102 research outputs found

    Secrecy Energy Efficiency of MIMOME Wiretap Channels with Full-Duplex Jamming

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    Full-duplex (FD) jamming transceivers are recently shown to enhance the information security of wireless communication systems by simultaneously transmitting artificial noise (AN) while receiving information. In this work, we investigate if FD jamming can also improve the systems secrecy energy efficiency (SEE) in terms of securely communicated bits-per- Joule, when considering the additional power used for jamming and self-interference (SI) cancellation. Moreover, the degrading effect of the residual SI is also taken into account. In this regard, we formulate a set of SEE maximization problems for a FD multiple-input-multiple-output multiple-antenna eavesdropper (MIMOME) wiretap channel, considering both cases where exact or statistical channel state information (CSI) is available. Due to the intractable problem structure, we propose iterative solutions in each case with a proven convergence to a stationary point. Numerical simulations indicate only a marginal SEE gain, through the utilization of FD jamming, for a wide range of system conditions. However, when SI can efficiently be mitigated, the observed gain is considerable for scenarios with a small distance between the FD node and the eavesdropper, a high Signal-to-noise ratio (SNR), or for a bidirectional FD communication setup.Comment: IEEE Transactions on Communication

    Power allocation and signal labelling on physical layer security

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    PhD ThesisSecure 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

    On Secrecy Metrics for Physical Layer Security over Quasi-Static Fading Channels

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    Theoretical studies on physical layer security often adopt the secrecy outage probability as the performance metric for wireless communications over quasi-static fading channels. The secrecy outage probability has two limitations from a practical point of view: a) it does not give any insight into the eavesdropper's decodability of confidential messages; b) it cannot characterize the amount of information leakage to the eavesdropper when an outage occurs. Motivated by the limitations of the secrecy outage probability, we propose three new secrecy metrics for secure transmissions over quasi-static fading channels. The first metric establishes a link between the concept of secrecy outage and the decodability of messages at the eavesdropper. The second metric provides an error-probability-based secrecy metric which is typically 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 collectively give a more comprehensive understanding of physical layer security over fading channels and enable one to appropriately design secure communication systems with different views on how secrecy is measured.ARC Discovery Projects Grant DP15010390

