Dissertations / Theses on the topic 'Physical-layer security'

Thesis (Ph.D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008.
Committee Chair: McLaughlin, Steven; Committee Member: Barros, Joao; Committee Member: Bellissard, Jean; Committee Member: Fekri, Faramarz; Committee Member: Lanterman, Aaron

Massive multiple-input multiple-output (MIMO) is one of the key technologies for the emerging fifth generation (5G) wireless networks, and has the potential to tremendously improve spectral and energy efficiency with low-cost implementations. While massive MIMO systems have drawn great attention from both academia and industry, few efforts have been made on how the richness of the spatial dimensions offered by massive MIMO affects wireless security. As security is crucial in all wireless systems due to the broadcast nature of the wireless medium, in this thesis, we study how massive MIMO technology can be used to guarantee communication security in the presence of a passive multi-antenna eavesdropper. Our proposed massive MIMO system model incorporates relevant design choices and constraints such as time-division duplex (TDD), uplink training, pilot contamination, low-complexity signal processing, and low-cost hardware components. The thesis consists of three main parts. We first consider physical layer security for a massive MIMO system employing simple artificial noise (AN)-aided matched-filter (MF) precoding at the base station (BS). For both cases of perfect training and pilot contamination, we derive a tight analytical lower bound for the achievable ergodic secrecy rate, and an upper bound for the secrecy outage probability. Both bounds are expressed in closed form, providing an explicit relationship between all system parameters, offering significant insights for system design. We then generalize the work by comparing different types of linear data and AN precoders in a secure massive MIMO network. The system performance, in terms of the achievable ergodic secrecy rate is obtained in closed form. In addition, we propose a novel low-complexity data and AN precoding strategy based on a matrix polynomial expansion. Finally, we consider a more realistic system model by taking into account non-ideal hardware components. Based on a general hardware impairment model, we derive a lower bound for the ergodic secrecy rate achieved by each user when AN-aided MF precoding is employed at the BS. By exploiting the derived analytical bound, we investigate the impact of various system parameters on the secrecy rate and optimize both the uplink training pilots and AN precoder to maximize the secrecy rate.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

Due to the open nature of the medium, wireless communications systems are highly vulnerable to security attacks. In recent years, security within the physical layer has gained attention, since the underlying techniques can add to those found in traditional cryptographic approaches. In this thesis, new mathematical models were developed for the analysis of secrecy capacity and outage probability in different scenarios, following which new optimisation problems were formulated and new algorithms devised to solve those problems. The process began by analysing secrecy performance for various cooperative communication scenarios in the presence of single and multiple eavesdropper(s). A multicast cooperative system was also analysed, based on distributed Alamouti space-time coding. Furthermore, the secrecy performance of a relay selection scheme was analysed in an independent but non-identically distributed (i.n.i.d) Rayleigh fading scenario. The second part of the thesis concentrated on optimisation methods for cooperative relaying and jamming techniques. For a dual-hop system, a joint cooperative beamforming and jamming scheme was created, considering both a perfect and an imperfect eavesdropper's channel state information (CSI). Optimal solutions to degrade the eavesdropper's interception by minimising its received signal to interference and noise ratio (SINR) were also presented whilst ensuring the legitimate receiver's SINR requirement. For the multi-hop scenario, the secrecy rate, with and without transmitting artificial noise, was considered for maximisation, and an optimal power splitting solution under limited power constraints at the transmitters was also proposed. In addition, an iterative solution for the joint optimisation of transmit power and power splitting coefficient at each transmitter was posited. The analyses and optimisation algorithms developed provide new insights into secrecy performance and optimal transmission schemes in various practical scenarios.

Over the last several decades, reliable communication has received considerable attention in the area of dynamic network con gurations and distributed processing techniques. Traditional secure communications mainly considered transmission cryptography, which has been developed in the network layer. However, the nature of wireless transmission introduces various challenges of key distribution and management in establishing secure communication links. Physical layer security has been recently recognized as a promising new design paradigm to provide security in wireless networks in addition to existing conventional cryptographic methods, where the physical layer dynamics of fading channels are exploited to establish secure wireless links. On the other hand, with the ever-increasing demand of wireless access users, multi-antenna transmission has been considered as one of e ective approaches to improve the capacity of wireless networks. Multi-antenna transmission applied in physical layer security has extracted more and more attentions by exploiting additional degrees of freedom and diversity gains. In this thesis, di erent multi-antenna transmit optimization techniques are developed for physical layer secure transmission. The secrecy rate optimization problems (i.e., power minimization and secrecy rate maximization) are formulated to guarantee the optimal power allocation. First, transmit optimization for multiple-input single-output (MISO) secrecy channels are developed to design secure transmit beamformer that minimize the transmit power to achieve a target secrecy rate. Besides, the associated robust scheme with the secrecy rate outage probability constraint are presented with statistical channel uncertainty, where the outage probability constraint requires that the achieved secrecy rate exceeds certain thresholds with a speci c probability. Second, multiantenna cooperative jammer (CJ) is presented to provide jamming services that introduces extra interference to assist a multiple-input multipleoutput (MIMO) secure transmission. Transmit optimization for this CJaided MIMO secrecy channel is designed to achieve an optimal power allocation. Moreover, secure transmission is achieved when the CJ introduces charges for its jamming service based on the amount of the interference caused to the eavesdropper, where the Stackelberg game is proposed to handle, and the Stackelberg equilibrium is analytically derived. Finally, transmit optimization for MISO secure simultaneous wireless information and power transfer (SWIPT) is investigated, where secure transmit beamformer is designed with/without the help of arti - cial noise (AN) to maximize the achieved secrecy rate such that satisfy the transmit power budget and the energy harvesting (EH) constraint. The performance of all proposed schemes are validated by MATLAB simulation results.

Visible-light communication (VLC) is an enabling technology that exploits the lighting infrastructure to provide ubiquitous indoor broadband coverage via high-speed short-range wireless communication links. On the other hand, physical-layer security has the potential to supplement conventional encryption methods with an additional secrecy measure that is provably unbreakable regardless of the computational power of the eavesdropper. The lack of wave-guiding transmission media in VLC channels makes the communication link inherently susceptible to eavesdropping by unauthorized users existing in areas illuminated by the data transmitters. In this thesis, we study transmission techniques that enhance the secrecy of VLC links within the framework of physical-layer security. Due to linearity limitations of typical light-emitting diodes (LEDs), the VLC channel is more accurately modelled with amplitude constraints on the channel input, rather than the conventional average power constraint. Such amplitude constraints render the prevalent Gaussian input distribution infeasible for VLC channels, making it difficult to obtain closed-form secrecy capacity expressions. Thus, we begin with deriving lower bounds on the secrecy capacity of the Gaussian wiretap channel subject to amplitude constraints. We then consider the design of optimal beamformers for secrecy rate maximization in the multiple-input single-output (MISO) wiretap channel under amplitude constraints. We show that the design problem is nonconvex and difficult to solve, however it can be recast as a solvable quasiconvex line search problem. We also consider the design of robust beamformers for worst-case secrecy rate maximization when channel uncertainty is taken into account. Finally, we study the design of linear precoders for the two-user MISO broadcast channel with confidential messages. We consider not only amplitude constraints, but also total and per-antenna average power constraints. We formulate the design problem as a nonconvex weighted secrecy sum rate maximization problem, and provide an efficient search algorithm to obtain a solution for such a nonconvex problem. We extend our approach to handle uncertainty in channel information. The design techniques developed throughout the thesis provide valuable tools for tackling real-world problems in which channel uncertainty is almost always inevitable and amplitude constraints are often necessary to accurately model hardware limitations.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

A cognitive radio is an intelligent wireless communication system that improves spectrum utilisation by allowing secondary users to use the idle radio spectrum from primary licensed networks or to share the spectrum with primary users. Due to several significant challenges for cryptographic approaches of upper layers in protocol stacks | for example, private key management complexity and key transmission security issues | physical layer (PHY) security has drawn significant attention as an alternative for cryptographic approaches at the upper layers of the protocol stack. Security threats may arise from passive eavesdropping node(s), which try to intercept communications between authenticated nodes. Most recent studies consider information theoretic secrecy to be a promising approach. The idea of information theoretic secrecy lies in exploiting the randomness of communication channels to ensure the secrecy of the transmitted messages. Due to the constraints imposed on cognitive radio networks by secondary networks, allocating their resources in an optimal way is a key to maximising their achievable secrecy rates. Therefore, in this thesis, optimal resource allocation and secrecy rate maximisation of cognitive radio networks (CRNs) are proposed. Cooperative jamming is proposed to enhance the primary secrecy rate, and a new chaos-based cost function is introduced in order to design a power control algorithm and analyse the dynamic spectrum-sharing issue in the uplink of cellular CRNs. For secondary users as the game players in underlay scenarios, utility/cost functions are defined, taking into account the interference from and interference tolerance of the primary users. The existence of the Nash equilibrium is proved in this power control game, which leads to significantly lower power consumption and a relatively fast convergence rate when compared to existing game algorithms. The simulation results indicate that the primary secrecy rate is significantly improved by cooperative jamming, and the proposed power control algorithm achieves low power consumption. In addition, an integrated scheme with chaotic scrambling (CS), chaotic artificial noise, and a chaotic shift keying (CSK) scheme are proposed in an orthogonal frequency division multiplexing (OFDM)-based CR system to enhance its physical layer security. By employing the chaos-based third-order Chebyshev map to achieve the optimum bit error rate (BER) performance of CSK modulation, the proposed three-layer integrated scheme outperforms the traditional OFDM system in an overlay scenario with a Rayleigh fading channel. Importantly, under three layers of encryption that are based on chaotic scrambling, chaotic artificial noise, and CSK modulation, a large key size can be generated to resist brute-force attacks and eavesdropping, leading to a significantly improved security rate. Furthermore, a game theory-based cooperation scheme is investigated to enhance physical layer (PHY) security in both the primary and secondary transmissions of a cognitive radio network (CRN). In CRNs, the primary network may decide to lease its own spectrum for a fraction of time to the secondary nodes in exchange for appropriate remuneration. The secondary transmitter (ST) is considered to be a trusted relay for primary transmission in the presence of the ED. The ST forwards a message from the primary transmitter (PT) in a decode-and-forward (DF) fashion and, at the same time, allows part of its available power to be used to transmit an artificial noise (i.e., jamming signal) to enhance secrecy rates. In order to allocate power between the message and jamming signals, the optimisation problem is formulated and solved for maximising the primary secrecy rate (PSR) and secondary secrecy rate (SSR) with malicious attempts from a single eavesdropper or multiple eavesdroppers. Cooperation between the primary and secondary transmitters is also analysed from a game-theoretic perspective, and their interaction modelled as a Stackelberg game. This study proves theoretically and computes the Stackelberg equilibrium. Numerical examples are provided to illustrate the impact of the Stackelberg game-based optimisation on the achievable PSR and SSR. The numerical results indicate that spectrum leasing, based on trading secondary access for cooperation by means of relay and a jammer, is a promising framework for enhancing primary and secondary secrecy rates in cognitive radio networks when the ED can intercept both the primary and secondary transmission. Finally, this thesis focuses on physical-layer security in cognitive radio networks where multiple secondary nodes assist multiple primary nodes in combating unwanted eavesdropping from malicious eavesdroppers. Two scenarios are considered: a single eavesdropper (scenario I) and multiple eavesdroppers (scenario II). The secondary users act as a relay and jammer in scenario I, whereas they act only as a jammer in scenario II. Furthermore, the multiple eavesdroppers are distributed according to a homogenous Poison Point Process (PPP) in scenario II. Closed forms are derived for the outage probability and mean secrecy rate for both the primary and secondary transmissions. Furthermore, the scalability and convergence of the matching theory are proved. Both the analytical and numerical results show that the proposed matching model is a promising approach for exploiting the utility functions of both primary and secondary users.

