Physical-Layer Security in Visible Light Communications

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Physical-Layer Security in Visible Light
Communications
Anil Yesilkaya†, Tezcan Cogalan†, Serhat Erkucuk∗, Yalcin Sadi∗,
Erdal Panayirci∗, Harald Haas†and H. Vincent Poor‡
†Institute for Digital Communications, LiFi R&D Centre, The University of Edinburgh, Edinburgh EH9 3JL, UK
Email: {a.yesilkaya, t.cogalan, h.haas}@ed.ac.uk
∗Department of Electrical and Electronics Engineering Kadir Has University, 34083, Istanbul, Turkey
Email: {serkucuk, yalcin.sadi, eepanay}@khas.edu.tr
‡Department of Electrical Engineering, Princeton University, NJ-08544, USA
Email: poor@princeton.edu
Abstract—Optical wireless communications (OWC) and its potential
to solve physical layer security (PLS) issues are becoming important
research areas in 6G communications systems. In this paper, an overview
of PLS in visible light communications (VLC), is presented. Then, two
new PLS techniques based on generalized space shift keying (GSSK)
modulation with spatial constellation design (SCD) and non-orthogonal
multiple access (NOMA) cooperative relaying are introduced. In the
first technique, the PLS of the system is enhanced by the appropriate
selection of a precoding matrix for randomly activated light emitting
diodes (LEDs). With the aid of a legitimate user’s (Bob’s) channel
state information (CSI) at the transmitter (CSIT), the bit error ratio
(BER) of Bob is minimized while the BER performance of the potential
eavesdroppers (Eves) is significantly degraded. In the second technique,
superposition coding with uniform signaling is used at the transmitter and
relays. The design of secure beamforming vectors at the relay nodes along
with NOMA techniques is used to enhance PLS in a VLC system. Insights
gained from the improved security levels of the proposed techniques are
used to discuss how PLS can be further improved in future generation
communication systems by using VLC.
Index Terms—Physical-layer security, Visible light communications
(VLC), Generalized space shift keying (GSSK), Non-orthogonal multiple
access (NOMA).
I. INTRODUCTION
The issue of security in wireless communication networks is of
paramount importance due to the widespread use of mobile devices
and their increasing capabilities. In addition, with the advent of ma-
chine type communications (MTC), the importance of physical layer
security (PLS) increase, for example, to prevent autonomous systems
to be turned into weapons. Security functions in a wireless network
are typically provided through each open systems interconnect (OSI)
layer separately to meet the confidentiality, integrity, availability and
non-repudiation requirements. Extensible Authentication Protocol
(EAP), Wired Equivalent Privacy (WEP) and Wireless Application
Protocol (WAP) could be given as example solutions for WiFi security
protocols operating in link and application layers. The confiden-
tiality of information is typically ensured by encryption in modern
wireless networks, which includes secret and public key encryption
This work was supported by the Scientific and Technical Research Coun-
cil of Turkey (TUBITAK) under the 1003-Priority Areas R&D Projects
support Program No. 218E034 and in part by KAUST under Grant No.
OSR-2016-CRG5-2958-02. A. Yesilkaya acknowledges the financial support
from Zodiac Inflight Innovations (TriaGnoSys GmbH). H. Haas acknowl-
edges support from the EPSRC under Established Career Fellowship Grant
EP/R007101/1. He also acknowledges the financial support of his research by
the Wolfson Foundation and the Royal Society.
methods. The theoretical foundations of secrecy by cryptography
were built in Shannon’s 1949 seminal paper [1]. To be specific,
in [1], mathematical foundations for symmetric key encryption are
provided. However, due to the broadcast nature of the wireless
channel, confining information to a specific area is challenging. The
key management in dynamic wireless networks, comprise of many
intermediate terminals between the source and the destination, could
create weaknesses in the entire system’s architecture. The internet-of-
things (IoT), device-to-device (D2D) and MTC applications as well
as ad hoc networks in the new generation wireless communication
systems could be good examples of this phenomena. Furthermore,
the encryption methods mainly rely on the computational hardness
of a mathematical problem and the assumption that the eavesdropper
has limited computational power. However, as cloud radio access
network (C-RAN) is envisioned to be a feasible solution for 5G
and beyond, remote radio heads (RRHs) will not be capable of
meeting the computational expectations. More importantly, by using
cloud/quantum computing and machine learning tools, computational
hardness can easily be overcome. Hence, the secrecy of encryption
based systems are compromised if a method of solving a non-trivial
mathematical problem can be implemented [2].
