In this dissertation, we present novel physical (PHY) and cross-layer design guidelines and resource adaptation algorithms to improve the security and user experience in the future wireless networks. Physical and cross-layer wireless security measures can provide stronger overall security with high efficiency and can also provide better flexibility in response to the typically time-varying wireless environment. To better utilize limited wireless resources, we present approaches and techniques based on new performance criteria for wireless system optimization by thoroughly exploiting user information including its Quality of Service (QoS) requests and channel state information (CSI) to improve network throughput and security.
In recent years, there has been a growing research interest in wireless system security from PHY layer perspective. In a wiretap channel model introduced in the seminal work by Wyner, a sender “Alice” wishes to transmit a secret message to the intended receiver “Bob” in presence of a passive eavesdropper “Eve”. Existing works have characterized maximum achievable secrecy rate or secrecy capacity for single- and multiple-antenna systems by applying Gaussian signaling and secrecy code. Despite the impracticality of Gaussian input, its compact closed-form expression of mutual information motivated the wide use of Gaussian input assumption in theoretical analysis. In contrast to the Gaussian codebook, practical wiretap codes must consist of finite-alphabet symbols. Because of this constraint, the achievable secrecy rate for a finite-alphabet input scenario would differ from the secrecy rate achievable by a Gaussian codebook. In this thesis, we quantify the effect of finite discrete-constellation on instantaneous and ergodic secrecy rate of multiple-antenna wire-tap channels. Our results demonstrate substantial performance difference between systems involving finite-alphabet inputs and systems with Gaussian inputs. In addition, we investigate the secrecy performance of multi-terminal wiretap systems when a codebook based transmission beamforming is implemented. We characterize the secrecy outage probability of a communication link under eavesdropping. We consider a limited feedback scenario where the transmitter uses a pre-defined codebook known to both the transmitter and the receiver for beamforming, and analyze the secrecy outage probability of the link when it being eavesdropped.
Next, we consider the security of wireless multi-hop networks. Certainly, network security is an overarching issue that transcends all layers of communication protocol stack. Therefore, we tackle the problem of network security by proper user scheduling, routing, and resource management. Our approach of secrecy is complementary to the traditional cryptography based techniques. Together, they can provide stronger overall security and flexibility in selecting the desired solution and can reduce bandwidth and energy overhead as much as possible. Multi-hop wireless networks frequently employ the traditional routing and resource allocation strategies. Such strategies often fail to take into account the additional need for protection against security vulnerabilities due to the wireless medium. Thus, to address security concerns, we investigate the problem of eavesdropping control by proposing security-aware power allocation strategies and routing algorithms for multi-hop networks.
The future wireless infrastructure will consists of a plethora of heterogeneous collections of applications and services. A differentiated service structure would therefore be an essential part of the future wireless infrastructure. In addition to the security consideration of wireless network, we focus on the heterogeneous structure of the network. We study the problem of cross-layer resource allocation and admission control in a multiuser heterogeneous OFDMA network providing both QoS-constrained user servicesand best-effort services. Such heterogeneous networks provide service to both High-Priority (HP) users and Best-Effort (BE) users. By clustering subcarriers, efficient algorithms for cluster and power allocation have been proposed. Our strategy maximizes the total network utility of the BE users while satisfying the QoS request for maximum number of HP users. The feasibility of the resource allocation problem depends on the number of HP users in the network. Since a large number of demanding HP users would render the resource allocation problem infeasible, a joint admission control and resource allocation scheme could be an efficient way of tackling both problems with less overhead. By incorporating admission control, we propose an efficient and optimal joint admission control and resource allocation algorithm.
In this dissertation, we present novel physical (PHY) and cross-layer design guidelines and resource adaptation algorithms to improve the security and user experience in the future wireless networks. Physical and cross-layer wireless security measures can provide stronger overall security with high efficiency and can also provide better flexibility in response to the typically time-varying wireless environment. To better utilize limited wireless resources, we present approaches and techniques based on new performance criteria for wireless system optimization by thoroughly exploiting user information including its Quality of Service (QoS) requests and channel state information (CSI) to improve network throughput and security.
