Physical-Layer Security in Multiuser Visible Light Communication Networks

This paper presents the first performance analysis of physical layer downlink secure transmissions in a scalable cell-free massive MIMO (SCF-mMIMO) system. A stochastic geometry approach is used to model the locations of the access points (APs), user equipments (UEs) and eavesdroppers (Eves) as independent homogeneous Poisson point processes (HPPPs). In addition to applying maximum ratio transmission (MRT) to send the confidential messages, null-space artificial noise is also injected for secrecy enhancement. We analytically characterize the secrecy performance in terms of both the outage-based secrecy transmission rate (STR) and the ergodic secrecy rate (ESR), appropriate for slow quasi-static fading channels and fast block-fading channels, respectively. By utilizing moment matching and Gil-Pelaez inversion theorem, we are able to obtain mathematically tractable approximations for the performance metrics. These approximations are shown to have high accuracy as compared to simulation results. Our numerical results reveal useful design insights that cannot be inferred from existing studies. These insights answer important questions such as whether it is best to deploy as many APs each with fewer antennas and to what extent the artificial noise insertion is beneficial.

In this paper, the impact of multiple active eavesdroppers on cooperative single carrier systems with multiple relays and multiple destinations is examined. To achieve the secrecy diversity gains in the form of opportunistic selection, a two-stage scheme is proposed for joint relay and destination selection, in which, after the selection of the relay with the minimum effective maximum signal-to-noise ratio (SNR) to a cluster of eavesdroppers, the destination that has the maximum SNR from the chosen relay is selected. To accurately assess the secrecy performance, exact and asymptotic expressions are obtained in closed form for several security metrics, including the secrecy outage probability, probability of nonzero secrecy rate, and ergodic secrecy rate in frequency selective fading. Based on the asymptotic analysis, key design parameters, such as secrecy diversity gain, secrecy array gain, secrecy multiplexing gain, and power cost, are characterized, from which new insights are drawn. In addition, it is concluded that secrecy performance limits occur when the average received power at the eavesdropper is proportional to the counterpart at the destination. In particular, for the secrecy outage probability, it is confirmed that the secrecy diversity gain collapses to zero with outage floor, whereas for the ergodic secrecy rate, it is confirmed that its slope collapses to zero with capacity ceiling.

Visible light communication (VLC) can accomplish the two-fold goal of illumination and short-range communication with the advantages of low cost, free spectrum, natural confidentiality, and low energy consumption, whereas interference management is a major challenge in VLC networks. In this paper, we propose a coalition formation framework for interference management in VLC networks. VLC access points (APs) are designed to self-organize into cooperation coalitions for interference cancellation through orthogonal time or frequency domain. The performances of VLC APs with and without forming coalitions are both analyzed. Based on coalition formation game theory, a simple merge-and-split algorithm based on Pareto order is proposed. The convergence, stability, and complexity of the proposed algorithm are further discussed. Numerical results are given to illustrate the effectiveness of the proposed framework.

In this paper, three different relay and jammers selection schemes are presented to improve the physical layer security of two-way cooperative networks in the presence of multiple cooperating and non-cooperating eavesdroppers. The presented selection schemes are; optimal selection scheme without jamming, optimal selection scheme with conventional jamming, and optimal selection scheme with controlled jamming. The proposed schemes select three intermediate nodes during two communication phases. In the first phase, a friendly jammer is selected to create intentional interference at the eavesdroppers' nodes. In the second phase, two relay nodes are selected; one node is selected to operate as a conventional relay and assists the sources to deliver their data to the corresponding destinations via a Decode-and-Forward (DF) strategy. While the other node behaves as a jammer node in order to confuse the eavesdroppers in this phase. The proposed schemes are analyzed in terms of ergodic secrecy rate and secrecy outage probability. The obtained results show that the jamming schemes outperform the non-jamming schemes in the eavesdroppers' environment. Moreover, cooperating eavesdroppers' further degrade system's secrecy performance than the non-cooperating ones. Finally, a comparison between relay and jammers selection schemes in both one-way and two-way cooperative networks is given in terms of both ergodic secrecy rate and secrecy outage probability metrics.

