In cooperative communication, a source communicates with a destination over indirect links through relays. Like most wireless systems, cooperative networks seriously suffer from secrecy related issues. In this thesis, cooperative jamming method is utilized to provide secrecy for cooperative networks in which there exist adversary receivers called eavesdroppers. The main idea is to broadcast noise signals from a selected relay to corrupt the signal-to-interference-plus-noise ratio (SINR) at eavesdroppers when the transmitter communicates with the receiver through another selected relay. Throughout the study, it is assumed that channel state information (CSI) of the links between the source, relays and the destination is available at the transmit and receive sides of links. Rich scattering channels are assumed and all channel gains are taken as independent. In contrast to the past studies in the literature, CSI of eavesdroppers’ channels exist at only themselves since they are accepted as passive devices in this study. The knowledge on channel gains except eavesdroppers’ ones are used to adaptively select the relays to support cooperative jamming. Cooperative jamming is investigated here from an outage probability perspective where probability of secure transmission is the parameter of interest for a fixed transmission rate. First, cooperative jamming is applied to cooperative networks with single-antenna nodes. The probability of secure communication increases to a degree for a range of the communication rate R with cooperative jamming and adaptive selection of relays. The results are generally encouraging, but may be insufficient for real-world cooperative networks. Second, cooperative jamming is implemented at cooperative networks with multiple-antenna nodes to increase the probability of secure communication benefiting from advantages of multiple antennas. Adaptive transmit precoding is applied at the source and the communicating relay to enhance the SINR at legitimate receivers. This advantage brings an edge over eavesdroppers. Moreover, adaptive noise generation is proposed at the noise emitting relays in order to minimize the effect of noise at legitimate receivers. It is shown that the probability of secure communication dramatically rises with these adaptive techniques. As a result, there is a certain transmission rate R at which the source is able to securely communicate with the destination with probability approaching one.
In cooperative communication, a source communicates with a destination over indirect links through relays. Like most wireless systems, cooperative networks seriously suffer from secrecy related issues. In this thesis, cooperative jamming method is utilized to provide secrecy for cooperative networks in which there exist adversary receivers called eavesdroppers. The main idea is to broadcast noise signals from a selected relay to corrupt the signal-to-interference-plus-noise ratio (SINR) at eavesdroppers when the transmitter communicates with the receiver through another selected relay. Throughout the study, it is assumed that channel state information (CSI) of the links between the source, relays and the destination is available at the transmit and receive sides of links. Rich scattering channels are assumed and all channel gains are taken as independent. In contrast to the past studies in the literature, CSI of eavesdroppers’ channels exist at only themselves since they are accepted as passive devices in this study. The knowledge on channel gains except eavesdroppers’ ones are used to adaptively select the relays to support cooperative jamming. Cooperative jamming is investigated here from an outage probability perspective where probability of secure transmission is the parameter of interest for a fixed transmission rate. First, cooperative jamming is applied to cooperative networks with single-antenna nodes. The probability of secure communication increases to a degree for a range of the communication rate R with cooperative jamming and adaptive selection of relays. The results are generally encouraging, but may be insufficient for real-world cooperative networks. Second, cooperative jamming is implemented at cooperative networks with multiple-antenna nodes to increase the probability of secure communication benefiting from advantages of multiple antennas. Adaptive transmit precoding is applied at the source and the communicating relay to enhance the SINR at legitimate receivers. This advantage brings an edge over eavesdroppers. Moreover, adaptive noise generation is proposed at the noise emitting relays in order to minimize the effect of noise at legitimate receivers. It is shown that the probability of secure communication dramatically rises with these adaptive techniques. As a result, there is a certain transmission rate R at which the source is able to securely communicate with the destination with probability approaching one.
