This paper investigates the secrecy performance of a keyhole-aided multi-user communication network in the presence of multiple eavesdroppers. The communication happens through the same keyhole for legitimate users and eavesdroppers. In this context, the secrecy performance is evaluated for a user scheduling technique by obtaining the exact closed-form expression of secrecy outage probability (SOP). Further, a simplified asymptotic SOP expression is derived assuming high signal-to-noise ratio (SNR) scenario for a better understanding of the impact of system parameters. The effect of the keyhole parameters, number of users, number of eavesdroppers, and threshold secrecy rate on the SOP performance are also investigated for the considered system model. In the high-SNR regime, the asymptotic SOP saturates to a constant value and does not depend on the keyhole parameter and the channel parameter of the source-to-keyhole channel.

In this paper, a joint optimal allocation of transmit power at the source and jamming power at the destination is proposed to maximize the average secrecy energy efficiency (SEE) of a wireless network within a finite time duration. The destination transmits the jamming signal to improve secrecy by utilizing full-duplex capability. The source and destination both have energy harvesting (EH) capability with limited battery capacity. Due to the Markov nature of the system, the problem is formulated as a finite-horizon reinforcement learning (RL) problem. We propose the finite-horizon joint power allocation (FHJPA) algorithm for the finite-horizon RL problem and compare it with a low-complexity greedy algorithm (GA). An infinite-horizon joint power allocation (IHJPA) algorithm is also proposed for the corresponding infinite-horizon problem. A comparative analysis of these algorithms is carried out in terms of SEE, expected total transmitted secure bits, and computational complexity. The results show that the FHJPA algorithm outperforms the GA and IHJPA algorithms due to its appropriate modelling in finite horizon transmission. When the source node battery has sufficient energy, the GA can yield performance close to the FHJPA algorithm despite its low-complexity. When the transmission time horizon increases, the accuracy of the infinite-horizon model improves, resulting in a reduced performance gap between FHJPA and IHJPA algorithms. The computational time comparison shows that the FHJPA algorithm takes 16.6 percent less time than the IHJPA algorithm.

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 at a high signal-to-noise ratio to a fixed value that depends on the ratio of the path-loss of the RIS-to-destination and RIS-to-eavesdropper links and the number of users. It is shown that the asymptotic SOP decreases exponentially with the number of RIS elements. We also compare the secrecy performance of 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 secrecy performance of the NOMA scheme is compromised and that our proposed scheduling scheme has better performance.

Abstract Due to the exponential growth of interconnected devices and reduced cell coverage, beyond fifth‐generation networks will be dense. Thus, instead of wired backhaul, wireless backhaul will be cost‐effective and flexible. For security in multi‐transmitter systems, sub‐optimal and optimal transmitter selection schemes exist. However, including backhaul activity knowledge available (BKA) and backhaul activity knowledge unavailable (BKU) cases and transmitter selection schemes, there is no generalized secrecy analysis method. Moreover, evaluation of the ergodic secrecy rate (ESR) of the optimal selection schemes is impossible using existing solution approaches. To address these, two sub‐optimal and optimal transmitter selection schemes are proposed for a small‐cell multi‐transmitter system in BKU or BKA cases in the presence of multiple colluding eavesdroppers. The distribution of the ratio of the destination channel SNR and eavesdropping channel SNR is derived, thereby providing different secrecy performance metrics uniformly irrespective of selection schemes and BKU or BKA cases. Simplified asymptotic expressions are provided to elucidate the influence of the system parameters and of the backhaul reliability. It is observed that the secrecy performance improves when the knowledge of backhaul link activity is utilized, and the improvement is most noticeable when the backhaul is highly unreliable. It is also observed that while the secrecy performance degrades with an increasing number of eavesdroppers, neither the asymptotic saturation value of the secrecy outage probability nor the rate of improvement of the ESR with signal‐to‐noise‐ratio depends on the number of eavesdroppers.

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 10–4by optimally allocating power compared to the case of equal power allocation.