PLS is an alternative way to achieve information theoretic secrecy
without encryption. The impairments in wireless channels, e.g.,
fading and noise could benefit from PLS systems to achieve better
secrecy. Accordingly, randomness in the channel can be harnessed to
guarantee that the messages are hidden from the adversarial users. In
[3], Wyner pioneered a new information theoretic path by providing
a mathematical description of a wiretap channel. Thus, the secret
key is not needed to ensure secrecy. Later, Csiszar and Korner
showed channel codes exist to simultaneously provide transmission
robustness and secrecy [4]. In 1978, Leung-Yan-Cheong et al. came
up with the notion of secrecy capacity [5]. In 2011, Oggier et al.
derived the secrecy capacity expressions for multiple-input-multiple-
output (MIMO) wiretap channels [6]. Both the legitimate user’s and
eavesdropper’s channel state information (CSI) are assumed to be
known in [6]. It is worth noting that in practical systems, the network
might be completely unaware of the passive eavesdropper’s potential
interceptions. Therefore, sub-optimal secure MIMO transmission is
mainly realized by transmitter preprocessing such as precoding [7],
friendly jamming [8], mapping [9] and selection [10].
In the above studies [1]–[10], radio frequency (RF) based systems
have been considered. In this paper, the concept of PLS will be

considered for visible light communications (VLC) systems. Ini-
tially, an overview of inherent advantages of the VLC for PLS
and some proposed PLS approaches will be presented. Then, two
recent techniques based on generalized space shift keying (GSSK)
modulation and non-orthogonal multiple access (NOMA) cooperative
relaying will be summarized and we will elaborate on the improved
PLS performance that these techniques can achieve. Finally, the
concluding remarks and future research directions will be presented.
II. OVE RVIEW: PHYSICAL LAYER SECURITY IN VISIBLE LIGHT
COMMUNICATIONS SYS TE MS
VLC and light-fidelity (LiFi)1utilize off-the-shelf light emitting
diodes (LEDs) for broadband data transmission. It is seen as an
efficient technology compared to conventional RF systems by means
of energy, deployment and cost. Moreover, the advantages of VLC
over conventional RF systems can be recalled as: (i) unregulated
and larger spectrum; (ii) applicability in electromagnetic interference
limited environments such as chemical plants, hospitals, aircrafts; and
(iii) enhanced security merits inherited from THz frequency band
characteristics.
In VLC, the information is encoded on the instantaneous light
intensity which could be detected at the receiver side by using photo-
diodes (PDs). Due to the spontaneous photon emission characteristics
of the conventional LEDs, the transmission and detection are realized
incoherently which is referred to as intensity-modulation and direct-
detection (IM/DD). Therefore, the transmit signal is limited to be real
and positive valued which restrains the unmodified application of RF
transmission techniques in VLC. In the PHY layer OWC literature,
efficient methods for obtaining both real and unipolar valued signals
are studied extensively. For example, in [11], the authors proposed a
signal shaping framework that considers the limitations imposed by
IM/DD transmission along with constraints based on the optical front-
ends. Accordingly, a comparison between single carrier modulation
techniques and optical orthogonal frequency division multiplexing
(OFDM) (O-OFDM) based multi carrier modulation techniques is
provided. In [12], an efficient MIMO transmission technique, spatial
modulation (SM), is combined with O-OFDM which is referred to
as optical spatial modulation (OSM) [13]. It is shown in [12] that
the Hermitian symmetry as well as unipolarity requirement in the
transmit signal can be mitigated by using LED indexes. In [14],
the effect of user mobility in OSM systems is investigated where
a power allocation is utilized to provide a certain bit error ratio
(BER) for a targeted user. Similarly, the technique proposed in [14]
is generalized to a single carrier MIMO-VLC transmission method
and termed optical GSSK (OGSSK) in [15].
