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Security and propagation issues and challenges in VLC and OCC
systems
Simona Riurean. *a, Robert Alexandru Dobre b, Alina-Elena Marcu b
aComputers, Automation and Electrical Engineering, University of Petroșani, 20 Universitatii Str.,
Petroșani 332006, Romania; bElectronic Technology and Reliability Department, Polytechnic
University of Bucharest, Bucharest, Romania
ABSTRACT
Both Visible Light Communication (VLC) and Optical Camera Communication (OCC), as technologies and applications
of the Optical Wireless Communication (OWC), have been intensely investigated in recent years. Because of their
advanced implementations prove to be reliable candidates as partners of the various wireless transmissions based on
radio frequency waves, and in some special situations, principal actors for different wireless communication scenarios in
particular situations such as the medical field, petrochemical industry or indoor spaces overcrowded where the radio
frequency spectrum crunch becomes obvious. We analyze several indoor setups and unique situations that incur alleged
security breaches and future challenges in secure VLC and OCC data communication. We also present here an
investigation of topologies and transmission scenarios with influences on a secure, reliable wireless transmission system
as well as vulnerabilities and threads in VLC and OCC networks. The optical properties of the visible light as a reliable
communication medium are explored here, as well. Both VLC and OCC have their own benefits, as well as drawbacks,
from the point of view of the overall security and performance, as well as the ability to keep the communication as a
continuous operational, intact, and trusty service. An experiment conducted using low frame rate cameras in OCC shown
the minimum distance between the centers of the LEDs that still allow an appropriate separation of the frames captured.
Keywords: LoS topology, NLoS, secure VLC, signal propagation OCC, eavesdropping
1. INTRODUCTION
Due to the most recent results and technological advances of the optical communication, implementing VLC and OCC
technologies in devices with different applications embedded, such as augmented reality, tracking systems with sensitive
applications, in the traffic as a real-time danger-awareness situation, a network of sensors for the environmental
surveillance in IoT or medical devices, are more worldwide demanded and used than ever. Alternative or partner
technologies have to be searched and implemented to cope with the unprecedented spectrum crunch because of the
growth of the number of systems and platforms with diverse wireless communication technologies embedded, most of
them based on radio frequencies (RF) [1].
Therefore, a general approach of data security during optical communication is required especially for sensitive data
transmitted between medical devices or in case of traffic-context awareness.
Although most of the authors that explore the wireless optical communication, consider the security as being one of its
benefits, when an extensive amount of important data is collected in some sensitive situations, data safety is compulsory
not only necessary. For example, in the case of the implanted medical devices, the networks of sensors placed on the
human body, or local communication of physiological data, there are intrinsic limitations in terms of devices' dimensions
or capabilities or dynamic scenarios in different setups or topologies that have to be taken into consideration along with
advanced safe communication techniques [2,3].
There is a classical, well-known model for information security that is defined by three main objectives: keeping the
confidentiality, integrity, and availability of the data transmitted during a communication session. Confidentiality refers
to protecting data from being accessed by anyone who is unauthorized. Any leak of confidential data is called a breach of
one's privacy, which is highly undesirable especially when sensitive or private information is transferred from source to
destination.
*simonariurean@upet.ro; phone 0040 744 517 396
The integrity of data means that data received at the destination must be the same as the one sent and hence has not been
altered during transmission. Information accessible to all the allowed (authorized) users is the third aim of a secure
transmission and refers to the availability of data.
The transmission security policy aims not only to protect its sensitive information but also its overall performance,
keeping at the same time its most important benefit: the ability to keep the communication as a continuous operational,
intact, and trusty service. A wireless communication is even more vulnerable to data theft, therefore the security
transmission refers to the defense policy adopted, tools and tactics implemented aiming to prevent, monitor, and respond
in due time to any intrusion during data communication while protecting digital assets.
