# Liquid Crystal-Based RIS for VLC Transmitters: Performance Analysis,
Challenges, and Opportunities

Sylvester Aboagye, Telex M. N. Ngatched, Alain R. Ndjiongue, Octavia A. Dobre,
and Hyundong Shin This paper has been accepted for publication by IEEE.
Copyright may be transferred without notice, after which this version may no
longer be accessible.

###### Abstract

This article presents a novel approach of using reconfigurable intelligent
surfaces (RISs) in the transmitter of indoor visible light communication (VLC)
systems to enhance data rate uniformity and maintain adequate illumination. In
this approach, a liquid crystal (LC)-based RIS is placed in front of the LED
arrays of the transmitter to form an LC-based RIS-enabled VLC transmitter.
This RIS-enabled transmitter is able to perform new functions such as transmit
light steering and amplification and demonstrates very high data rate and
illumination performance when compared with traditional VLC transmitters with
circular and distributed LED arrays and the more recent angle diversity
transmitter. Simulation results reveal the strong potential of LC-based RIS-
aided transmitters in satisfying the joint illumination and communication
needs of indoor VLC systems and positions VLC as a critical essential block
for next generation communication networks. Several challenging and exciting
issues related to the realization of such transmitters are discussed.

###### Index Terms:

LED array arrangement, angle diversity transmitter, liquid crystal,
reconfigurable intelligent surfaces, illumination, data rate.

##  I Introduction

Visible light communication (VLC) has emerged as one of the key revolutionary
technologies to support energy-efficient, secure, and high data rate
transmissions with low deployment costs in the next generation of
communication networks [1]. This is because of the huge and unlicensed
bandwidth availability in the visible light spectrum and the rapid development
of light-emitting diodes (LEDs), which serve as transmitters in VLC. However,
the practical deployment of VLC systems, especially in indoor environments,
has been faced with unique challenges such as non-uniform illumination and
data rate coverage [2, 3, 4, 5], line-of-sight (LoS) blockages, random device
orientation, and loss of incident signal power in VLC receivers [6]. The focus
of this article relates to the non-uniform illumination and data rate coverage
design issue. Although this challenge can be considered the most important
since there can be no successful communication without adequate network
coverage, it has received the least attention. Specifically, large variations
in illumination and data rate, especially in the corners of indoors, limit the
system’s ability to provide ubiquitous services and high data rates to
multiple users located at different places. Moreover, the quality of any
user’s experience should not be defined solely or affected by the location in
a room as all users should enjoy high communication quality.

The authors in [2] examined the illumination properties of different LED array
arrangements and experimentally demonstrated the communication and
illumination performance of a phosphor-based VLC system. In [3], the authors
investigated the bit error rate and channel capacity performance of circular
and centered LED arrangements. In [5], the authors analyzed the effects of LED
array layout on the illumination uniformity of VLC systems. An optimization
problem to determine the optimal placement of LED arrays to maximize the
average area spectral efficiency has been considered in [7]. The results
indicated that LED arrays must be deployed in the middle of the room to
support high spectral efficiency and illumination. The authors in [8, 9, 10]
studied the use of angle diversity transmitters (ADTs) to reduce illumination
fluctuation and provide a more uniform data rate in indoor VLC systems.

Recently, a number of studies such as [1, 4, 11, 12, 13, 6] have investigated
the use of optical reconfigurable intelligent surfaces (RISs) to solve the
design problems (i) to (iv). In [4], the authors proposed a novel approach of
using mirrors to enhance the illumination uniformity and data rate of an
indoor multi-cell VLC system. The studies in [1, 11, 12] proposed RIS-aided
VLC system models to combat the blockage and random device orientation issues
in an indoor environment. In [6, 13], the authors examined a liquid crystal
(LC)-based RIS to enhance signal detection in VLC receivers by performing
incident light amplification and light steering. In the studies mentioned
above, the considered system models involved the deployment of optical RISs
either in the transmission channel [1, 11, 12, 4] or in front of the
photodetector (PD) of the VLC receiver [6, 13]. To the best of the authors’
knowledge, there has yet to be a study on the application of optical RISs at
the transmitter side to boost the coverage and enhance key performance metrics
of VLC systems.