This paper investigates cooperative security in wireless networks, where a source (Alice) intends to transmit a confidential message to a legitimate destination (Bob), with the help of a cooperative jammer (Charlie), coexisting with multiple passive eavesdroppers (Eves). In particular, by assuming knowledge of Bob's perfect channel state information (CSI) but only Eves’ statistical CSIs, secrecy beamforming with artificial noise (AN) is utilized for secure transmission, and cooperative jamming (CJ) is explored to further enhance secrecy. We first derive an accurate closed-form expression for the secrecy outage probability (SOP), and establish the condition under which positive secrecy rate is achievable. Then, we provide a secure transmit design for maximizing the SOP constrained secrecy rate. Moreover, based on a strict mathematical analysis, we characterize the impact of the main channel quality and the number of Eves on transmit design and secrecy performance. Specifically, optimal power allocation ratio between the information-bearing signal and the AN signal increases as the main channel quality improves, and decreases with the number of Eves. Numerical results confirm that our design achieves performance improvement in terms of both secrecy rate and secure energy efficiency, as compared to the approach without CJ.
The broadcast nature of wireless communications makes the propagation medium vulnerable to security attacks such as eavesdropping and jamming from adversarial or unauthorized users. Applying physical layer secrecy approaches will enable the exchange of confidential messages over a wireless medium in the presence of unauthorized eavesdroppers, without using any secret keys. However, physical layer security approaches are typically feasible only when the source-eavesdropper channel is weaker than the source-destination channel. Cooperative jamming can be used to overcome this challenge and increase the secrecy rate. In this paper, the security of two-phase relaying system with multiple intermediate nodes and in the presence of an eavesdropper is investigated. To enhance the system secrecy rate, a joint cooperative beamforming and jamming combined with relay and jammer selections is proposed. In phase I, the source node broadcasts a signal to relays while three intermediate nodes (which act as jammers) help the source node by transmitting random jamming signals to confuse the eavesdropper. Since the friendly (cooperative) jammers create interference for both the intended relays and the eavesdropper, optimal beamforming is applied such that no interference is caused at two preselected desired relays (that are going to receive the confidential massage in phase I). In phase II, two preselected relays transmit the source message with beamforming coefficients such that the received signal at the eavesdropper is completely nulled out. Our goal in this paper is to minimize the received SNR at the eavesdropper while increasing it at the destination as much as possible by applying different methods such as cooperative beamforming, cooperative jamming and relay selection. To avoid operational complexity, we consider the minimum number of intermediate nodes that are necessary without losing the performance. Numerical results demonstrate the advantage of our proposed scheme compared with the scheme with no cooperative jamming.
Motivated by recent developments in wireless communication, this thesis aims to characterize the secrecy performance in several types of typical wireless networks. Advanced techniques are designed and evaluated to enhance physical layer security in these networks with realistic assumptions, such as signal propagation loss, random node distribution and non-instantaneous channel state information (CSI). The first part of the thesis investigates secret communication through relay-assisted cognitive interference channel. The primary and secondary base stations (PBS and SBS) communicate with the primary and secondary receivers (PR and SR) respectively in the presence of multiple eavesdroppers. The SBS is allowed to transmit simultaneously with the PBS over the same spectrum instead of waiting for an idle channel. To improve security, cognitive relays transmit cooperative jamming (CJ) signals to create additional interferences in the direction of the eavesdroppers. Two CJ schemes are proposed to improve the secrecy rate of cognitive interference channels depending on the structure of cooperative relays. In the scheme where the multiple-antenna relay transmits weighted jamming signals, the combined approach of CJ and beamforming is investigated. In the scheme with multiple relays transmitting weighted jamming signals, the combined approach of CJ and relay selection is analyzed. Numerical results show that both these two schemes are effective in improving physical layer security of cognitive interference channel. In the second part, the focus is shifted to physical layer security in a random wireless network where both legitimate and eavesdropping nodes are randomly distributed. Three scenarios are analyzed to investigate the impact of various factors on security. In scenario one, the basic scheme is studied without a protected zone and interference. The probability distribution function (PDF) of channel gain with both fading and path loss has been derived and further applied to derive secrecy connectivity and ergodic secrecy capacity. In the second scenario, we studied using a protected zone surrounding the source node to enhance security where interference is absent. Both the cases that eavesdroppers are aware and unaware of the protected