During the last few years, PLS in VLC has become a promising
research area to enhance the privacy of wireless networks and to
complement encryption techniques applied at higher layers of the
network stack. The secrecy capacity of a LiFi network is shown to be
at least an order of magnitude higher than that of a Wireless Fidelity
(WiFi) network due to the inherent characteristics of the THz band
[16]. As light cannot travel through walls, the area spectral efficiency
and secrecy can simply be engineered according to the necessity in
LiFi networks. Furthermore, the lack of small scale fading effects in
OWC also create channel conditions which are mainly determined by
1LiFi extends the concept of VLC and describes a fully wireless network.
It supports bi-directional wireless networking with dense access point (AP)
deployment, multi-user transmission and seamless handover capabilities. In
this paper, LiFi is used interchangeably with VLC to represent an indoor
optical wireless communications (OWC) system that operates on infra-red
(IR) and visible light (VL) bands.
the geometry between the transmit LEDs and receive PDs. Thus, the
channel impulse response (CIR) of the legitimate user (Bob) could
be determined to some extend if the opto-electronic characteristics
of LEDs, PDs and Bob’s 3-D position parameters are known at the
evasdropper’s (Eve’s) side. In the literature, the problem of PLS in
VLC can be handled by, (i) beamforming; (ii) friendly jamming; (iii)
mapping and combinations of these techniques.
The early investigation for beamforming based PLS in VLC
systems is presented by Ayman et al. in [17]. In beamforming,
spatially distributed transmitters are utilized. Each of these have
a power scaling factor namely, the beamforming coefficients. The
emission pattern of the transmitters could be focused on Bob by
selecting the power scaling coefficients with the aid of its CSI at
the transmitter (CSIT). Thus, the confidential information becomes
hidden from Eve as long as the leakage is kept to a minimum. The
maximum secrecy could be achieved if both Bob’s and Eve’s CSIT
is assumed to be known. However, the network might be completely
unaware of malicious users which makes this assumption impractical.
Hence, a sub-optimal beamformer is employed in [15], [18]–[20]. It
can be noted that to apply beamforming, there must be a central
controller to convey necessary information such as user data and
beamforming coefficients to the distributed transmitters. However,
another LiFi-specific beamforming approach exist, named angular
diversity transmitter (ADT) [21]. ADT consists of multiple LEDs
that are directed (with narrow-beam angle) to different orientations.
Therefore, each LED has its own confined coverage area. Activating
a single or set of these narrow-beam LEDs in the transmitter with
the aid of Bob’s CSIT mimics the beamforming and effectively
reduces the leakage of confidential information. Hence, deploying
ADT mitigates the necessity of a central controller that is needed for
applying beamforming and can be seen as one of the LiFi-specific
PLS techniques.
Friendly jamming is another PLS technique that is based on the
transmission of both useful messages and interfering jamming signals,
simultaneously. Since the friendly jamming signal is designed to be in
the null space of the legitimate user’s channel matrix, it only effects
the adversarial user which has its channel spanning on the null space
of the legitimate user’s channel. It is noted in [17], [22] that the
friendly jamming and beamforming systems are only effective in
systems with multiple transmit units. Thus, as an efficient MIMO
transmission alternative, OSM-based VLC security has attracted a
significant attention in the literature [15], [23]–[25].
Transmit signal mapping is another technique which combines
encryption with conventional PLS methods for securing MIMO
transmissions, specifically SM [26]–[29]. In the mapping based PLS
method, a specific rule is devised by using a legitimate user’s CSI
which will be utilized for encoding the message signal. At the
legitimate user’s side, a de-mapping function is applied to obtain the
intended message back. However, it is assumed that the mapping rule
is known in both transmitting AP (Alice) and legitimate user’s (Bob)
sides. Similar to the encryption techniques, this assumption does not
always hold and creates a weakness in the system security. Hence,
beamforming, friendly jamming and a combination of beamforming
and friendly jamming methods are accepted as the main PLS methods
in VLC literature.