In VLC systems, it is highly desirable to ensure illumination uniformity and
high speed data transmission in the indoor environment. This is necessary to
ensure that quality-of-service is not dependent on the location of the user.
However, the current approaches of deploying light fixtures (e.g., LEDs or LED
arrays) indoors do not achieve uniform illumination coverage as, most often,
the corners of various rooms have less illumination. An LED with a large semi-
angle can generate wider beam angles to ensure more uniform illumination
indoors. However, the generation of such broad beams results in a decrease in
the intensity of the optical signals. Only a few studies have investigated
methods of providing even illumination and high data rates in indoor VLC
systems. Furthermore, the impact of LED/LED array arrangement on the
performance of indoor VLC systems has yet to receive much attention. Note that
the LED array arrangement can play a vital role in the overall system
performance of VLC. Unlike [6] and [13], for the first time this article
explores the use of LC-based RISs in the transmitter of a VLC system to
jointly improve the communication and illumination performances in indoor VLC
systems. In this article, a novel approach of using LC-based RIS to enhance
illumination and data rate uniformity of an indoor VLC system is presented,
bringing out the following main contributions:

  * •

We propose a novel application of optical RISs in VLC systems. Specifically,
an LC-based RIS-enabled transmitter design is introduced and the impact of the
RIS on the emerging light from the LED is analyzed.

  * •

We examine the propagation characteristics of a VLC system with RIS-enabled
transmitters as optical signals travel from the LEDs to the receivers using
geometric optics. In addition, we provide an expression to characterize the
illumination distribution.

  * •

We use simulations to quantify the potential gains of deploying RISs in front
of LED arrays and present a performance comparison with a VLC system equipped
with ADTs and other popular LED array arrangement schemes.

  * •

Finally, we discuss other potential applications of this novel transmitter
design and present several exciting and challenging research opportunities
that can further improve its performance and accelerate its realization for
future generation optical wireless networks.

##  II Traditional Deployment of LEDs Indoors

This section describes the various and modern ways of LED array placement
indoors. Although the deployment of indoor light fixtures could depend on
design factors such as size and space, occupant’s age and preference, ceiling
height and shape, to mention only a few, a general description is provided.

As the primary purpose of LED arrays is to provide sufficient indoor
illumination, their traditional deployment has always focused solely on
lighting and illumination. Due to the Lambertian radiation pattern of LEDs and
the fact that most human interactions or conversations occur at the center of
indoor environments (i.e., the task or activity area), LED arrays are
typically deployed as ceiling fixtures at/or near the center to provide
adequate functional illumination. Such a typical LED array arrangement is
illustrated in Fig. 1 (a) where, under the point source assumption, 4 LED
arrays are deployed as a ceiling light fixture that directs light downwards.
Figure 1 (b) depicts another popular LED array arrangement where the 4 LED
arrays are distributed on the ceiling in a rectangular/square shape, with each
LED array serving as a point source transmitter. The LED array arrangements in
Figs. 1 (a) and (b) have been extensively considered in earlier studies on
indoor VLC systems such as [3, 2, 10, 4, 12, 13, 7, 11].

Figure 1: Typical LED arrangement indoors and the proposed LC-based RIS-
enabled VLC transmitter: (a) centralized LED array placement; (b) distributed
LED array placement; (c) ADT LED array placement; (d) centralized LED arrays
with an LC-based RIS at the front end; (e) geometry of light signal
propagation and amplification through the LC-based RIS element.

A more recent approach for LED array placement, referred to as ADT arrays, is
shown in Fig. 1 (c). In this figure, each LED array is inclined at an
elevation angle $\tau$ to point toward a particular direction to improve the
illumination level throughout. ADTs have been considered in the design of
multi-cell VLC systems in a few recent studies (e.g., [8, 9, 10]). These
studies demonstrated the ADTs superior data rate and energy efficiency
performance compared to the centralized and distributed LED array placement
schemes. However, illumination and data rate uniformity assessments have yet
to be reported on ADTs.

