In this paper, we study the role of cooperative relays to provide and improve secure communication rates through decode-and-forward (DF) strategies in a full-duplex multiple relay network with an eavesdropper. We consider the DF scheme as a basis for cooperation and propose several strategies that implement different versions of this scheme suited for cooperation with multiple relays. Our goal is to give an efficient cooperation paradigm based on the DF scheme to provide and improve secrecy in a multiple relay network. We first study the DF strategy for secrecy in a single relay network. We propose a suboptimal DF with zero forcing (DF/ZF) strategy for which we obtain the optimal power control policy. Next, we consider the multiple relay problem. We propose three different strategies based on DF/ZF and obtain their achievable secrecy rates. The first strategy is a single hop strategy whereas the other two strategies are multiple hop strategies. In the first strategy, we show that it is possible to eliminate all the relays' signals from the eavesdropper's observation (full ZF), however, the achievable secrecy rate is limited by the worst source-relay channel. Our second strategy overcomes the drawback of the first strategy, however, with the disadvantage of enabling partial ZF only. Our third strategy provides a reasonable compromise between the first two strategies. That is, in this strategy, full ZF is possible and the rate achieved does not suffer from the drawback of the first strategy. We conclude our study by a set of numerical results to illustrate the performance of each of the proposed strategies in terms of the achievable rates in different practical scenarios.
In this paper, we study the role of cooperative relays to provide and improve secure communication rates through decode-and-forward (DF) strategies in a full-duplex multiple relay network with an eavesdropper. We consider the DF scheme as a basis for cooperation and propose several strategies that implement different versions of this scheme suited for cooperation with multiple relays. Our goal is to give an efficient cooperation paradigm based on the DF scheme to provide and improve secrecy in a multiple relay network. We first study the DF strategy for secrecy in a single relay network. We propose a suboptimal DF with zero forcing (DF/ZF) strategy for which we obtain the optimal power control policy. Next, we consider the multiple relay problem. We propose three different strategies based on DF/ZF and obtain their achievable secrecy rates. The first strategy is a single hop strategy whereas the other two strategies are multiple hop strategies. In the first strategy, we show that it is possible to eliminate all the relays' signals from the eavesdropper's observation (full ZF), however, the achievable secrecy rate is limited by the worst source-relay channel. Our second strategy overcomes the drawback of the first strategy, however, with the disadvantage of enabling partial ZF only. Our third strategy provides a reasonable compromise between the first two strategies. That is, in this strategy, full ZF is possible and the rate achieved does not suffer from the drawback of the first strategy. We conclude our study by a set of numerical results to illustrate the performance of each of the proposed strategies in terms of the achievable rates in different practical scenarios.
In this paper, we first study the Decode-and-Forward strategy for secrecy in a single-relay network. We propose a suboptimal Decode-and-Forward with Zero Forcing (DF/ZF) strategy for which we obtain the optimal power control policy. Next, we consider the multiple relay problem. We propose three different strategies based on DF/ZF. The first strategy is a single-hop strategy in which all the relays decode the source message at the same time, then perform beamforming such that all the relays' signals are eliminated from the eavesdropper's observation (full zero-forcing). We give the achievable rate by this strategy and derive the optimal power control policy. We show that, in this strategy, the relays which are far from the source create a bottleneck and limit the achievable rate. The second strategy is a multiple hop strategy that overcomes the drawback of the first strategy, however, with the disadvantage of enabling partial zero-forcing only, assuming that all the relays are required to transmit fresh information in every transmission block. The third strategy is also a multiple hop strategy in which full zero-forcing is possible and the rate achieved does not suffer from the drawback of the first strategy.
In this paper, we consider secure communications of one source-destination pair in wireless multi-hop decode-and-forward (DF) relay networks. In the presence of an eavesdropper, we derive an optimal power allocation strategy to maximize achievable secrecy rates under an overall transmit power constraint assuming that a single relay is located at each individual hop. We demonstrate that the optimal power allocation is obtained when the overall power constraint and the DF-relaying constraints necessary to guarantee that each DF relay correctly decodes the information signals hold with equality, which rely on channel conditions between the source, relays, and destination. The channel conditions for the eavesdropper are shown to only influence relays that may be inactive for secrecy rate maximization. We also consider multiple cooperative relays at each hop to perform cooperative beamforming in the presence of multiple eavesdroppers. To improve the secrecy rate in multi-hop relay networks, which include more than two hops, we propose an iterative cooperative beamformer design and transmit power allocation scheme with relay selection. Numerical results are presented to investigate the secrecy capacity obtained by the optimal power allocation in multi-hop DF relay networks and to confirm the secrecy rate enhancement of the proposed cooperative beamforming scheme.