To enhance the system security beyond the noted VLC PLS
methods, one can benefit from the accurate localization capabilities of
VLC systems by virtue of the properties of light propagation. Very
precise localization information of mobile devices can be obtained
in indoor VLC systems. Hence, (i) the location of devices can be
continuously recorded; (ii) statistics can be created to establish the

Bob
Eve
Alice
GSSK
Modulation
Spatial
Constellation
Design
Zero
Forcing
Precoder
Power
Scaling
Fig. 1: A block diagram for secure OGSSK transmission technique.
typical movement pattern of the device; and (iii) machine learning
techniques could be developed to flag any anomaly. Furthermore,
as the light signals are significantly better spatially containable
compared to radio signals, it is possible to effectively use the concept
of dual-gate locking. Accordingly, a single username and password
are not sufficient to gain access to the user’s account, whereas a
key is also required. The key is specific to the AP that the user is
assigned to, which generally is the closest AP. Because of the spatial
containment of light, the coverage region could already be within
a few meters from the location of the legitimate user and hence,
dual-gate locking provides an extra level of protection. In order to
support mobility of the legitimate users, typically, the system would
allow handover to the immediate next neighbour APs. This creates a
dynamic geo-fence as some APs will be accessible and others would
be outside the ring of immediate next neighbours. This concept is
referred to as geo-fencing. Depending on the number of legitimate
users and eavesdroppers, different cases in PLS techniques can be
classified as one legitimate user and one eavesdropper in a single
cell VLC system or multi-user in large-scale VLC networks where
multiple legitimate users communicate in the presence of multiple
eavesdroppers scattered randomly in an indoor facility.
III. EFFIC IE NT PHYSICAL LAYE R SECURITY FOR OGSSK
In this section, we present a novel spatial constellation design
(SCD) based PLS technique for OGSSK systems. When the trans-
mitter is equipped with multiple LEDs and the eavesdropper’s CSI is
not known at the transmitter, some of the new modulation techniques
such as OSM and its variants can be employed. OSM is a promising
MIMO transmission method to reduce inter-channel interference de-
tection complexity and power consumption of the system. Our recent
research results have shown that PLS for MIMO-VLC systems could
be ensured by SCD in OGSSK [15], [30]. It has been shown that OSM
along with suitably designed precoding at the transmitter has the
capability to generate very strong interference to the eavesdropper and
zero interference to a legitimate user, without consuming additional
power to generate the jamming signal. Furthermore, in SCD-based
beamforming, the source does not need to know the CSIT of Eve.
In Fig. 1, a single cell VLC system consisting of one legitimate
user and one eavesdropper over a MIMO wiretap channel is shown.
For the given system model, it is assumed that Alice is equipped
with Nttransmit LEDs and both Bob and Eve are equipped with Nr
receive PDs. In conventional GSSK, Naout of NtLEDs are activated
during the transmission period. The active LED index set for the kth
transmission symbol may be given by Ik∈ {Ik,1,Ik,2,· · · ,INa}.
Therefore, the received signal model in conventional GSSK is as
follows:
y=˜
Hk˜
sk+n,(1)
where ˜
skand ˜
Hk∈RNr×Nt
+denotes effective constellation symbols
vector and effective channel matrix for the kth symbol, respectively.