The fast development of wireless networks, which are intrinsically exposed to eavesdropping, has created a growing concern for confidentiality. While classical cryptographic schemes require a key provided by the end-user, physical-layer security leverages the randomness of the physical communication medium as a source of secrecy. The main benefit of physical-layer security techniques is their relatively low cost and their ability to combine with any existing security mechanisms. This dissertation provides an analysis including the theoretical study of the two-way wiretap channel to obtain a better insight into how to design coding mechanisms, practical tests with experimental systems, and the design of actual codes. From a theoretical standpoint, the study confirms the benefits of combining several multi-user coding techniques including cooperative jamming, coded cooperative jamming and secret key generation. For these different mechanisms, the trade-off between reliability, secrecy and communication rate is clarified under a stringent strong secrecy metric. Regarding the design of practical codes, spatially coupled LDPC codes, which were originally designed for reliability, are modified to develop a coded cooperative jamming code. Finally, a proof-of-principle practical wireless system is provided to show how to implement a secret key generation system on experimental programmable radios. This testbed is then used to assess the realistic performance of such systems in terms of reliability, secrecy and rate.

Smart antenna technique has emerged as one of the leading technologies for enhancing the quality of service in wireless networks. Because of its ability to concentrate transmit power in desired directions, it has been widely adopted by academia and industry to achieve better coverage, improved capacity and spectrum efficiency of wireless communication systems. In spite of its popularity in applications of performance enhancement, the smart antenna\'s capability of improving wireless network security is relatively less explored. This dissertation focuses on exploiting the smart antenna technology to develop physical layer solutions to anti-eavesdropping and location security problems.

We first investigate the problem of enhancing wireless communication privacy. A novel scheme named "artificial fading" is proposed, which leverages the beam switching capability of smart antennas to prevent eavesdropping attacks. We introduce the optimization strategy to design a pair of switched beam patterns that both have high directional gain to the intended receiver. Meanwhile, in all the other directions, the overlap between these two patterns is minimized. The transmitter switches between the two patterns at a high frequency. In this way, the signal to unintended directions experiences severe fading and the eavesdropper cannot decode it. We use simulation experiments to show that the artificial fading outperforms single pattern beamforming in reducing the unnecessary coverage area of the wireless transmitter.

We then study the impact of beamforming technique on wireless localization systems from the perspectives of both location privacy protection and location spoofing attack.

For the location privacy preservation scheme, we assume that the adversary uses received signal strength (RSS) based localization systems to localize network users in Wireless LAN (WLAN). The purpose of the scheme is to make the adversary unable to uniquely localize the user when possible, and otherwise, maximize error of the adversary\'s localization results. To this end, we design a two-step scheme to optimize the beamforming pattern of the wireless user\'s smart antenna. First, the user moves around to estimate the locations of surrounding access points (APs). Then based on the locations of the APs, pattern synthesis is optimized to minimize the number of APs in the coverage area and degenerate the localization precision. Simulation results show that our scheme can significantly lower the chance of being localized by adversaries and also degrade the location estimation precision to as low as the coverage range of the AP that the wireless user is connected to.

As personal privacy preservation and security assurance at the system level are always conflictive to some extent, the capability of smart antenna to intentionally bias the RSS measurements of the localization system also potentially enables location spoofing attacks. From this aspect, we present theoretical analysis on the feasibility of beamforming-based perfect location spoofing (PLS) attacks, where the attacker spoofs to a target fake location by carefully choosing the beamforming pattern to fool the location system. The PLS problem is formulated as a nonlinear feasibility problem, and due to its intractable nature, we solve it using semidefinite relaxation (SDR) in conjunction with a heuristic local search algorithm. Simulation results show the effectiveness of our analytical approach and indicate the correlation between the geometry of anchor deployment and the feasibility of PLS attacks. Based on the simulation results, guidelines for guard against PLS attacks are provided.
Ph. D.

This paper is persuaded to understand the physical layer security in wireless commu-nications utilizing NOMA (Non Orthogonal Multiple Access) concepts in the presence of an eavesdropper. Physical layer security maintains the confidentiality and secrecyof the system against eavesdroppers. We use the power domain in this paper, where NOMA allows many users to share resources side by side. Power allocation concern-ing channel condition is taken into consideration where user whose channel condition is weak is allocated with eminent power to directly decode the signal, whereas theuser with better channel condition applies successive interference cancellation (SIC)to decode the signal. Here, the base station communicates with the users and sends data signals while the eavesdropper secretly eavesdrops on the confidential informa-tion simultaneously. In this thesis, to improve the physical layer security, jamming method was usedwhere users are assumed to be in full duplex, send jamming signals to degrade the performance of the eavesdropper. Analytic expressions of CDF, PDF, outage proba-bility and secrecy capacity are obtained from analyzing the NOMA jamming scheme. The numerical results are evaluated with the simulations results and analysed theeffect of jamming on improving the performance of the NOMA system in presenceof an eavesdropper.

Wireless communications offer data transmission services anywhere and anytime, but with the inevitable cost of introducing major security vulnerabilities. Indeed, an eavesdropper can overhear a message conveyed over the open insecure wireless media putting at risk the confidentiality of the wireless users. Currently, the way to partially prevent eavesdropping attacks is by ciphering the information between the authorised parties through complex cryptographic algorithms. Cryptography operates in the upper layers of the communication model, bit it does not address the security problem where the attack is suffered: at the transmission level. In this context, physical layer security has emerged as a promising framework to prevent eavesdropping attacks at the transmission level. Physical layer security is based on information-theoretic concepts and exploits the randomness and the uniqueness of the wireless channel. In this context, this thesis presents signal processing techniques to secure wireless networks at the physical layer by optimising the use of multiple-antennas. A masked transmission strategy is used to steer the confidential information towards the intended receiver, and, at the same time, broadcast an interfering signal to confuse unknown eavesdroppers. This thesis considers practical issues in multiple-antenna networks such as limited transmission resources and the lack of accurate information between the authorised transmission parties. The worst-case for the security, that occurs when a powerful eavesdropper takes advantage of any opportunity to put at risk the transmission confidentiality, is addressed. The techniques introduced improve the security by offering efficient and innovative transmission solutions to lock the communication at the physical layer. Notably, these transmission mechanisms strike a balance between confidentiality and quality to satisfy the practical requirements of modern wireless networks.

Abstract. We examine the physical layer security for machine type communication networks and highlight a secure communication scenario that consists of a transmitter Alice, which employs Transmit Antenna Selection, while a legitimate receiver Bob that uses Maximum Ratio Combining, as well as an eavesdropper Eve. We provide a solution to avoid eavesdropping and provide ways to quantify security and reliability. We obtain closed-form expressions for Multiple-Input Multiple-Output and Multi-antenna Eavesdropper (MIMOME) scenario. The closed{-}form expressions for three useful variations of MIMOME scenario, i.e., MISOME, MIMOSE, and MISOSE are also provided. A low cost and less complex system for utilizing the spatial diversity in multiple antennas system, while guaranteeing secrecy and reliability. Similarly, it is also assumed that Alice, Bob, and Eve can estimate their channel state information, and then we evaluate the performance of closed-form expressions in terms of secrecy outage probability and provide Monte Carlo simulations to corroborate the proposed analytical framework.

It has been well established that multiple-input multiple-output (MIMO) transmission using multiple conductors can improve the data rate of power line communication (PLC) systems. In this thesis, we investigate whether the presence of multiple conductors could also facilitate the communication of confidential messages by means of physical layer security methods. In particular, this thesis focuses on the secrecy capacity of MIMO PLC. Numerical experiments show that multi-conductor PLC networks can enable a more secure communication compared to the single conductor case. On the other hand, we demonstrate that the keyhole property of PLC channels generally diminishes the secure communication capability compared to what would be achieved in a similar wireless communications setting. Furthermore, we consider the cases of unknown and partially known channel state information (CSI) about the eavesdropper channel. For this purpose, we provide deterministic channel uncertainty model parameters for PLC networks via the bottom-up channel modelling method. Numerical results show how imperfect CSI has a negative impact on secure communication, and enable us to analyze the tradeoff between choosing different transmission strategies that correspond to unknown CSI and partially known CSI.
Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate

Security and privacy have become increasingly significant concerns in wireless communication networks, due to the open nature of the wireless medium which makes the wireless transmission vulnerable to eavesdropping and inimical attacking. The emergence and development of decentralized and ad-hoc wireless networks pose great challenges to the implementation of higher-layer key distribution and management in practice. Against this background, physical layer security has emerged as an attractive approach for performing secure transmission in a low complexity manner. This thesis concentrates on physical layer security design and enhancement in wireless networks. First, this thesis presents a new unifying framework to analyze the average secrecy capacity and secrecy outage probability. Besides the exact average secrecy capacity and secrecy outage probability, a new approach for analyzing the asymptotic behavior is proposed to compute key performance parameters such as high signal-to-noise ratio slope, power offset, secrecy diversity order, and secrecy array gain. Typical fading environments such as two-wave with diffuse power and Nakagami-m are taken into account. Second, an analytical framework of using antenna selection schemes to achieve secrecy is provided. In particular, transmit antenna selection and generalized selection combining are considered including its special cases of selection combining and maximal-ratio combining. Third, the fundamental questions surrounding the joint impact of power constraints on the cognitive wiretap channel are addressed. Important design insights are revealed regarding the interplay between two power constraints, namely the maximum transmit at the secondary network and the peak interference power at the primary network. Fourth, secure single carrier transmission is considered in the two-hop decode-andi forward relay networks. A two-stage relay and destination selection is proposed to minimize the eavesdropping and maximize the signal power of the link between the relay and the destination. In two-hop amplify-and-forward untrusted relay networks, secrecy may not be guaranteed even in the absence of external eavesdroppers. As such, cooperative jamming with optimal power allocation is proposed to achieve non-zero secrecy rate. Fifth and last, physical layer security in large-scale wireless sensor networks is introduced. A stochastic geometry approach is adopted to model the positions of sensors, access points, sinks, and eavesdroppers. Two scenarios are considered: i) the active sensors transmit their sensing data to the access points, and ii) the active access points forward the data to the sinks. Important insights are concluded.

