wavelengths. The second
method uses RGB LEDs. With the correct mixing of the three
colours, red, green and blue, white light can be created such as
in colour TV. The lower cost and complexity of the first method
makes it more popular. However, the modulation bandwidth is
limited to tens of MHz due to the slow response of the phosphor
which limits the communication data rate. Both techniques have
been demonstrated in conjunction with VLC [107], [108]. Fig.
11 depicts the methods used to generate white light from the
LEDs.
Fig. 11: Two approaches used to produce white emissions from LEDs, (A)
phosphor LED method, (B) RGB method.
2. LDs
Laser diodes (LDs) can be used as sources for illumination
and communication in VLC systems. The experiment in [37]
shows that mixing different LD colors can provide a white light
source. Fig. 12 shows the experimental setup used to generate
white color using 4 different LDs colors. The four laser colors
are combined using chromatic beam-splitters. The combined
laser colors reflected by a mirror pass through a ground-glass
diffuser which reduces speckle before the illumination.
Fig. 12: Generating white light using 4 different LD colors [37]
A simple physical layer block diagram of a VLC system is
shown in Fig. 13. Digital and analogue components are used in
the transmitter and the receiver. The data stream, baseband
modulator and digital to analogue convertor (DAC) are the
digital components in the transmitter. Similarly, the digital
components in the receiver are the analogue to digital converter
(ADC), demodulator and data sink. The trans-conductance
amplifier (TCA), bias Tee and LEDs are the analogue
components in the transmitter. The receiver includes the
photodiode (PD), trans-impedance amplifier (TIA) and band
pass filter (BP). The AC signal in the transmitter is added onto
the DC current by a bias Tee (a device used in a high frequency
transmission line to produce a DC offset). Since LEDs work in
a region with unipolar driving currents, the absolute driving
current (AC+DC) has to be larger than zero. The total current is
fed to the LEDs to emit the modulated output power. The power
received by the PD is converted into a current (I-PD), and this
current consists of two components, the AC and DC parts. The
AC component is amplified and then filtered using TIA and BP
10
respectively. Finally, the digital signal is demodulated after
conversion using ADC.
Fig. 13: Block diagram of VLC system.
Brightness (light dimming) control for the LEDs is required
in VLC systems as these sources are also used for illumination.
Many methods have been proposed to implement dimming
control [109]–[112]. The simplest method is amplitude
modulation (AM) dimming: the luminous flux is controlled by
controlling the input DC current. However, the chromaticity
coordinates of the emitted light can change [113]. Pulse width
modulation (PWM) is another method that controls the width of
the current pulse. The main feature of PWM dimming is that the
amplitude of the pulse is constant. The width of the pulse varies
according to the dimming level, thus resulting in the emitted
light spectrum being constant. Binary code modulation or bit
angle modulation (BAM) is a good dimming method that was
introduced by artistic licence engineering (ALE). It is a new
LED drive technique that can be used in VLC systems [114].
The main advantages in using this technique are
implementation simplicity, the potential to reduce flicker
compared to PWM, lower processing power and simple data
recovery operations.
BAM encoding for LED dimming makes use of the binary
data pattern shown in Fig. 14. In the 8 bit BAM system, the
pulse width of bit 7 (most significant bit, (MSB)) is equal to
27=128, whereas the pulse width of bit 0 (least significant bit,
LSB) is equal to 20=1 (a unit width). This allows continuous
brightness levels to be defined where the granularity of the
dimming levels depends on the number of bits used in BAM.
Fig. 14: BAM signal waveform used in dimming control.
As mentioned earlier, the primary function of LEDs is to
provide illumination, so it is essential to define the luminous
intensity, the quantity used to determine the illumination level.
To get comfortable office lighting, a certain amount of
luminance is required (300 lx -1500 lx) [106]. The brightness
of the LED is expressed as luminous intensity. Luminous
intensity is defined as the luminous flux per solid angle as [96]:
𝐼 =𝑑∅
𝑑Ω (12)
where Ω is the spatial angle and ∅ is the luminous flux. From
the energy flux (emission spectrum), ∅e and ∅ can be calculated
as [96]:
∅ = 𝐹 ∫ 𝐴(𝜆)780
380∅𝑒 (𝜆) 𝑑𝜆 (13)
where 𝐴(𝜆) is the eye sensitivity function and 𝐹 is the
maximum visibility, which is equal to 683 lumen/Watt (lm/W)
at 555 nm wavelength. Assuming that the LED light has a
Lambertian radiation pattern, the direct luminance at any given
point in an office can be estimated as:
𝐸 = 𝐼(0)𝑐𝑜𝑠𝑛(𝜃) 𝑐𝑜𝑠(𝛾)
𝐷2 (14)
where 𝐼(0) is the centre luminous intensity of the LED
measured in candela (cd), 𝜃 is the irradiance angle, 𝐷 is the
distance between the LED and the point of interest, 𝛾 is the
angle of incidence and 𝑛 is the Lambertian emission order,
defined as [22]:
𝑛 = −𝑙𝑛 (2)
𝑙𝑛 (𝑐𝑜𝑠 (𝛷12
))
(15)
where 𝛷1
2
is the semi angle at half power of the LED.
Using the relations above, the illuminance distribution is
determined in a room with dimensions 5m×5m×3m, where four
LED light units with specifications similar to those in [96] were
used. Fig. 15 illustrates the illumination distribution. Note that
the illumination is within the acceptable range of (300 lx -1500
lx).
Fig. 15: The distribution of illuminance, Min. 315 lx, Max. 1220 lx.