Also, nis the additive white Gaussian noise (AWGN) vector. In order
to achieve secrecy with enhanced error performance on Bob’s side by
using SCD, the transmit signal is chosen to be, ˜
sk=ρPkvk+b. The
parameters ρand bdenote power normalization and DC bias coeffi-
cients, respectively [15]. Furthermore, vkrepresents the transmitted
signal points chosen from a uniform constellation with maximized
minimum Euclidean distance. The design of the precoding matrix for
the kth transmit symbol Pkplays a vital role in the secrecy capacity
of the VLC system. The standard procedure is to use zero forcing
precoding (ZFP) to avoid inter-channel leakages,
Pk=˜
HT
k,B˜
Hk,B−1˜
HT
k,B∀k. (2)
where ˜
Hk,Brepresents Bob’s effective channel matrix for the kth
transmission signal. Consequently, the receiver signals on both Bob’s
and Eve’s sides is given by
yB=sB+nB,and yE=sB+J+nE,(3)
where sB
∆
=ρ˜
Hk,BPkvk,B=ρvk,B. As can be seen from (3), a
destructive jamming signal vector, J, occurs at Eve’s location that
enhances the PLS of the OGSSK system. The jamming vector could
also be expressed by:
J=sE−sB=ρ˜
Hk,E−˜
Hk,BPkvk,B(4)
It is important to note from (4) that Jemerges naturally in the
proposed method while SCD for Bob is utilized. Thus, PLS is
achieved without the knowledge of Eve’s CSIT information and
without spending additional power for enhanced secrecy. At the
receiver, the transmitted GSSK symbols can be recovered by detecting
the active LED indexes at both Bob’s and Eve’s sides. Thus, the
maximum-likelihood (ML) detection at Bob and Eve’s sides are as
follows:
b
Ik,B= arg max
IkkyB−sBk2},
b
Ik,E= arg max
Ikky
0
E−s
0
Ek2}.(5)
Hence, the achievable secrecy capacity bounds for the SCD-based
OGSSK system can be derived as,
CGSSK ≤Nr
2log2 det (Cw)(1/Nr)
σ2
B!−ζU,
CGSSK ≥Nr
2log2 det (Cw)(1/Nr)
σ2
B!−ζL(6)
where
ζL=Nr
2log2 1 + det Cw−σ2
BINr(1/Nr)
σ2
BK(2/Nr)!,(7)
ζU=Nr
2log2 exp(1) det (Dw)(1/Nr)
2σ2
B!−log2(K)(8)
+ log21 + (K−1) exp −ρ2
4σ2
B
d2
min,
and Dw=diag(Cw). Interested readers are referred to [15] for the
proofs and additional details about the notation conventions.
The secrecy performance for the proposed SCD-based OGSSK
PLS technique is investigated for three different scenarios to simulate
various Bob-Eve separations. Accordingly, a 6m×6m×3m

0510 15 20 25 30
SNR [dB]
10-6
10-4
10-2
100
BER
Scenario 1 - Bob
Scenario 1 - Eve
Scenario 2 - Bob
Scenario 2 - Eve
Scenario 3 - Bob
Scenario 3 - Eve
Fig. 2: BER vs. SNR performance of the proposed SCD-based GSSK.
standard room is considered. Moreover, it is assumed that 8LEDs
are uniformly distributed and located on the ceiling of the room. In
Fig. 2, the secrecy performance of the proposed SCD-based OGSSK
technique is evaluated for the low, mid and high separation of Bob-
Eve. As can be seen from the figure, the proposed system reduces
the effective signal-to-noise-ratio (SNR) by introducing jamming
which destructs the achievable BER at Eve’s side. Furthermore, the
maximization of the minimum Euclidean distance by SCD assists Bob
in achieving the minimum BER within transmit power constraints.
Similarly, in Fig. 3, the secrecy performance of the proposed system
is evaluated by the long term average of the BER. Accordingly, the
random mobility of both Bob and Eve are considered in Fig. 3. It can
be inferred from Fig. 3 that the proposed system ensures the BER of
Eve is kept at a fixed level for all the practical SNR regimes even
when the users are mobile.
IV. PHY LAYE R SECURITY FOR MULTI FRIENDLY USE RS W IT H
COO PE RATI VE RE LAYI NG (NOMA AP PRO ACH )
In this section, we investigate the implementation of two possible
key enabling technologies for 6G mobile networks, NOMA and
VLC. Particularly, the application of beamforming to VLC-NOMA
systems is considered for PLS [31]. A number of trusted cooperative
half-duplex relay luminaries are deployed to achieve the secrecy
in the transmitted data. Transmitters are equipped with single light
fixtures, containing multiple LEDs, and receiving nodes are equipped
with single PDs, rendering the considered setting as a single-input-
single-output (SISO) system. Transmission is amplitude constrained
to maintain operation within the LED’s dynamic range. Achievable
secrecy rate regions are derived under such amplitude constraints for
this multi-receiver wiretap channel, initially for direct transmission
without the relays, and then for multiple relaying schemes.