In recent years, there has been a growing research interest in wireless system security from PHY layer perspective. In a wiretap channel model introduced in the seminal work by Wyner, a sender “Alice” wishes to transmit a secret message to the intended receiver “Bob” in presence of a passive eavesdropper “Eve”. Existing works have characterized maximum achievable secrecy rate or secrecy capacity for single- and multiple-antenna systems by applying Gaussian signaling and secrecy code. Despite the impracticality of Gaussian input, its compact closed-form expression of mutual information motivated the wide use of Gaussian input assumption in theoretical analysis. In contrast to the Gaussian codebook, practical wiretap codes must consist of finite-alphabet symbols. Because of this constraint, the achievable secrecy rate for a finite-alphabet input scenario would differ from the secrecy rate achievable by a Gaussian codebook. In this thesis, we quantify the effect of finite discrete-constellation on instantaneous and ergodic secrecy rate of multiple-antenna wire-tap channels. Our results demonstrate substantial performance difference between systems involving finite-alphabet inputs and systems with Gaussian inputs. In addition, we investigate the secrecy performance of multi-terminal wiretap systems when a codebook based transmission beamforming is implemented. We characterize the secrecy outage probability of a communication link under eavesdropping. We consider a limited feedback scenario where the transmitter uses a pre-defined codebook known to both the transmitter and the receiver for beamforming, and analyze the secrecy outage probability of the link when it being eavesdropped.
Next, we consider the security of wireless multi-hop networks. Certainly, network security is an overarching issue that transcends all layers of communication protocol stack. Therefore, we tackle the problem of network security by proper user scheduling, routing, and resource management. Our approach of secrecy is complementary to the traditional cryptography based techniques. Together, they can provide stronger overall security and flexibility in selecting the desired solution and can reduce bandwidth and energy overhead as much as possible. Multi-hop wireless networks frequently employ the traditional routing and resource allocation strategies. Such strategies often fail to take into account the additional need for protection against security vulnerabilities due to the wireless medium. Thus, to address security concerns, we investigate the problem of eavesdropping control by proposing security-aware power allocation strategies and routing algorithms for multi-hop networks.
The future wireless infrastructure will consists of a plethora of heterogeneous collections of applications and services. A differentiated service structure would therefore be an essential part of the future wireless infrastructure. In addition to the security consideration of wireless network, we focus on the heterogeneous structure of the network. We study the problem of cross-layer resource allocation and admission control in a multiuser heterogeneous OFDMA network providing both QoS-constrained user servicesand best-effort services. Such heterogeneous networks provide service to both High-Priority (HP) users and Best-Effort (BE) users. By clustering subcarriers, efficient algorithms for cluster and power allocation have been proposed. Our strategy maximizes the total network utility of the BE users while satisfying the QoS request for maximum number of HP users. The feasibility of the resource allocation problem depends on the number of HP users in the network. Since a large number of demanding HP users would render the resource allocation problem infeasible, a joint admission control and resource allocation scheme could be an efficient way of tackling both problems with less overhead. By incorporating admission control, we propose an efficient and optimal joint admission control and resource allocation algorithm.