C. Design challenges in VLC systems
VLC systems are a potential candidate to achieve multi
gigabits per second data rates in indoor wireless systems. There
are however several impairments that VLC systems have to deal
with. These impairments include the ISI caused by multipath
propagation, the low modulation bandwidth of the transmitter
(i.e. LEDs), and the provision of an uplink connection for VLC
systems.
1. ISI in VLC systems
In real environments, there is more than one transmitter
(LEDs), and these transmitters are used for illumination as well
as communication. The VLC receiver therefore collects signals
from more than one transmitter, and each signal arrives using
11
potentially a different path with a different delay profile. In
addition, due to the reflections from the walls, ceiling and other
objects, the received optical signal suffers dispersion due to
multipath propagation. This results in ISI, especially at high
data rates. In the literature, many techniques have been
proposed to mitigate ISI in VLC systems. Among the most
notable solutions is the use of return-to-zero OOK (RZ-OOK)
modulation with the space between pulses being used as a guard
interval to reduce the effect of the delay spread in low data rate
applications [115]; while, in [116] the authors used OFDM to
reduce the ISI. Spread spectrum has also been considered to
combat ISI, however, it reduces the bandwidth efficiency as
well [117]. The authors in [118] used zero forcing (ZF)
equalization with transmitter (i.e. LED) arrangements to reduce
the effects of ISI. The bit error rate (BER) achieved using this
technique was similar to the channel without ISI. In [96] it was
found that decreasing the receiver field of view leads to reduced
ISI, whereas increasing the data rate leads to an increase in ISI.
The use of adaptive equalization using the least mean square
algorithm has the ability to reduce the effects of ISI and was
investigated at data rates up to 1 Gbps [119]. The authors in
[120] used MIMO approaches to reduce shadowing effects.
Other approaches can be used to reduce ISI in VLC systems.
For instance, using pre and post equalization techniques,
coding, angle diversity receivers, and imaging receivers [10]–
[13].
2. Low modulation bandwidth of the LEDs
The modulation bandwidth of the transmitters (i.e. LEDs) is
typically less than the VLC channel bandwidth, which means
that the former limits the VLC data rates.
To achieve high data rates in VLC systems a number of
different techniques can be used, for instance, optical filters, pre
and post equalization (or both), complex modulation techniques
and parallel communication (optical MIMO, (OMIMO)). A
blue optical filter at the receiver can be used to filter the slow
response yellowish component, and this approach is probably
the simplest and most cost effective approach to increase the
VLC data rates [121]. However, the achieved bandwidth is
insignificant (8 MHz). The drop in the white LED response can
be compensated for by using a simple analogue pre-equalizer at
the transmitter, and this technique was shown to offer 40 Mbps
without the use of a blue filter [122]. However, the data rate
achieved is still very low compared to the available VLC
spectrum. A moderate data rate (80 Mbps) can be achieved
using more complex pre-equalization [123]. Pre-equalization
however has the drawback that the drive circuit of the LED
needs to be modified, and this leads to higher costs and lower
emitter efficiency (i.e. not all the input power is converted to
light). By combining simple pre and post equalization, 75 Mbps
can be achieved [124]. Both analogue and digital equalization
techniques can be applied. Analogue equalization techniques
are suitable for OOK modulation, while digital techniques are
preferable for OFDM [125]. Using least mean square (LMS)
and decision feedback (DFE) equalizers at data rates above 200
Mbps was shown to be very effective to mitigate ISI [119]. The
use of such equalizers however leads to a computationally
complex receiver structure. A data rate of 100 Mbps - 230 Mbps
was the maximum data rate achieved using phosphorescent
LEDs with simple OOK modulation [126]. The modulation
scheme used is very important in VLC systems because it
affects the achievable data rate, BER and luminance. For
example, multiple PPM was used for modulation and dimming
control [111]. Different types of modulation were evaluated in
terms of their combined modulation and dimming control
capabilities. The results showed that overlapping PPM (OPPM)
can achieve flexible luminance and good performance [127].
Higher data rates can be achieved when complex modulation
approaches are used. For example, Discrete Multi-tone
modulation (DMT) can provide data rates of about 1 Gbps
[108]. This type of modulation requires however a complex
transceiver. In terms of parallel transmission, VLC systems
have naturally favorable features for OMIMO as many LEDs
(transmitters) are used for illumination and different data
streams can be sent on every single LED to maximize the
throughput. At the same time, an array of photodetectors can be
used at the receiver. This setup offers improvements in security,
link range and data rates while the power required is unaltered
[128], [129]. Detailed study of OMIMO revealed that channel
cross talk is a critical limiting factor in VLC systems [130]. A
significant enhancement in the data rates can be achieved with
RGB LEDs. A 1.25 Gbps system was reported in [131] using
RGB LEDs in a single color transmission mode, and 1.5 Gbps
was achieved using a new micro LED (μLED) array with non-
return-to-zero OOK (NRZ-OOK) as the modulation scheme.
The 3 dB modulation bandwidth of this LED was 150 MHz
[132]. The maximum data rate achieved using commercial RGB
LEDs with low complexity modulation (OOK) was 500 Mbps
[133]. Recently, a 3 Gbps VLC system based on a single μLED
with OFDM modulation was successfully demonstrated [134].
The highest throughput achieved by LEDs was reported in
[107], which reported an aggregate throughput of 3.4 Gbps
using DMT, wavelength division multiplexing (WDM) and
RGB LEDs. The design and implementation complexity are a
major concern in these systems however.