In this paper, we investigate a two-stage cooperative network consisting of one source, one corresponding destination, multiple eavesdroppers, and several intermediate nodes. In the first stage, the source selects several relays from the intermediate nodes and broadcasts its message to them. In the second stage, the selected nodes operating as the decode-and-forward (DF) relays perform collaborative beamforming for the secure data transmission that prevents eavesdropping. We study the relay selection and secure beamforming problem considering both the achievable rate in the first stage and the secrecy rate constraint in the second stage, whereas the existing works leave out of the transmission in the first stage. We propose two combined relay selection and beamforming schemes. The first scheme is to maximize the secrecy rate of the relay selection and beamforming with the achievable rate constraint in the first stage. To reduce the complexity of the first scheme, the second scheme combines relay selection and a suboptimal secure beamforming scheme, in which we hypothesize a virtual eavesdropper instead of all eavesdroppers and maximize the secrecy rate according to the virtual eavesdropper. Simulation results show that the performance of our first scheme is the same as that of the exhaustive search scheme, and the second scheme significantly reduces the complexity with acceptable performance degradation.
We consider a cooperative wireless network in the presence of one or more eavesdroppers, and exploit node cooperation for achieving physical (PHY) layer based security. Two different cooperation schemes are considered. In the first scheme, cooperating nodes retransmit a weighted version of the source signal in a decode-and-forward (DF) fashion. In the second scheme, referred to as cooperative jamming (CJ), while the source is transmitting, cooperating nodes transmit weighted noise to confound the eavesdropper. We investigate two objectives: i) maximization of the achievable secrecy rate subject to a total power constraint and ii) minimization of the total power transmit power under a secrecy rate constraint. For the first design objective, we obtain the exact solution for the DF scheme for the case of a single or multiple eavasdroppers, while for the CJ scheme with a single eavesdropper we reduce the multivariate problem to a problem of one variable. For the second design objective, existing work introduces additional constraints in order to reduce the degree of difficulty, thus resulting in suboptimal solutions. Our work raises those constraints, and obtains either an analytical solution for the DF scheme with a single eavesdropper, or reduces the multivariate problem to a problem of one variable for the CJ scheme with a single eavesdropper. Numerical results are presented to illustrate the proposed results and compare them to existing work.
Physical (PHY) layer security approaches for wireless communications can prevent eavesdropping without upper layer data encryption. However, they are hampered by wireless channel conditions: absent feedback, they are typically feasible only when the source-destination channel is better than the source-eavesdropper channel. Node cooperation is a means to overcome this challenge and improve the performance of secure wireless communications. This paper addresses secure communications of one source-destination pair with the help of multiple cooperating relays in the presence of one or more eavesdroppers. Three cooperative schemes are considered: decode-and-forward (DF), amplify-and-forward (AF), and cooperative jamming (CJ). For these schemes, the relays transmit a weighted version of a reencoded noise-free message signal (for DF), a received noisy source signal (for AF), or a common jamming signal (for CJ). Novel system designs are proposed, consisting of the determination of relay weights and the allocation of transmit power, that maximize the achievable secrecy rate subject to a transmit power constraint, or, minimize the transmit power subject to a secrecy rate constraint. For DF in the presence of one eavesdropper, closed-form optimal solutions are derived for the relay weights. For other problems, since the optimal relay weights are difficult to obtain, several criteria are considered leading to suboptimal but simple solutions, i.e., the complete nulling of the message signals at all eavesdroppers (for DF and AF), or the complete nulling of jamming signal at the destination (for CJ). Based on the designed relay weights, for DF in the presence of multiple eavesdroppers, and for CJ in the presence of one eavesdropper, the optimal power allocation is obtained in closed-form; in all other cases the optimal power allocation is obtained via iterative algorithms. Numerical evaluation of the obtained secrecy rate and transmit power results show that the proposed design can significantly improve the performance of secure wireless communications.
We investigate the problem of optimizing the secrecy rate of a link between a single source and a single destination in the presence of multiple relays and multiple eavesdroppers. Multiple relays assist the source by either retransmitting a weighted version of the source signal in a decode-and-forward (DF) fashion, or by performing cooperative jamming (CJ) i.e., transmitting weighted noise in order to confound the eavesdropper. In each protocol, the relay weights are designed to maximize the secrecy rate subject to a total power constraint. For the case of a single eavesdropper we obtain expressions for the optimal weights under both protocols in closed form, or propose the algorithms to research for the solution. For the DF protocol with multiple eavesdroppers we propose an algorithm to search for the solution. Numerical results illustrate that cooperation can significantly improve the system performance as compared to direct transmission.