zone boundary are investigated. Based on the above scenarios, further deployment of the protected zones at legitimate receivers is designed to convert detrimental interference into a beneficial factor. Numerical results are investigated to check the reliability of the PDF for reciprocal of channel gain and to analyze the impact of protected zones on secrecy performance. In the third part, physical layer security in the downlink transmission of cellular network is studied. To model the repulsive property of the cellular network planning, we assume that the base stations (BSs) follow the Mat´ern hard-core point process (HCPP), while the eavesdroppers are deployed as an independent Poisson point process (PPP). The distribution function of the distances from a typical point to the nodes of the HCPP is derived. The noise-limited and interference-limited cellular networks are investigated by applying the fractional frequency reuse (FFR) in the system. For the noise-limited network, we derive the secrecy outage probability with two different strategies, i.e. the best BS serve and the nearest BS serve, by analyzing the statistics of channel gains. For the interference-limited network with the nearest BS serve, two transmission schemes are analyzed, i.e., transmission with and without the FFR. Numerical results reveal that both the schemes of transmitting with the best BS and the application of the FFR are beneficial for physical layer security in the downlink cellular networks, while the improvement due
In this paper, we investigate a security transmission scheme at the physical layer for cooperative wireless relay networks in the presence of a passive eavesdropper.While the security scheme has been previously investigated with perfect channel state information(CSI) in the presence of a passive eavesdropper, this paper focuses on researching the robust cooperative relay beamforming mechanism for wireless relay networks which makes use of artificial noise (AN) to confuse the eavesdropper and increase its uncertainty about the source message.The transmit power used for AN is maximized to degrade the signal-to-interference-plus-noise-ratio (SINR) level at the eavesdropper, while satisfying the individual power constraint of each relay node and worst-case SINR constraint at the desired receiver under a bounded spherical region for the norm of the CSI error vector from the relays to the destination.Cooperative beamforming weight vector in the security scheme can be obtained by using S-Procedure and rank relaxation techniques.The benefit of the proposed scheme is showed in simulation results.
In this paper, the security of two-phase relaying system with multiple intermediate nodes in the presence of a malicious eavesdropper is investigated. To enhance the secrecy, a joint cooperative beamforming and jamming combined with relay and jammer selection is proposed. First, the source broadcasts its signal to the relays that are located close to source in a cluster, i.e., the source node uses a small amount of power to broadcast its message locally to other nodes of the cluster, while destination and the eavesdropper are located outside this cluster. In the second phase, two relays transmit re-encoded signal with optimal beamforming such that the secrecy rate is maximized. Simultaneously, two other intermediate nodes (which act as friendly jammers) transmit random jamming signals to degrade the eavesdropper’s channel. Our goal in this paper is to maximize the secrecy rate by applying different methods such as cooperative beamforming, cooperative jamming and relay and jammer selection. To avoid operational complexity, we consider the minimum number of intermediate nodes needed for relaying and jamming without losing the performance, i.e., achieving a non-zero secrecy rate. Cooperative beamforming with multiple relays demands high amount of information exchange and therefore increases the operational complexity. Thus, we aim to reduce the number of relays which take part in cooperative beamforming. Limiting the number of relays may have a bad effect on the coding gain which we compensate it with a proposed selection gain scheme. Numerical results demonstrate the advantage of our proposed scheme compared to the scheme with no cooperative jamming. The main contribution of this work is combining cooperative beamforming and jamming with relay and jammer selection to enhance the physical layer security.
Cooperative jamming for multiuser multiple input multiple output (MIMO) broadcast channel is studied to enhance the physical layer security with the help of a friendly jammer. We assume the base station transmits multiple independent data streams to multiple legitimate users. During the transmission, however, there are multiple eavesdroppers with multiple antennas that have interests in the streams from the base station. In order to wiretap the desired streams, the eavesdroppers may collude or not, and maximize the signal-to-interference-plus-noise ratio (SINR) of the desired streams using receive beamforming. The optimal cooperative jammer is designed to keep the achieved SINR at eavesdroppers below a threshold to guarantee that the transmission from the base station to legitimate users is confidential. One main advantage of the proposed cooperative jamming scheme is that no modification is needed for the existing precoding schemes at the base station and decoding schemes at legitimate users. Thus, any existing practical precoding/decoding schemes for multiuser MIMO broadcast channel can be applied directly with the help of a friendly jammer using the proposed cooperative jamming.