Security has become an increasingly significant and urgent issue in wireless networks. The growing computational capability of the eavesdropper (Eve) and more and more complicated secure key management in large scale and heterogeneous networks, pose more and more stringent requirement to the cryptographic protocols. As a new design paradigm, physical layer security has been recognised as a promising solution to enhance the security of wireless links relying on the channel properties and advanced signal processing, and can act as either an alternative or a complementary solution to the conventional cryptographic methods. Without complicated secure key generation and management, physical layer security is quite suitable for the large scale distributed networks. In this thesis, we aim to improve the secrecy performance in emerging wireless communication systems relying on different physical layer security techniques. Firstly, the security of an amplify-and-forward successive relaying network with multiple untrusted relay nodes is investigated, where the conventional detrimental inter-relay interference is exploited to jam the untrusted nodes without assistance of external helpers. Considering different complexity requirements, several relay selection schemes are proposed, and the closed-form expressions of the lower bound of the secrecy outage probability and the maximum secrecy diversity order are derived accordingly. It is shown that the maximum secrecy diversity order of N-1 can be achieved for an N relay nodes network. Secondly, the artificial noise (AN) aided secure transmission strategy for a multiple-input single-output multiple-Eve (MISOME) system with a secure user (Bob) and a normal user (NU) is investigated. The power allocations among Bob, NU and AN, as well as the wiretap code rates, are jointly optimized to maximize the effective secrecy throughput (EST), under the average throughput constraint of NU. An alternative optimization algorithm with guaranteed convergence is proposed to obtain the optimal parameters. It is shown that the EST increases with the transmitting power and the number of transmit antennas, and decreases with the throughput constraint of NU, and the EST can be improved through injecting AN and concurrent transmission of Bob and NU. Thirdly, physical layer security in a multi-antenna small-cell network is investigated, where the multi-antenna base stations (BSs), cellular users, and Eves are all randomly distributed according to independent Poisson point processes. Stochastic geometry is applied to analyze the connection and secrecy outage probabilities and the average achievable secrecy rate. The impact of different parameters, including power allocation, BS and Eve density, and the adaptive eavesdropping or jamming on the secrecy performance is analyzed. It is shown that the average secrecy rate is a quasi-concave function of the power allocation factor and monotonically decreases with the ratio of the Eve-BS density. Finally, we study the physical layer security in a large-scale heterogeneous network consisting of both sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot spoofing attacks from the Eves, the coverage and secrecy probabilities are derived using stochastic geometry and the conditions under which the millimeter wave tier outperforms the sub-6 GHz counterpart are discussed in terms of both coverage and secrecy.It is shown that the mmWave small cell can provide better coverage performance in the high transmission code-rate region, and the secrecy performance of the mmWave system outperforms the sub-6 GHz counterpart in the low redundant rate region, which reveals the advantage of using mmWave for secure communication.
With the rapid development of wireless communications, security becomes extremely important. In many applications, each transmitter desires to send independent and confidential message to its intended receiver while ensuring mutual information-theoretic secrecy. In this dissertation, I study strategies for enhanced secrecy in wireless communication systems with limited rate feedback and transmitter cooperation. The two-user Gaussian channel model is considered under different scenarios. The transmitters both require channel state information (CSI), which is quantized at the receiver and fed back through the sum-rate-limited feedback channels. The quantization errors reduce the beamforming gain from the direct transmitter, and cause interference leakage from the cross transmitter. In the first scenario, I introduce the wiretap channel model where one receiver is a known eavesdropper, and a second transmitter is used to send a cooperative jamming signal to degrade the eavesdropper's channel. I consider two cases, one where no information about the eavesdroppers is available, and one where statistical CSI is available. With no information about the eavesdroppers, I show how to choose the allocation of feedback bits to the transmitters in order to maximize the amount of jamming power available to interfere with the eavesdroppers, subject to maintaining the lower bound on the rate at a minimum quality-of-service level. For the case of statistical CSI, I derive an approximate lower bound on the average secrecy rate, and optimize the bound to find a suitable bit allocation and the transmit power allocated to the transmitters. In the second scenario, I consider the interference channel model where the two transmitters are amenable to cooperation for improving the overall secrecy performance of the system. I derive an approximation for the average secrecy rate of each link, and optimize the sum secrecy rate over the transmit power and feedback bits allocated to the transmitters. Interestingly, increasing the transmit power beyond a certain point decreases the secrecy performance. When the transmitters have the same number of antennas, I derive the results in closed form. Simulations validate the theoretical analysis and demonstrate the significant performance gains that result from the use of optimal transmit power control and intelligent feedback bit allocation.