Secure communications between legitimate users have received considerable attention recently. Transmission cryptography, which introduces secrecy on the network layer, is heavily relied on conventionally to secure communications. However, it is theoretically possible to break the encryption if unlimited computational resource is provided. As a result, physical layer security becomes a hot topic as it provides perfect secrecy from an information theory perspective. The study of physical layer security on real communication system model is challenging and important, as the previous researches are mainly focusing on the Gaussian input model which is not practically implementable. In this thesis, the physical layer security of wireless networks employing finite-alphabet input schemes are studied. In particular, firstly, the secrecy capacity of the single-input single-output (SISO) wiretap channel model with coded modulation (CM) and bit-interleaved coded modulation (BICM) is derived in closed-form, while a fast, sub-optimal power control policy (PCP) is presented to maximize the secrecy capacity performance. Since finite-alphabet input schemes achieve maximum secrecy capacity at medium SNR range, the maximum amount of energy that the destination can harvest from the transmission while satisfying the secrecy rate constraint is computed. Secondly, the effects of mapping techniques on secrecy capacity of BICM scheme are investigated, the secrecy capacity performances of various known mappings are compared on 8PSK, 16QAM and (1,5,10) constellations, showing that Gray mapping obtains lowest secrecy capacity value at high SNRs. We propose a new mapping algorithm, called maximum error event (MEE), to optimize the secrecy capacity over a wide range of SNRs. At low SNR, MEE mapping achieves a lower secrecy rate than other well-known mappings, but at medium-to-high SNRs MEE mapping achieves a significantly higher secrecy rate over a wide range of SNRs. Finally, the secrecy capacity and power allocation algorithm (PA) of finite-alphabet input wiretap channels with decode-and-forward (DF) relays are proposed, the simulation results are compared with the equal power allocation algorithm.

Wireless communication technology has evolved rapidly during the last 20 years. Nowadays, there are huge networks providing communication infrastructures to not only people but also to machines, such as unmanned air and ground vehicles, cars, household appliances and so on. There is no doubt that new wireless communication technologies must be developed, that support the data traffic in these emerging, large networks. While developing these technologies, it is also important to investigate the vulnerability of these technologies to different malicious attacks. In particular, spoofing and jamming attacks should be investigated and new countermeasure techniques should be developed. In this context, spoofing refers to the situation in which a receiver identifies falsified signals, that are transmitted by the spoofers, as legitimate or trustable signals. Jamming, on the other hand, refers to the transmission of radio signals that disrupt communications by decreasing the signal-to-interference-and-noise ratio (SINR) on the receiver side.  In this thesis, we analyze the effects of spoofing and jamming both on global navigation satellite system (GNSS) and on massive multiple-input multiple-output (MIMO) communications. GNSS is everywhere and used to provide location information. Massive MIMO is one of the cornerstone technologies in 5G. We also propose countermeasure techniques to the studied spoofing and jamming attacks.  More specifically, in paper A we analyze the effects of distributed jammers on massive MIMO and answer the following questions: Is massive MIMO more robust to distributed jammers compared with previous generation’s cellular networks? Which jamming attack strategies are the best from the jammer’s perspective, and can the jamming power be spread over space to achieve more harmful attacks? In paper B, we propose a detector for GNSS receivers that is able to detect multiple spoofers without having any prior information about the attack strategy or the number of spoofers in the environment.

The major challenges facing resource constraint wireless devices are error resilience, security and speed. Three joint schemes are presented in this research which could be broadly divided into error correction based and cipher based. The error correction based ciphers take advantage of the properties of LDPC codes and Nordstrom Robinson code. A cipher-based cryptosystem is also presented in this research. The complexity of this scheme is reduced compared to conventional schemes. The securities of the ciphers are analyzed against known-plaintext and chosen-plaintext attacks and are found to be secure. Randomization test was also conducted on these schemes and the results are presented. For the proof of concept, the schemes were implemented in software and hardware and these shows a reduction in hardware usage compared to conventional schemes. As a result, joint schemes for error correction and security provide security to the physical layer of wireless communication systems, a layer in the protocol stack where currently little or no security is implemented. In this physical layer security approach, the properties of powerful error correcting codes are exploited to deliver reliability to the intended parties, high security against eavesdroppers and efficiency in communication system. The notion of a highly secure and reliable physical layer has the potential to significantly change how communication system designers and users think of the physical layer since the error control codes employed in this work will have the dual roles of both reliability and security.

In modern society, the communications sector is a critical enabler of economic and social activity. Despite the benefit of the improved ubiquitousness, the rapid diffusion of communications technologies is driving one to question the security of communications networks and systems. It is this situation which has motivated the research and development work to be reported in this thesis. Especially, this thesis consists of two parts: The first part focuses on our main research under the umbrella of physical-layer security (PLS), and the second part presents our work on security and data management in Smart Grid communication networks (SGCNs). Security technologies embedded at the physical layer of the communication systems can provide additional countermeasure against the inherent interception threat associated with a wireless transmission medium. Unlike traditional cryptographic solutions, which usually handle security at the network and application layer, the PLS techniques exploit the randomness that is intrinsic to the physical communication channel. Specifically, the first part of this thesis addresses the problem of defeating passive eavesdroppers in wireless-powered communication networks (WPCNs). The primary concern is to develop and analyze secure transmission protocols based on PLS and radio frequency energy harvesting techniques in WPCNs. This thesis starts with investigating the problem of secure transmission between a wireless-powered transmitter and a receiver in the presence of multiple eavesdroppers. To counteract eavesdropping, a transmission protocol named accumulate-then-transmit (ATT) is proposed. Specifically, the proposed protocol employs a multi-antenna power beacon (PB) to assist the transmitter with secure transmission. First, the PB transfers wireless power to charge the transmitter's battery. After accumulating enough energy, the transmitter sends confidential information to the receiver, and simultaneously, the PB emits jamming signals (i.e., artificial noise) to interfere with the eavesdroppers. A key element of the protocol is the perfect channel state information (CSI), with which the jamming signals can be deliberately designed to avoid disturbing the intended receiver. Based on the assumption that the eavesdroppers do not collude, the secrecy performance of the proposed protocol is evaluated in terms of secrecy outage probability and secrecy throughput. This study reveals that cooperative jamming (CJ) is a critical enabler of physical-layer security in WPCNs. After investigating the use of a multi-antenna PB with perfect CSI, this thesis exploits the employment of a wireless-powered full-duplex (FD) jammer to enhance the secrecy in the presence of CSI errors. Noteworthy, due to imperfect CSI, the jamming signals transmitted by the jammer yield undesired interference at the receiver. This study analyzes the impact of channel estimation error on the secrecy performance. Besides, due to the FD capability, the jammer is able to perform simultaneous jamming and energy harvesting. It hence makes the energy storage of the jammer experience concurrent charging and discharging. A hybrid energy storage system with finite capacity is adopted, and its long-term stationary distribution of the energy state is characterized through a finite-state Markov Chain. The secrecy performance of the proposed accumulate-and-jam (AnJ) protocol is evaluated to reveal its merits. Moreover, an alternative energy storage model with infinite capacity and the use of a wireless-powered half-duplex (HD) jammer are also exploited to serve as benchmarks. In the second part of the thesis, security and data management issues are investigated in SGCNs. Due to the integrations of communications and information technologies with the power system, data security and management play a crucial role in the Smart Grid. First, the problem of the unauthorized real-time pricing (RTP) information redistribution between advanced metering infrastructure (AMI) participants and nonparticipants is addressed via an evolutionary game model. The objective is to find the optimal AMI subscription price associated with the maximal proportion of participating consumers. Second, a voluntary real-time incentive scheme is proposed to promote the participation of electricity consumers in reporting their power demand. Simulation results demonstrate that the proposed voluntary scheme can achieve satisfactory social welfare as compared with compulsory demand upload schemes. Finally, time-varying attacks in the SGCNs are studied, and a time-correlated attacker-defender model is developed and analyzed to ensure attack detection while maintaining low defense expense.