##  III LC-Based RIS-Enabled VLC System

###  III-A LC-Based RIS-Enabled Transmitter Design

Figure 1 (d) depicts the proposed RIS-enabled VLC transmitter design. In this
figure, an LC-based RIS is deployed in front of a centralized LED array
arrangement.111Note that the discussions on channel gain expression and
analysis on the performance for this transmitter design applies to other LED
array arrangements such as ADT and distributed LED arrays. The motivation for
considering LC-based RIS is its light steering and amplification capabilities
when subjected to an external electric field. More specifically, LC-based RISs
are characterized by their electronically tunable physico-chemical properties
(e.g., refractive index, emission, and attenuation coefficients) that can be
easily controlled by the arrangement of the LC molecules via an external
electric field. Moreover, LCs, in general, have received significant interest
in optical wireless networks and have been considered in the development of
optical filters for communications, next generation light detection and
ranging sensors for self-driving vehicles, and optical receivers [6].

Figure 1 (e) illustrates the principles of light steering and amplification in
the LC-based RIS when an external voltage, $v_{\rm e}$, greater than the
threshold voltage, $v_{\rm th}$, is applied. The threshold voltage is a
critical voltage at which the re-orientation of the LC’s molecules begins. As
shown in this figure, the emitted light beam from the LED arrays impinges on
the LC-based RIS. At the surface of the LC-based RIS, part of the incident
light beam gets reflected while the remaining beam undergoes refraction as it
passes from the air medium with a refractive index $n_{a}$ into the LC element
with a refractive index $n_{c}$. Inside the LC-based RIS element, the light
beam’s photons interact with the LC’s excited molecules (due to the presence
of the external electric field). This causes the excited molecules to drop to
a lower energy level, resulting in the generation of identical new photons
through the principle of stimulated emission. The direction of the resulting
light beam (i.e., incoming photons and the generated photons) as it propagates
through and exits the LC-based RIS element with thickness $d$ can be
controlled through an electric field-induced molecular reorientation [13, 6].
More particularly, the transmission coefficient that characterizes light
propagation in the LC-based RIS element is a function of the refractive index
$n_{c}$. This refractive index can be tuned electronically by changing the
orientation of the LC-based RIS element’s molecules through the external
voltage $v_{\rm e}$. Thus, incident light steering and amplification are
obtained by subjecting the LC-based RIS to the external electric field.

Figure 2: Application scenarios of the proposed LC-based RIS-enabled VLC
transmitter: (a) transmit signal amplification and coverage enhancement; (b)
dynamic beam steering; (c) transmit signal amplification and beam focusing;
(d) secured and interference-free VLC.

Figure 2 illustrates the various application scenarios of the LC-based RIS-
enabled VLC transmitter for indoor VLC systems. Figure 2 (a) shows how the LC-
based RIS can be used to amplify incoming light from LEDs to produce high-
intensity emitted visible light signal to jointly enhance coverage,
illumination, and received signal strength. Figure 2 (b) illustrates how the
LC-based RIS can provide dynamic beam steering, which is useful in indoor
environments where users are mobile. Figure 2 (c) demonstrates the joint light
amplification and beam focusing capabilities of an LC-based RIS-enabled VLC
transmitter. Finally, Fig. 2 (d) highlights how the LC-based RIS-enabled VLC
transmitter can produce light beams with narrow beamwidth to reduce
interference, provide multiple beams with different intensities via controlled
amplification and beam focusing to support differentiated quality-of-service
requirements for multiple users, and enhance the security of indoor VLC
systems. These application scenarios reveal how higher communication (e.g.,
energy efficiency, data rate, and security) and illumination performances can
be achieved without any additional resources, i.e., no extra transmit power or
bandwidth resources. Moreover, this novel RIS-enabled VLC transmitter can
assist in addressing the LoS blockage issue through light amplification and
beam focusing onto an RIS (e.g., mirror array) in the channel.