3. Provision of an uplink connection in VLC systems
Using white illumination LEDs for communication is
naturally one directional communication (downlink). So,
providing an uplink connection can be a major challenge. Many
techniques have been proposed to provide an uplink for VLC
systems. In [135], the use of retro reflecting transceivers with a
corner cube modulator to provide low data rate uplinks was
proposed for visible light communication as shown in Fig. 16A.
In [136], [137] an IR transmitter was proposed to provide an
uplink for VLC systems. In [138] the authors suggested that RF
systems can be employed in conjunction with the VLC system
to provide reliable bidirectional communication as shown in
Fig. 16B. However, all the above techniques add complexity to
the VLC systems and do not provide a high data rate uplink
connection. Recent research demonstrated high data rate bi-
directional VLC transmission with 575 Mbps downlink and 225
Mbps uplink using SCM with WDM [139]. In addition, other
recent research proposed a solution for the uplink VLC system
using IRC. As a result, the uplink data rate reached 2.5 Gbps
[140].
12
Fig. 16: Two techniques used to provide an up-link connection in VLC
systems, (A) retro reflecting transceivers and (B) bidirectional VLC system
combined with an RF system.
D. VLC system architectures
Popular wireless system architectures used in RF wireless
systems have also been explored in VLC systems. Fig. 17
illustrates four system architectures: single input single output
(SISO), single input multiple output (SIMO), multiple input
single output (MISO) and multiple input multiple output
(MIMO). The number of inputs and outputs in RF
communication systems is dictated by the number of antennas
at the sender and the receiver, while, in VLC systems it is based
on the number of sources (LEDs, or LDs) and the number of
optical receivers.
Fig. 17: VLC system architectures (A) SISO (B) SIMO (C) MISO and (D)
MIMO.
MIMO approaches were widely investigated in VLC systems
to provide high data rates and to support multiple users [117] –
[122]. Imaging and non-imaging receivers are investigated in
VLC systems [141]. A hybrid diffusing IR transmitter was
studied to support VLC systems when the light is turned off or
dimmed [52]. In addition, the authors in [142] developed
MIMO VLC systems by combining transceivers and dimming
control.
E. Multiplexing techniques
OFDM and WDM are two of the most widely considered
multiplexing techniques in VLC systems. Their main features
are summarised here.
1. OFDM
The use of OFDM in VLC systems offers the ability to
combat ISI. High data rates can be achieved by dividing the
available bandwidth into sets of orthogonal subcarriers [147].
However, OFDM signals suffer in-band and out-band distortion
due to the high peak to average power ratio (PAPR). This flicker
in the power leads to reduced device lifetime and eye safety
problems [148].
Numerous approaches have been proposed to realize optical
OFDM (O-OFDM), ie approaches that satisfy the IM/DD
constraints. These include direct-current (DC) O-OFDM
(DCO-OFDM) [36], asymmetrically clipped (AC) O-OFDM
(ACO-OFDM) [149], Flip-OFDM [150] and Hermitian
symmetry (HS) free (HSF)-OFDM (HSF-OFDM) [151].
2. WDM
WDM allows the multiplexing of different data sources on
different wavelengths and then into a single channel. It
promises great flexibility and bandwidth efficiency, as it did in
the case of fibre optic communication systems. It is feasible to
apply WDM in VLC systems, as white light is composed of a
combination of colors which can be produced using colored
LEDs (RGB) or colored LDs (BGYR). Colored LEDs can be
individually modulated and an optical multiplexer then
transmits the parallel data. At the receiver the signal is passed
through a de-multiplexer (optical filters used as de-multiplexer)
and then optical receivers are used (each optical receiver thus
detects a certain wavelength) for data recovery. Fig. 18 shows
a WDM VLC system. By using LEDs (RGB), a WDM data rate
of 3.22 Gbps was reported in [152]. Another demonstration was
reported in [153] of a VLC system that employs WDM. The use
of LDs (BGYR), in VLC systems was proposed in [11]
resulting in a data rate of 10 Gbps.
Fig. 18: WDM used in VLC systems (A) using LEDs and (B) using LDs.
The effect of the change in the correlated colour temperature
due to modulation has not been substantially explored. An
important feature of the source of light is its apparent colour as
a perfectly white source when viewed. This feature can be
quantified using chromaticity coordinates on a chromaticity
diagram. The International Commission on Illumination (CIE)
13
created in 1931 the RGB colour space diagram shown in Fig.
19. The circumference of the area includes the coordinates of
all the real colours. A black body radiator at different
temperatures produces different colours. The white colour is
defined by the region close to the black body radiators locus
(starting at approximately 2,500 K). RGB hues are located
within regions that span from the white region towards the
corresponding corners of the figure. Sources that are very close
to the Planckian locus may be described by the colour
temperature (CT) [154].
The chromaticity coordinates can be defined as [154]:
𝑋 =∑ 𝑥𝑖
𝑛𝑖=1 ∅𝑖
∑ ∅𝑖 𝑛𝑖=1
(16)
and
𝑌 =∑ 𝑦𝑖
𝑛𝑖=1 ∅𝑖
∑ ∅𝑖 𝑛𝑖=1
(17)
where ∅𝑖 is the radiant flux and 𝑥𝑖 and 𝑦𝑖 are the primary
sources’ coordinates. From Fig. 19, a white colour can be
obtained from two colour sources (blue and yellow, see dotted
line in Fig. 19). In addition, a white colour can be obtained from
three sources (see solid triangle in Fig. 19).
Fig. 19: 1931 CIE chromaticity diagram [154].
