Physical layer security has recently been regarded as an emerging technique to complement and improve the communication security in future wireless networks. The current research and development in physical layer security are often based on the ideal assumption of perfect channel knowledge or the capability of variable-rate transmissions. In this paper, we study the secure transmission design in more practical scenarios by considering channel estimation errors at the receiver and investigating both fixed-rate and variable-rate transmissions. Assuming quasi-static fading channels, we design secure on-off transmission schemes to maximize the throughput subject to a constraint on secrecy outage probability. For systems with given and fixed encoding rates, we show how the optimal on-off transmission thresholds and the achievable throughput vary with the amount of knowledge on the eavesdropper's channel. In particular, our design covers the interesting case where the eavesdropper also uses the pilots sent from the transmitter to obtain imperfect channel estimation. An interesting observation is that using too much pilot power can harm the throughput of secure transmission if both the legitimate receiver and the eavesdropper have channel estimation errors, while the secure transmission always benefits from increasing pilot power when only the legitimate receiver has channel estimation errors but not the eavesdropper. When the encoding rates are controllable parameters to design, we further derive both a non-adaptive and an adaptive rate transmission schemes by jointly optimizing the encoding rates and the on-off transmission thresholds to maximize the throughput of secure transmissions.
Physical layer security has recently been regarded as an emerging technique to complement and improve the communication security in future wireless networks. The current research and development in physical layer security are often based on the ideal assumption of perfect channel knowledge or the capability of variable-rate transmissions. In this paper, we study the secure transmission design in more practical scenarios by considering channel estimation errors at the receiver and investigating both fixed-rate and variable-rate transmissions. Assuming quasi-static fading channels, we design secure on-off transmission schemes to maximize the throughput subject to a constraint on secrecy outage probability. For systems with given and fixed encoding rates, we show how the optimal on-off transmission thresholds and the achievable throughput vary with the amount of knowledge on the eavesdropper's channel. In particular, our design covers the interesting case where the eavesdropper also uses the pilots sent from the transmitter to obtain imperfect channel estimation. An interesting observation is that using too much pilot power can harm the throughput of secure transmission if both the legitimate receiver and the eavesdropper have channel estimation errors, while the secure transmission always benefits from increasing pilot power when only the legitimate receiver has channel estimation errors but not the eavesdropper. When the encoding rates are controllable parameters to design, we further derive both a non-adaptive and an adaptive rate transmission schemes by jointly optimizing the encoding rates and the on-off transmission thresholds to maximize the throughput of secure transmissions.
This paper studies physical-layer secure transmissions from a transmitter to a legitimate receiver against an eavesdropper over slow fading channels, taking into account the impact of finite blocklength secrecy coding. A comprehensive analysis and optimization framework is established to investigate secrecy throughput for both single- and multi-antenna transmitter scenarios. Both adaptive and non-adaptive design schemes are devised, in which the secrecy throughput is maximized by exploiting the instantaneous and statistical channel state information of the legitimate receiver, respectively. Specifically, optimal transmission policy, blocklength, and code rates are jointly designed to maximize the secrecy throughput. Additionally, null-space artificial noise is employed to improve the secrecy throughput for the multi-antenna setup with the optimal power allocation derived. Various important insights are developed. In particular, 1) increasing blocklength benefits both reliability and secrecy under the proposed transmission policy; 2) secrecy throughput monotonically increases with blocklength; 3) secrecy throughput initially increases but then decreases as secrecy rate increases, and the optimal secrecy rate maximizing the secrecy throughput should be carefully chosen in order to strike a good balance between rate and decoding correctness. Numerical results are eventually presented to verify theoretical findings.
In this paper, we investigate the secure transmission design with practical assumptions on the channel state information (CSI). Specifically, the CSI of the legitimate link is imperfectly estimated and the CSI of the eavesdropper is unknown to the legitimate users. We derive both a fixed-rate and an adaptive-rate transmission schemes by explicitly considering the rate parameters of the wiretap code in the secure transmission design. The derived schemes achieve the maximum throughput subject to constraints on the secrecy performance against eavesdropping and the reliability performance against channel estimation error. For a given target throughput requirement, our numerical results illustrate how the security performance needs to be compromised when the accuracy of channel estimation reduces. One interesting finding is that the security cost of having a small amount of channel estimation error is low, while the security cost increases fast when the channel estimation error becomes considerable.
The issue of physical layer security for the wireless communication with slow fading is addressed in this paper, and the secrecy outage performance is concerned. The system is modeled as a wiretap channel, and each of the transmitter and receivers is equipped with a single antenna. The channel state information (CSI) about the channel from the transmitter to the legitimate receiver (the main channel) is assumed to be perfectly known by the transmitter and the receivers, while the transmitter does not know the CSI about the channel from the transmitter to the eavesdropper (the eavesdropper's channel) but its probability density function (PDF). Differing from the fixed secrecy rate scheme and the on/off scheme in the literature, this paper adaptively chooses the secrecy rate. The secrecy rate for each value of the main channel gain is determined by the required secrecy outage probability and the PDF of the eavesdropper's channel, and the secrecy throughput is maximized under the constraint of the transmit power for the required secrecy outage probability. The quasi-static Rayleigh fading channel is used to compare our scheme with the two existing schemes, and simulation results show that our scheme outperforms them when small secrecy outage probabilities are required.
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.
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.
In this paper, we investigate the physical-layer security of slow fading wiretap channel with channel estimation errors. We aim to maximize the reliable secrecy rate under constraints of secrecy outage probability and reliability outage probability. To this end, we propose an adaptive (ADP) transmission scheme in which the communication rate and secrecy rate are adaptively adjusted based on the estimated imperfect channel state information (CSI) of the legitimate channel. The optimal solution is obtained by solving a single-variable maximization problem, where the objective function is log-concave. Numerical results are provided to verify our approach as well as to show the impact of imperfect CSI on the secrecy throughput.