    Robust Optimization of Private Communication in Multi-Antenna Systems

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    The thesis focuses on the privacy of communication that can be ensured by means of the physical layer, i.e., by appropriately chosen coding and resource allocation schemes. The fundamentals of physical-layer security have been already formulated in the 1970s by Wyner (1975), Csiszár and Körner (1978). But only nowadays we have the technical progress such that these ideas can find their way in current and future communication systems, which has driven the growing interest in this area of research in the last years. We analyze two physical-layer approaches that can ensure the secret transmission of private information in wireless systems in presence of an eavesdropper. One is the direct transmission of the information to the intended receiver, where the transmitter has to simultaneously ensure the reliability and the secrecy of the information. The other is a two-phase approach, where two legitimated users first agree on a common and secret key, which they use afterwards to encrypt the information before it is transmitted. In this case, the secrecy and the reliability of the transmission are managed separately in the two phases. The secrecy of the transmitted messages mainly depends on reliable information or reasonable and justifiable assumptions about the channel to the potential eavesdropper. Perfect state information about the channel to a passive eavesdropper is not a rational assumption. Thus, we introduce a deterministic model for the uncertainty about this channel, which yields a set of possible eavesdropper channels. We consider the optimization of worst-case rates in systems with multi-antenna Gaussian channels for both approaches. We study which transmit strategy can yield a maximum rate if we assume that the eavesdropper can always observe the corresponding worst-case channel that reduces the achievable rate for the secret transmission to a minimum. For both approaches, we show that the resulting max-min problem over the matrices that describe the multi-antenna system can be reduced to an equivalent problem over the eigenvalues of these matrices. We characterize the optimal resource allocation under a sum power constraint over all antennas and derive waterfilling solutions for the corresponding worst-case channel to the eavesdropper for a constraint on the sum of all channel gains. We show that all rates converge to finite limits for high signal-to-noise ratios (SNR), if we do not restrict the number of antennas for the eavesdropper. These limits are characterized by the quotients of the eigenvalues resulting from the Gramian matrices of both channels. For the low-SNR regime, we observe a rate increase that depends only on the differences of these eigenvalues for the direct-transmission approach. For the key generation approach, there exists no dependence from the eavesdropper channel in this regime. The comparison of both approaches shows that the superiority of an approach over the other mainly depends on the SNR and the quality of the eavesdropper channel. The direct-transmission approach is advantageous for low SNR and comparably bad eavesdropper channels, whereas the key generation approach benefits more from high SNR and comparably good eavesdropper channels. All results are discussed in combination with numerous illustrations.Der Fokus dieser Arbeit liegt auf der Abhörsicherheit der Datenübertragung, die auf der Übertragungsschicht, also durch geeignete Codierung und Ressourcenverteilung, erreicht werden kann. Die Grundlagen der Sicherheit auf der Übertragungsschicht wurden bereits in den 1970er Jahren von Wyner (1975), Csiszár und Körner (1978) formuliert. Jedoch ermöglicht erst der heutige technische Fortschritt, dass diese Ideen in zukünftigen Kommunikationssystemen Einzug finden können. Dies hat in den letzten Jahren zu einem gestiegenen Interesse an diesem Forschungsgebiet geführt. In der Arbeit werden zwei Ansätze zur abhörsicheren Datenübertragung in Funksystemen analysiert. Dies ist zum einen die direkte Übertragung der Information zum gewünschten Empfänger, wobei der Sender gleichzeitig die Zuverlässigkeit und die Abhörsicherheit der Übertragung sicherstellen muss. Zum anderen wird ein zweistufiger Ansatz betrachtet: Die beiden Kommunikationspartner handeln zunächst einen gemeinsamen sicheren Schlüssel aus, der anschließend zur Verschlüsselung der Datenübertragung verwendet wird. Bei diesem Ansatz werden die Abhörsicherheit und die Zuverlässigkeit der Information getrennt voneinander realisiert. Die Sicherheit der Nachrichten hängt maßgeblich davon ab, inwieweit zuverlässige Informationen oder verlässliche Annahmen über den Funkkanal zum Abhörer verfügbar sind. Die Annahme perfekter Kanalkenntnis ist für einen passiven Abhörer jedoch kaum zu rechtfertigen. Daher wird hier ein deterministisches Modell für die Unsicherheit über den Kanal zum Abhörer eingeführt, was zu einer Menge möglicher Abhörkanäle führt. Die Optimierung der sogenannten Worst-Case-Rate in einem Mehrantennensystem mit Gaußschem Rauschen wird für beide Ansätze betrachtet. Es wird analysiert, mit welcher Sendestrategie die maximale Rate erreicht werden kann, wenn gleichzeitig angenommen wird, dass der Abhörer den zugehörigen Worst-Case-Kanal besitzt, welcher die Rate der abhörsicheren Kommunikation jeweils auf ein Minimum reduziert. Für beide Ansätze wird gezeigt, dass aus dem resultierenden Max-Min-Problem über die Matrizen des Mehrantennensystems ein äquivalentes Problem über die Eigenwerte der Matrizen abgeleitet werden kann. Die optimale Ressourcenverteilung für eine Summenleistungsbeschränkung über alle Sendeantennen wird charakterisiert. Für den jeweiligen Worst-Case-Kanal zum Abhörer, dessen Kanalgewinne einer Summenbeschränkung unterliegen, werden Waterfilling-Lösungen hergeleitet. Es wird gezeigt, dass für hohen Signal-Rausch-Abstand (engl. signal-to-noise ratio, SNR) alle Raten gegen endliche Grenzwerte konvergieren, wenn die Antennenzahl des Abhörers nicht beschränkt ist. Die Grenzwerte werden durch die Quotienten der Eigenwerte der Gram-Matrizen beider Kanäle bestimmt. Für den Ratenanstieg der direkten Übertragung ist bei niedrigem SNR nur die Differenz dieser Eigenwerte maßgeblich, wohingegen für den Verschlüsselungsansatz in dem Fall keine Abhängigkeit vom Kanal des Abhörers besteht. Ein Vergleich zeigt, dass das aktuelle SNR und die Qualität des Abhörkanals den einen oder anderen Ansatz begünstigen. Die direkte Übertragung ist bei niedrigem SNR und verhältnismäßig schlechten Abhörkanälen überlegen, wohingegen der Verschlüsselungsansatz von hohem SNR und vergleichsweise guten Abhörkanälen profitiert. Die Ergebnisse der Arbeit werden umfassend diskutiert und illustriert
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