Exploiting the randomness embedded in wireless channels, the physical-layer security techniques can provide additional security robustness against malicious eavesdropping at the physical layer of communication systems. To further this line of study, this thesis focuses on developing advanced multi-antenna transmission strategies for enhancing the secrecy performance of wireless communication systems. Assuming perfect channel feedback from the desired receiver and none from the eavesdropper, we consider beamforming with smartly injected artificial noise, aiming to confuse the eavesdropper whilst leaving the desired receiver unaffected. The system parameters are either chosen to be fixed for all transmissions, or adaptively adjusted based on the channel feedback from the desired receiver. For both cases, we provide novel and explicit design solutions for achieving the maximum throughput subject to a secrecy outage constraint. We then derive accurate approximations for the maximum throughput, and give new insights into the additional power cost for achieving a higher security level and the throughput gain of adaptive transmission over non-adaptive transmission. Having investigated the perfect feedback scenario, we then relax the feedback assumption and consider practical effects imposed by limited feedback constraints on the channel from the desired receiver to the transmitter. To deal with quantization error due to limited feedback whilst also accounting for unknown channel of the eavesdropper, we specify a connection outage constraint to guarantee the reliability of desired communication and a secrecy outage constraint to guard against malicious eavesdropping. Under dual outage constraints, we derive a novel optimized rate-adaptive transmission design to maximize the average secrecy throughput. New insights regarding the optimal power/feedback allocation and the number of feedback bits required for efficient quantization are obtained through our analysis. Our preceding study focused on isolated point-to-point transmission without any interference. However, the performance of contemporary wireless networks is often limited by interference from concurrent transmissions. To characterize the impact of interference on secure communication, we consider an interference-limited large-scale network. To confuse the malicious eavesdroppers, in addition to information-bearing signals, the transmitters are designed to generate artificial noise through either beamforming or antenna sectoring in a distributed manner. For both cases, using tools from stochastic geometry, we provide a statistical characterization of the reliability performance of desired communication and the secrecy performance against eavesdropping. We then investigate the networkwide secrecy throughput performance and optimize the transmit power allocation. Our analysis reveals interesting differences between the beamforming and sectoring approaches in terms of the optimal power allocation and the maximum networkwide secrecy throughput.
Inherent openness of the wireless medium imposes stronger challenges on the security of wireless communications. Information-theoretic security addresses these challenges at the physical layer by using tools from wireless communication theory, signal processing and information theory. In information-theoretic security, physical layer communication is intelligently designed to exploit the characteristics of the wireless medium, such as fading, interference, cooperation, and multi-dimensional signaling, in order to provide or improve security. In this dissertation, we study the security of several fundamental wireless network configurations from an information-theoretic perspective. First, we study the Gaussian multiple-input multiple-output (MIMO) wiretap channel. In this channel, the transmitter sends a common message to both the legitimate user and the eavesdropper. In addition to the common message, a private message is sent only to the legitimate user, which needs to be kept hidden as much as possible from the eavesdropper. We obtain the entire capacity-equivocation region for this channel model. In particular, we show the sufficiency of jointly Gaussian auxiliary random variables and channel input to evaluate the existing single-letter description of the capacity-equivocation region due to Csiszar-Korner. Next, we study the secure broadcasting problem, where a transmitter wants to have secure communication with multiple legitimate users in the presence of an external eavesdropper. We study several special cases of the secure broadcasting problem. First, we consider the degraded multi-receiver wiretap channel, and establish its secrecy capacity region. Second, we consider the parallel less noisy multi-receiver wiretap channel, and obtain its common message secrecy capacity and sum secrecy capacity. Third, we consider the parallel degraded multi-receiver wiretap channel for the two-user and two-sub-channel case, and obtain its entire secrecy capacity region. Finally, we consider a parallel channel model with two sub-channels, where the transmitter can use only one of the subchannels at any time, and characterize its secrecy capacity region. Then, we study the two-user Gaussian MIMO broadcast channel with common and confidential messages. In this channel model, the transmitter sends a common message to both users, and a confidential message to each user which needs to be kept perfectly secret from the other user. We obtain the entire capacity region of this channel. We also explore the connections between this channel model and its non-confidential counterpart, i.e., the Gaussian MIMO broadcast channel with common and private message. Next, we consider the Gaussian MIMO multi-receiver wiretap channel and obtain its secrecy capacity region for the most general case. We first show that even for the single-input single-output (SISO) case, existing converse techniques fall short of proving the secrecy capacity region, to emphasize the need for a new proof technique, which we develop by using the relationships between the Fisher information and the differential entropy. Using this new proof technique, we obtain the secrecy capacity region of the degraded MIMO channel. We then establish the secrecy capacity region of the