La sécurité de la couche physique (PLS) est un paradigme émergent qui se concentre sur l'utilisation des propriétés de la communication sans fil, telles que le bruit, l'évanouissement, la dispersion, l'interférence, la diversité, etc. pour assurer la sécurité entre les utilisateurs légitimes en présence d'un espion. Comme le PLS utilise des techniques de traitement du signal et de codage, il intervient au niveau de la couche physique et peut donc garantir le secret quelle que soit la puissance de calcul de l'espion. Cela en fait une approche intéressante pour compléter la cryptographie traditionnelle dont le principe de sécurité est basé sur la dureté informatique de l'algorithme de cryptage qui ne peut pas être facilement cassé par un espion. En outre, les récents progrès rapides des technologies de communication sans fil ont permis l'émergence et l'adoption de technologies telles que l'internet des objets, les communications ultra-fiables et à faible latence, les communications massives de type machine, les véhicules aériens sans pilote, etc. La plupart de ces technologies sont décentralisées, limitées en ressources de calcul et de puissance, et sensibles aux délais. La plupart de ces technologies sont décentralisées, limitées en ressources de calcul et de puissance, et sensibles aux délais. Cela fait du PLS une alternative très intéressante pour assurer la sécurité dans ces technologies. À cette fin, dans cette thèse, nous étudions les limites de la mise en œuvre pratique de la PLS et proposons des solutions pour relever ces défis. Tout d'abord, nous étudions le défi de l'efficacité énergétique de la PLS par l'injection de bruit artificiel (AN) dans un contexte massif d'entrées multiples et de sorties multiples (MIMO). La grande matrice de précodage dans le contexte MIMO massif contribue également à un signal d'émission avec un rapport élevé entre la puissance de crête et la puissance moyenne (PAPR). Cela nous a incités à proposer un nouvel algorithme, appelé PAPR-Aware-Secure-mMIMO. Dans ce schéma, les informations instantanées sur l'état du canal (CSI) sont utilisées pour concevoir un AN tenant compte du PAPR qui assure simultanément la sécurité tout en réduisant le PAPR. Ensuite, nous considérons le PLS par adaptation du canal. Ces schémas PLS dépendent de la précision de la CSI instantanée et sont inefficaces lorsque la CSI est imprécise. Toutefois, la CSI peut être inexacte dans la pratique en raison de facteurs tels qu'un retour d'information bruyant, une CSI périmée, etc. Pour résoudre ce problème, nous commençons par proposer un schéma PLS qui utilise le précodage et la diversité pour fournir le PLS. Nous proposons ensuite un réseau neuronal autoencodeur peu complexe pour débruiter la CSI imparfaite et obtenir des performances PLS optimales. Les modèles d'autoencodeur proposés sont appelés respectivement DenoiseSecNet et HybDenoiseSecNet. Enfin, nous étudions les performances de la PLS dans le cas d'une signalisation à alphabet fini. Les signaux gaussiens ont une grande complexité de détection parce qu'ils prennent un continuum de valeurs et ont des amplitudes non limitées. Dans la pratique, on utilise des entrées de canal discrètes parce qu'elles permettent de maintenir une puissance de transmission de crête et une complexité de réception modérées. Cependant, elles introduisent des contraintes qui affectent de manière significative la performance du PLS, d'où la contribution de cette thèse. Nous proposons d'utiliser des clés dynamiques pour partitionner les espaces de modulation de manière à ce qu'ils profitent à un récepteur légitime et non à un espion. Ces clés sont basées sur le canal principal indépendant et leur utilisation pour la partition conduit à des régions de décision plus grandes pour le récepteur prévu et plus petites pour l'espion. Ce système est appelé modulation partitionnée par index (IPM)
Physical layer security (PLS) is an emerging paradigm that focuses on using the properties of wireless communication, such as noise, fading, dispersion, interference, diversity, etc., to provide security between legitimate users in the presence of an eavesdropper. Since PLS uses signal processing and coding techniques, it takes place at the physical layer and hence can guarantee secrecy irrespective of the computational power of the eavesdropper. This makes it an interesting approach to complement legacy cryptography whose security premise is based on the computational hardness of the encryption algorithm that cannot be easily broken by an eavesdropper. The advancements in quantum computing has however shown that attackers have access to super computers and relying on only encryption will not be enough. In addition, the recent rapid advancement in wireless communication technologies has seen the emergence and adoption of technologies such as Internet of Things, Ultra-Reliable and Low Latency Communication, massive Machine-Type Communication, Unmanned Aerial Vehicles, etc. Most of these technologies are decentralized, limited in computational and power resources, and delay sensitive. This makes PLS a very interesting alternative to provide security in such technologies. To this end, in this thesis, we study the limitations to the practical implementation of PLS and propose solutions to address these challenges. First, we investigate the energy efficiency challenge of PLS by artificial noise (AN) injection in massive Multiple-Input Multiple-Output (MIMO) context. The large precoding matrix in massive MIMO also contributes to a transmit signal with high Peak-to-Average Power Ratio (PAPR). This motivated us to proposed a novel algorithm , referred to as PAPR-Aware-Secure-mMIMO. In this scheme, instantaneous Channel State Information (CSI) is used to design a PAPR-aware AN that simultaneously provides security while reducing the PAPR. This leads to energy efficient secure massive MIMO. The performance is measured in terms of secrecy capacity, Symbol Error Rate (SER), PAPR, and Secrecy Energy Efficiency (SEE). Next, we consider PLS by channel adaptation. These PLS schemes depend on the accuracy of the instantaneous CSI and are ineffective when the CSI is inaccurate. However, CSI could be inaccurate in practice due to such factors as noisy CSI feedback, outdated CSI, etc. To address this, we commence by proposing a PLS scheme that uses precoding and diversity to provide PLS. We then study the impact of imperfect CSI on the PLS performance and conclude with a proposal of a low-complexity autoencoder neural network to denoise the imperfect CSI and give optimal PLS performance. The proposed autoencoder models are referred to as DenoiseSecNet and HybDenoiseSecNet respectively. The performance is measured in terms of secrecy capacity and Bit Error Rate (BER). Finally, we study the performance of PLS under finite-alphabet signaling. Many works model performance assuming that the channel inputs are Gaussian distributed. However, Gaussian signals have high detection complexity because they take a continuum of values and have unbounded amplitudes. In practice, discrete channel inputs are used because they help to maintain moderate peak transmission power and receiver complexity. However, they introduce constraints that significantly affect PLS performance, hence, the related contribution in this thesis. We propose the use of dynamic keys to partition modulation spaces in such a way that it benefits a legitimate receiver and not the eavesdropper. This keys are based on the independent main channel and using them to partition leads to larger decision regions for the intended receiver but smaller ones for the Eavesdropper. The scheme is referred to as Index Partitioned Modulation (IPM). The performance is measured in terms of secrecy capacity, mutual information and BER

In the last decade, OFDM has been chosen as the physical layer solution for a large variety of wireless, high data rates communication standards. The reasons for this success are found in the possibility of coping with frequency selective channels with simple and efficiently implemented transceivers, and achieving high spectral efficiency. In order to push the performance of these systems close to their limit, emerging wireless networks need efficient methods for time and frequency synchronization, since an erroneous choice of the symbol timing and residual offsets in the clock and carrier frequencies at the transmitter and the receiver can highly impair transmissions due to the effects of ISI and ICI. In the first part of this thesis we focus on time and frequency synchronization algorithms for UWB MB OFDM systems. First, we consider the time synchronization problem and we analyze the case when the channel delay spread is larger than the cyclic prefix length determined by the standard, that is when ISI and ICI can not be completely avoided with a proper symbol timing. In this case, we identify as an appropriate target for synchronization the maximization of the ratio of the total useful received power over all subcarriers to the total power of ISI and ICI for a given channel realization. We also present a practical low-complexity synchronization scheme and show that its performance tops the results obtained by the best existing correlation-based timing estimators. Moreover, the very high transmission rate of MB-OFDM architectures asks for carrier and clock frequency offset estimators with moderate complexity and fast acquisition times. Then, we formulate algorithms that are based upon the received frequency domain symbols, where the effects of both offsets can be observed, and jointly estimate them with either a linear least squares or a maximum likelihood approach. The performance of the algorithms is assessed through simulation in a realistic UWB channel scenario and compared with previous literature results. In parallel with the request for large transmission rates, the use of sensitive data over wireless communications has raised an increasing demand for confidentiality (and security in general) of the transmitted information. Physical layer security is emerging as a valuable tool for future wireless networks to secure information directly at the physical layer by exploiting the channel diversity at the legitimate nodes and the attacker. In the second part of this thesis, we consider how information-theoretic security results can be adapted and applied to systems that adopt OFDM as the modulation format. The multicarrier architecture is modeled as a particular instance of a MIMO channel. In this way, we are able to evaluate the capabilities of the OFDM system in transmitting confidential messages by leveraging the results presented in the literature for the MIMO Gaussian wiretap channel. We evaluate the information-theoretic secrecy rates that are achievable by an OFDM transmitter/receiver pair in the presence of an eavesdropper that might either use an OFDM structure or choose a more complex receiver architecture. The physical layer features of the communication channels can be used not only to transmit secret messages but also to share secret keys. We consider the general case of secret-key agreement over MIMO channels (of which OFDM can be seen as a particular case) and we establish closed-form expressions for the secret-key capacity in the low-power and high-power regimes. In the low-power regime, we show that the optimal signaling strategy is independent of the eavesdropper's fading realization. By combining this signaling strategy with reconciliation and privacy amplification, one obtains a semi-blind key-distillation strategy in which the knowledge of the eavesdropper's fading is required for privacy amplification alone. A similar approach is used to study the jamming game in an OFDM environment. In this scenario, the attacker is active and its objective is to disrupt the communication between the legitimate users by transmitting a jamming signal. The setup is modeled as a zero-sum game in which the payoff function is represented by the mutual information between the transmitter and the receiver. Optimal signaling strategies are determined for both the transmitter and the jammer, and the Nash equilibrium of the game is found for both DMT and FMT architectures.
Nell'ultimo decennio, la tecnica nota come orthogonal frequency division multiplexing (OFDM) è stata scelta come soluzione di strato fisico per diversi sistemi di trasmissione wireless ad alto bit rate. Le ragioni di tale successo sono riscontrabili nella capacità di sfruttare canali selettivi in frequenza con dispositivi di semplice ed efficiente implementazione, nonché nella possibilità di ottenere un'elevata efficienza spettrale. Allo scopo di ottimizzare le prestazioni di questi sistemi, le reti wireless di nuova generazione necessitano di metodi efficaci per la sincronizzazione di tempo e di frequenza, dato che un'errata scelta del sincronismo di simbolo ed offset di frequenza residui di portante e campionamento possono disturbare fortemente le trasmissioni a causa dell'introduzione di interferenza di intersimbolo (ISI) ed interferenza di intercanale (ICI). Nella prima parte della tesi il lavoro si è concentrato sulla formulazione e l'analisi di algoritmi di sincronizzazione di tempo e frequenza per sistemi ultrawide band (UWB) multiband (MB) OFDM. Per prima cosa è stato considerato il problema della sincronizzazione di simbolo ed è stato analizzato il caso in cui la lunghezza del canale dispersivo è maggiore di quella del prefisso ciclico previsto dallo standard, ovvero quando sia ISI che ICI non possono essere completamente annullate scegliendo in maniera opportuna l'epoca di campionamento. In questo caso, un opportuno obiettivo per la sincronizzazione di simbolo è stato identificato nella massimizzazione del rapporto fra la potenza utile totale su tutte le sottoportanti e la potenza totale di ISI ed ICI, per una data realizzazione di canale. E' stato anche derivato uno schema di sincronizzazione pratico a bassa complessità computazionale, che offre prestazioni migliori di quelle fornite dagli stimatori a correlazione esistenti in letteratura. Inoltre, l'alto rate di trasmissione dei sistemi MB-OFDM richiede che la stima degli offset di campionamento e di portante venga effettuata con metodi a complessità moderata e con tempi di acquisizione ridotti. Pertanto, sono stati formulati algoritmi che operano sui simboli ricevuti nel dominio della frequenza, dai quali è possibile rilevare gli effetti di entrambi gli offset, che vengono così stimati congiuntamente mediante approcci ai minimi quadrati o a massima verosimiglianza. Le prestazioni di questi algoritmi sono state valutate mediante simulazioni in uno scenario UWB realistico e sono state confrontate con i risultati ottenuti dagli stimatori presentati precedentemente in letteratura. In parallelo con la richiesta di elevati rate di trasmissione, l'imponente traffico di dati sensibili attraverso comunicazioni wireless ha generato un bisogno crescente di segretezza (e sicurezza in generale) per l'informazione trasmessa su tali canali. La sicurezza a livello di strato fisico si sta rivelando un utile strumento per proteggere l'informazione trasmessa su reti wireless di nuova generazione, e sfrutta la diversità esistente fra le realizzazioni di canale dei terminali legittimi rispetto a quelle dell'attaccante. Quindi, nella seconda parte della tesi, si è valutato come i risultati ottenuti nell'ambito dell'information-theoretic security possano essere adattati ad un sistema che adotta OFDM come tipo di modulazione. L'architettura multiportante è stata quindi rappresentata come un caso particolare di un canale multiple-input multiple-output (MMO). In questo modo è stato possibile valutare le potenzialità del sistema OFDM nel trasmettere messaggi riservati applicando ed adattando a questo caso i risultati presentati in letteratura riguardo canali wiretap MIMO gaussiani. Si sono così caratterizzati i rate di segretezza raggiungibili da una coppia trasmettitore/ricevitore OFDM in presenza di un ascoltatore indesiderato, sia nel caso in cui esso adotti un ricevitore di tipo OFDM, sia nel caso in cui esso possa implementare architetture di ricezione più complesse e sofisticate. Le caratteristiche fisiche del canale di trsmissione possono essere sfruttate non solo per trasmettere messaggi in maniera che il loro contenuto sia segreto per eventuali attaccanti, ma anche per condividere in maniera sicura chiavi segrete, da utilizzare in sistemi di crittografia classica. In particolare, si è studiato il caso generale di condivisione di chiavi segrete mediante trasmissioni su canali MIMO (di cui OFDM può essere pensato come istanza particolare) e si sono ricavate in forma chiusa le espressioni per la capacità di chiavi segreta (ovvero il massimo rate con cui può essere scambiata una chiave segreta in presenza di un attaccante) nei regimi asintotici di basso ed alto SNR. Per bassi SNR, è stato dimostrato come la strategia ottima di trasmissione sia indipendente dalla realizzazione di canale dell'attaccante. Pertanto, combinando questa strategia di trasmissione con le fasi di information reconciliation e privacy amplification, è possibile ottenere uno schema di condivisione di chiavi di tipo semi-blind, per il quale la conoscenza del canale dell'attaccante è rischiesta solo nella fase finale di privacy amplification. Un approccio simile a quelli descritti finora è stato applicato per studiare il problema della robustezza dei sistemi OFDM ad attacchi di jamming. In questo scenario l'attaccante è attivo, ed il suo scopo è quello di disturbare la comunicazione fra i terminali legittimi mediante la trasmissione di un segnale di jamming, appunto. Tale setup è stato modellato mediante strumenti ricavati dalla Teoria dei Giochi, in particolare come gioco a somma zero in cui la funzione di payoff è rappresentata dall'informazione mutua scambiata fra trasmettitore e ricevitore. In quseto modo sono state determinate le strategie di trasmissione ottime sia per il trasmettitore legittimo che per l'attaccante, ed è stato trovato il punto di equilibrio di Nash del gioco sia per sistemi OFDM di tipo discrete multitone (DMT) che filtered multitone (FMT).