Intensity modulation (IM) is used to superimpose the source’s data
signal x∈Ron top of a fixed positive bias current that drives its
LEDs. Superposition coding is used to convey two messages (x1, x2)
to the legitimate users:
x=αx1+ (1 −α)x2,0≤α≤1.(9)
The weak user decodes its message first by treating the interference
caused by the other user as noise, whereas the strong user decodes
its message via successive interference cancellation. The amplitude
constraint imposed to maintain operation within LEDs’ dynamic
range is
0510 15 20 25 30
SNR [dB]
10-4
10-3
10-2
10-1
100
BER
Bob
Eve
Fig. 3: Average BER performance of Bob and Eve when they are located
randomly in the room.
source
relays
eavesdropperstrong userweak user
cooperative jamming
Fig. 4: An indoor VLC system model in which a source luminary communi-
cates with two legitimate users in the presence of an eavesdropper.
α|x1|+ (1 −α)|x2| ≤ Aa.s.,||d||1≤1.(10)
An illustration of a source communicating with two legitimate users
in the presence of an eavesdropper is depicted in Fig. 4.
The relays cooperatively transmit a jamming signal Jz,simultane-
ously with the source’s transmission. Both the beamforming vector
J∈RKand relay’s common signal z, which is a random variable,
are chosen to satisfy the following conditions:
|z| ≤ ¯
Aa.s. |J|  1K.(11)
The parameter zis distributed uniformly within −¯
A, ¯
A. In order
not to harm the legitimate users, the beamforming vector should be
chosen in the null spaces of the legitimate users. That is,
gT
1Jo=gT
2Jo= 0 (12)
The achievable secrecy rate pairs via cooperative jamming decode-
and-forward, and amplify-and-forward have been obtained and for
each relaying scheme, secure beamforming signals have been de-
signed to maximize the achievable rates under the relays’ amplitude
constraints [31]. The design of the beamforming signals was based
on formulating optimization problems that were inferred from the
derived achievable secrecy rates. Finally, achievable secrecy regions
of the proposed schemes have been obtained and the effect of the
eavesdropper’s distance from the source on the achievable secrecy
sum rate have been investigated. In Fig. 5, we plot the achievable
secrecy rate regions for the schemes proposed in this paper, along
with the direct transmission scheme. The solid lines are when the

0
0.05
0.1
0.15
0.2
0.25
0.3
0 0.1 0.2 0.3 0.4 0.5 0.6
weak user’s secrecy rate (nats/s/Hz)
strong user’s secrecy rate (nats/s/Hz)
direct transmission
cooperative jamming
decode-and-forward
amplify-and-forward
Fig. 5: The eavesdropper at (0, 1.75, 0.7) [solid] and at (0, 2, 0.7) [dashed].
eavesdropper is located at (0, 1.75, 0.7). We see in this case that
all the proposed schemes perform better than the direct transmission.
The dashed lines in Fig. 5 illustrate when the eavesdropper is located
a bit further away from the source (and the relays) at (0, 2, 0.7).
We see from this that all the achievable secrecy rates are higher, yet
the direct transmission outperforms both the decode-and-forward and
amplify-and-forward schemes, since the channels from the relays to
the eavesdropper are worse. We also notice the slight improvement
of the cooperative jamming scheme in this case over the direct
transmission. It is clear from this figure that the best relaying scheme
depends on both the source-Eve and relay-Eve locations.
V. CONCLUSION
In this paper, PLS has been considered for VLC. Following an
overview of VLC based PLS approaches, two recently proposed
VLC-specific PLS techniques named OGSSK-based modulation with
SCD and superposition coding based NOMA have been elaborated
on. The computer simulations have showed that the proposed tech-
niques effectively enhanced the secrecy of the information transmis-
sion by exploiting the properties of the VLC channel. Therefore,
further enhancements on the PLS performance of VLC systems could
also be achieved by other VLC-specific techniques such as (i) em-
ploying SCD along with IM-based MIMO-NOMA; (ii) manipulating
the geometry of the ADT array to confine each LEDs coverage area;
and (iii) constructing a geo-fenced area to protect the legitimate user.
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