Keeping the entire security strategy of VLC and OCC systems at the physical layer, might be one of the confidentiality
enablers in wireless optical connectivity based on visible light. The present features of the visible-light wireless
communication front-ends hardware, combined with the advances in neural networks or artificial intelligence algorithms,
can be exploited to meet the security requirements when dealing with simple but sensitive devices unable to implement
cryptographic methods, for example, medical nano-devices where the human bodies become nodes of the future
augmented and integrated local and remote medical support systems [4,5].
2. CHARACTERISTICS CONSIDERED FOR SECURE OPTICAL COMMUNICATION
2.1 VLC and OCC setups
There are different topologies (Fig 1) used for communication between transmitter (oTx) and receiver (oRx) for both
VLC and OCC systems, starting with a directed line of sight (LoS), non-directed LOS (NLoS), diffuse or quasi-diffuse
links that are more or less secure. When the security transmission is particularly analyzed from this point of view, there
are several degrees of risk regarding data theft during the data wireless transmission [6].
Figure 1. a. Direct Line of Side (LoS) link, b. NLoS link, c. Diffuse link d. Quasi-diffuse link.
In a multi-channel reflection environment, the total data rate of the VLC link depends on the delay of the propagation
path. Moreover, total power gain can be increased by narrowing the angle of field of view FoV of the LED. The LED’s
radiation light beam-angle (usually VLC systems use Lambertian emitters that, by definition, are LEDs with uniform
luminous distribution of light for all directions) as well as the photodiode’s (PD’s) FoV influence the optical link range
and the optical power for each communication scheme. The PD’s FoV is one of the important characteristics that affect
the optical signal quality at destination, as well as the data theft risk. The larger the beam-angle of LED, its high optical
power, correlated with a high FoV at the PD, and the optical beamforming (focusing light emitted from multiple LEDs
on the active area of a PD [7,8]), the data communication vulnerabilities become possible. Color optical filters can
change the light propagation range, which has an effect on the effective region of the receiver or FoV.
The possible threats in optical wireless communication can be eavesdropping/snooping, jamming and data alteration.
Each threat has different characteristics depending on the communication structure as for example lighting fixture to
mobile device (LFtM), mobile device to lighting fixture (MtLF) or mobile device to mobile device (MtM).
It is easier to eavesdrop on LFtM or MtLF than MtM. A common method of realizing eavesdropping attacks is directly
accessing the VLC or OCC channel link.
Although the current wireless RF communication technologies rely on advanced coded and encrypted communication
methods, the exposure of the optical networks to eavesdropping poses a considerable security vulnerability. The
eavesdropping method aims to gain unauthorized access to data transmitted to collect and/or analyze traffic when attacks
can be with direct access to the non-encrypted optical channel and those based on breaching the encryption key in
encrypted optical systems.
In case of any of the VLC or OCC topology, an unauthorized oRx or more can be easily introduced into the environment
without being recognized. The goal of service degradation attacks is to degrade the quality of optical communication link
or cause service denial, by insertion of additional and/or harmful signals into the network. Snooping on VLC or OCC
wireless transmission is obviously limited by physical factors, and is far more difficult than Wi-Fi snooping, for
example, but there are no evident reasons why it should not be possible. A Men in the Middle (MitM) attack that aims to
actively manipulate transmitted data in addition to the common sniffing and injection of packets is also possible [8].
These MitM type of attack aims to elide the encryption algorithms as well as the handshake used in the security
algorithms of WPA2 like most key reinstallation attacks [9,10]. In practical systems, the network might be completely
unaware of the passive eavesdroppers and potential interceptions.
2.2 Light properties and optical signal quality due to surrounding elements indoor
Light' properties considered in a VLC or OCC wireless system are intrinsic and apparent. The intrinsic optical properties
(IOPs) of light depend exclusively by the communication medium, while its apparent optical Properties (AOPs) are
dependent both of the medium and the environment indoor. The environment indoor refers to the surrounding within its
space particularities: natural and/or artificial light, geometrics, type of materials and color of the objects within the space
where the VLC or OCC wireless systems are considered. The IOPs are conservative properties and hence the magnitude
of the absorption coefficient linearly varies with the concentration of the absorbing material [11,12].