F. Multiple access schemes
Several multiple access (MA) schemes have been studied for
use in VLC systems. Some of the MA schemes that have been
used in RF systems are also used in VLC systems. These
include time division multiple access (TDMA), frequency
division multiple access (FDMA), space division multiple
access (SDMA), code division multiple access (CDMA) and
non-orthogonal multiple access (NOMA).
1. TDMA
TDMA for multi-user VLC systems was proposed and
investigated in [155]. It is also sometimes used for preventing
collisions where other MA schemes may fail, such as situations
where two transmitters have the same distance to the receiver
[29]. A controller can share time between the transmitters in a
medium access control (MAC) protocol, which may also allow
uneven time splitting between trasnceivers. The throughput of
TDMA schemes decreases if the number of users increases. As
a result the system cost may rise due to the need for
synchronization between transmitters [156].
2. FDMA
FDMA is a scheme that can support multiple access based on
orthogonal frequency bands for example. In VLC systems,
there are four techniques based on FDMA that have been
investigated [157]–[159].
The first method uses single carrier FDMA (SC-FDMA)
which is based on frequency division multiplexing. SC-FDMA
was proposed in [159]. It reduces the value of the peak-to-
average power ratio (PAPR) and that is one of its main
advantages [160].
The second technique is orthogonal frequency division
multiple access (OFDMA) which was studied in [157].
OFDMA is a multi-user approach based on OFDM modulation.
It is an extension of OFDM which means each user can employ
a group of subcarriers in each time slot. OFDMA was evaluated
in a number of studies [161]–[163]. The data rate achieved
when using OFDMA can be reduced because of spectrum
partitioning [164]. The third technique is orthogonal frequency
division multiplexing interleave division multiple access
(OFDM-IDMA), which was proposed in [157]. OFDM-IDMA
is also a multi-user technique based on OFDM modulation. It is
a scheme that combines OFDM and IDMA technologies. Both
techniques: OFDMA and OFDM-IDMA are asymmetrically
clipped at zero level after OFDM modulation. OFDM-IDMA
provides good power efficiency compared to OFDMA. In
addition, it was shown in [157] that signal to noise ratios (SNR)
above 10 dB in a system with modulation size equal to 16,
OFDM-IDMA provides better results compared to OFDMA. It
is shown in [157] however that the decoding complexity and
PAPR are lower in OFDMA compared to OFDM-IDMA.
The last technique is interleaved frequency division multiple
access (IFDMA) which was proposed in [158]. IFDMA was
introduced to reduce PAPR compared to OFDMA. In addition,
the effect of the non-linear characteristics of LEDs is lower in
IFDMA which can provide a higher power efficiency. The
computational complexity needed is lower in IFDMA
compared to OFDMA as IFDMA does not include discrete
Fourier transform or inverse discrete Fourier transform
operations. Moreover, it was shown in [165] that IFDMA is a
promising multiple access technique in VLC due to its ability
to mitigate multi-path distortion and reduce synchronization
errors.
3. CDMA
CDMA is a non-orthogonal multiple access scheme which
can offer high spectral efficiency compared to OFDMA and
TDMA. Each user in CDMA uses a special code to provide
simultaneous transmission and reception. The special code used
by each user is typically an optical code; an example was
proposed in [166]. In addition, random optical codes were
proposed in [167], [168] to support a large number of users by
spreading the signal bandwidth. A synchronization technique
was reported in [169] to help prevent the undesired correlation
characteristics over random optical codes. As a result, a good
performance was achieved.
The authors in [170] proposed color shift keying (CSK) as a
technique based on CDMA that can increase the multiple access
capacity. Consequently, each transmitter can gain 3dB
compared to OOK.
Multi-carrier (MC) is another approach investigated in [171],
[172] based on CDMA. MC-CDMA combines features of
CDMA and OFDM. MC-CDMA diffuses the data symbols of
each user in the frequency domain over OFDM subcarriers.
14
Subsequently, the sum of the data symbols from multiple users
are re-modulated in the time domain using OFDM.
Additionally, the authors in [171] showed that the transmitted
optical power can be reduced by using sub-carrier selection.
Another multiple access technique based on CDMA was
proposed in [173] which provides interference-free links while
transferring information. The technique allows users to decode
their signals without pre-confirmation. However, this technique
was investigated in a small area less than one square meter
which reduces its practical utility.
The power allocation issue in CDMA was studied in [174].
Two types of distributed power allocation were proposed which
are partial and weighted distributed power allocation, which can
be optimized. The result of the study showed that the proposed
techniques offer low computational complexity compared to the
optimized optimal centralized power allocation approach.
Different CDMA schemes were proposed in [175] that utilize
micro-LED arrays. These schemes provide a higher modulation
bandwidth which can also support a large number of users.
4. SDMA
SDMA can provide multi-user support in VLC systems. It
was shown in [176] that increasing the number of transmitters
in SDMA VLC systems increases the throughput. Moreover,
the system capacity was shown to improve more than ten times
when SDMA is used compared to TDMA VLC systems. In
[177] a low complexity suboptimal algorithm was introduced
for coordinated multi-point VLC systems with SDMA
grouping. The proposed technique offers an improvement in the
system performance and fairness in throughput.
5. NOMA
NOMA was studied in [178], [179], [180] as a promising
technique that can provide multiple access in VLC systems. It
is also called power domain multiple access. NOMA differs
from other multiple access techniques which provide
orthogonal access for multiple users either in frequency, time,
code or phase. Each user in NOMA can use the same band at
the same time. Thus, the power level can be used to help
distinguish users. In addition, the transmitter in NOMA applies
superposition coding to simplify the operation at the receiver.