How many small cell (SC) access points (APs) are required to guarantee a chosen quality of service in a heterogeneous network? In this chapter, we answer this question considering two different network models. The first is the downlink of a finite-area SC network where the locations of APs within the chosen area are uniformly distributed. A key step in obtaining the closed-form expressions is to generalize the well-accepted moment matching approximation for the linear combination of lognormal random variables. For the second model, we focus on a two-layer downlink heterogeneous network with frequency reuse-1 hexagonal macro cells (MCs), and SC APs that are placed at locations that do not meet a chosen quality of service from macro base stations (BSs). An important property of this model is that the SC AP locations are coupled with the MC coverage. Here, simple bounds for the average total interference within an MC makes the formulation possible for the percentage of MC area in outage, as well as the required average number of SCs (per MC) to overcome outage, assuming isolated SCs. Introduction Heterogeneous cellular networks (HCNs) are being considered as an efficient way to improve system capacity as well as effectively enhance network coverage [1, 2]. Comprising multiple layers of access points (APs), HCNs encompass a conventional macro cellular network (first layer) overlaid with a diverse set of small cells (SCs) (higher layers). Cell deployment is an important problem in heterogeneous networks, both in terms of the number and positioning of the SCs. Traditional network models are either impractically simple (such as the Wyner model [3]) or excessively complex (e.g., the general case of random user locations in a hexagonal lattice network [4]) to accurately model SC networks. A useful mathematical model that accounts for the randomness in SC locations and irregularity in the cells uses spatial point processes, such as Poisson point process (PPP), to model the location of SCs in the network [5–10]. The independent placement of SCs from the MC layer, has the advantage of analytical tractability and leads to many useful SINR and/or rate expressions. However, even assuming that wireless providers would deploy SCs to support mobile broadband services, the dominant assumption remains that SCs are deployed randomly and independent of the MC layer [11].

Handover management in Visible Light Communications (VLC) networks is an open issue due to the need of maintaining alignment between a transmitting Light Emitting Diode (LED) and a receiver PhotoDetector (PD). Mobility strongly affects VLC connectivity links, as well as occlusions that may obstruct the Line-of-Sight (LoS) propagation. In this paper we present a proactive handover solution, which is able to switch connectivity links from a serving VLC “lighting” cell to a candidate one, in case of blockage that affects the VLC link. Differently from conventional handover mechanisms, where a handover is trigged by quality of service metrics or localization information, our technique is dedicated to VLC networks which can be affected by occlusions causing blockages. The proposed approach has been implemented both in hard and soft mode, and assessed in terms of achievable data rate and hand over latency for a user walking in a given reference room at different speeds.

We have considered a dual-hop wireless network consisting of a decode-and-forward (DF) relay in presence of an eavesdropper. Relay can correctly decode and retransmit only if the received signal-to-noise ratio (SNR) meets a particular threshold. The eavesdropper can tap signals both from the relay and the source. The performance of the maximal ratio combining (MRC) and selection combining (SC) diversity at the eavesdropper is investigated. Whether knowledge of channel state information (CSI) at the transmitter is available or not is considered for the performance evaluation in secrecy outage probability and ergodic secrecy rate. Asymptotic analyses of performances are provided. We show that direct link has a significant impact on system secrecy. It is found that secrecy performances are the best if the relay is always able to decode correctly. We also found that knowledge of CSI helps to achieve better secrecy at lower rate. It is observed that unbalance created due to constraint on either of the source-relay link average SNR or relay-destination link average SNR does not yield similar secrecy outage performance but both can limit the secrecy outage performance. On the contrary, it is found that only source-relay link quality can limit the ergodic secrecy rate performance.