In this paper, a communication network using single carrier with cyclic prefix modulation over frequency selective channels is considered, where an access point provides connectivity to a legitimate destination through multiple transmitters with unreliable backhaul links in the presence of an eavesdropper. A sub-optimal and an optimal transmitter selection scheme are proposed to improve the secrecy of the system, depending on whether the active backhaul channel knowledge is available a priori or not. The secrecy outage probability (SOP) and its asymptotic limit are presented in closed-form. This provides some insights regarding how knowledge of the active backhaul links affects the secrecy performance of the network. Our results show that the optimal transmitter selection scheme obtains a larger benefit than the sub-optimal scheme from the knowledge of the active backhaul links, resulting in a significantly improved system performance; however, the sub-optimal transmitter selection scheme can reduce the complexity and feedback overhead.

A small-cell network with multiple transmitters and unreliable wireless backhaul is considered for secrecy enhancement. The small-cell network is operating under a spectrum sharing agreement with a primary network in a cognitive radio system. A constraint on the desired outage probability at the primary receiver is assumed as a part of the spectrum sharing agreement. The reliability of the wireless backhaul links are modeled by a set of independent and identically distributed Bernoulli random variables. A sub-optimal and an optimal small-cell transmitter selection (TS) scheme is proposed to improve the performance of the system, depending on the availability of channel state information. Selection schemes are designed for the scenario where knowledge is available regarding which backhaul links are active. The corresponding secrecy outage probabilities along with their asymptotic expressions are derived. It is shown that the secrecy performance is significantly improved compared to the case where knowledge of the active backhaul links is unavailable.

In this paper, we apply the long short-term memory (LSTM), an advanced recurrent neural network based machine learning (ML) technique, to the problem of transmitter selection (TS) for secrecy in an underlay small-cell cognitive radio network with unreliable backhaul connections. The cognitive communication scenario under consideration has a secondary small-cell network that shares the same spectrum of the primary network with an agreement to always maintain a desired outage probability constraint in the primary network. Due to the interference from the secondary transmitter common to all primary transmissions, the secrecy rates for the different transmitters are correlated. LSTM exploits this correlation and matches the performance of the conventional technique when the number of transmitters is small. As the number grows, the performance degrades in the same manner as other ML techniques such as support vector machine, k-nearest neighbors, naive Bayes, and deep neural network. However, LSTM still significantly outperforms these techniques in misclassification ratio and secrecy outage probability. It also reduces the feedback overhead against conventional TS.

In this study, the secrecy performance of cooperative half‐duplex cognitive relay networks (CRNs) comprised of multiple primary users (PUs) and multiple eavesdroppers under the effect of unreliable wireless backhauls is studied. The authors consider relay networks with single relay assisting multiple sources under power constraints at PUs and secondary transmitters, and based on the availability of channel‐state information at the sources, they propose two best source selection schemes, namely: (i) optimal source selection (OSS) and (ii) sub‐optimal source selection (SoSS). For both scenarios, they obtain exact closed‐form and asymptotic expressions for the system's outage probability and carry out numerical simulations to justify their analyses. From the results, source selection is proven to be capable of counteracting the negative impact of unreliable backhaul on CRNs. The secrecy performance limitations are also verified to be influenced by the backhaul reliability. Furthermore, OSS is more desirable compared to SoSS in low signal‐to‐noise ratio (SNR) regime, while the secrecy performance for both schemes converge to an asymptotic limit in high‐SNR ranges.

In this research, the authors investigate the outage probability (OP) of the secondary network in a cognitive wireless powered communication network. Energy‐constrained secondary users harvest energy from a hybrid access‐point (H‐AP) and the primary transmitter in the first phase. In the second phase, they select a secondary user based on two different schemes, namely the best uplink channel selection (UCS) and the minimal interference channel selection (MICS), to transfer information to the H‐AP. The secondary network can share the same spectrum with the primary network ensuring that a desired OP constraint in the primary network is always met. This constraint represents the quality‐of‐service (QoS) of the primary network. The analytical expressions and asymptotic expressions of the OP of the secondary network are derived. The result shows that increasing the number of secondary users can considerably improve system performance. The transmit power of the selected secondary user, energy harvesting time and relaxing the QoS constraint of the primary network have a significant impact on the OP of the secondary network. The results show that UCS outperforms MICS.