As a result, channels are separated at the receiver for both
uplink and downlink users [179]. NOMA was also studied in
some recent work as a means to support multiple users at high
data rates [164], [179], [181], [182].
NOMA outperforms OFDMA in terms of the achievable data
rate in VLC as was shown in [179]. In addition, it was shown
in [183] that the system capacity is improved when NOMA is
employed. MIMO NOMA VLC systems were studied in [160].
The results indicated that this approach can offer high data
rates, high capacity and higher spectral efficiency. An
experimental study was reported in [164] where a MIMO
NOMA VLC system was demonstrated. A normalized gain
difference power allocation (NGDPA) method was proposed in
the experiment to provide an efficient and low complexity
power allocation approach. The experimental results showed
that NOMA with NGDPA can result in up to 29% sum rate
improvement compared to NOMA with the gain ratio power
allocation scheme.
Table 4 provides a summary of the multiple access schemes
studied in the context of VLC.
Table 2: Summary of VLC multiple access schemes
Scheme Summary
TDMA
TDMA can help prevent collisions between users in VLC
systems. The per user throughput decreases when the number of users increases and resources can be wasted during user
inactivity. Accurate synchronization is needed between the
transmitters which increases cost and complexity [156].
FDMA
Four main FDMA techniques have been studied in VLC
systems: SC-FDMA, OFDMA, OFDM-IDMA and IFDMA.
SC-FDMA reduces the PAPR. However, it may not be the best approach at high data rates and with a large number of users.
OFDMA and OFDM-IDMA can provide better throughput.
OFDMA offers lower decoding complexity and lower PAPR compared to OFDM-IDMA, however it can introduce spectrum
partitioning which may reduce the data rate. The power
efficiency in OFDMA is high compared to OFDM-IDMA. Moreover, at some SNRs and modulation sizes, OFDM-IDMA
has better throughput compared to OFDMA.
IFDMA can mitigate multi-path distortion and can help reduce the impact of synchronization errors. The computational
complexity and PAPR in IFDMA are lower compared to their
values in OFDMA.
CDMA
Special and random codes are utilized. Random code were
proposed to increase the number of users. Synchronization
techniques were added over Random codes to improve the system performance. CSK is another technique that was
proposed in conjunction with CDMA. It can provide 3dB gain
for each user compared to OOK. Techniques were proposed based on CDMA to provide interference-free links, but in small
areas while other approaches were reported to increase the
achievable data rates and number of users.
SDMA
SDMA is not widely studied in VLC systems to date, but is a very promising scheme. One technique based on SDMA was
evaluated and shown to offer an improvement in the system
performance and throughput fairness.
NOMA
NOMA can provide non-orthogonal multiple access in VLC
systems. It can outperform OFDMA in terms of data rate and
system capacity. MIMO NOMA provides further improvement in data rate, capacity and spectral efficiency [144].
G. Mobility in VLC systems
One of the requirements in indoor communication systems is
to provide support for the expected forms of user mobility. To
do this with minimal impact on the system performance
adaptation may be needed in the transmitter and receiver. In
IRC systems mobility can be supported through the use of
adaptive LSMS, adaptive BCM, angle diversity receivers and
imaging receivers [28], [43]–[45], [48] which when used
together can enable the IRC system to adapt to the environment.
However, adaptive techniques such as LSMS and BCM cannot
be directly applied in VLC systems due to the dual functionality
of the VLC source (i.e. illumination and communication).
Mobility may be supported in VLC systems by using improved
receivers such as angle diversity receivers or imaging receivers
[10]–[13].
Beam steering approaches are already applied in FSO
systems and can also be utilized in VLC systems. VLC beam
steering can be achieved by using electronically controlled
mirrors in front of the receiver and receiver tilting. One
inexpensive approach used to provide good link quality during
mobility uses mirrors with piezoelectric actuators in front of the
receiver [184]. This method however, leads to a bulky receiver
and cannot be used in mobile devices. Fig. 20 illustrates this
15
technique. Another approach is the tilting of the transmitter and
receiver together using piezoelectric actuators that are
controlled electronically. As with the mirrors method, the tilting
method also needs to be controlled using electronic circuits.
The tilting approach has two different structures as shown in
Fig. 21A. In the first structure all of the transceiver (LED and
photodetector) are placed on actuators and in the second
structure, in Fig. 21B only the lenses of the transceiver are
placed on the actuators. The advantage of the second structure
is the ability to zoom in and out, which means the FOV can be
changed [184].
Fig. 20: Beam steering using a mirror.
Fig. 21: Beam steering using tilting method, (A) all body placed on
actuators, (B) only lenses placed on actuators.
H. Hologram Design for VLC Systems
Holograms were recently studied in VLC systems to provide
a high data rate and to improve the SNR [12], [13], [185], [186].
A hologram is a transparent or reflective device that can be used
with VLC sources. It modulates the amplitude or phase of the
signals that passes through it. Consequently, the light rays are
diffused by splitting the light into beams to cover the required
area, as shown in Fig. 22. The authors in [12] used a CGH in
their study to achieve a data rate of up to 20 Gbps. In addition,
fast CGH (FCGH) adaptation was proposed in [13] as a new
method that can support user mobility. The data rate was also
increased up to 25 Gbps, and the SNR was improved.
Fig. 22: Hologram and illumination produced.
I. Security in VLC Systems
Security in VLC system can be divided into two categories,
namely physical layer security and data layer security, ie
encryption.