###  III-B Channel Model, Data Rate, and Illumination Expression

The channel model of a VLC system with an LC-based RIS-enabled transmitter
characterizes the propagation of light beams from the LED arrays, through the
LC-based RIS in front of the LED arrays, and finally through air to the PD of
the VLC receiver. The channel gain can be expressed as
$H=\alpha_{LC}\times{G_{\rm LoS}}$, where $\alpha_{LC}$ denotes the LC’s
transmission coefficient defined in [13] and ${G_{\rm LoS}}$ is the direct
current gain of the LoS propagation [4]. The LC’s transmission coefficient can
be tuned by optimizing the refractive index of the LC in the presence of an
external electric field to control the emerged light direction as revealed in
[13]. Note that for the VLC transmitters (i.e., LED array arrangements) in
Fig. 1, $H={G_{\rm LoS}}$.

The achievable data rate for any user in an indoor environment served by a VLC
system with an LC-based RIS-enabled transmitter with the channel gain, $H$,
the optical transmit power, $P$, and the amplification gain coefficient,
$\Gamma$, which is defined in [14] can be determined using the rate expression
in [13].

Uniform lighting distribution is essential to the reliability of the VLC
systems and, as a result, needs to be considered in RIS-aided VLC systems. The
reason is to ensure that the RIS placed in front of the LED arrays does not
degrade the illumination properties of the VLC transmitter or cause any health
risk. The illumination intensity at a surface inside a room with an LC-based
RIS-enabled transmitter can be measured by the surface illuminance which is
expressed as [4, 13]

$I=\exp\left({\Gamma d}\right)\times P\times\alpha_{\rm
LC}\times\frac{{\left({m+1}\right)}}{{2\pi{l^{2}}\delta}}{\cos^{m}}\left({{\Phi}}\right)\cos\left({{\varphi}}\right)G\left({{\varphi}}\right),$

where $\Gamma$ denotes the amplification gain coefficient defined in [13],
$\alpha_{\rm LC}$ is the transition coefficient,
$m=-{{{{\log}_{2}}\big{(}{\cos\big{(}{{\phi_{{1}/{2}}}}\big{)}}\big{)}}}^{-1}$
is the Lambertian emission order with ${{\phi_{{1}/{2}}}}$ as the LED’s
semi–angle at half power, $l$ is the distance between the transmitter and the
surface, $\delta$ is the optical to luminous flux conversion factor, ${\Phi}$
is the angle of irradiance, $\varphi$ is the angle of incidence, and
$G\left({{\varphi}}\right)=f^{2}/\sin^{2}\varphi,0\leq\varphi\leq{\phi_{{1}/{2}}}$
is the gain of the non-imaging concentrator which focuses the light from the
LEDs into the LC-based RIS with ${\phi_{{1}/{2}}}$ as the LED’s semi-angle at
half power and $f$ as the internal refractive index of the concentrator. In
this expression, $P$ is the optical transmit power from the LED array which is
incident on the LC-based RIS and $\exp\left({\Gamma d}\right)\times
P\times\alpha_{\rm LC}$ represents the emerged optical power from the LC-based
RIS after the incident light has propagated through the LC module and
undergone amplification in the presence of an external electric field. The
remaining part of the expression denotes the luminous flux of a unit optical
power.

Figure 3: Illumination distribution comparison for VLC transmitters
characterized by different LED array arrangement and LC-based RIS-enabled VLC
transmitter: (a) centralized LED array arrangement (Min. 10 lux, Max. 107
lux); (b) distributed LED array arrangement (Min. 15 lux, Max. 43 lux); (c)
ADT LED array placement (Min. 28 lux, Max. 81 lux); (d) proposed centralized
LED array with LC-based RIS (Min. 222 lux, Max. 910 lux). The $x$, $y$, and
$z$ axes represent the length (m), width (m), and illumination (lux),
respectively.