In this work, a multilayer security solution for digital communication systems is provided by considering the joint effects of physical-layer security channel codes with application-layer cryptography. We address two problems: first, the cryptanalysis of error-prone ciphertext; second, the design of a practical physical-layer security coding scheme. To our knowledge, the cryptographic attack model of the noisy-ciphertext attack is a novel concept. The more traditional assumption that the attacker has the ciphertext is generally assumed when performing cryptanalysis. However, with the ever-increasing amount of viable research in physical-layer security, it now becomes essential to perform the analysis when ciphertext is unreliable. We do so for the simple substitution cipher using an information-theoretic framework, and for stream ciphers by characterizing the success or failure of fast-correlation attacks when the ciphertext contains errors. We then present a practical coding scheme that can be used in conjunction with cryptography to ensure positive error rates in an eavesdropper's observed ciphertext, while guaranteeing error-free communications for legitimate receivers. Our codes are called stopping set codes, and provide a blanket of security that covers nearly all possible system configurations and channel parameters. The codes require a public authenticated feedback channel. The solutions to these two problems indicate the inherent strengthening of security that can be obtained by confusing an attacker about the ciphertext, and then give a practical method for providing the confusion. The aggregate result is a multilayer security solution for transmitting secret data that showcases security enhancements over standalone cryptography.

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 to the application of the FFR is limited by the capacity of the legitimate channel.

This thesis discusses the design of two Physical Layer Security (PLS) techniques on Software Defined Radios (SDRs). PLS is a classification of security methods that take advantage of physical properties in the waveform or channel to secure communication. These schemes can be used to directly obfuscate the signal from eavesdroppers, or even generate secret keys for traditional encryption methods. Over the past decade, advancements in Multiple-Input Multiple-Output systems have expanded the potential capabilities of PLS while the development of technologies such as the Internet of Things has provided new applications. As a result, this field has become heavily researched, but is still lacking implementations. The design work in this thesis attempts to alleviate this problem by establishing SDR designs geared towards Over-the-Air experimentation. The first design involves a 2x1 Multiple-Input Single-Output system where the transmitter uses Channel State Information from the intended receiver to inject Artificial Noise (AN) into the receiver's nullspace. The AN is consequently not seen by the intended receiver, however, it will interfere with eavesdroppers experiencing independent channel fading. The second design involves a single-carrier Alamouti coding system with pseudo-random phase shifts applied to each transmit antenna, referred to as Phase-Enciphered Alamouti Coding (PEAC). The intended receiver has knowledge of the pseudo-random sequence and can undo these phase shifts when performing the Alamouti equalization, while an eavesdropper without knowledge of the sequence will be unable to decode the signal.
Master of Science

In questa tesi vengono analizzate due applicazioni della codifica di canale per fini di sicurezza dell'informazione: I) utilizzare il problema di decodifica come primitiva crittografica in maniera efficiente, sia dal punto di vista della dimensione delle chiavi che della complessità computazionale II) l'utilizzo di tecniche di codifica per prevenire il furto di informazioni su un canale insicuro, non protetto da schemi crittografici. Le transazioni commerciali su internet ed il web-banking sono basati su sistemi crittografici (principalmente RSA) che non resisteranno all'avvento dei computer quantistici. D'altro canto, decodificare un codice lineare generico è un problema NP e non è noto alcun algoritmo in grado di risolverlo efficientemente neanche attraverso l'utilizzo di computer quantistici . In questa tesi viene studiata l'adozione di alcune famiglie di codici lineari (GRS e QC-LDPC), come sostituti dei codici di Goppa, nel crittosistema di McEliece, che si basa su tale problema di decodifca. Verrà discusso come l'utilizzo di tali codici permetta di superare il problema che, fino ad oggi, ne ha impedito un pratico utilizzo: l'eccessiva dimensione delle chiavi pubbliche. Attraverso l'utilizzo di questi codici, e modificando il sistema originale, la dimensione delle chiavi può essere ridotta fino a valori prossimi a quelli delle soluzioni non-quantistiche (RSA), con l'ulteriore vantaggio di un costo computazionale ridotto. Tecniche simili (l'uso di codici lineari e scrambling dell'informazione) possono essere adottate anche in schemi di physical layer security. In tali schemi non ci sono chiavi segrete: gli utenti, legittimo e non autorizzato, vengono distinti sulla base delle differenze fisiche dei loro canali. Se consideriamo, come esempio, una comunicazione wireless e due utenti, è possibile utilizzare tali tecniche per ridurre drasticamente la distanza relativa tra utente autorizzato e attaccante, alla quale essi devono essere, per far si che l'utente legittimo possa ricevere dati dal router e l'attaccante non sia in grado di ottenere informazione.
This thesis presents a study of channel coding techniques for security issues. Two possible applications are investigated: I) using the decoding problem as a cryptographic trapdoor in an efficient way, from the key size and complexity point of view II) the usage of channel coding techniques aimed to prevent the information eavesdropping on a public insecure channel without encryption. A quantum computer will be able to break the worldwide implemented number-theory-based cryptographic primitives, such as RSA. This could have dramatic consequences, since all the web-banking and e-commerce transactions are protected by this kind of primitives. Decoding a random linear code, is a problem that have been shown to be NP-complete and no algorithm is known to solve it, in reasonable time, even using quantum algorithm. This thesis analyzes the possible introduction of different families of linear codes (GRS and QC-LDPC) in the main code-based cryptosystem: the McEliece cryptosystem. We show how to avoid its biggest problem, the huge key size, by modifying the original system and adopting different codes, obtaining keys that are comparable with non-quantum solutions as RSA, with a reduced decrypting complexity. Moreover, the adoption of similar techniques, that is, a proper mix of scrambling and linear coding, can be used for physical layer security. In a physical layer security scheme, all the users are completely aware of all the components of the communication system, there are no secret keys: the differentiation between legitimate and unauthorized users is based only on the physical channel randomness. As an example, we can imagine a wireless router transmission, the legitimate user is inside the router's room and the eavesdropper is outside of it. The channel deterioration experienced by the attacker, since he is farther than the legitimate receiver, is small and he can correctly receive router's data. The proposed scheme allows to greatly improve the security of the system in such a way to prevent the information leakage, even if the channel difference between the two users is very small.

The Internet of Things (IoT) aims to connect all things to the internet to facilitate intelligent identifying, locating, tracking, monitoring, and managing of objects and information. The IoT paradigm will be supported by advanced wireless communications technologies and fifth-generation (5G) cellular networks. As such, new network architectures and protocols should be developed to ensure successful deployments of IoT communications systems in many domains including smart homes, transportation, logistics, smart grids and healthcare. In order to extend existing wireless network coverage and support the robustness and connectivity of IoT, unmanned aerial vehicle (UAV) has been proposed as an ideal IoT platform due to its flexibility and mobility. Moreover, mobile edge computing (MEC) which provides considerable computing resource at the edge of the network to support IoT applications and services in real time is another key enabling technology for IoT. However, realizing the vision of IoT is challenging due to many difficulties that need to be addressed. Particularly, physical layer security (PLS) and low-latency design are the two main challenges. On the one hand, due to the openness of wireless transmission medium, the communications between the legitimate transmitter-receiver pair can be readily attacked by eavesdroppers or spoofers where the confidentiality and integrity of the information cannot be guaranteed. On the other hand, with the tremendous amount and variety of data to be collected and processed in IoT networks, achieving low-latency communications is challenging and has become one of the major bottlenecks of IoT. In this thesis, we develop new efficient algorithms to improve the PLS and latency in IoT communications based on UAV and MEC technologies to address the aforementioned challenges of IoT. We first develop a secure UAV-enabled communication framework by exploiting a UAV-based friendly jammer which emits artificial noise to prevent eavesdropping attacks on legitimate ground nodes when the eavesdropper location is unknown. We propose an efficient algorithm to maximize the security performance by jointly optimizing the 3D deployment and jamming power of the UAV jammer. Numerical results show that our proposed iterative algorithm performs close to an exhaustive search with significantly reduced complexity. Next, we propose a more general IoT communications model where both the latency and PLS performance are jointly considered in an MEC network. We formulate a latency minimization problem by jointly optimizing the user's transmit power, computing capacity allocation, and user association subject to PLS performance and computing resource constraints. Numerical results show that our proposed solution outperforms baseline strategies over a wide range of computing capacities and highlight a fundamental tradeoff between latency and security in an MEC network. Subsequently, we extend the latency-security tradeoff analysis to a UAV-enabled MEC network, where multiple ground users offload large computing tasks to a nearby legitimate UAV in the presence of multiple eavesdropping UAVs with imperfect locations. For this system, we design a low-complexity iterative algorithm to maximize the minimum secrecy capacity subject to latency, minimum offloading and total power constraints. Numerical results show that our proposed algorithm significantly outperforms baseline strategies over a wide range of UAV self-interference (SI) efficiencies, locations and packet sizes of ground users. Furthermore, we show that there exists a fundamental tradeoff between the security and latency of UAV-enabled MEC systems. Finally, to protect wireless communications from potential spoofing attacks, we investigate received signal strength (RSS)-based physical layer authentication (PLA) in UAV-enabled communication systems based on game theory. We first model an authentication hypothesis test based on the RSS distance and derive the false alarm rate and miss detection rate. We then formulate a zero-sum PLA game to model the interactions between the spoofer and the UAV receiver. The Nash equilibrium (NE) and its existence condition for the proposed PLA game are also derived. Monte Carlo simulation results accurately verify our analytical expressions for the false alarm rate and miss detection rate.