1. Physical Layer Security
VLC systems offer physical layer security for the user
information, as light signals cannot travel through opaque
objects such as walls. Therefore, an attacker cannot get access
to the medium unless they are in the visible range for the VLC
communication system. This makes VLC systems more secure
than an RF communication system [29]. VLC physical layer
security was studied in [163.5], [163.6].
2. Encryption
Quantum key distribution (QKD) can help secure data in
OWC systems. QKD can use photons to exchange a secret key
between two points. Therefore, it can increase security in OWC
system by encrypting the transferred data. The secret key can
be exchanged for example using a one-time pad (OTP)
cryptographic algorithm as well as the characteristics of
quantum physics. Eve is restricted by the quantum theory from
storing or copying transferred photons via the quantum channel
because of the changes in the photon characteristics. After
exchanging the secret key, the data are encrypted by the secret
key and transferred via classical communication. A wireless
QKD approach was examined in an indoor environment [187].
J. Future trends for VLC
VLC systems are rapidly developing, however, before VLC
becomes widespread, many issues should be resolved, such as
the identification of the optimum modulation techniques and
the best dimming control methods, together with the optimum
multiple access approaches and standardization. Currently,
VLC systems do not fit with most types of LED lighting
systems, as there are different drive mechanisms and
architectures. So, collaboration between communications
organizations and the solid state lighting (SSL) industry is
necessary to establish standards for SSL systems to be used for
illumination and communications [188].
16
VII. MAIN APPLICATIONS OF OWC SYSTEMS
OW is generally used in three applications area: office
interconnection, last mile broadband access and personal
communications.
A. Main applications of IRC systems
To date, there have been many applications that use infrared
OW systems. A key application area is personal
communications. OW personal communications (short range
applications) are widely studied due to their low complexity,
high bandwidth and low cost. Two main factors that make short
range OW systems preferable from the users’ point of view are
the security features (light is contained in the environment
where it originates) and the potentially high data rates these
systems can achieve. In addition, lower transmission powers
can be used and a data rate of 1 Gbps has already been
standardized in the Giga IR standard with data rates up to 10
Gbps planned [189]. A range of devices incorporated IrDA
ports, however these were short range and relatively low data
rate. This led to the development of many subsequent standards,
most notably the recent effort by the IEEE to develop an OW
physical layer option in IEEE 802.11, namely the planned IEEE
802.11bb standard [190]. The IrDA standard can be considered
obsolete as it was not recently updated and the maximum data
rate is low. The IrDA standardized short range infrared OW for
point to point communication (half duplex mode), with direct
LOS being the preferred configuration for IrDA links. The
IrDA link architecture does not cater for user mobility. The
standard was created in 1993 by IrDA, which includes more
than 160 members with the majority of these being
manufacturers of hardware, peripherals, components and
software [191]. This standard provides interoperability between
different software and hardware devices using wireless IR.
between 2004 and 2008 more than 600 million devices were
produced based on IrDA technology, and this number used to
increase by 20% every year according to [191], [192]. Personal
communication devices that use IrDA transceivers include:
printers, PDAs, electronic books, digital cameras and
notebooks (most kinds of communications and computing
devices using IR). Other OW indoor systems were developed,
for example those that use Optical Camera Communications
[193].
B. Main applications of VLC systems
Indoor VLC applications include applications for data
communications, indoor location estimation, indoor navigation
for visually impaired people and accurate position
measurements.
• Indoor applications
Like IRC systems, VLC systems can be used to transfer data
in offices and indoor spaces. The user location can be estimated
in an indoor environment using white LEDs. For example,
indoor spaces can be illuminated by LEDs with a unique ID for
each LED. White LEDs in conjunction with geometric sensors
integrated in smart phones can help visually impaired people
move inside buildings [194]. Fig. 23 shows a prototype
navigation system for visually impaired people [194]. RF
signals can be undesirable inside hospital environments,
especially in operating theatres and magnetic resonance
imaging (MRI) scanners, therefore VLC systems can be a
potential solution in such scenarios.
Fig. 23: Indoor VLC navigation system for visually impaired people.
Recently, various methods for VLC indoor positioning were
studied [195], [196]. VLC systems are strong candidates for
indoor positioning due to several features. Firstly, better
positioning accuracy (few millimetres) can be achieved, which
is better than the accuracy achieved by radio wave systems. The
added accuracy is mainly attributed to the lower interference
and multipath effects suffered by VLC systems and the short
wavelength used. Secondly, VLC positioning systems can be
used in environments where radio positioning systems are
restricted, such as industrial settings with large metallic
structures [197] or in hospitals [198].
VIII. VLC experimental demonstrations and products
This section provides a brief overview of recent VLC projects
and products.
A. VLC experimental demonstration
1. Ultra-parallel visible light communications (UP-VLC)
project
The UP-VLC project started in October 2012 and concluded in
February 2017. It involved six research groups at 5 institutions
and was funded by the UK Engineering and Physical Sciences
Research Council (EPSRC). Its vision was built on the unique
capabilities of gallium nitride (GaN) LEDs which can provide
good illumination and high data rate optical communications.
New forms of spatial multiplexing were implemented where
independent communications channels were provided by
individual elements in high-density arrays of GaN based LEDs
[199]. The project combined the strengths of researches in the
UK in complementary metal-oxide-semiconductors (CMOS),
nitrides, organic semiconductors and communications systems
engineering to provide a new wireless communication
technology that transfers data through visible light instead of
RF (Wi-Fi) [200]. Multi-Gb/s VLC was shown as a possibility
for the first time by using GaN LEDs and novel multiplexing
schemes were explored [200].
