Wireless communications system designs are sought that rely on the potential of the physical (PHY) layer to allow a transmitter-receiver pair communicate securely in the presence of one or more eavesdroppers. Communication is performed with the help of multiple trusted relays, via the decode-and-forward (DF) protocol, and assuming knowledge of global channel state information (CSI). Instead of dealing with information-theoretic security, we assume that eavesdroppers have bounded resources, and therefore we need to assure that the transmitted information is computationally secure; this is shown to impose constraints on the maximum power of the signals received by the eavesdroppers. The optimal relay weights that maximize the signal-to-noise ratio (SNR) difference SNRd- SNRebetween the destination and the eavesdroppers are determined under various (transmit, security, reliability) power constraints. Closed-form solutions are obtained in some problem formulations for one eavesdropper, whereas an iterative algorithm is proposed if more eavesdroppers are present. Simulation results indicate that a higher SNR value (compared to the non-cooperative case) is attained at the destination, while satisfying the security constraints.
Wireless communications system designs are sought that rely on the potential of the physical (PHY) layer to allow a transmitter-receiver pair communicate securely in the presence of one or more eavesdroppers. Communication is performed with the help of multiple trusted relays, via the decode-and-forward (DF) protocol, and assuming knowledge of global channel state information (CSI). Instead of dealing with information-theoretic security, we assume that eavesdroppers have bounded resources, and therefore we need to assure that the transmitted information is computationally secure; this is shown to impose constraints on the maximum power of the signals received by the eavesdroppers. The optimal relay weights that maximize the signal-to-noise ratio (SNR) difference SNRd- SNRebetween the destination and the eavesdroppers are determined under various (transmit, security, reliability) power constraints. Closed-form solutions are obtained in some problem formulations for one eavesdropper, whereas an iterative algorithm is proposed if more eavesdroppers are present. Simulation results indicate that a higher SNR value (compared to the non-cooperative case) is attained at the destination, while satisfying the security constraints.
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.
The feasibility of physical-layer-based security approaches for wireless communications in the presence of one or more eavesdroppers is hampered by channel conditions. In this paper, cooperation is investigated as an approach to overcome this problem and improve the performance of secure communications. In particular, a decode-and-forward (DF) based cooperative protocol is considered, and the objective is to design the system for secrecy capacity maximization or transmit power minimization. System design for the DF-based cooperative protocol is first studied by assuming the availability of global channel state information (CSI). For the case of one eavesdropper, an iterative scheme is proposed to obtain the optimal solution for the problem of transmit power minimization. For the case of multiple eavesdroppers, the problem of secrecy capacity maximization or transmit power minimization is in general intractable. Suboptimal system design is proposed by adding an additional constraint, i.e., the complete nulling of signals at all eavesdroppers, which yields simple closed-form solutions for the aforementioned two problems. Then, the impact of imperfect CSI of eavesdroppers on system design is studied, in which the ergodic secrecy capacity is of interest.
Relying on physical layer security is an attractive alternative of utilizing cryptographic algorithms at upper layers of protocol stack for secure communications. In this paper, we consider a two-hop wireless relay network in the presence of an eavesdropper. Our scenario of interest spans over a four-node network model including a source, a destination, a trusted relay, and an untrusted eavesdropper in which the relay forwards the source message in a decode-and-forward (DF) fashion. The source and relay are allowed to use some of their available power to transmit jamming signals in order to create interference at the eavesdropper. The relay and destination are assumed to have the knowledge of the jamming signals. An important question is how to allocate the transmission power of the message signal and that of the jamming signal. First, we propose an optimal power allocation solution in which the knowledge of global channel state information (CSI) is required. To facilitate practical system design, two simple yet sub-optimal power allocation solutions are proposed which do not rely on eavesdropper's channels. For the purpose of performance comparisons, power allocation problems for two benchmark schemes without jamming are also analyzed.
A cooperative jamming protocol is studied in this paper and its ability to protect the communications of a pair of users in the presence of an eavesdropper. Communication of users is assisted by many helping interferers, assuming knowledge of channel state information. Closed form expressions are given for the optimal weights and power allocation maximizing the difference in the SNR between destination and eavesdropper; these are determined under transmit, reliability, and security constraints. Simulations show that noticeable improvements, of more than 30dB, may be attained in the SNR difference compared to the non-cooperative case.
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.
This paper addresses the physical layer security of a two-hop decode-and-forward (DF) wireless relay network in the presence of several eavesdroppers. The wiretap channel model consists of a source, several eavesdroppers, couples of trusted relays and a destination. For this DF based cooperative relay scheme, the relays use different code words independent of the source code words to transmit a weighted version of the recoded messages as with the source messages to the legitimate receiver and jam the eavesdropper. We fix the transmit power of the source ( Ps ) to obtain the weights for secrecy rate maximization, assuming that global channel state information (CSI) is available. The maximization problem can be made to a generalized eigenvector problem, and it is easy to obtain the corresponding eigenvector that is the optimal relay weight vector by using the matrix computations. The closed-form optimal solution is derived for the relay weights that maximize the achievable secrecy rate.