We consider the problem of secure communications in a Gaussian two-way relay network where two nodes exchange confidential messages only via an untrusted relay. The relay is assumed to be honest but curious, i.e., an eavesdropper that conforms to the system rules and applies the intended relaying scheme. We analyze the achievable secrecy rates by applying network coding on the physical layer or the network layer and compare the results in terms of complexity, overhead, and efficiency. Further, we discuss the advantages and disadvantages of the respective approaches.

We consider the problem of secure communications in a Gaussian two-way relay network where two nodes exchange confidential messages only via an untrusted relay. The relay is assumed to be honest but curious, i.e., an eavesdropper that conforms to the system rules and applies the intended relaying scheme. We analyze the achievable secrecy rates by applying network coding on the physical layer or the network layer and compare the results in terms of complexity, overhead, and efficiency. Further, we discuss the advantages and disadvantages of the respective approaches.

The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.

Data privacy in traditional wireless communications is accomplished by cryptography techniques at the upper layers of the protocol stack. This thesis aims at contributing to the critical security issue residing in the physical-layer of wireless networks, namely, secrecy rate in various transmission environments. Physical-layer security opens the gate to the exploitation of channel characteristics to achieve data secure transmission. Precoding techniques, as a critical aspect in pre-processing signals prior to transmission has become an effective approach and recently drawn significant attention in the literature. In our research, novel non-linear precoders are designed focusing on the improvement of the physical-layer secrecy rate with consideration of computational complexity as well as the Bit Error Ratio (BER) performance. In the process of designing the precoder, strategies such as Lattice Reduction (LR) and Artificial Noise (AN) are employed to achieve certain design requirements. The deployment and allocation of resources such as relays to assist the transmission also have gained significant interest. In multiple-antenna relay networks, we examine various relay selection criteria with arbitrary knowledge of the channels to the users and the eavesdroppers. Furthermore, we provide novel effective relay selection criteria that can achieve a high secrecy rate performance. More importantly they do not require knowledge of the channels of the eavesdroppers and the interference. Combining the jamming technique with resource allocation of relay networks, we investigate an opportunistic relaying and jamming scheme for Multiple-Input Multiple-Output (MIMO) buffer-aided downlink relay networks. More specifically, a novel Relaying and Jamming Function Selection (RJFS) algorithm as well as a buffer-aided RJFS algorithm are developed along with their ability to achieve a higher secrecy rate. Relying on the proposed relay network, we detail the characteristics of the system, under various relay selection criteria, develop exhaustive search and greedy search-based algorithms, with or without inter-relay Interference Cancellation (IC).

Among various means of energy harvesting (EH) for green communications, radio-frequency (RF)-enabled wireless energy harvesting (WEH), inter alia, has recently drawn significant interest for its long operational distance and effective energy multicasting; it thus motivates the paradigm of wireless powered communication network (WPCN). Nevertheless, besides benefitting from the broadcast nature of wireless channels, WPCN is also vulnerable in terms of confidentiality and privacy of the data transmission, since legitimate information may be eavesdropped by unauthorized parties. To resolve this issue, physical-layer security (PLS) has been proposed as a promising solution to achieve information-theoretic security. This thesis is devoted to addressing some major challenges encountered in enhancing PLS for WPCN while exploiting opportunities gained from WPCN by pragmatic and prominent transmitting and/or cooperative strategies along with corresponding optimal (suboptimal) resource allocations. This thesis begins with considering a three node single-input-single-output (SISO) fading wiretap channel, where the confidential messages sent to the information receivers (IRs) may be eavesdropped by the energy receivers (ERs) that are usually deployed nearer to the transmitter because of their high power receiving sensitivity. In this case, an artificial noise (AN)-aided transmission scheme, where the transmit power is split into two parts, to send the confidential message to the IR and an AN to interfere with the ER respectively, is proposed to facilitate the secrecy information transmission and yet meet the EH requirement. The fundamental challenges in balancing the goals between achieving PLS and satisfying ER’s EH requirement are modeled by various secrecy performances versus harvested energy trade-offs, the regions of which are enlarged by both dual decomposition-based optimal solutions and alternating optimization-based suboptimal solutions. On the other hand, under circumstances where some ERs are trustful, their self-sustaining features can also be favourable to providing PLS by means of cooperative jamming (CJ). In the second part of the thesis, a novel harvest-and-jam (HJ) relaying protocol is proposed for multiple multi-antenna ERs to assist in the secrecy information transmission via one multi-antenna amplify-and-forward (AF) relay. Joint optimization of the CJ covariance and AF-relay beamforming is studied using semidefinite relaxation (SDR) under perfect and imperfect channel state information (CSI) respectively. In particular, for the imperfect CSI case, a novel approach that jointly models channel imperfections induced by an arbitrary number of CJ helpers is proposed to equivalently reformulate the worst-case robust optimization problem into the convex optimization framework. Following the trend of WEH-enabled cooperative secrecy transmission, a more general wiretap channel with multiple WEH-enabled AF relays in the presence of multiple eavesdroppers all equipped with single antenna is studied in the last part of the thesis. To the end of combing the benefit of CJ and cooperative beamforming (CB), a new hybrid power splitting (PS) relaying strategy is proposed. In the first transmission phase each AF relay employs a PS receiver that splits a fraction of the received power for EH and consumes the rest for information receiving. In the second transmission phase the relay further divides its harvested power to forward the confidential information and to generate the jamming signals. The formulated secrecy rate maximization problems turn out to be very challenging due to the multiplicative variables in the relay weights. Under the centralized scheme, the global optimum joint CB and CJ solution is obtained for the static power splitting (SPS) case, while for the generalized dynamic power splitting (DPS) case, the global optimum CB-only solution is provided by utilizing SDR, which is then developed into a suboptimal joint CB and CJ design based on alternating optimization.

Physical layer security using optical signal processing in optical networks has been researched over recent decades to preserve data transmission confidentiality and privacy. One of the solutions that has been studied is optical steganography, which is used to conceal the data transmission in a host signal. However, only hiding the signal is not enough. Therefore, this research aims to demonstrate optical steganography using encoding techniques as cyber security solutions in the physical layer to guarantee privacy and confidentiality of the data transmitted. Laboratory experiments and software simulations were used to demonstrate the feasibility of the solutions proposed. Further, with an experimental approach using a quantitative method, the independent variables were manipulated to test the viability of the proposed circuits, and their effect measured and recorded for data analysis. This research has proposed two optical network solutions. First, an Amplified Spontaneous Emission (ASE) noise source was used to create a stealth channel using variable optical time delay and polarisation control creating a two-factor technique to encode the data transmitted. This signal is then hidden in the additional ASE noise and the public channel for transmission. Second, a Spectral-Amplitude Code (SAC) Optical Code- Division Media-Access (OCDMA) with multiple user channels using fibre Brag grating (FBG) as encoders of the optical steganography to create the stealth channel, which is also hidden in the ASE noise, and the public channel. Polarisation control and Dispersion Compensation Fibre (DCF) are also used for creating a three-factor encoding technique for an optical security solution. Particularly, the ASE noise is a side effect present in most of the fibre optic communication networks, which aids in hiding the presence of the data transmission, thus the presence of the ASE noise in the signal is not suspicious. This research has made the following contributions in the field of cyber security solutions in the physical layer: 1) demonstrated feasibility in combining different encoding techniques with optical steganography; 2) three-factor security solutions requirement, which are employed to enhance confidentiality and privacy of specific applications; 3) presented cyber security solutions in the physical layer as effective alternatives to software and protocol encryption.

"In a wireless network environment, all the users are able to access the wireless channel. Thus, if malicious users exploit this feature by mimicking the characteristics of a normal user or even the central wireless access point (AP), they can intercept almost all the information through the network. This scenario is referred as a Man-in-the-middle (MITM) attack. In the MITM attack, the attackers usually set up a rogue AP to spoof the clients. In this thesis, we focus on the detection of MITM attacks in Wi-Fi networks. The thesis introduces the entire process of performing and detecting the MITM attack in two separate sections. The first section starts from creating a rogue AP by imitating the characteristics of the legitimate AP. Then a multi-point jamming attack is conducted to kidnap the clients and force them to connect to the rogue AP. Furthermore, the sniffer software is used to intercept the private information passing through the rogue AP. The second section focuses on the detection of MITM attacks from two aspects: jamming attacks detection and rogue AP detection. In order to enable the network to perform defensive strategies more effectively, distinguishing different types of jamming attacks is necessary. We begin by using signal strength consistency mechanism in order to detect jamming attacks. Then, based on the statistical data of packets send ratio (PSR) and packets delivery ratio (PDR) in different jamming situations, a model is built to further differentiate the jamming attacks. At the same time, we gather the received signal strength indication (RSSI) values from three monitor nodes which process the random RSSI values employing a sliding window algorithm. According to the mean and standard deviation curve of RSSI, we can detect if a rogue AP is present within the vicinity. All these proposed approaches, either attack or detection, have been validated via computer simulations and experimental hardware implementations including Backtrack 5 Tools and MATLAB software suite. "

Doctor of Philosophy
Department of Electrical and Computer Engineering
Balasubramaniam Natarajan
Widely recognized security vulnerabilities in current wireless radio access technologies undermine the benefits of ubiquitous mobile connectivity. Security strategies typically rely on bit-level cryptographic techniques and associated protocols at various levels of the data processing stack. These solutions have drawbacks that have slowed down the progress of new wireless services. Physical layer security approaches derived from an information theoretic framework have been recently proposed with secret key generation being the primary focus of this dissertation. Previous studies of physical layer secret key generation (PHY-SKG) indicate that a low secret key generation rate (SKGR) is the primary limitation of this approach. To overcome this drawback, we propose novel SKG schemes to increase the SKGR as well as improve the security strength of generated secret keys by exploiting multiple input and multiple output (MIMO), cooperative MIMO (co-op MIMO) networks. Both theoretical and numerical results indicate that relay-based co-op MIMO schemes, traditionally used to enhance LTE-A network throughput and coverage, can also increase SKGR. Based on the proposed SKG schemes, we introduce innovative power allocation strategies to further enhance SKGR. Results indicate that the proposed power allocation scheme can offer 15% to 30% increase in SKGR relative to MIMO/co-op MIMO networks with equal power allocation at low-power region, thereby improving network security. Although co-op MIMO architecture can offer significant improvements in both performance and security, the concept of joint transmission and reception with relay nodes introduce new vulnerabilities. For example, even if the transmitted information is secured, it is difficult but essential to monitor the behavior of relay nodes. Selfish or malicious intentions of relay nodes may manifest as non-cooperation. Therefore, we propose relay node reliability evaluation schemes to measure and monitor the misbehavior of relay nodes. Using a power-sensing based reliability evaluation scheme, we attempt to detect selfish nodes thereby measuring the level of non-cooperation. An overall node reliability evaluation, which can be used as a guide for mobile users interested in collaboration with relay nodes, is performed at the basestation. For malicious behavior, we propose a network tomography technique to arrive at node reliability metrics. We estimate the delay distribution of each internal link within a co-op MIMO framework and use this estimate as an indicator of reliability. The effectiveness of the proposed node reliability evaluations are demonstrated via both theoretical analysis and simulations results. The proposed PHY-SKG strategies used in conjunction with node reliability evaluation schemes represent a novel cross-layer approach to enhance security of cooperative networks.

