Opportunistic User Scheduling for Secure RIS-aided Wireless Communications

In this paper, we provide expressions for the secrecy outage probability (SOP) for suboptimal and optimal opportunistic scheduling schemes in a reconfigurable intelligent surface (RIS) aided system with multiple eavesdroppers in approximate closed form. A suboptimal scheduling (SS) scheme is analyzed, which is used when the channel state information (CSI) of the eavesdropping links is unavailable, and the optimal scheduling (OS) scheme is also analyzed, which is used when the global CSI is available. For each scheme, we provide a simplified expression for the SOP in the high signal-to-noise ratio (SNR) regime to demonstrate its behavior as a function of the key system parameters. At high SNR, the SOP saturates to a constant level which decreases exponentially with the number of RIS elements in the SS scheme and with the product of the number of RIS elements and the number of users in the OS scheme. We compare the performance of the opportunistic user scheduling schemes with that of a non-orthogonal multiple access (NOMA) based scheduling scheme which chooses a pair of users in each time slot for scheduling and we show that the opportunistic schemes outperform the NOMA-based scheme. We also derive a closed-form expression for the SOP of a decode-and-forward (DF) relay-aided scheduling scheme in order to compare it with that of the RIS-aided system. It is found that the RIS-aided system outperforms the relay-aided systems when the number of RIS elements is sufficiently large. An increased number of RIS elements is required to outperform the relay-aided system at higher operating frequencies.

Destination Scheduling for Secure Pinhole-Based Power-Line Communication

We propose an optimal destination scheduling scheme to improve the physical layer security (PLS) of a power-line communication (PLC) based Internet-of-Things system in the presence of an eavesdropper. We consider a pinhole (PH) architecture for a multi-node PLC network to capture the keyhole effect in PLC. The transmitter-to-PH link is shared between the destinations and an eavesdropper which correlates all end-to-end links. The individual channel gains are assumed to follow independent log-normal statistics. Furthermore, the additive impulsive noise at each node is modeled by an independent Bernoulli-Gaussian process. Exact computable expressions for the average secrecy capacity (ASC) and the probability of intercept (POI) performance over many different networks are derived. Approximate closed-form expressions for the asymptotic ASC and POI are also provided. We find that the asymptotic ASC saturates to a constant level as transmit power increases. We observe that the PH has an adverse effect on the ASC. Although the shared link affects the ASC, it has no effect on the POI. We show that by artificially controlling the impulsive to background noise power ratio and its arrival rate at the receivers, the secrecy performance can be improved.

Secrecy of Opportunistic User Scheduling in RIS-Aided Systems: A Comparison with NOMA Scheduling

In this paper, we propose an opportunistic user scheduling scheme in a multi-user reconfigurable intelligent surface (RIS) aided wireless system to improve secrecy. We derive the secrecy outage probability (SOP) and its asymptotic expression in approximate closed form. The asymptotic analysis shows that the SOP does not depend on the transmitter-to-RIS distance and saturates to a fixed value depending on the ratio of the path loss of the RIS-to-destination and RIS-to-eavesdropper links and the number of users at high signal-to-noise ratio. It is shown that increasing the number of RIS elements leads to an exponential decrease in the SOP. We also compare our scheme with that of a non-orthogonal multiple access (NOMA) scheduling scheme, which chooses a pair of users to schedule in each time slot. The comparison shows that the SOP of all of the NOMA users is compromised, and that our proposed scheduling scheme has better performance.

Transmitter Selection for Secrecy Against Colluding Eavesdroppers with Backhaul Uncertainty

In this paper, the secrecy performance of a multi-transmitter system with unreliable wireless backhaul links in the presence of multiple colluding eavesdroppers is considered. To improve the secrecy performance, two sub-optimal and optimal transmitter selection schemes are proposed. A generalized approach for the secrecy performance analysis is adopted to integrate the backhaul link reliability factor with the channel signal-to-noise ratio (SNR) through a mixture distribution depending on whether the backhaul reliability knowledge is unavailable or available. Non-zero secrecy rate (NZSR), secrecy outage probability (SOP), and ergodic secrecy rate (ESR) are evaluated for each selection schemes via a unified channel SNR ratio distribution based approach. Simplified asymptotic expressions are provided in each case to elucidate the infulence of the system parameters and of the backhaul reliability factor. We observe that the unreliable backhaul limits the secrecy performance of transmitter selection schemes by saturating the NZSR and SOP performance and by reducing the rate of change of the ESR with SNR. The performance improves when the knowledge of backhaul link activity is utilized, and the improvement is most noticeable when the backhaul is highly unreliable. We also observe that while the secrecy performance degrades with an increasing number of eavesdroppers, neither the asymptotic saturation value of the NZSR and SOP nor the rate of ESR improvement with SNR depends on the number of eavesdroppers.

Ergodic Secrecy Rate of Optimal Source-Destination Pair Selection in Frequency-Selective Fading

Node selection is a simple technique to achieve diversity and thereby enhance the physical layer security in future wireless communication systems which require low complexity. High-speed data transmission often encounters frequency selective fading. In this context, we evaluate the exact closed-form expression for the ergodic secrecy rate (ESR) of the optimal source-destination pair selection scheme with single-carrier cyclic-prefix modulation, where the destination and eavesdropper channels both exhibit independent frequency selective fading with an arbitrary number of multipath components. A simplified analysis in the high-SNR scenario along with an asymptotic analysis is also provided. We also derive and compare the corresponding results for the sub-optimal source-destination pair selection scheme. We show that our analysis produces the corresponding ESR results under narrowband independent Nakagami-$m$ fading channel with any arbitrary integer parameter $m$. The effect of transmitters, destination and eavesdropping paths correlation on the ESR is also demonstrated. Our solution approach is general and can be used to find the ESR of a wider variety of transmitter selection schemes.