##  IV Analysis of Achievable Data Rate and Illumination Performance

In this section, a performance comparison of the different LED arrays
arrangements and the proposed RIS-enabled VLC transmitter is performed. The
performance metrics used in our analysis are the achievable data rate, surface
illumination, illumination uniformity, and data rate uniformity. Illumination
uniformity can be defined as the ratio between the minimum and the average
illumination among all surfaces or sensing points in an indoor environment [4,
5, 15]. Similarly, data rate uniformity refers to the ratio between the
minimum and the average data rate among all surfaces or users. According to
[5], values close to 1 indicate uniform lighting or data rate condition and,
in general, values more than 0.7 are desired. However, a uniformity value of
0.4 is considered the least acceptable for illumination purposes [15].

Without loss of generality, a $5$ m $\times$ $5$ m $\times$ $3$ m room size is
considered [2, 3, 4, 5]. For the distributed LED array placement scheme, the
entire room is assumed to be divided into four equal quadrants, each with an
LED array ceiling lamp at the center. For the remaining LED array arrangement
schemes and the proposed approach, the LED arrays are deployed at the center
of the room. The illumination and data rates are measured at a typical desktop
height (i.e., receiver plane) of 0.85 m above the floor. To better determine
the performances of the various schemes across the whole room, the receiver
plane is divided into $100\,\times 100\,$ grid points which serve as sensing
points. This is in accordance with the European lighting standard [15], which
requires the use of the grid specification to calculate and measure
illumination averages and uniformity. For the considered simulations, the
information carrying bandwidth is set as 200 MHz, $\tau=45^{\circ}$, $d=0.75$
mm, $v_{\rm th}=1.34$ V, and $P=1$ W. The LEDs operate at a wavelength of 510
nm. The analysis for the RIS-enabled transmitter is carried out for an LC-
based RIS element with a fixed refractive index $n_{c}=1.55$ ($v_{\rm e}=2.1$
V is the required voltage). All other simulation parameters are set according
to [13].

Figure 4: Data rate distribution comparison for VLC transmitters characterized
by different LED array arrangement and an LC-based RIS-enabled VLC
transmitter: (a) centralized LED array arrangement (Min. 0.3595 Gbps, Max.
1.6399 Gbps); (b) distributed LED array arrangement (Min. 0.4351 Gbps, Max.
1.5373 Gbps); (c) ADT LED array placement (Min. 0.9179 Gbps, Max. 2.8024
Gbps); (d) proposed centralized LED array with LC-based RIS (Min. 2.6115 Gbps,
Max. 3.7363 Gbps). The $x$, $y$, and $z$ axes represent the length (m), width
(m), and data rate (bps), respectively.

Figure 3 compares the illumination distribution in an indoor environment for a
VLC system with a transmitter characterized by typical LED array arrangement
schemes with that of the proposed RIS-enabled VLC transmitter. This figure is
generated by partitioning the receiver plane into small grids and determining
the illumination in each grid. Several key observations can be made from Fig.
3. To begin with, the illumination is highest at the center of the room and
lowest at the corners for all the different LED array arrangement schemes and
the proposed transmitter. This observation can be explained by the Lambertian
radiation pattern of LEDs. The highest illumination for the centralized,
distributed, and ADT arrangement schemes are significantly below the
recommended minimum indoor lighting requirement of 150 lux for any
office/gym/home activities as described in [15]. As can be observed, the
illumination at the task or activity area, which is typically the center of
the room, and the room corners is very low. As a result, these approaches may
not always satisfy the illumination function in VLC. However, this is not the
case for the proposed transmitter, as illumination is greatly enhanced due to
the light amplification and beam focusing capabilities of the LC-based RIS.
The minimum illumination requirement is satisfied for the task or activity
area and the corners of the room. Secondly, the proposed approach satisfies
the minimum required illumination of 400 lux for reading/office work according
to the European lighting standard [15], as more than 50$\%$ of the total room
area has illumination above the threshold of 400 lux. Moreover, the
illumination of the immediate surrounding area of the task area and that of
the background area fall within the recommended values (i.e., over $500$ lux
for the immediate surrounding and above 200 lux for the background
area).222According to the European lighting standard [15], the immediate
surrounding area refers to the space just around the task or activity area and
the area outside the immediate surrounding is referred to as the background
area. The illumination of the immediate surrounding area should be related to
the illumination of the task area, and the illumination of the background area
should be related to that of the immediate surrounding to avoid significant
spatial variations in indoor illumination. Thus, it is acceptable that the
illumination of the room corners is lower than that of the task or activity
area. The above analyses reveal that the proposed LC-based RIS-enabled
transmitter has superior illumination performance and fulfills the visual
comfort needs. Furthermore, compared with the centralized LED array
arrangement scheme (i.e., without the LC-based RIS), up to 2100$\%$ gain in
illumination is obtained with the deployment of an RIS at the transmitter
side. Note that a similar performance gain is expected when an LC-based RIS is
used with the distributed and ADT LED array arrangement schemes. In addition,
the proposed approach obtains illumination performance gain of up to 2000$\%$
and 1325$\%$ when compared to the distributed and ADT placement schemes,
respectively.

