Security is one of the most important issues in wireless communication.Because using wireless channels entails broadcasting data transmissions, malicious nodes can eavesdrop on wireless transmission, which leads to insecure data transmission.Traditional security methods are based on cryptography.Recently, physical-layer security (PLS) has gained much attention as an efficient method of obtaining secure information without using cryptography.PLS protocols employ the physical characteristics of wireless channels such as channel state information (CSI) and the distances between connection links in order to enhance secrecy performances in wireless systems.To enhance secrecy performances, the dual-hop/multi-hop relay protocols can be used efficiently.However, according to my best knowledge, there are few published works considering the impact of hardware impairments on secrecy performances for the wireless relaying networks.Hence, this dissertation examines the diversity-based relay protocols for wireless communication networks in order to improve secrecy performances in presence of hardware imperfections.First, new dual-hop and multi-hop cooperative relaying schemes with various relay selection methods are proposed in order to obtain improved secrecy performance compared with the existing schemes.Second, new joint relay and jammer selection protocols are also considered in order to improve the channel capacity for data links while reducing the channel capacity of eavesdropping links.Finally, this dissertation proposes harvest-to-transmit and harvest-to-jam protocols, where the relay and jammer nodes harvest energy from the radio frequency signal for use in the data transmission and the jamming process, respectively.To evaluate the performance of the proposed protocols over fading channels, this dissertation focuses on deriving the closed-form expressions for secrecy performances.Then, the derived expressions are verified by Monte Carlo simulations, which also present the advantages of the proposed protocols.
Security is one of the most important issues in wireless communication.Because using wireless channels entails broadcasting data transmissions, malicious nodes can eavesdrop on wireless transmission, which leads to insecure data transmission.Traditional security methods are based on cryptography.Recently, physical-layer security (PLS) has gained much attention as an efficient method of obtaining secure information without using cryptography.PLS protocols employ the physical characteristics of wireless channels such as channel state information (CSI) and the distances between connection links in order to enhance secrecy performances in wireless systems.To enhance secrecy performances, the dual-hop/multi-hop relay protocols can be used efficiently.However, according to my best knowledge, there are few published works considering the impact of hardware impairments on secrecy performances for the wireless relaying networks.Hence, this dissertation examines the diversity-based relay protocols for wireless communication networks in order to improve secrecy performances in presence of hardware imperfections.First, new dual-hop and multi-hop cooperative relaying schemes with various relay selection methods are proposed in order to obtain improved secrecy performance compared with the existing schemes.Second, new joint relay and jammer selection protocols are also considered in order to improve the channel capacity for data links while reducing the channel capacity of eavesdropping links.Finally, this dissertation proposes harvest-to-transmit and harvest-to-jam protocols, where the relay and jammer nodes harvest energy from the radio frequency signal for use in the data transmission and the jamming process, respectively.To evaluate the performance of the proposed protocols over fading channels, this dissertation focuses on deriving the closed-form expressions for secrecy performances.Then, the derived expressions are verified by Monte Carlo simulations, which also present the advantages of the proposed protocols.
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
The open broadcast nature of the wireless networks makes it vulnerable to the eavesdropper attacks. The physical layer security (PLS) is a grown research field that exploits the characteristics of the physical wireless channel to prevent the eavesdropper interruptions and attacks. The use of the cooperative relay transmission is a method to approach the physical layer security. This paper studies the secrecy performance metrics of the half-duplex two-way multi-relaying technique. A cooperative jamming protocol is proposed to enhance the physical layer security, where the destination sends jamming signals to the eavesdropper node to degrade its channel capacity. However, the relays can act as a jammer and send jamming signals to the eavesdropper node. Moreover, Simulations of the performance metrics namely the secrecy capacity (Cs) and the secrecy outage probability (SOP) applied on the technique are presented and discussed to measure the secrecy level of the proposed technique compared with an existing technique namely half-duplex one-way multi-relaying technique. Based on simulations and discussions the proposed technique outperforms the existing technique in term of secrecy performance.
Physical Layer Security (PLS) provides a promising complementary approach to cryptographic methods for protecting wireless communications from eavesdropping. This paper investigates secrecy enhancement through cooperative relaying and jamming in a wireless network with two legitimate users, one eavesdropper, and five potential relay nodes operating in amplify-and-forward mode. Three relay–jammer selection strategies are considered: random relay and jammer selection, best-SNR (signal-to-noise ratio) relay with random jammer selection, and best-SNR relay with a jammer chosen based on the strongest channel toward the eavesdropper. The secrecy performance of these strategies are evaluated in terms of secrecy rate and secrecy outage probability. The results demonstrate that intelligent relay and jammer selection can significantly improve PLS compared to random selection, highlighting the benefits of cooperation in secure wireless systems.