In this paper, we consider an active eavesdropping scenario in a cooperative system consisting of a source, a destination, and an active eavesdropper with multiple decode-and-forward relays. Considering an existing assumption in which an eavesdropper is also a part of network, a proactive relay selection by the eavesdropper is proposed. The best relay which maximizes the eavesdropping rate is selected by the eavesdropper. A relay selection scheme is also proposed to improve the secrecy of the system by minimizing the eavesdropping rate. Performances of these schemes are compared with two passive eavesdropping scenarios in which the eavesdropper performs selection and maximal ratio combining on the relayed links. A realistic channel model with independent non-identical links between nodes and direct links from the source to both the destination and eavesdropper are assumed. Closed- form expressions for the secrecy outage probability (SOP) of these schemes in Rayleigh fading channel are obtained. It is shown that the relay selection by the proactive eavesdropper is most detrimental to the system as not only the SOP increases with the increase in the number of relays, but its diversity also remains unchanged.

In this paper, an incremental selective decode-and-forward (ISDF) relay strategy is proposed for power line communication (PLC) systems to improve the spectral efficiency. Traditional decode-and-forward (DF) relaying employs two time slots by using half-duplex relays which significantly reduces the spectral efficiency. The ISDF strategy utilizes the relay only if the direct link quality fails to attain a certain information rate, thereby improving the spectral efficiency. The path gain is assumed to be log-normally distributed with very high distance dependent signal attenuation. Furthermore, the additive noise is modeled as a Bernoulli-Gaussian process to incorporate the effects of impulsive noise contents. Closed-form expressions for the outage probability and the fraction of times the relay is in use, and an approximate closed-form expression for the average bit error rate (BER) are derived for the binary phase-shift keying signaling scheme. We observe that the fraction of times the relay is in use can be significantly reduced compared to the raditional DF strategy. It is also observed that at high transmit power, the spectral efficiency increases while the average BER decreases with increase in the required rate.

In this paper, relay selection is considered to enhance security of a cooperative system with multiple threshold-selection decode-and-forward (DF) relays. Threshold-selection DF relays are the relays in which a predefined signal-to-noise ratio is set for the condition of successful decoding of the source message. We focus on the practical and general scenario where the channels suffer from independent non-identical Rayleigh fading and where the direct links between the source and destination and source and eavesdropper are available. Based on channel state information knowledge, three relay selection strategies, namely traditional, improved traditional, and optimal, are studied. In particular, the secrecy outage probability of all three strategies are obtained in closed-form. It is found that the diversity of secrecy outage probability of all strategies can improve with increasing the number of relays. It is also observed that the secrecy outage probability is limited by either the source to relay or relay to destination channel quality.

In this paper, we investigate the secure communication of cognitive full-duplex relay networks in the presence of multiple eavesdroppers and multiple primary receivers. In the considered network, multiple full-duplex relays are deployed to transfer information in the secondary network, under the malicious attempts of non- colluding/colluding eavesdroppers. Meanwhile, the transmit powers of secondary transmitters are constrained by the quality-of-service of the primary network. The optimal relay selection scheme is proposed to enhance the secrecy performance of the considered system. We study the secrecy performance by providing the exact closed- form and asymptotic expressions of the proposed system secrecy outage probability. We have demonstrated that increasing the number of full- duplex relays can improve the security performance. At the illegitimate side, using colluding eavesdroppers and increasing the number of eavesdroppers put information confidentiality at a greater risk. Besides, the transmit power and the desired outage probability of the primary network have great influences on the secrecy outage probability of the secondary network.

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.

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.