Figure 4 illustrates the data rate distribution for the considered LED array
arrangement schemes and the proposed RIS-enabled VLC transmitter. This figure
is generated by partitioning the receiver plane into small grids and
determining the data rate in each grid. It can be observed from this figure
that the proposed approach achieves up to 626$\%$, 500$\%$, and 184$\%$
improvement in data rate when compared to the centralized, distributed, and
ADT LED array arrangement schemes, respectively. Moreover, only the proposed
approach has a minimum data rate on the order of Gbps, as the remaining
approaches could only support Mbps download speeds. This reveals that all
users can enjoy Gbps download speed irrespective of their location (i.e., in
the corner of the room or at the center) for the same number of LED arrays,
transmit power, and bandwidth resources. Thus, in addition to the significant
increase in the peak data rates, higher data rates can be guaranteed for the
vast majority of the locations in the indoor environment. This significant
performance enhancement is due to the new reconfigurable and transmit light
amplification capability of the RIS-enabled transmitter.

Table I shows the illumination and data rate uniformity for the different LED
array arrangement schemes and the proposed approach based on the results in
Figs. 3 and 4. The data rate (illumination) uniformity value for any LED array
arrangement scheme is the ratio between the minimum value of the data rates
(illumination) for all the grid points and the average data rate (average
illumination) among all the grid points. The centralized LED array arrangement
has the worst illumination and data rate uniformity. Its uniformity values of
0.2371 and 0.3535 are below the acceptable values of 0.40 for illumination and
0.7 for data rate, respectively. Hence, the centralized scheme is
characterized by high illumination and data rate fluctuations over the entire
room and exhibits an unacceptable performance level in achieving the dual role
of communication and illumination. The two remaining LED array arrangement
schemes and the proposed approach demonstrate uniform illumination and, hence,
would experience the least fluctuation. Besides, these uniformity values
(i.e., 0.4378, 0.4755, and 0.4628) are practical since it is acceptable for
some parts of the room space to have lower illumination. Moreover, a
comparison of the data rate uniformity values of 0.3937, 0.4379, and 0.8168
for the distributed, ADT, and the proposed schemes, respectively, suggests
that the proposed approach can support high data rates across the entire room.
More reliable and high quality communication can be guaranteed for all users
in an indoor environment with the proposed RIS-enabled transmitter. Unlike
illumination, data rate uniformity values of 0.3535, 0.3937, and 0.4379 are
not acceptable for VLC. For such values, the data demands of all users may not
be satisfied since the achievable data rate becomes highly dependent on the
user’s location (i.e., extremely high data rate for users in the center of the
room and extremely low data rate for users far away from the center).