Nowadays, information security is a very important topic. In particular, wireless networks are experiencing an ongoing widespread diffusion, also thanks the increasing number of Internet Of Things devices, which generate and transmit a lot of data: protecting wireless communications is of fundamental importance, possibly through an easy but secure method. Physical Layer Security is an umbrella of techniques that leverages the characteristic of the wireless channel to generate security for the transmission. In particular, the Physical Layer based-Key generation aims at allowing two users to generate a random symmetric keys in an autonomous way, hence without the aid of a trusted third entity. Physical Layer based-Key generation relies on observations of the wireless channel, from which harvesting entropy: however, an attacker might possesses a channel simulator, for example a Ray Tracing simulator, to replicate the channel between the legitimate users, in order to guess the secret key and break the security of the communication. This thesis work is focused on the possibility to carry out a so called Ray Tracing attack: the method utilized for the assessment consist of a set of channel measurements, in different channel conditions, that are then compared with the simulated channel from the ray tracing, to compute the mutual information between the measurements and simulations. Furthermore, it is also presented the possibility of using the Ray Tracing as a tool to evaluate the impact of channel parameters (e.g. the bandwidth or the directivity of the antenna) on the Physical Layer based-Key generation. The measurements have been carried out at the Barkhausen Institut gGmbH in Dresden (GE), in the framework of the existing cooperation agreement between BI and the Dept. of Electrical, Electronics and Information Engineering "G. Marconi" (DEI) at the University of Bologna.

While significant research effort has been dedicated to wireless position location over the past decades, most location security aspects have been overlooked. Recently, with the proliferation of diverse wireless devices and the desire to determine their position, there is an increasing concern about the security of location information which can be spoofed or disrupted by adversaries or unreliable signal sources. This dissertation addresses the problem of securing a radio location system against location spoofing, specifically the characterization, analysis, detection, and localization of location spoofing attacks by focusing on fundamental location estimation issues. The objective of this dissertation is four-fold. First, it provides an overview of fundamental security issues for position location, particularly associated with range-based localization. Of particular interest are security risks and vulnerabilities in location estimation, types of localization attacks, and their impact. The second objective is to characterize the effects of signal strength and beamforming attacks on range estimates and the resulting position estimate. The characterization can be generalized to a variety of location spoofing attacks and provides insight into the anomalous behavior of range and location estimators when under attack. Through this effort we can also identify effective attacks that are of particular interest to attack detection and localization. The third objective is to develop an effective technique for attack detection which requires neither prior environmental nor statistical knowledge. This is accomplished by exploiting the bilateral behavior of a hybrid framework using two received signal strength (RSS) based location estimators. We show that the resulting approach is effective at detecting attacks with the detection rate increasing with the severity of the induced location error. The last objective of this dissertation is to develop a localization method resilient to attacks and other adverse effects. Since the detection and localization approach relies solely on RSS measurements in order to be applicable to a wide range of wireless systems and scenarios, this dissertation focuses on RSS-based position location. Nevertheless, many of the basic concepts and results can be applied to any range-based positioning system.
Ph. D.

The broadcast nature of wireless medium has made information security as one of the most important and critical issues in wireless systems. Physical layer security, which is based on information-theoretic secrecy concepts, can be used to secure the wireless channels by exploiting the noisiness and imperfections of the channels. Massive multiple-input multiple-output (MIMO) systems, which are equipped with very large antenna arrays at the base stations, have a great potential to boost the physical layer security by generating the artificial noise (AN) with the exploitation of excess degrees-of-freedom available at the base stations. In this thesis, we investigate physical layer security provisions in the presence of passive/active eavesdroppers for single-hop massive MIMO, dual-hop relay-assisted massive MIMO and underlay spectrum-sharing massive MIMO systems. The performance of the proposed security provisions is investigated by deriving the achievable rates at the user nodes, the information rate leaked into the eavesdroppers, and the achievable secrecy rates. Moreover, the effects of active pilot contamination attacks, imperfect channel state information (CSI) acquisition at the base-stations, and the availability of statistical CSI at the user nodes are quantified. The secrecy rate/performance gap between two AN precoders, namely the random AN precoder and the null-space based AN precoder, is investigated. The performance of hybrid analog/digital precoding is compared with the full-dimensional digital precoding. Furthermore, the physical layer security breaches in underlay spectrum-sharing massive MIMO systems are investigated, and thereby, security provisions are designed/analyzed against active pilot contamination attacks during the channel estimation phase. A power-ratio based active pilot attack detection scheme is investigated, and thereby, the probability of detection is derived. Thereby, the vulnerability of uplink channel estimation based on the pilots transmitted by the user nodes in time division duplexing based massive MIMO systems is revealed, and the fundamental trade-offs among physical layer security provisions, implementation complexity and performance gains are discussed.

Cognitive Radio (CR) is a paradigm shift in wireless communications to resolve the spectrum scarcity issue with the ability to self-organize, self-plan and self-regulate. On the other hand, wireless devices that can learn from their environment can also be taught things by malicious elements of their environment, and hence, malicious attacks are a great concern in the CR, especially for physical layer security. This thesis introduces a data-driven Self-Awareness (SA) module in CR that can support the system to establish secure networks against various attacks from malicious users. Such users can manipulate the radio spectrum to make the CR learn wrong behaviours and take mistaken actions. The SA module consists of several functionalities that allow the radio to learn a hierarchical representation of the environment and grow its long-term memory incrementally. Therefore, this novel SA module is a way forward towards realizing the original vision of CR (i.e. Mitola's Radio) and AI-enabled radios. This thesis starts with a basic SA module implemented in two applications, namely the CR-based IoT and CR-based mmWave. The two applications differ in the data dimensionality (high and low) and the PHY-layer level at which the SA module is implemented. Choosing an appropriate learning algorithm for each application is crucial to achieving good performance. To this purpose, several generative models such as Generative Adversarial Networks, Variational AutoEncoders and Dynamic Bayesian Networks, and unsupervised machine learning algorithms such as Self Organizing Maps Growing Neural Gas with different configurations are proposed, and their performances are analysed. In addition, we studied the integration of CR and UAVs from the physical layer security perspective. It is shown how the acquired knowledge from previous experience within the Bayesian Filtering facilitates the radio spectrum perception and allows the UAV to detect any jamming attacks immediately. Moreover, exploiting the generalized errors during abnormal situations permits characterizing and identifying the jammer at multiple levels and learning a dynamic model that embeds its dynamic behaviour. Besides, a proactive consequence can be drawn after estimating the jammer's signal to act efficiently by mitigating its effects on the received stimuli or by designing an efficient resource allocation for anti-jamming using Active Inference. Experimental results show that introducing the novel SA functionalities provides the high accuracy of characterizing, detecting, classifying and predicting the jammer's activities and outperforms conventional detection methods such as Energy detectors and advanced classification methods such as Long Short-Term Memory (LSTM), Convolutional Neural Network (CNN) and Stacked Autoencoder (SAE). It also verifies that the proposed approach achieves a higher degree of explainability than deep learning techniques and verifies the capability to learn an efficient strategy to avoid future attacks with higher convergence speed compared to conventional Frequency Hopping and Q-learning.

This thesis focuses on generalized moment problems and their applications in the framework of information engineering. Its contribution is twofold. The first part of this dissertation proposes two new techniques for tackling multivariate spectral estimation, which is a key topic in system identification: Relative entropy rate estimation and multivariate circulant rational covariance extension. The former provides a very natural multivariate extension of a state-of-the-art approach for scalar parametric spectral estimation with a complexity bound, known as THREE (Tunable High-Resolution Estimator). It allows to take into account available a priori information on the spectral density. It exhibits high resolution features and it is robust in case of short data records. As for multivariate circulant rational covariance extension, it is a new convex optimization approach to spectral estimation for periodic multivariate processes, in which the computation of the solution can be tackled efficiently by means of Fast Fourier Transform. Numerical examples show that this procedure also provides an efficient tool for approximating regular covariance extension for multivariate processes. The second part of this dissertation considers the problem of deriving a universal performance bound for a message source authentication scheme based on channel estimates in a wireless fading scenario, where an attacker may have correlated observations available and possibly unbounded computational power. Under the assumption that the channels are represented by multivariate complex Gaussian variables, it is proved that the tightest bound corresponds to a forging strategy that produces a zero mean signal that is jointly Gaussian with the attacker observations. A characterization of their joint covariance matrix is derived through the solution of a system of two nonlinear matrix equations. Based upon this characterization, the thesis proposes an efficient iterative algorithm for its computation: The solution to the matricial system appears as fixed point of the iteration. Numerical examples suggest that this procedure is effective in assessing worst case channel authentication performance.
La tesi affronta il problema dei momenti generalizzato e le sue applicazioni nell'ambito dell’ingegneria dell’informazione. Nella prima parte della tesi sono proposte due nuove tecniche per affrontare efficientemente il problema della stima spettrale multivariata, che è molto rilevante nel contesto dell'identificazione dei sistemi dinamici: la stima basata sul tasso di entropia relativa tra processi e l’estensione di covarianza razionale per processi multivariati e periodici. La stima spettrale basata sul tasso di entropia relativa estende in modo molto naturale un approccio che rappresenta lo stato dell’arte per quanto riguarda la stima spettrale per processi scalari con un vincolo sulla massima complessità della soluzione, noto come THREE - Tunable High Resolution Estimator. La tecnica proposta permette all'utente di tenere in considerazione le informazioni sulla densità spettrale del processo eventualmente disponibili. Inoltre, essa permette di ottenere stime ad elevata risoluzione ed è robusta nel caso di ridotta numerosità campionaria dei dati. Per quanto riguarda l'estensione circolare di covarianza per processi multivariati, essa fornisce un nuovo approccio di ottimizzazione convessa alla stima spettrale per processi multivariati periodici, nel quale la soluzione può essere ricavata in modo efficiente ricorrendo agli algoritmi per il calcolo della trasformata di Fourier veloce (FFT - Fast Fourier Transform). Alcuni esempi numerici illustrano che questa procedura fornisce uno strumento efficace per approssimare l’estensione di covarianza per processi in generale non periodici. La seconda parte della tesi si occupa del problema dell'autenticazione a livello fisico della sorgente di un messaggio in un sistema di comunicazione wireless. Nello scenario considerato l'attaccante ha accesso ad alcune informazioni sul canale tra sorgente legittima e ricevitore e può avere potenza di calcolo illimitata. Sotto l'ipotesi che i canali d'interesse siano descritti da vettori gaussiani a valori complessi, si dimostra che la strategia d'attacco ottima corrisponde alla generazione di un segnale congiuntamente gaussiano con le osservazioni dell'attaccante. La distribuzione congiunta è caratterizzata mediante la soluzione di un sistema di equazioni matriciali non lineari, che può essere calcolata per mezzo di un algoritmo iterativo. Alcuni esempi numerici illustrano l'efficacia della procedura proposta nel valutare le performance al caso pessimo dello schema di autenticazione di canale.