This paper investigates the physical layer security of indoor visible light communication (VLC) systems in the presence of randomly located eavesdroppers using a stochastic geometry framework. Unlike traditional LOS-only or perfect-CSI assumptions, this work incorporates a more realistic setting where the eavesdropper’s channel state information (CSI) is unknown, and both line-of-sight (LOS) and first-order non-line-of-sight (NLOS) reflections are accounted for in the channel modelling. The analysis further integrates device orientation randomness and evaluates security under eavesdroppers employing selection combining (SC) and maximum ratio combining (MRC). Closed-form expressions for the channel gain, SNR distribution, and secrecy outage probability (SOP) are derived by exploiting the spatial statistics of a bounded Poisson point process (PPP). Results show that incorporating NLOS components significantly enhances legitimate SNR and improves secrecy performance, while eavesdropper collaboration using maximum ratio combining presents the worst-case security threat. Monte Carlo simulations validate the analytical derivations, demonstrating close agreement across various system parameters. The study offers an accurate and practical secrecy analysis framework for next-generation VLC deployments in realistic indoor environments.

This paper analyzes the ergodic capacity of a user-centric virtual cell (VC) dense network, where multiple access points (APs) form a VC for each user equipment (UE) and transmit data cooperatively over millimeter wave (mmWave) channels. Different from traditional microwave radio communications, blockage phenomena have an important effect on mmWave transmissions. Accordingly, we adopt a distance-dependent line-of-sight (LOS) probability function and model the locations of the LOS and non-line-of-sight (NLOS) APs as two independent non-homogeneous Poisson point processes (PPP). Invoking this model in a VC dense network, new expressions are derived for the downlink ergodic capacity, accounting for: blockage, small-scale fading and AP cooperation. In particular, we compare the ergodic capacity for different types of fading distributions, including Rayleigh and Nakagami. Numerical results validate our analytical expressions and show that AP cooperation can provide notable capacity gain, especially in low-AP-density regions.

Visible light communication (VLC) networks, consisting of multiple light-emitting diodes (LEDs) acting as optical access points (APs), can provide low-cost high-rate data transmission to multiple users simultaneously in indoor environments. However, the performance of VLC networks is severely limited by the interference between different users. In this paper, we establish a distributed user-centric scheduling framework based on stable marriage theory, and propose a novel decentralized scheduling method to manage interference by forming flexible amorphous cells for all users. The proposed scheduling method has provable low computational complexity and requires only the exchange of a few 1-bit messages between the APs and the users but not the feedback of the channel state information of the entire network. We further show that the proposed method can achieve both user-wise and system-wise optimality as well as a certain level of fairness. Simulation results indicate that our decentralized user-centric scheduling method outperforms existing centralized approaches in terms of throughput, fairness, and computational complexity.

By taking a variety of realistic hardware imperfections into consideration, we propose an optimal power allocation (OPA) strategy to maximize the instantaneous secrecy rate of a cooperative wireless network comprised of a source, a destination, and an untrusted amplify-and-forward relay. We assume that either the source or the destination is equipped with a large-scale multiple antennas' system, while the rest are equipped with a single antenna. To prevent the untrusted relay from intercepting the source message, the destination sends an intended jamming noise to the relay, which is referred to as destination-based cooperative jamming. Given this system model, novel closed-form expressions are presented in the high signal-to-noise ratio regime for the ergodic secrecy rate and the secrecy outage probability. We further improve the secrecy performance of the system by optimizing the associated hardware design. The results reveal that by beneficially distributing the tolerable hardware imperfections across the transmission and reception radio-frequency front ends of each node, the system's secrecy rate may be improved. The engineering insight is that equally sharing the total imperfections at the relay between the transmitter and the receiver provides the best secrecy performance. Numerical results illustrate that the proposed OPA together with the most appropriate hardware design significantly increases the secrecy rate.