Optimal Friendly Jamming and Transmit Power Allocation in RIS-assisted Secure Communication

This paper analyzes the secrecy performance of a reconfigurable intelligent surface (RIS) assisted wireless communication system with a friendly jammer in the presence of an eavesdropper. The friendly jammer enhances the secrecy by introducing artificial noise towards the eavesdropper without degrading the reception at the destination. Approximate secrecy outage probability (SOP) is derived in closed form. We also provide a simpler approximate closed-form expression for the SOP in order to understand the effect of system parameters on the performance and to find the optimal power allocation for the transmitter and jammer. The optimal transmit and jamming power allocation factor is derived by minimizing the SOP assuming a total power constraint. It is shown that the SOP performance is significantly improved by the introduction of the jammer and a gain of approximately $3$ dB is achieved at an SOP of $10^{-4}$ by optimally allocating power compared to the case of equal power allocation.

Secrecy Outage Probability of Cognitive Small-Cell Network with Unreliable Backhaul Connections

In this paper, we investigate the secrecy performance of underlay cognitive small-cell radio network with unreliable backhaul connections. The secondary cognitive small-cell transmitters are connected to macro base station by wireless backhaul links. The small-cell network is sharing the same spectrum with the primary network ensuring that a desired outage probability constraint in the primary network is always satisfied. We propose an optimal transmitter selection (OTS) scheme for small-cell network to transfer information to the destination. The closed-form expression of secrecy outage probability are derived. Our result shows that increasing the primary transmitter's transmit power and the number of small-cell transmitter can improve the system performance. The backhaul reliability of secondary and the desired outage probability of the primary also have significant impact on the system.

Incremental Relaying for Power Line Communication: Performance Analysis and Power Allocation

In this paper, incremental decode-and-forward (IDF) and incremental selective decode-and-forward (ISDF) relaying are proposed to improve the spectral efficiency of power line communication. Contrary to the traditional decode-and-forward (DF) relaying, IDF and ISDF strategies utilize the relay only if the direct link ceases to attain a certain information rate, thereby improving the spectral efficiency. The path gain through the power line is assumed to be log-normally distributed with high distance-dependent attenuation and the additive noise is from a Bernoulli-Gaussian process. Closed-form expressions for the outage probability, and approximate closed-form expressions for the end-to-end average channel capacity and the average bit error rate for binary phase-shift keying are derived. Furthermore, a closed-form expression for the fraction of times the relay is in use is derived as a measure of the spectral efficiency. Comparative analysis of IDF and ISDF with traditional DF relaying is presented. It is shown that IDF is a specific case of ISDF and can obtain optimal spectral efficiency without compromising the outage performance. By employing power allocation to minimize the outage probability, it is realized that the power should be allocated in accordance with the inter-node distances and channel parameters.

Secrecy Performance of Small-Cell Networks with Transmitter Selection and Unreliable Backhaul under Spectrum Sharing Environment

We investigate the secrecy performance of an underlay small-cell cognitive radio network under unreliable backhaul connections. The small-cell network shares the same spectrum with the primary network, ensuring that a desired outage probability constraint is always met in the primary network. {To improve the security of the small-cell cognitive network, we propose three sub-optimal small-cell transmitter selection schemes,} namely sub-optimal transmitter selection, minimal interference selection, and minimal eavesdropping selection. Closed-form expressions of the non-zero secrecy rate, secrecy outage probability, and ergodic secrecy capacity are provided for the schemes along with asymptotic expressions. {We also propose an optimal selection scheme and compare performances with the sub-optimal selection schemes.} {Computable expressions for the non-zero secrecy rate and secrecy outage probability are presented for the optimal selection scheme.} Our results show that by increasing the primary transmitter's power and the number of small-cell transmitters, the system performance improves. The selection scheme, the backhaul reliability, and the primary user quality-of-service constraint also have a significant impact on secrecy performance.

Secrecy Outage of Dual-hop Regenerative Multi-relay System with Relay Selection

Relay selection is considered to enhance the secrecy of a dual-hop regenerative multi-relay system with an eavesdropper. Without assuming perfect decoding at the relays, the secrecy outage probability of a single relay system is obtained first. Secrecy outage of optimal, traditional and suboptimal relay selection schemes is then evaluated. To reduce the power consumption, partial relay selection schemes based only on either of the source-relay or relay-destination instantaneous channel state information (ICSI) are introduced. Its secrecy outage is evaluated and compared with the other schemes. Secrecy outage of all the selection schemes are obtained in closed-form. An optimal relay selection scheme is proposed using secrecy outage which does not require any ICSI. Asymptotic and diversity gain analysis of the secrecy outage is presented when source-relay and relay-destination average SNRs are same or different. We observe that the improvement in eavesdropper link quality affects the secrecy outage more when required secrecy rate is low as compared to the case when rate is high. We also observe that relay selection improves performance more when number of relays are more. It is important to note that either of the source-relay or the relay-destination link quality can equally limit the secrecy outage performance even if the other link quality is infinitely good.