1

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

ml15ozma@leeds.ac.uk, asdftaha@yahoo.com, malresheedi@ksu.edu.sa, j.m.h.elmirghani@leeds.ac.uk

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

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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.

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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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