Energy efficiency is a major issue in the energy‐constrained wireless sensor network to ensure longer battery life. In this study, minimisation of total energy per bit for multi‐hop decode‐and‐forward network employing M‐ary quadrature amplitude modulation is considered. An average bit error rate (ABER) requirement at the destination over an independent Rayleigh fading channel is assumed. A realistic energy model is considered. The authors have proposed joint optimisation of constellation size, energy allocation per bit and relay location assuming fixed end‐to‐end distance. The authors have considered two scenarios: one, that each hop employs the same constellation size per hop and two, it adapts independently to take different constellation size. A novel solution is implemented following least square method to find the approximate closed‐form solution of the optimal constellation size, which otherwise would not have been possible. It is observed that joint optimisation technique where the constellation size adapts independently per hop outperforms the one where the constellation size is same. Least amount of total energy is consumed per bit for balanced link. Finally, the analysis shows that for a realistic energy model, the dual hop network provides significant energy saving of 58% at 10 − 4 ABER over multi‐hop network with four hops.

A joint optimisation of power allocation (PA) and relay location is proposed for decode‐and‐forward relaying over multi‐hop, independent, log‐normal fading channels. Optimisation is carried out at high signal‐to‐noise ratio (SNR) and low SNR scenarios to minimise the outage probability with a constraint on total transmit power and end‐to‐end relaying distance. End‐to‐end average bit error rate is also minimised assuming the same constraints. Optimisation is carried out to find optimal PA (OPA) for a given relay location, optimal relay location (ORL) for a given PA and joint OPA and relay location. All the optimisation problems reformulated from the original problems are shown to be convex except relay location optimisation minimising outage probability at low SNR scenario. It is then solved using properties of concave minimisation. Optimal allocation factors do not depend on the instantaneous channel state information. It also does not depend on total available power in the system except allocation factors evaluated by optimising outage at high SNR. It is observed that OPA allocates more power to weaker links and ORL allocates less distance to weaker links. It is also observed that performance improvement increases with increase in link unbalance and distance‐based allocation is most effective.

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. The 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 the 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.

We evaluate the bounds on the secrecy outage probability of a dual hop Amplify-and-forward (AF) system with an eavesdropper tapping the second hop, when all the links undergo Rayleigh fading. An approximate expression of outage is also obtained when the signal-to-noise ratio of the first hop is high. Based on the developed outage expressions we evaluate the bounds and the approximate outage for optimal relay selection when full instantaneous channel state information (FCSI) i.e. of all the links is available. We also propose a novel relay selection scheme when only statistical channel state information (SCSI) of all the links is available. The proposed scheme can be used in networks where feedback of FCSI to the decision making node is power intensive and CSI of the eavesdropper cannot be obtained at all instants. In the results section we show that the approximate expression follows the simulations exactly and the outage for relay selection in FCSI decreases with an increase in the number of relays.

In this paper, a joint optimization of power allocation (PA) and relay location is proposed for Amplify-and-Forward (AF) multihop relaying over independent lognormal fading channels. The proposed optimization method maximizes the end-to-end (e2e) average signal-to- noise ratio (ASNR) having constraints on total transmit power with no per terminal maximum limit and assuming fixed e2e distance. Optimal parameters do not depend on instantaneous channel state information (ICSI) and can reduce complexity and power consumption. Optimal PA for maximizing e2e instantaneous SNR (ISNR) is also derived considering same constraints. Outage performance of joint optimization method based on ASNR and optimization method based on ISNR is compared. It is observed that joint optimization not only performs better than equal allocation, it can also outperform PA based on ISNR when link unbalance is large.

In this paper, we consider the problem of fusing decisions in a distributed detection system when the local binary decisions made at the sensors, relative to observations of a common binary phenomenon, are transmitted over wireless links subject to Rayleigh flat fading and additive noise. A training-based channel estimator is used at the fusion center (FC) to estimate the complex Gaussian fading coefficients characterizing the channels between the sensors and the FC. We use channel state information (CSI) on the fading coefficients for censoring the sensors. Locally optimal decision threshold is considered for binary quantization at the sensors. The detection error probability, using a maximal ratio combining (MRC) fusion rule, is selected as a qualitative measure of system performance and is evaluated by means of simulations. We also use majority logic fusion at the FC to compare detection error probability with MRC after censoring sensors. We study the effects, on the system performance, of the channel estimation error, the channel signal-to-noise ratio (SNR), the sensor SNR, and the number of selected sensors.