Support of many different services, approximately 1000x increase of current data rates, ultra-reliability, low latency and energy/cost efficiency are among the demands from upcoming 5G standard. In order to meet them, researchers investigate various potential technologies involving different network layers and discuss their trade-offs for possible 5G scenarios. Waveform design is a critical part of these efforts and various alternatives have been heavily discussed over the last few years. Besides that, wireless technology is expected to be deployed in many critical applications including the ones involving with daily life activities, health-care and vehicular traffic. Therefore, security of wireless systems is also crucial for a reliable and confidential deployment. In order to achieve these goals in future wireless systems, physical layer (PHY) algorithms play a vital role not only in waveform design but also for improving security. In this dissertation, we draft the ongoing activities in PHY in terms of waveform design and security for providing spectrally efficient and reliable services considering various scenarios, and present our algorithms in this direction. Regarding the waveform design, orthogonal frequency division multiplexing (OFDM) is mostly considered as the base scheme since it is the dominant technology in many existing standards and is also considered for 5G new radio. We specifically propose two approaches for the improvement of OFDM in terms of out-of-band emission and peak to average power ratio. We also present how the requirements of different 5G RAN scenarios reflect on waveform parameters and explore the motivations behind designing advanced frames that include multiple waveforms with different parameters, referred to as numerologies by the 3GPP community, as well as the problems that arise with such coexistence. On the security aspect, we firstly consider broadband communication scenarios and propose practical security approaches that suppress the cyclic features of OFDM and single carrier-frequency domain equalization based waveforms and remove their vulnerability to the eavesdropping attacks. Additionally, an authentication mechanism in PHY is presented for wireless implantable medical devices. Thus, we address the security issues for two critical wireless communication scenarios in PHY to contribute a confidential and reliable deployment of wireless technologies in the near future.

Near-field contact less payments using contactless cards or NFC devices are quickly becoming a quicker and more convenient alternative to conventional means of carrying out small value purchases. Along with their increased popularity, there are rising concerns regarding their security. Existing research has shown that certain attacks can be used successfully against contact less technology, but it is unclear how such attacks can be translated into a feasible and clear threat to a user's privacy and financial security. Therefore there is a need for an evaluation to determine whether physical layer based attacks could be used by attackers to cause financial or anonymity loss to an individual. This dissertation presents the design and implementation of an inconspicuous, easily concealable and portable system that could be used to reliably eavesdrop contactless transactions. This includes guidelines on the effective and efficient design of eavesdropping antennas, including the use of large metallic structures already within the vicinity of such an attack, along with the assembly of a communications receiver consisting of readily available electronics with a moderate cost.

The cognitive radio network (CRN) concept has been proposed as a solution to the growing demand and underutilization of the radio spectrum. To improve the radio spectrum utilization, CRN technology allows the coexistence of licensed and unlicensed systems over the same spectrum. In an underlay spectrum sharing system, secondary users (SUs) transmit simultaneously with the primary users (PUs) in the same frequency band given that the interference caused by the SU to the PU remains below a tolerable interference limit. Besides the transmission power limitation, a secondary network is subject to distinct channel impairments such as fading and interference from the primary transmissions. Also, CRNs face new security threats and challenges due to their unique cognitive characteristics.This thesis analyzes the performance of underlay CRNs and underlay cognitive relay networks under spectrum sharing constraints and security constraints. Distinct SU transmit power policies are obtained considering various interference constraints such as PU outage constraint or PU peak interference power constraint. The thesis is divided into an introduction and two research parts based on peer-reviewed publications. The introduction provides an overview of radio spectrum management, basic concepts of CRNs, and physical layer security. In the first research part, we study the performance of underlay CRNs with emphasis on a multiuser environment.In Part I-A, we consider a secondary network with delay-tolerant applications and analyze the ergodic capacity. Part I-B analyzes the secondary outage capacity which characterises the maximum data rate that can be achieved over a channel for a given outage probability. In Part I-C, we consider a secondary network with delay constrained applications, and derive expressions of the outage probability and delay-limited throughput. Part I-D presents a queueing model that provides an analytical tool to evaluate the secondary packet-level performance with multiple classes of traffic considering general interarrival and service time distributions. Analytical expressions of the SU average packet transmission time, waiting time in the queue, andtime spent in the system are provided.In the second research part, we analyze the physical layer security for underlay CRNs and underlay cognitive relay networks. Analytical expressions of the probability of non-zero secrecy capacity and secrecy outage probability are derived.Part II-A considers a single hop underlay CRN in the presence of multiple eavesdroppers (EAVs) and multiple SU-Rxs. In Part II-B, an underlay cognitive relay network in the presence of multiple secondary relays and multiple EAVs is studied.Numerical examples illustrate that it is possible to exploit the physical layer characteristics to achieve both security and quality of service in CRNs while satisfying spectrum sharing constraints.

In the area of computer and network security, due to the insufficiency, high costs, and user-unfriendliness of existing defending methods against a number of cyber attacks, focus for developing new security improvement methods has shifted from the digital to analog domain. In the analog domain, devices are distinguished based on the present variations and characteristics in their physical signals. In fact, each device has unique features in its signal that can be used for identification and monitoring purposes. In this regard, the term physical layer identification (PLI) or device fingerprinting refers to the process of classifying different electronic devices based on their analog identities that are created by employment of signal processing and data analysis methods. Due to the fact that a device behavior undergoes changes due to variations in external and internal conditions, the available PLI techniques might not be able to identify the device reliably. Therefore, a tracking system that is capable of extracting and explaining the present variations in the electrical signals is required to be developed. In order to achieve the best possible tracking system, a number of prediction models are designed using certain statistical techniques. In order to evaluate the performance of these models, models are run on the acquired data from five different fabrications of the same device in four distinct experiments. The results of performance evaluation show that the surrounding temperature of a device is the best option for predicting its signal. The last part of this research project belongs to the security evaluation of a PLI system. The leveraged security examination technique exposes the PLI system to different types of attacks and evaluates its defending strength accordingly. Based on the mechanism of the employed attack in this work, the forged version of a device’s signal is generated using an arbitrary waveform generator (AWG) and is sent to the PLI system. The outcomes of this experiment indicate that the leveraged PLI technique is strong enough in defeating this attack.

Dans cette thèse on s’est intéressé aux méthodes de génération de clés secrètes symétriques en utilisant la couche physique ultra large bande impulsionnelle (IR-UWB). Les travaux ont été réalisés selon trois axes, les deux premiers concernant la communication point-à-point et le dernier, les communications coopératives. Tout d’abord, la quantification des signaux typiques IR-UWB (soit directement échantillonnés, soit estimés) a été investiguée, principalement du point de vue du compromis entre la robustesse (ou réciprocité) des séquences binaires obtenues et leur caractère aléatoire. Différents algorithmes de quantification valorisant l’information temporelle offerte par les canaux IR-UWB pour améliorer ce compromis ont alors été proposés. Ensuite, des études concernant les échanges publics nécessaires à l’étape de réconciliation (visant la correction d’éventuels désaccords entre les séquences binaires générées de part et d’autre du lien) ont montré qu’il était possible d’être plus robuste face aux attaques passives en utilisant des informations de plus haut niveau, inhérentes à cette technologie et disponibles à moindre coût (ex. via une estimation précise du temps de vol aller-retour). Finalement, une nouvelle méthode a été développée afin d’étendre les schémas de génération de clé point-à-point à plusieurs nœuds (trois dans nos études) en utilisant directement la couche physique fournie par les liens radio entre les nœuds
Emerging decentralized wireless systems, such as sensor or ad-hoc networks, will demand an adequate level of security in order to protect the private and often sensitive information that they carry. The main security mechanism for confidentiality in such networks is symmetric cryptography, which requires the sharing of a symmetric key between the two legitimate parties. According to the principles of physical layer security, wireless devices within the communication range can exploit the wireless channel in order to protect their communications. Due to the theoretical reciprocity of wireless channels, the spatial decorrelation property (e.g., in rich scattering environments), as well as the fine temporal resolution of the Impulse Radio - Ultra Wideband (IR-UWB) technology, directly sampled received signals or estimated channel impulse responses (CIRs) can be used for symmetric secret key extraction under the information-theoretic source model. Firstly, we are interested in the impact of quantization and channel estimation algorithms on the reciprocity and on the random aspect of the generated keys. Secondly, we investigate alternative ways of limiting public exchanges needed for the reconciliation phase. Finally, we develop a new signal-based method that extends the point-to-point source model to cooperative contexts with several nodes intending to establish a group key

The unmanned aerial vehicles (UAVs) are also known as drones. Recently, UAVs have attracted the next generation researchers due to their flexible, dynamic, and cost-effective deployment, etc. Moreover, the UAVs have a wide range of application domains, such as rescue operation in the remote area, military surveillance, emergency application, etc. Given the UAVs are appropriately deployed, the UAVs provide continuous and reliable connectivity, on-demand, and cost-effective features to the desired destination in the wireless communication system. Thus, the UAVs can be a great choice to deploy as a mobile relay in co-existence with the base stations (BSs) on the ground to serve the 5G wireless users. In this thesis, the UAV-assisted mobile relay (UAV-MR) in the next generation wireless networks has been studied, which also considers the UAV-MR physical layer security. The proposed system also considers one ground user, one BS on the ground, and active presence of multiple eavesdroppers, situated nearby the ground user. The locations of these nodes (i.e., the ground user, the BS, and the eavesdroppers) are considered fixed on the ground. Moreover, the locations of the eavesdroppers are not precisely known to the UAV-MR. Thus, this thesis aims to maximize the achievable secrecy rate, while the BS sends the secure information to the ground user via the UAV-MR. However, the UAV-MR has some challenges to deploy in wireless networks, such as 3D deployment, robust resource allocation, secure UAV-MR to ground communication, the channel modeling, the UAV-MR flight duration, and the UAV-MR robust trajectory design, etc. Thus, this project investigates the UAV-MR assisted wireless networks, which addresses those technical challenges to guarantee efficient UAV-MR communication. Moreover, the mathematical frameworks are formulated to support the proposed model. An efficient algorithm is proposed to maximize the UAV-MR achievable secrecy rate. Finally, the simulation results show the improved performance for the UAV-MR assisted next-generation networks.