general MIMO channel by using the channel enhancement technique in conjunction with the capacity result we obtained for the degraded MIMO channel. For the general MIMO channel, we show that dirty-paper coding (DPC) combined with stochastic encoding attains the entire secrecy capacity region. Then, we study the multi-receiver wiretap channel for a more general scenario, where, in addition to confidential messages, the transmitter sends public messages to the legitimate users, on which there are no secrecy constraints. First, we consider the degraded discrete memoryless channel, and obtain inner and outer bounds for the capacity region. These inner and outer bounds match for certain cases, providing the capacity region. Second, we obtain an inner bound for the general discrete memoryless channel by using Marton's inner bound. Third, we consider the degraded Gaussian MIMO channel, and show that jointly Gaussian auxiliary random variables and channel input are sufficient to exhaust the inner and outer bounds. Finally, we provide an inner bound for the capacity region of the general Gaussian MIMO channel. Next, we focus on the multiple access wiretap (MAC-WT) channel whose capacity region is unknown. We consider a special class of MAC-WT channels which we call the weak eavesdropper class, where each user's link to the legitimate receiver is stronger than its link to the eavesdropper. For this class of channels, we develop an outer bound for the secrecy capacity region, which partially matches the achievable region in an n-letter form. We evaluate a looser version of our outer bound for the Gaussian case, and show that our outer bound is within 0.5 bits/channel use of the achievable rates along the individual secrecy rates for all weak eavesdropper Gaussian MAC-WT. Then, we investigate the effects of user cooperation on the secrecy of broadcast channels by considering the cooperative relay broadcast channel (CRBC). We propose an achievable scheme that combines Marton's coding scheme for broadcast channels and Cover and El Gamal's compress-and-forward (CAF) scheme for relay channels. For the Gaussian CRBC, we show that both users can have positive secrecy rates, which is not possible for scalar Gaussian broadcast channels without cooperation. We further investigate the effects of user cooperation on secrecy by considering the multiple access channel with generalized feedback (MAC-GF), which can be viewed as the MAC-dual of the CRBC. We propose a CAF-based achievable secrecy rate region for the MAC-GF. Specializing our results to a Gaussian MAC-GF, we present numerical results which demonstrate that cooperation can improve secrecy for the MAC-GF. Next, we study the two-user one-eavesdropper discrete memoryless compound wiretap channel, and provide the best known lower bound for the secrecy capacity of this compound channel. We evaluate this achievable secrecy rate for the Gaussian MIMO case by using DPC. We show that this achievable secrecy rate achieves at least half of the secrecy capacity of this Gaussian MIMO compound wiretap channel, and also attains the secrecy capacity when the eavesdropper is degraded with respect to one of the two users. Then, we study the degraded compound multi-receiver wiretap channel (DCMRWC), which, in addition to a group of eavesdroppers, has two groups of users, namely the stronger group and the weaker group. We study two different communication scenarios for this channel. In the first scenario, there is only one eavesdropper, and the transmitter sends a confidential message to each group of legitimate users while keeping both messages secret from the eavesdropper. In the second scenario, we study the DCMRWC with layered messages without any restriction on the number of eavesdroppers. For both scenarios, we obtain the secrecy capacity region for the discrete memoryless channel, the parallel channel, and the Gaussian parallel channel. For the Gaussian MIMO channel, we obtain the secrecy capacity region when there is only one user in the second group. Next, we study the two-user fading broadcast channel and obtain its ergodic secrecy capacity region. We show that, thanks to fading, both users can have simultaneous secure communication with the transmitter, although this is not possible in the scalar non-fading Gaussian broadcast channel where only one user can have secure communication. This simultaneous secrecy of both users is achieved by an opportunistic communication scheme, in which, at each time instant, the transmitter communicates with the user having a better channel gain. Then, we study the secure lossy transmission of a vector Gaussian source to a legitimate user in the presence of an eavesdropper, where both the legitimate user and the eavesdropper have vector Gaussian side information. We obtain an outer bound for the rate, equivocation and distortion region. Moreover, we obtain the maximum equivocation at the eavesdropper when there is no constraint on the transmission rate. By using this maximum equivocation result, we show two facts. First, for this problem, in general, Wyner-Ziv scheme is suboptimal, although, it is optimal in the absence of an eavesdropper. And, second, even when there is no transmission rate constraint, an uncoded transmission scheme is suboptimal; the presence of an eavesdropper necessitates the use of a coded scheme to attain the maximum equivocation. Finally, we revisit the secure lossy source coding problem. In all works on this problem, either the equivocation of the source at the eavesdropper or the equivocation of the legitimate user's reconstruction of the source at the eavesdropper is used to measure secrecy. We first propose the relative equivocation of the source at the eavesdropper with respect to the legitimate user as a new secrecy measure. We argue that this new secrecy measure is the one that corresponds to the natural generalization of the equivocation in a wiretap channel to the context of secure lossy source coding. Under this new secrecy measure, we provide a single-letter description of the rate, relative equivocation and distortion region, as well as its specializations to degraded and reversely degraded cases. We investigate the relationships between the optimal scheme that attains this region and the Wyner-Ziv scheme.