Other important characteristic, indoors, where the VLC system is set-up, that have to be carefully taken into
consideration, refers to the surrounding elements: walls, ceiling, floor and any kind of object/obstacle inside the room.
The light beam invariably suffers, from absorption, reflection, diffraction, scattering. All these physical properties of
light depend on a number of factors. That is why, the type of material the surroundings and objects are made of and their
properties, mate/glossy, smooth/rough as well as their colors, highly influence the optical signals’ quality that reach the
oRx [13,14].
The mobile oRx and/or oTx as well as the moving obstacles can cause the signals to fade randomly due to the multipath
components and their path loss. Additional noises coming from any other source of natural or artificial light, namely the
additional white Gaussian noise (AWGN) can also influence in a negative way the optical signal's quality at the oRx
[15]. As long as the signal power decreases in a long optical link outside of the FoV, the optical network vulnerabilities
decrease as well.
2.3 Optical channel model
In terms of channel modeling approaches, for years already, there are many channel models for conventional wireless
infrared communication, however, channel modeling for VLC or OCC although there are many works on the subject,
because of the system’ complexity, a general model of an optical channel in visible light is not yet defined. The research
in this area is still at an early stage, although have been reported some theoretical works related to VLC / OCC channel
modeling such as Geometry-Based Deterministic or Non Geometry-Based Stochastic Models. Not even the IEEE
802.17.5 standard (2011) dedicated to VLC wireless communication, does not specify any VLC channel models that can
be used for the technology evaluation, hence the security subject was not deep investigated, since it has not been
considered relevant so far.
2.4 Modulation and security techniques
Modulation techniques developed for wireless communication in the radiofrequency (RF) spectrum cannot be applied
straight for optical communication since the optical signal is real and non-negative. Many single carrier modulation
techniques (such as OOK – On Off Keying, M-PAM – multi level Pulse Amplitude Modulation M-PPM - multi level
Pulse Position Modulation or CAP – Carrierless Amplitude Modulation) or multi-carrier modulation techniques (DCO-
OFDM, ACO –OFDM, and many others), have been theoretically demonstrated and some of them practically proved.
They all have benefits and drawbacks, depending on many particularities of the VLC and OCC systems that have to be
carefully considered.
The security issues at the physical layer in VLC networks, that has emerged as a promising approach to complement the
conventional encryption techniques used in RF wireless communication, aim to provide a first line of defense against
eavesdropping attacks. The key idea is to use the intrinsic properties of the VLC channel to realize enhanced physical
layer security. The evolution toward 6G wireless communications brings along new and technical challenges that remain
unsolved for physical layer security in VLC research, including physical layer security coding, massive multiple-input
multiple-output, non-orthogonal multiple access, full-duplex technology and so on. Since the most communication
scheme, practically demonstrated, proved to be in VLC systems the intensity modulation and direct detection (IM/DD),
the conventional security techniques used in RF communications is not possible to apply straightforward in wireless
optical communication networks. Moreover, as the main conveyor of data in optical wireless communication is light, due
to its nature, the intensity-modulating data signal must satisfy a positive-valued amplitude constraint [16-20].
A friendly jamming can be used in the optical link channel, aiming to create an artificial noise which included in the null-
space of a legitimate user. The method relies on the confidential information that is combined with the jamming signal at
the oTx side, thus, only the eavesdropper will experience damaging effects from the jamming signal. In this case both the
components of the confidential message (spatial and constellation parts) must be encoded in order to decrease the
network vulnerability. In case of a precoding method, the secrecy lies intrinsically in both domains due to the matrix
multiplication but the spatial information is not hidden from the eavesdropper in jamming systems. Therefore, all the
transmitters are fed with artificial noise to ensure the spatial domain secrecy in jamming based systems. Compared with
the precoding method, friendly jamming can achieve better secrecy because the channel state information of the
legitimate user is not sufficient in avoiding artificial noise [21,22].
