A common approach for introducing security at the physical layer is to rely on the channel variations of the wireless environment. This type of approach is not always suitable for wireless networks where the channel remains static for most of the network lifetime. For these scenarios, a channel independent physical layer security technique is more appropriate which will rely on a secret known to the sender and the receiver but not to the eavesdropper. In this paper, we propose CD-PHY, a physical layer security technique that exploits the constellation diversity of wireless networks which is independent of the channel variations. The sender and the receiver use a custom bit sequence to constellation symbol mapping to secure the physical layer communication which is not known a priori to the eavesdropper. Through theoretical modeling and experimental simulation, we show that this information theoretic construct can achieve Shannon secrecy and any brute force attack from the eavesdropper incurs high overhead and minuscule probability of success. Our results also show that the high bit error rate also makes decoding practically infeasible for the eavesdropper, thus securing the communication between the sender and receiver.
A common approach for introducing security at the physical layer is to rely on the channel variations of the wireless environment. This type of approach is not always suitable for wireless networks where the channel remains static for most of the network lifetime. For these scenarios, a channel independent physical layer security technique is more appropriate which will rely on a secret known to the sender and the receiver but not to the eavesdropper. In this paper, we propose CD-PHY, a physical layer security technique that exploits the constellation diversity of wireless networks which is independent of the channel variations. The sender and the receiver use a custom bit sequence to constellation symbol mapping to secure the physical layer communication which is not known a priori to the eavesdropper. Through theoretical modeling and experimental simulation, we show that this information theoretic construct can achieve Shannon secrecy and any brute force attack from the eavesdropper incurs high overhead and minuscule probability of success. Our results also show that the high bit error rate also makes decoding practically infeasible for the eavesdropper, thus securing the communication between the sender and receiver.
Security is a critical element in the design of communication systems. However, despite its essential role in enabling low-cost implementations and robust theoretical foundations, physical layer security is often overlooked in developing countermeasures against eavesdropping. In this work, we propose practical physical-layer countermeasures that enhance secrecy with low computational overhead. Our approach decomposes the constellation diagram using basis vectors, constructing the original message symbol in the main lobe aimed at the intended receiver, while projecting randomized constellation points in other directions to mislead potential eavesdroppers. When the eavesdropper’s location is known, we use static basis vectors, achieving a higher level of secrecy in that direction. When the location is unknown, we dynamically change the basis vectors to provide information-theoretic security in all directions through basis vector randomization. This design enables the intended receiver to operate without knowledge of the basis vectors, reducing receiver overhead. We evaluate the scheme’s secrecy performance through bit error rate and secrecy rate analyses, demonstrating its potential to improve physical layer security.
In this paper, a hybrid scheme to provide wireless physical layer security based on encryption and wireless channel pre-compensation is presented. The physical layer security is provided by converting the signal constellation into a higher order constellation by using conventional encryption (symmetric key encryption) and then using channel state information (i.e. amplitude and phase variation), which is location specific, to further encrypt the constellation before transmission. Since detection and demodulation are based upon the known signal constellation, which is not known in advance in our case, it becomes difficult for the eavesdropper to determine the original signal constellation. The data intercepted by the eavesdropper is unintelligible due to the combined effect of encryption and pre-compensation. Using bit error rate (BER) as performance metric, it is found that the eavesdropper suffers from high BER as compared to its legitimate counterpart hence providing secure communication. The developed scheme was tested with existing wireless communication standard IEEE 802.11 to show the practical application of the scheme.
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.
This paper proposes a physical-layer security scheme for wireless networks, aiming to achieve communication secrecy by making the eavesdropper incapable of decoding the secret wireless message. The considered scenario features one user within the range of two access points, API and AP2. The APs are assumed to be connected through an alternative secure (e.g. wired) connection. The goal is to secure the wireless link between the user and API. While the user transmits to API, AP2 simultaneously transmits an interfering signal, which is a priori provided to API, such that API is likely the only node capable of decoding the user's entire transmission. Evaluation is done through simulation by measuring the upper bound of the information-theoretic secrecy and error performance. In the latter case it is shown that the eavesdropper experiences significantly higher error rates than the intended receiver, thus providing evidence of practical security.
Wireless communications are naturally exposed to important security risks due to their broadcast nature. In contrast with wired communications, no physical medium is isolating the transmission surroundings to a specific path. Traditional methods to reach the goals of communications security rely on algorithms at protocol stack level, which are based on mathematically derived cryptographic protocols. With the tremendous proliferation of wireless devices, and the increase in computational power available for decryption, traditional cryptographic measures may be compromised. To protect communications at the lowest level, physical layer (PHY) security methods have been proposed recently. These techniques aim to ensure secure communications by exploiting the unique characteristics of the physical medium between communicating nodes, effectively reducing the decryption capabilities of an eavesdropper to intercept transmissions. This thesis explains within a general framework the most important PHY layer security schemes. First we explore approaches which utilize the knowledge of the unique channel between communicating nodes to distort a transmitted signal with various kinds of randomizations, in such a way that the intended receivers are unaffected by the distortion, but any other observer will be exposed to it. These techniques, however, introduce several trade-offs between transmission resources and the effectiveness in achieving secrecy. Of particular interest for this investigation is a class of methods which can be referred to as "security enhancing techniques", in which minimal priors about the potential eavesdroppers locations and capabilities are assumed. In this thesis, a wave-based approach to the communication secrecy problem is introduced. An interesting perspective is provided by studying the relation between electromagnetic theory and information theory, the so-called electromagnetic information theory. This allows to properly identify the unavoidable limitations imposed by the laws of electromagnetics to our ability to communicate securely. The proposed model can be extended to the analysis of different signal distortion methodologies and different channel conditions. The resulting approach is highly relevant to the study of PHY layer security methods since it is physically fundamental as well as architecture independent, which leads to a consolidating point of view of the secrecy problem. In addition, the thesis presents computer simulation results which shed insight on the security possibilities and limitations of three different security scenarios, which are termed MISO (multiple-input, single-output) (with single eavesdropper), MISOME (multiple-input, single-output, multiple eavesdropper), and MIMOME (multiple-input, multiple-output, multiple-eavesdropper).
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.