The use of wireless networks has grown significantly in contemporary \ntimes, and continues to develop further. The broadcast nature of \nwireless communications, however, makes them particularly vulnerable \nto eavesdropping. Unlike traditional solutions, which usually handle \nsecurity at the application layer, the primary concern of this \ndissertation is to analyze and develop solutions based on coding \ntechniques at the physical-layer. \n \nFirst, in chapter 2, we consider a scenario where a source node \nwishes to broadcast two confidential messages to two receivers, \nwhile a wire-tapper also receives the transmitted signal. This model \nis motivated by wireless communications, where individual secure \nmessages are broadcast over open media and can be received by any \nillegitimate receiver. The secrecy level is measured by the \nequivocation rate at the eavesdropper. We first study the general \n(non-degraded) broadcast channel with an eavesdropper, and present \nan inner bound on the secrecy capacity region for this model. This \ninner bound is based on a combination of random binning, and the \nGelfand-Pinsker binning. We further study the situation in which the \nchannels are degraded. For the degraded broadcast channel with an \neavesdropper, we present the secrecy capacity region. Our achievable \ncoding scheme is based on Cover's superposition scheme and random \nbinning. We refer to this scheme as the Secret Superposition Scheme. \nOur converse proof is based on a combination of the converse proof \nof the conventional degraded broadcast channel and Csiszar Lemma. We \nthen assume that the channels are Additive White Gaussian Noise and \nshow that the Secret Superposition Scheme with Gaussian codebook is \noptimal. The converse proof is based on Costa's entropy power \ninequality. Finally, we use a broadcast strategy for the slowly \nfading wire-tap channel when only the eavesdropper's channel is \nfixed and known at the transmitter. We derive the optimum power \nallocation for the coding layers, which maximizes the total average \nrate. \n \n \nSecond, in chapter 3 , we consider the \nMultiple-Input-Multiple-Output (MIMO) scenario of a broadcast \nchannel where a wiretapper also receives the transmitted signal via \nanother MIMO channel. First, we assume that the channels are \ndegraded and the wiretapper has the worst channel. We establish the \ncapacity region of this scenario. Our achievability scheme is the \nSecret Superposition Coding. For the outerbound, we use notion of \nthe enhanced channels to show that the secret superposition of \nGaussian codes is optimal. We show that we only need to enhance the \nchannels of the legitimate receivers, and the channel of the \neavesdropper remains unchanged. We then extend the result of the \ndegraded case to a non-degraded case. We show that the secret \nsuperposition of Gaussian codes, along with successive decoding, \ncannot work when the channels are not degraded. We develop a Secret \nDirty Paper Coding scheme and show that it is optimal for this \nchannel. We then present a corollary generalizing the capacity \nregion of the two receivers case to the case of multiple receivers. \nFinally, we investigate a scenario which frequently occurs in the \npractice of wireless networks. In this scenario, the transmitter and \nthe eavesdropper have multiple antennae, while both intended \nreceivers have a single antenna (representing resource limited \nmobile units). We characterize the secrecy capacity region in terms \nof generalized eigenvalues of the receivers' channels and the \neavesdropper's channel. We refer to this configuration as the MISOME \ncase. We then present a corollary generalizing the results of the \ntwo receivers case to multiple receivers. In the high SNR regime, we \nshow that the capacity region is a convex closure of rectangular \nregions. \n \n \nFinally, in chapter 4, we consider a K-user secure Gaussian \nMultiple-Access-Channel with an external eavesdropper. We establish \nan achievable rate region for the secure discrete memoryless MAC. \nThereafter, we prove the secrecy sum capacity of the degraded \nGaussian MIMO MAC using Gaussian codebooks. For the non-degraded \nGaussian MIMO MAC, we propose an algorithm inspired by the \ninterference alignment technique to achieve the largest possible \ntotal