Subsequently, color-tunable smart displays were developed
using CMOS electronics combined with GaN based micro-
LEDs. The modulation bandwidth of the LED pixels of these
color-tunable displays is 100 MHz which provides a
wavelength-agile source simultaneously for high-speed VLC
[199].
2. Visible light based interoperability and networking
(VisIoN) project
The VisIoN project started in September 2017 and will
conclude in September 2021 and is led by Centrale Marseille in
17
France. The project focuses on synergizing optical
communications technologies with wireless communication
technologies. It is built on three key technical work packages
(WPs) with a proof-of-concept demonstration for each WP. The
first WP focuses on smart cities, offices and homes where
everything in the home or city is connected wirelessly. As such,
the communication network must provide high data rates, high
reliability, high availability and low latency. VLC is proposed
in this project instead of RF communication systems as the
project expects VLC to provide improvements in terms of the
number of devices that can be supported and the achievable
capacity. Also, VLC is expected to offer localization and
diverse control with motion detection for these devices. The
second WP has a focus on smart transportation, with vehicle to
vehicle (V2V) and vehicle to infrastructure (V2I)
communication through VLC. The last WP considers
manufacturing and medical uses of VLC. In manufacturing,
VLC can provide unique safety and security features. In
medical applications, VLC is expected to offer interference
immunity compared to RF. In addition, it is expected that the
VLC systems studied will offer data communication,
localization and sensing together with high bandwidth, high
reliability, robustness and low latency [201].
3. Super Receivers for Visible Light Communications project
The project was established in August 2017 and will conclude
July 2020, led by University of Oxford. It focuses on creating a
more sensitive new super receiver. This super receiver is
designed to overcome sensitivity and data rate limitations and
will offer easy alignment. It can be easily integrated onto any
device due to its characteristics (thin and flexible). The project
plan is to test the receiver in free space visible light
communications links to quantify its performance. The project
estimates that the new super receiver will lead to a hundred
times improvement in the performance compared to the existing
receiver by offering higher data transmission rates, longer
transmission distance and lower noise [202].
4. Multifunctional Polymer Light-Emitting Diodes with
Visible Light Communications (MARVEL) project
The project started December 2016 and is due to conclude
November 2019. The project will introduce new Polymer Light-
Emitting Diode designs with new approaches that can
maximize the light efficiency [203].
5. Terabit Bidirectional Multi-user Optical Wireless System
(TOWS) for 6G LiFi project
The project will take place between January 2019 and
December 2023, and is led by the University of Leeds working
with the Universities of Cambridge and Edinburgh. The project
will develop and experimentally demonstrate multiusers Tbps
optical wireless systems. The developed systems will provide
capacities at least two orders of magnitude higher than the
existing and planned 5G optical and radio wireless systems. In
addition, a roadmap will be developed to achieve capacities up
to four orders of magnitude higher than those of existing 5G
wireless systems [204].
6. Designing Future Optical Wireless Communication
Networks (DETERMINE) project
The project started in June 2013 and concluded in May 2017,
and was led by Linkopings University in Sweden. The project
focused on developing concepts, algorithms, models and
architectures that exploit the optical spectrum and had a focus
on EU exchange of knowledge between the partners of the
consortium in the physical layer and the other OW layers [205].
7. Wireless Optical/Radio TErabit Communications
(WORTECS) project
The project, with a time window September 2017 to August
2020, led by Orange, France will develop ultra-high data rate
wireless systems by combining high radio frequency
communications with optical wireless communications in the
regions of infrared and visible optical spectrum by utilizing
heterogeneous networking concepts. Two Proof-of-Concept
demonstrators will be delivered in the project: an ultra-high
density LiFi/Radio network with multi-Gbps data rates, and an
ultra-high data rate system capable of delivering Tbps
networking [206].
B. VLC’s products
Light Fidelity (Li-Fi) helped propel OW exploitation bringing
it closer to end users and standards. pureLiFi has a commercial
product, LiFi-XC, that can transfer data using indoor lights. The
system is full duplex and bi-directional. It uses visible light for
downlink communication and infrared for the uplink. It can
offer up to 43 Mbps with fully networked Li-Fi with mobility
allowing users to move around the environment. The pureLiFi
device can be separated from, or integrated into the computer
[207]. In addition other products include those from Oledcomm
which provides a receiver, LiFiNET® Dongle. This receiver
can connect the computer to the Internet and can offer a data
rate around 10 Mbps [208]. Signify, a new Philips lighting
company, offers a Li-Fi system with excellent light quality and
30 Mbps data rate for two way communications [10].
VLNComm provides an advanced Li-Fi adapter called
LUMISTICK LIFI ADAPTER. This adapter connects devices
to the Li-Fi network offering 108 Mbps for downlink
communication and 53 Mbps for uplink communication [209].
IX. OPEN RESEARCH ISSUES
This section summarizes new areas of research that deserve
further attention in IRC and VLC systems.
A. Open research issues in IRC systems
Despite the notable solutions introduced for IRC systems to
provide mobility, mitigate ISI and achieve high data rates, low
complexity and easily implementable techniques are still
required. Areas of further investigation in IRC systems are
listed below.
• Mobility can degrade the performance of CDS and LSMS
systems. However, using certain techniques, such as beam
delay adaptation, power adaptation, beam angle adaptation
and imaging receivers, can enhance the system
performance. However, experimental verification of these
techniques has not been carried out yet. Real time
18
adaptation in these systems may lead to very complex
solutions. Therefore, investigation is needed to assess these
techniques in realistic IRC systems to provide mobility and
to achieve multi-gigabit per second data rates. In addition,
further investigations are needed to find new approaches to
support mobility with minimum complexity.