TABLE I: Illumination and data rate uniformity for indoors. LED Array Arrangement Type | Uniformity Distribution
---|---
Illumination | Data Rate
Centralized LED Array | 0.2371 | 0.3535
Distributed LED Array | 0.4378 | 0.3937
ADT LED Array | 0.4755 | 0.4379
Proposed | 0.4628 | 0.8168

Finally, Fig. 5 illustrates the achievable data rate and illumination
performance of an indoor VLC system for different transmit power values. In
this figure, the minimum data rate and illumination have been plotted for
different transmit power values for the centralized, distributed, ADT, and the
proposed LED array arrangement schemes. It can be observed that the minimum
data rate and minimum illumination increase with the transmit power for all
the considered approaches. However, growth trends for the data rate and
illumination distribution are different because, unlike surface illumination,
data rate is a log function. The proposed design outperforms the remaining
approaches in terms of mimimum data rate when the transmit power is less than
4 W. For transmit power values of 4 W and above, the ADT LED array arrangement
scheme achieves a higher minimum data rate than the LC-based RIS-enabled
transmitter. The reason for this observation is that, while the data rate for
the proposed scheme begins to saturate, that of the ADT approach keeps
increasing for transmit power values greater than 4 W. This growth trend of
the ADT approach, which is similar to that of the distributed LED array
arrangement, is due to the spatial configuration of its LED arrays and the
fact that the received power depends on the position of the LED arrays and the
angle of irradiance. Although the ADT LED arrays arrangement scheme
demonstrates the best minimum data rate performance for transmit power values
above 4 W, its illumination remains worse than that of the proposed approach.
Thus, even for higher transmit power values, the centralized, distributed, and
ADT LED array arrangement schemes fail to provide acceptable joint data rate
and illumination performance. Note that the proposed approach involves placing
the LC-based RIS in front of the centralized LED array arrangement scheme. It
can be inferred from the simulation results that higher performance gains
could be expected when the LC-based RIS is placed in front of the ADT LED
arrays since the ADT arrangement scheme outperforms the centralized LED arrays
placement scheme.

Figure 5: Impact of transmit power on the performance of a VLC system for the
considered approaches: (a) minimum achievable data rate vs. transmit power;
(b) minimum illumination vs. transmit power.

##  V Open Research Opportunities and Challenges

Unlike the deployment of RISs in the channel, which has received great
research attention, the application of RISs in VLC transceiver design and
performance optimization has yet to be explored. This article has demonstrated
the great potential of LC-based RIS-enabled VLC transmitter. By presenting a
novel approach of using LC-based RIS to (i) achieve a much higher level of
illumination, (ii) significantly improve data rate, and (iii) jointly enhance
illumination and data rate uniformity, the following promising research
opportunities and challenges have been identified:

1\. Performance Optimization under Joint Illumination and Communication
Constraints: The performance of LC-based RISs is highly dependent on the
electronic tuning of the refractive index since its value determines both the
amplification and transmission gain coefficients of the LC-based RIS. Thus,
the expression for the achievable data rate in VLC systems with an RIS-enabled
transmitter differs from a VLC system without the RIS. Moreover, it is vital
to consider the joint optimization of the LC’s refractive index and the
available transmit power of the VLC system to enhance the illumination and
communication performances further. Several objective functions such as data
rate maximization, energy efficiency maximization, load balancing, max-min
rate, transmit power minimization, and secrecy rate maximization need to be
examined under illumination and communication related constraints (e.g.,
quality-of-service, quality-of-experience, and LC-based RIS tuning time).
Furthermore, novel constraints related to the amplification gain should be
studied as a network operator or a customer might want to emphasize or de-
emphasize the amplification capability of the LC-based RIS-enabled VLC
transmitter. Due to the resulting unique channel gain, illumination, and data
rate expressions, the potential high dimensionality of the optimization
problems, and the new design constraints, traditional approaches for
optimizing the performance of VLC systems cannot be directly adopted. New
mathematical optimization techniques with low complexity and machine learning-
driven approaches should be explored in further research, proof-of-concept
studies, and, finally, the realization of LC-based RIS-enabled transmitters in
VLC. These developments could be extended into other VLC applications, such as
vehicle-to-vehicle and underwater wireless communication since their channel
models differ from that of the indoor environment.