Secure-Degrees-of-Freedom . When all the terminals are \nequipped with a single antenna, Gaussian codebooks have shown to be \ninefficient in providing a positive S-DoF. Instead, we propose a \nnovel secure coding scheme to achieve a positive S-DoF in the single \nantenna MAC. This scheme converts the single-antenna system into a \nmultiple-dimension system with fractional dimensions. The \nachievability scheme is based on the alignment of signals into a \nsmall sub-space at the eavesdropper, and the simultaneous separation \nof the signals at the intended receiver. We use tools from the field \nof Diophantine Approximation in number theory to analyze the \nprobability of error in the coding scheme. We prove that the total \nS-DoF of fracK−1K can be achieved for almost all channel \ngains. For the other channel gains, we propose a multi-layer coding \nscheme to achieve a positive S-DoF. As a function of channel gains, \ntherefore, the achievable S-DoF is discontinued.
The current research on physical layer security is far from implementations in practical networks, arguably due to impractical assumptions in the literature and the limited applicability of physical layer security. Aiming to reduce the gap between theory and practice, this thesis focuses on wireless physical layer security towards practical assumptions and requirements. In the first half of the thesis, we reduce the dependence of physical layer security on impractical assumptions. The secrecy enhancements and analysis based on impractical assumptions cannot lead to any true guarantee of secrecy in practical networks. The current study of physical layer security was often based on the idealized assumption of perfect channel knowledge on both legitimate users and eavesdroppers. We study the impact of channel estimation errors on secure transmission designs. We investigate the practical scenarios where both the transmitter and the receiver have imperfect channel state information (CSI). Our results show how the optimal transmission design and the achievable throughput vary with the amount of knowledge on the eavesdropper's channel. Apart from the assumption of perfect CSI, the analysis of physical layer security often ideally assumed the number of eavesdropper antennas to be known. We develop an innovative approach to study secure communication systems without knowing the number of eavesdropper antennas by introducing the concept of spatial constraint into physical layer security. That is, the eavesdropper is assumed to have a limited spatial region to place (possibly an infinite number of) antennas. We show that a non-zero secrecy rate is achievable with the help of a friendly jammer, even if the eavesdropper places an infinite number of antennas in its spatial region. In the second half of the thesis, we improve the applicability of physical layer security. The current physical layer security techniques to achieve confidential broadcasting were limited to application in single-cell systems. The primary challenge to achieve confidential broadcasting in the multi-cell network is to deal with not only the inter-cell but also the intra-cell information leakage and interference. To tackle this challenge, we design linear precoders performing confidential broadcasting in multi-cell networks. We optimize the precoder designs to maximize the secrecy sum rate with based on the large-system analysis. Finally, we improve the applicability of physical layer security from a fundamental aspect. The analysis of physical layer security based on the existing secrecy metric was often not applicable in practical networks. We propose new metrics for evaluating the secrecy of transmissions over fading channels to address the practical limitations of using existing secrecy metrics for such evaluations. The first metric establishes a link between the concept of secrecy outage and the eavesdropper's ability to decode confidential messages. The second metric provides an error-probability-based secrecy metric which is often used for the practical implementation of secure wireless systems. The third metric characterizes how much or how fast the confidential information is leaked to the eavesdropper. We show that the proposed secrecy metrics enable one to appropriately design secure communication systems with different views on how secrecy is measured.