Wireless networks have experienced rapid evolutions toward sustainability, scalability and interoperability. The digital economy is driven by future networked societies to a more holistic community of intelligent infrastructures and connected services for a more sustainable and smarter society. Furthermore, an enormous amount of sensitive and confidential information, e.g., medical records, electronic media, financial data, and customer files, is transmitted via wireless channels. The implementation of higher layer key distribution and management was challenged by the emergence of these new advanced systems. In order to resist various malicious abuses and security attacks, physical layer security (PLS) has become an appealing alternative. The basic concept behind PLS is to exploit the characteristics of wireless channels for the confidentiality. Its target is to blind the eavesdroppers such that they cannot extract any confidential information from the received signals. This thesis presents solutions and analyses to improve the PLS in wireless networks. \n \nIn the second chapter, we investigate the secrecy capacity performance of an amplify-andforward (AF) dual-hop network for both distributed beamforming (DBF) and opportunistic relaying (OR) techniques. We derive the capacity scaling for two large sets; trustworthy relays and untrustworthy aggressive relays cooperating together with a wire-tapper aiming to intercept the message. We show that the capacity scaling in the DBF is lower bounded by a value which depends on the ratio between the number of the trustworthy and the untrustworthy aggressive relays, whereas the capacity scaling of OR is upper bounded by a value depending on the number of relays as well as the signal to noise ratio (SNR). \n \nIn the third chapter, we propose a new location-based multicasting technique, for dual phase AF large networks, aiming to improve the security in the presence of non-colluding passive eavesdroppers. We analytically demonstrate that the proposed technique increases the security by decreasing the probability of re-choosing a sector that has eavesdroppers, for each transmission time. Moreover, we also show that the secrecy capacity scaling of our technique is the same as for broadcasting. Hereafter, the lower and upper bounds of the secrecy outage probability are calculated, and it is shown that the security performance is remarkably enhanced, compared to the conventional multicasting technique. \n \nIn the fourth chapter, we propose a new cooperative protocol, for dual phase amplify-andforward large wireless sensor networks, aiming to improve the transmission security while taking into account the limited capabilities of the sensor nodes. In such a network, a portion of the K relays can be potential passive eavesdroppers. To reduce the impact of these untrustworthy relays on the network security, we propose a new transmission protocol, where the source agrees to share with the destination a given channel state information (CSI) of source-trusted relay-destination link to encode the message. Then, the source will use this CSI again to map the right message to a certain sector while transmitting fake messages to the other sectors. Adopting such a security protocol is promising because of the availability of a high number of cheap electronic sensors with limited computational capabilities. For the proposed scheme, we derived the secrecy outage probability (SOP) and demonstrated that the probability of receiving the right encoded information by an untrustworthy relay is inversely proportional to the number of sectors. We also show that the aggressive behavior of cooperating untrusted relays is not effective compared to the case where each untrusted relay is trying to intercept the transmitted message individually. \n \nFifth and last, we investigate the physical layer security performance over Rayleigh fading channels in the presence of impulsive noise, as encountered, for instance, in smart grid environments. For this scheme, secrecy performance metrics were considered with and without destination assisted jamming at the eavesdropper’s side. From the obtained results, it is verified that the SOP, without destination assisted jamming, is flooring at high signal-to-noise-ratio values and that it can be significantly improved with the use of jamming.
In the past few years, there has been a rapid increase in demand on transmitting data over the open wireless medium. Severe issues of information security and privacy thus arise, as the wireless transmissions may be intercepted by any unintended receiver in the network. There is a need to design the transmission system in an efficacious way, such that the available space and time can be used adequately. This becomes a challenging job as there are random variations in the channel quality due to deleterious effects of the multi-path fading. Hence, to improve the transmission reliability, coverage probability and throughput, relay aided communication draw in great attention from both academia and industry. Relays provide the advantage of cooperative diversity, where the directly transmitted signal and the relayed signal can be combined at the receiver using various diversity combining schemes to mitigate the fading effect. In this paper, we review the secrecy performance of dual-hop cooperative relay system with diversity reception and relay selection. The significance and secrecy effect of channel knowledge at the transmitter is also studied. We have corroborated that CSI knowledge at the transmitter can improve secrecy.
Cooperative communications is obviously an evolution in wireless networks due to its noticeable advantages such as increasing the coverage as well as combating fading and shadowing effects. However, the broadcast characteristic of a wireless medium which is exploited in cooperative communications leads to a variety of security vulnerabilities. As cooperative communication networks are globally expanded, they expose to security attacks and threats more than ever. Primarily, researchers have focused on upper layers of network architectures to meet the requirements for secure cooperative transmission while the upper-layer security solutions are incapable of combating a number of security threats, e.g., jamming attacks. To address this issue, physical-layer security has been recommended as a complementary solution in the literature. In this thesis, physical layer attacks of the cooperative communication systems are studied, and corresponding security techniques including cooperative jamming, beamforming and diversity approaches are investigated. In addition, a novel security solution for a two-hop decode-and-forward relaying system is presented where the transmitters insert a random phase shift to the modulated data of each hop. The random phase shift is created based on a shared secret among communicating entities. Thus, the injected phase shift confuses the eavesdropper and secrecy capacity improves. Furthermore, a cooperative jamming strategy for multi-hop decode-and-forward relaying systems is presented where multiple non-colluding illegitimate nodes can overhear the communication. The jamming signal is created by the transmitter of each hop while being sent with the primary signal. The jamming signal is known at the intended receiver as it is according to a secret common knowledge between the communicating entities. Hence, artificial noise misleads the eavesdroppers, and decreases their signal-to-noise-ratio. As a result, secrecy capacity of the system is improved. Finally, power allocation among friendly jamming and main signal is proposed to ensure that suggested scheme enhances secrecy.