2\. Analysis of the Impact of VLC and RIS Parameters on Illumination and
Communication Performance: VLC transmitters and systems generally have several
design parameters such as the LED’s semi-angle at half power, available
transmit power and subchannels, room dimension and shape, wall reflection
coefficients, and the distance between LED arrays. On the other hand, an LC-
based RIS element has parameters that can affect its performance gains. Such
parameters include the threshold voltage, thickness of the LC, and
transmission wavelength. Due to page limitations, this article considered
fixed values for the parameters mentioned above. It would be worth examining
how different system parameters will impact the system performance and
potentially yield further improvements in data rate and illumination.
Moreover, optimization problems with such parameters as decision variables
need to be considered to determine the optimal values for various performance
metrics.

3\. Noise Effect on the Performance of the LC-based RIS: The LC-based RIS
performs transmit light amplification through the process of stimulated
emission that generates new photons. In practice, the photon generation
process can limit the data rate and illumination performances. This is because
tuning LCs takes time and is characterized by the response time (which is
typically in the order of milliseconds). In addition, the photon generation
process might result in noise affecting the system’s performance. It is
therefore important to investigate ways of decreasing the response time, how
often to perform LC tuning, and noise effects on the performance of LC-based
RIS-enabled VLC transmitters.

4\. Adaptive Cell Formation for VLC: The ability of LC-based RIS-enabled
transmitters to focus, amplify, and steer emitted light due to their
electronically adjustable refractive index make them suitable for the design
of adaptive cell formation algorithms. Unlike ADTs, where the resulting cells
formed by the highly directional LED arrays cannot be altered with changes in
the indoor environment (e.g., change in user density and distribution, data
rate and illumination requirements, and mobility), LC-based RIS-enabled
transmitters permit the design of algorithms to control both the refractive
index and transmit power to move illumination coverage areas in indoors and
create adaptive communication cells. The impact of such joint power allocation
and cell formation algorithms on the illumination and communication
performance of LC-based RIS-enabled VLC systems is another exciting research
area for future work.

5\. Impact on Cost of VLC: The development of RISs, in general, and optical
RISs, in particular, is still at an early stage. As a result, much has yet to
be known about the cost of RISs. Inserting an LC-based RIS into an LED package
can affect its cost. Since low deployment cost is one of the key features that
network operators look out for, it is critical to investigate how RISs would
affect the cost of LEDs. Such investigations should focus on theoretical
studies and practical deployments that consider imperfections in the system
model to accurately examine the cost-benefit analysis.

6\. Enhanced VLC for joint communication, illumination, and sensing: VLC has
been identified as a key technology for indoor joint communication and
sensing. The quality of the received signal plays a critical role in the
communication performance and sensing accuracy of such an integrated sensing
and communication system. The signal focusing and amplification capabilities
of the proposed LC-based RIS-enabled VLC transmitter can be leveraged to
enhance the received signal strength in VLC-based integrated sensing and
communication systems.

##  VI Conclusion

This article investigated a novel approach of using LC-based RIS to improve
the data rate and illumination distribution in indoor VLC systems.
Specifically, the illumination and data rate performances of popular
approaches for LED array arrangement were first analyzed. Then, a novel VLC
transmitter design that involves placing LC-based RIS in front of the LED
arrays was introduced, and the channel gain and data rate expressions were
presented. Simulation results revealed that, unlike popular LED array
arrangement schemes, the proposed LC-based RIS-enabled VLC transmitter
achieves a higher data rate and illumination distribution. Moreover, it
enhances data rate uniformity and demonstrates that a much higher level of
data rate and illumination can be jointly obtained from the same number of LED
arrays and transmit power value when an LC-based RIS with a 1.34 V - 10 mA
source is employed. Thus, more energy efficient RIS-aided VLC systems can be
designed due to the nearly-passive nature of the LC-based RIS. Furthermore,
several open challenges have been highlighted. Since the use of RISs at the
transmitter side in VLC remains unexplored, this article can provide a helpful
guide and inspire further studies on optical RIS-aided communications.

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