We present a secure multi-user multi-antenna transmission framework based on artificial-noise-aided linear zero-forcing beamforming, with limited channel state information feedback from multiple distributed legitimate users (LUs). The secrecy performance of the proposed scheme is analytically investigated and optimized. We develop a new accurate closed-form expression of a lower bound on the ergodic secrecy rate (ESR) of each LU without assuming asymptotes for any system parameter. To make system design tractable, we develop another lower bound on ESR which is so analytically amenable that it enables one to not only extend the results of the previous related works but also explore some untouched aspects of the well known artificial-noise-aided scheme. We derive the optimized power allocation coefficient to message-bearing signals which maximizes the latter ESR lower bound. Furthermore, we theoretically study respectively the impacts of the two main parameters, i.e., transmit power $P$ and the number of feedback bits of each LU $B$ , on the power allocation coefficient, and show the asymptotic results for the high-power and high-quantization-resolution systems. We also develop a sufficient condition on $P$ and $B$ under which a positive ESR of each LU can be achieved. We study some important parameters called the minimum required transmit power (MRTP) and the minimum required number of feedback bits (MRFBs) for each LU to achieve a positive ESR or to achieve a target ESR, which have rarely been touched before. Besides, we propose the algorithms to obtain the MRTP and MRFBs. Numerical results are also provided to verify our theoretical results.

In indoor positioning system, the user’s location is estimated based on user fingerprints database collected. The received signal from a user at a certain location is compared with all database entries and the best match is selected as the correct estimation. For a given number of access points (APs), optimizing the locations of APs can significantly improve the performance of user location estimation. In this work, the probability of correct estimation is analyzed based on APs location. Both analytical expressions and numerical results are presented and discussed. It has been shown that the probability of correct estimation is highly affected by the APs locations. Furthermore, a new technique is proposed to remarkably reduce the search-cost when comparing a received user signal to the data-base. The performance of the system is analyzed considering different APs locations, different user locations, and different noise variances. It is shown that user’s location estimation is significantly improved when compared to existing techniques.

This research paper considers a wireless system that assumes massive multiple-input multiple-output (MIMO) technical solutions for the Base Station (BS), while both near and far users have just one antenna. This study primarily aims to examine the security implications of the physical layer in this system using Non-Orthogonal Multiple Access (NOMA) along with an Intelligent Reconfigurable Surface (IRS). It is assumed that the BS broadcasts signals to pairs of NOMA users, and both categories of users also receive signals from the IRS. Additionally, the far user is regarded as untrusted, while the jammer is present. Moreover, to assess the confidentiality performance of the studied system, the Ergodic Secrecy Rate (ESR) and Secrecy Outage Probability (SOP) are considered, and their closed-form expressions are provided. Our findings indicate that the integration of IRS leads to an exponential enhancement in the ESR, consequently reducing the SOP. Additionally, our observations highlight the superior efficiency of networks featuring a jamming mechanism. Furthermore, the employment of a massive multi-antenna setup amplifies the transfer Signal-to-Noise Ratio (SNR), leading to heightened ESR and a simultaneous reduction in the SOP.

In this paper, the secrecy of a cooperative system consisting of a single source, relay, destination and eavesdropper is analyzed. The threshold-selection decode-and-forward relay is considered, where the relay can correctly decode and forward only if it satisfies a threshold signal-to-noise ratio (SNR). Both destination and eavesdropper take advantage of the direct and relayed transmissions through maximal ratio diversity combining. The secrecy outage probability (SOP) and ergodic secrecy rate are derived in closed-form for different channel state information (CSI) availability. It was observed that when the required rate is low, having CSI knowledge is more advantageous than in the case of higher rate. An increase in the required threshold SNR at the relay can increase the SOP if the relayed link SNR is relatively higher than the direct link SNR. It was also shown that SOP cannot be improved beyond a certain value when keeping either dual-hop link average SNR fixed and increasing the other link SNR, whereas the ergodic secrecy rate can be increased by keeping the source to destination average SNR fixed.