• Power, angle and beam delay adaptation have been
evaluated for a single user scenario. An open area of
research is the use of these techniques in multiuser
scenario. Multiuser scenarios are necessary for the wide
use and adoption of these OW systems.
• In multi-user OW systems, the optimum allocation of
resource such as wavelengths, time slots, beams, imaging
receiver pixels, and ADR facets is an open research area.
• In the case of a multiuser scenario, scheduling can also be
used with the techniques above to maximise throughput,
reduce interference, reduce power consumption, increase
reliability, reduce delay or optimize with respect to other
metrics.
• Efficient use of the optical transmitted power is key in IRC
systems due to the power transmitted in IRC systems being
restricted by eye safety. Increasing the transmit power can
however lead to improved SNR if interference is properly
managed. High SNRs can allow simple receiver structures
to be used reducing the system complexity. Therefore,
effort is required to identify methods that can enable the
use of high transmit power in a safe way, such as
holographic techniques.
• The optimization of an imaging receiver can lead to low
complexity and easier implementation. To the best of our
knowledge, there are no high-speed imaging receivers to
date that have been specially designed for mobile IRC
systems. At high data rates most of the components will
probably be adopted from the optical fibre domain, which
is not ideal for IRC systems; thus, more research is needed
in this area to design and fabricate new devices for IRC
systems.
• Applying new techniques, such as OMIMO, in
combination with advanced modulation (Optical OFDM
and other) may further enhance the data rates achieved in
IRC systems.
B. Open research issues in VLC systems
The main current research focuses on modulation techniques,
mobility, uplink connectivity, dimming control and
illumination, increasing the achievable data rate, improving the
modulation bandwidth of transmitters and mitigating ISI.
• Uplink channel provision is an important issue. The
proposed solutions, such as the use of IRC, corner cube
reflectors or RF systems, unfortunately do not fit well with
the VLC system goal to provide high data rate in the uplink
and down link as these techniques may conflict with
providing mobility and minimizing the size of the VLC
devices (portable device). Therefore, new schemes to
provide an uplink for VLC are required. Intermediate
stations (relays) have been studied previously for use in a
variety of networks to increase the coverage area and the
capacity. Relay stations may be used for VLC systems to
provide an up link connection for users and enable
communication between users.
• Further studies are necessary to answer questions relating
to the optimum dimming control and optimum modulation
for VLC that provide a good balance between
communication and illumination.
• Providing mobility for VLC systems is vital for indoor
users. The ADR and imaging receiver evaluated in IRC
systems can be used to offer mobility for VLC systems and
to mitigate the ISI problem at the same time. Our previous
work in this area has shown that significant enhancements
can be achieved when using an imaging receiver [24].
• LDs can be used for illumination and communication in
VLC systems instead of LEDs in order to enable higher
data rates. Initial results have shown that data rates of tens
of gigabits per second per user and more can be achieved
[10], [24], [25]. Further work is needed in this area to
optimize the transmitter and receiver devices, their
properties and architecture.
• Due to the energy efficiency, scalability and flexibility of
VLC systems, they can be used in next generation data
centres replacing hundreds of metres of cables or optical
fibre. Investigations in this area are required to examine the
benefits of applying VLC systems in data centres.
• The main frontier in VLC and OWC now is networking.
This includes networking and MAC techniques applied in
the optical medium to optimize beams, receiver detectors
fields of view and orientations, and to determine the
optimal allocation of resources that include wavelengths,
space and time resources. The optimum front haul and
backhaul network architectures are open questions that
deserve substantial attention to deliver and collect data
from optical luminaires or from IRC sources and detectors.
• The use of machine learning and artificial intelligence in
VLC and OWC is important and timely to carry out the
optimum resource allocation described above, to enable the
network to self-configure and self-adapt taking into
account interference, capacity, power consumption,
throughput and reliability and availability into account.
X. CONCLUSIONS
In this paper, we have presented an overview of OW systems
that emphasized their deployment to provide indoor
communication. We illustrated how OW technologies can offer
a complementary technology for future wireless
communications in indoor applications, co-existing potentially
with RF systems.
Two distinct categories of OW systems (IRC and VLC)
systems were discussed. Link design, modulation techniques,
transmitter/receiver structures, possible challenges, the main
applications and open research issues for each category have
been identified.
IRC systems can provide extremely high data rates and can
support mobility. These are however still significantly
challenging due to the indoor OW channel nature. However,
suitable techniques such as the use of the space dimension
through the introduction of suitable beams, beam angle, beam
power adaptation and beam delay adaptation, together with the
19
use of imaging receivers can be considered to provide mobility
and enhance the performance of the indoor IRC systems.
VLC systems are among the promising solutions to the
bandwidth limitation problem faced by RF systems. Significant
research effort is being directed towards the development of
VLC systems due to their numerous advantages over radio
systems. Networking in VLC systems is a key outstanding
challenge.
XI. ACKNOWLEDGMENTS
We would like to acknowledge funding from the Engineering
and Physical Sciences Research Council (EPSRC) for the
INTERNET (EP/H040536/1) and STAR (EP/K016873/1)
projects. All data are provided in full in the results section of
this paper. The authors extend their appreciation to the
International Scientific Partnership Program ISPP at King Saud
University, Kingdom of Saudi Arabia for funding this research
work through ISPP#0093.
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