3. OCC SETUP AND EXPERIMENTAL RESULTS
Recent results [23,24] have shown that low frame rate cameras can be used in OCC. To achieve automatic code
synchronization and avoid capturing uncertain states of the LEDs caused by capturing a frame exactly in the moment
when the LED is turned on or off, at least two LEDs are used. The same code is used to control both LEDs, but a slight
time difference between the two waveforms is inserted. In this way, if one LED is in an uncertain state when the frame is
captured, the other is clearly on or off. To properly process the captured frames, the two LEDs must be separable in the
images. The cameras featured on smartphones nowadays rarely capture video at a resolution lower than 1080×720 pixels.
Therefore, a certain distance must exist between LEDs, depending on the distance to camera.
In this paper we conducted an experiment to determine the minimum distance between the centers of the LEDs that
would allow to properly separate them in the captured frames, depending on the LEDs to camera distance. For this, a
Nikon D7200 camera was used coupled with a Nikon 18-105 mm f1:3.5-5.6 lens. The video resolution was set to
1080×720, the shutter speed was set to 1/8000, the aperture was set to 1:5, the ISO value to 25,600, and the focal length
was kept 18 mm through all the experiments. Because a camera with an APS-C sensor was used, the full-frame (35 mm
sensor) equivalent focal length is 27 mm.
The LEDs to camera distance was varied from 15 cm to 150 cm with a 15 cm step. The LEDs were fit on a breadboard,
to properly control the distance between them. The distance between their centers was varied between 0.3 inches and 1.7
inches with a step equal to 0.1 inch. The LEDs were driven with the codes presented in [24]. From the experiments, we
have determined that the minimum distance between the edges of the LEDs facing to one-another that allowed a proper
detection was 15 pixels. For each of the LEDs to camera distance we have determined the distance between the LEDs
centers that would determine a distance of at least 15 pixels between the edges of the LEDs facing to one-another in the
captured frames. The distance between the LEDs’ edges was determined by extracting the luminance on a line that
crosses both LEDs, as shown in Fig. 2.
The luminance is further investigated by setting a threshold value equal to 30% of the maximum luminance, then
identifying the LEDs and measure the number of pixels between them where the luminance is below the set threshold, as
shown in Fig. 3. The results are shown in Fig. 4. A linear approximation of the variation was estimated and illustrated on
the same graph. Using these results, an OCC solution can be properly designed, knowing an estimate of the LEDs to
camera distance.
Figure 2. Example of a captured frame. LEDs to camera distance
was 30 cm. Distance between LEDs’ centers was 0.5 inch.
Figure 3. The luminance on the luminance line illustrated in Fig.
2 along with the threshold line.
Figure 4. The required distance between the LEDs’ centers depending on the distance to camera, to assure proper separation
in the captured images.
4. CONCLUSIONS
VLC, as well as OCC networks are vulnerable to various local types of attacks as eavesdropping/snooping, jamming,
data alteration, and service disruption that can, each or all, lead to high data losses. Due to the tendency of shifting the
security issues to the software programmable and flexible node architectures, new security vulnerabilities are created in
optical communication networks. All of these have to be identified and taken into account during network design and
operation. This paper provides an overview of some of the current security vulnerabilities in optical networks and
identifies possible attack methods as well as security techniques to be applied in wireless communication links based on
visible light. The experiment conducted using OCC aims to determine the minimum distance between the centers of the
LEDs that would allow to properly separate the captured frames. The distance between the two LEDs’ edges was
determined by extracting the luminance on a line that crosses both LEDs. Depending on the LEDs to camera distance,
results detection of 15 pixels, reliable for a proper optical communication.
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