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.
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.
Visible light communication (VLC) is a new enabling communication technology that exploits illumination devices, mostly high-brightness light-emitting diodes (LEDs), to establish high-speed short-range wireless communication links. VLC is a promising technology that has gained significant interest in the last decade due to its high data rates and low cost of deployment. Although VLC systems are less susceptible to eavesdropping than RF systems since light does not penetrate through walls, they become as vulnerable as their radio-frequency (RF) counterparts when their nodes are deployed in public areas and/or when there are large windows. Thus, security for VLC systems is as important as it is for RF systems. Security in wireless communication systems may be enhanced by introducing physical layer security (PLS) techniques. In fact, PLS has been applied to a wide range of RF applications in an effort to improve the overall security by complementing existing cryptography-based security techniques. The potential of PLS stems from its ability of leveraging features of the surrounding environments via sophisticated encoding techniques at the physical layer. Indeed, PLS schemes can be applied in the same spirit to VLC systems. \n \nThere exist many specificities that characterize VLC systems, leading to major differences compared to RF systems. Specifically, VLC channels are quasi-static and real valued channels, which seemingly simplify the application of PLS techniques. However, due to the limited dynamic range of the LEDs, VLC systems impose an amplitude constraint on the channel input, which makes unbounded inputs not admissible. As a result, the performance of PLS schemes must be revised in the VLC context due to its different operating constraints. \n \nWe propose in this thesis enhancing the secrecy performance of VLC systems through PLS techniques. Firstly, we consider the degraded single-input single-output (SISO) VLC system. We derive achievable secrecy rates using continuous and discrete input distributions and we demonstrate the substantial improvement in the secrecy performance by using discrete distributions. Secondly, we consider the multiple-input single-output (MISO) VLC systems. We consider at first the case of randomly located eavesdroppers, in which we develop an associated achievable secrecy rate and we develop a beamforming-based precoding scheme that enhances the secrecy performance of this class of systems. In the second part, we consider the multi-user (MU) MISO broadcast system with confidential messages and we develop linear precoding schemes that enhance typical secrecy performance measures, including the max-min fairness and the weighted fairness. Finally, we consider the multiple-input multiple-output (MIMO) VLC system. We derive achievable secrecy rates using continuous and discrete distributions and we develop linear precoding schemes that improve its secrecy performance.
We consider the physical layer security (PLS) of multi-user (MU) multiple-input-multiple-output visible light communication (VLC) systems with an eavesdropper (Eve) and propose a novel spatial constellation design technique based on generalized space shift keying (MU-GSSK-SCD). The received signals of the legitimate users are optimized jointly, such that their bit error ratios (BERs) are minimized and Eve's BER is significantly degraded. The emission power of randomly selected light-emitting diodes is adjusted, by exploiting users' channel state information at the transmitter. Our strategy ensures that legitimate users receive confidential messages fully in an undistorted fashion, while any meaningful leakage to Eve is strongly prohibited, without any artificial noise addition. Every user can decode only its information, hence inter-user security is also guaranteed. The PLS improvements are presented in terms of both BERs and achievable secrecy rates in practical VLC scenarios. For various user configurations, it is shown that MU-GSSK-SCD increases the BER at Eve to the 0.5 level, while providing minimized BERs to the legitimate users. The achievable secrecy rate region is derived for MU-GSSK-SCD and it is shown that full secrecy can be achieved at 0 dB signal-to-noise ratio (SNR) level with a user separation as small as 90 cm.
In order to improve the security performance of multiuser visible light communication (VLC) and facilitate the secure application of optical wireless communication technology in Internet-of-Things, we investigate the physical-layer security in a multiuser VLC system with non-orthogonal multiple access (NOMA). When the light-emitting diode (LED) transmitter communicates with multiple legitimate users by downlink NOMA, both single eavesdropper and multi-eavesdropper scenarios are considered. In the presence of single eavesdropper, based on transmission characteristics of the optical wireless channel, with known instantaneous channel state information (CSI) of the NOMA legitimate channels and statistical CSI of the eavesdropper channel, an exact expression of secrecy outage probability (SOP) is derived, which acts as a benchmark of the security performance to guide selecting or optimizing parameters of the LED transmitter and the photodiode receiver of NOMA legitimate users. In the multi-eavesdropper case, based on the spatial distribution of legitimate users and eavesdroppers, the SOP is obtained via a stochastic geometry theory, so as to guide the NOMA legitimate users to keep away from the area with high eavesdropper density. For typical parameters of the indoor LED transmitter and the PD receiver, simulation results show that the SOP performance improves with the increasing of LED transmission power or transmission signal-to-noise ratio (SNR) in both scenarios. Specifically, in the single eavesdropper case, enlarging the channel condition difference of user groups or deviating the eavesdropper from the given user group can improve the SOP performance, and for a given NOMA legitimate user, the SOP eventually settles around 0.2 while the semiangle at half illuminance of the LED varies between 15° to 60°. In the multi-eavesdropper case, we can get a better SOP performance when reducing the eavesdropper density or the semi-angle at half illuminance of the LED for a given eavesdropper density.
As the existing radio-frequency (RF) networks cannot meet the ever-increasing data rate demand, the optical wireless communication (OWC), which uses a wide untapped unregulated spectrum, has been proposed as a promising technology to overcome the RF spectrum limitations. On the other hand, with the increasing demand for high data rate and the prevalence of the broadcast-nature networks, researchers have recently come up with new mechanisms to improve secure communication using physical layer techniques. Compared to RF networks, the OWC networks are more secure and less susceptible to the interception because of the small coverage provided by LEDs, and because they work properly only in the presence of the line-of-sight (LoS) components. However, the security in OWC networks is still an issue, especially in visible light communication (VLC) when the transmitted information can be accessed by multiple users as in public areas, meeting rooms, laboratories, and libraries. That means potential eavesdroppers may be existing to gather confidential messages. This paper reviews all the conducted work on physical layer security (PLS) in two types of OWC networks, which are the VLC and free space optical (FSO) networks. Furthermore, the paper proposes several open problems in these networks to optimize and enhance the security performance.
Spatial modulation (SM) is a promising technique that reduces inter-channel interference while providing high power efficiency and detection simplicity. In order to ensure the secrecy of SM, precoding and friendly jamming are widely adopted in the literature. However, neither of those methods can take advantage of SM. In this paper, a novel spatial constellation design (SCD) technique is proposed to enhance the physical layer security (PLS) of optical generalized space shift keying (GSSK), which can retain some benefits of SM. Due to the lack of small-scale fading, the quasi-static characteristics of the optical channel is used to tailor the received signal at the legitimate user's (Bob's) side. The PLS of the system is guaranteed by the appropriate selection of the power allocation coefficients for randomly activated light emitting diodes (LEDs). With the aid of Bob's channel state information at the transmitter, the bit error ratio (BER) of Bob is minimized while the BER performance of the potential eavesdroppers (Eves) is significantly degraded. Monte-Carlo simulation results show that the proposed SCD-zero forcing precoding (ZFP) forces Eve to experience a BER of around 0.5 by outperforming both the conventional and ZFP based GSSK for all practical signal-to-noise-ratio regimes and Bob-Eve separations.