Visible light communication (VLC) is a promising technology for next-generation indoor wireless broadband communication systems. Physical layer security (PLS) is one proposed method to ensure data security for VLC in public areas. However, most PLS studies in the literature neglect the reflections in the VLC channel and do not consider inter-symbol interference (ISI). This paper investigates the ISI effects on the secrecy rate in multiple-input single-output VLC systems where the reflections are not neglected for the cases of perfect and imperfect knowledge of the eavesdropper’s channel state information. We aim to compensate for the ISI effects by designing zero-forcing (ZF) and artificial noise (AN) beamformers. In addition, we formulate optimization problems to maximize the secrecy rate resulting in non-convex and NP-hard problems. Thus, we utilize the particle swarm optimization algorithm for solutions and also convert non-convex optimization problems into easily solvable linear problems. Furthermore, robust ZF and AN beamformers for the worst-case scenario are considered for the cases of ISI and no ISI. The results reveal that ISI severely degrades the secrecy rate; however, well-designed beamformers can reverse the ISI effects.
Visible light communication (VLC) is a promising technology for next-generation indoor wireless broadband communication systems. Physical layer security (PLS) is one proposed method to ensure data security for VLC in public areas. However, most PLS studies in the literature neglect the reflections in the VLC channel and do not consider inter-symbol interference (ISI). This paper investigates the ISI effects on the secrecy rate in multiple-input single-output VLC systems where the reflections are not neglected for the cases of perfect and imperfect knowledge of the eavesdropper’s channel state information. We aim to compensate for the ISI effects by designing zero-forcing (ZF) and artificial noise (AN) beamformers. In addition, we formulate optimization problems to maximize the secrecy rate resulting in non-convex and NP-hard problems. Thus, we utilize the particle swarm optimization algorithm for solutions and also convert non-convex optimization problems into easily solvable linear problems. Furthermore, robust ZF and AN beamformers for the worst-case scenario are considered for the cases of ISI and no ISI. The results reveal that ISI severely degrades the secrecy rate; however, well-designed beamformers can reverse the ISI effects.
For simplicity, most works on visible light communications (VLC) neglect the non-line-of-sight (NLoS) channel gain and the inter-symbol interference (ISI). This letter investigates the optimal and robust secure beamforming problems for multiple-input single-output VLC systems, where both the NLoS channel gain and ISI are considered. Under perfect channel state information (CSI), we formulate an optimal beamforming problem to maximize the secrecy rate under amplitude constraints on beamformers. The problem is shown to be nonconvex. We transform it into multiple tractable quasiconvex subproblems, and propose an iterative algorithm to solve it. To be more practical, we then consider the imperfect CSI and design the robust secure beamformers. Numerical results verify the efficiency of the proposed beamforming schemes. Moreover, it is also shown that the NLoS channel gain can improve the secrecy rate and well-designed beamformers can reverse the effect of ISI.
In this paper, we consider the physical-layer security for single-input single-output (SISO) indoor visible light communication (VLC) wiretap systems in the presence of multipath reflections. We derive both the lower and upper bounds on the secrecy capacity in the context of both the symbol- and block-based transmission policies. To enhance the secrecy performance relying on block transmission policy, we propose a low-complexity amplitude scaling (AS) scheme by scaling the amplitudes of different symbols in each block to maximize the achievable secrecy rate. We further provide the upper bound on the optimal secrecy rate achieved by precoding for the sake of validating the effectiveness of the proposed AS scheme. Numerical results suggest that the secrecy performance is severely degraded by inter-symbol interference (ISI) imposed by multipath reflections, which yet can be alleviated by long-block based transmission. Moreover, the proposed AS scheme can enhance the secrecy performance significantly and achieve a secrecy rate very close to the optimal solution.
Visible-light communication (VLC) is an enabling technology that exploits the lighting infrastructure to provide ubiquitous indoor broadband coverage via high-speed short-range wireless communication links. On the other hand, physical-layer security has the potential to supplement conventional encryption methods with an additional secrecy measure that is provably unbreakable regardless of the computational power of the eavesdropper. The lack of wave-guiding transmission media in VLC channels makes the communication link inherently susceptible to eavesdropping by unauthorized users existing in areas illuminated by the data transmitters. In this thesis, we study transmission techniques that enhance the secrecy of VLC links within the framework of physical-layer security. Due to linearity limitations of typical light-emitting diodes (LEDs), the VLC channel is more accurately modelled with amplitude constraints on the channel input, rather than the conventional average power constraint. Such amplitude constraints render the prevalent Gaussian input distribution infeasible for VLC channels, making it difficult to obtain closed-form secrecy capacity expressions. Thus, we begin with deriving lower bounds on the secrecy capacity of the Gaussian wiretap channel subject to amplitude constraints. We then consider the design of optimal beamformers for secrecy rate maximization in the multiple-input single-output (MISO) wiretap channel under amplitude constraints. We show that the design problem is nonconvex and difficult to solve, however it can be recast as a solvable quasiconvex line search problem. We also consider the design of robust beamformers for worst-case secrecy rate maximization when channel uncertainty is taken into account. Finally, we study the design of linear precoders for the two-user MISO broadcast channel with confidential messages. We consider not only amplitude constraints, but also total and per-antenna average power constraints. We formulate the design problem as a nonconvex weighted secrecy sum rate maximization problem, and provide an efficient search algorithm to obtain a solution for such a nonconvex problem. We extend our approach to handle uncertainty in channel information. The design techniques developed throughout the thesis provide valuable tools for tackling real-world problems in which channel uncertainty is almost always inevitable and amplitude constraints are often necessary to accurately model hardware limitations.
In this paper, we address the secure data transmission through visible light communication (VLC) using physical layer security (PLS) techniques and particularly, optical beamforming with the zero-forcing algorithm. More precisely, we consider the secrecy capacity of classical multiple-input single-output VLC so that the system can deal with eavesdroppers by minimizing the secrecy outage probability (SOP). The considered wireless channel is modeled by the Gaussian distribution, which is subject to amplitude constraints. We quantity the achievable secrecy capacity and SOP for the conventional line-of-sight VLC link and show that how the beamforming can determine the optimal placement of the transmitters. We also show that for a given SOP, the proposed optimal placement offers a signal-to-noise ratio gain of up to 6 dB compared to classical methods such as uniform placement of the transmitters. Our numerical results indicate that the proposed optimal LED arrangement can achieve an SOP of 10−10 while the SOP with uniform arrangement is equal to 10−4.
This paper analyzes the secrecy rate and designs the secure beamforming for an indoor visible light communication (VLC) network consisting of one transmitter, one legitimate receiver, one eavesdropper, and several friendly jammers. In the VLC network, both the main and wiretapping channels have a uniformly distributed signal input and corrupted with additive white Gaussian noise. The imperfect channel state information (CSI) is considered by using the ellipsoidal approximation method. Under the amplitude constraints of the data and the interfering signal, the secrecy rate for the VLC network is analyzed. A beamforming optimization problem is proposed to maximize the secrecy rate with the given constraints. It is shown that the optimization problem is non-convex. In this paper, it is transformed into a solvable quasi-convex linear search problem. Numerical results demonstrate the performance gain of the optimal beamformer compared to the zero-forcing scheme. Moreover, the secrecy performance under different CSI uncertainties is also provided.
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.