Wireless networks are susceptible to eavesdropping due to the inherent broadcast nature of the wireless medium. However, it is also this broadcast nature of wireless communications that allows cooperation among multiple users or relay nodes. Thus, it is unclear how cooperation and secrecy interacts, particularly in the scenario when each node in the network is equipped with multiple antennas (MIMO). In this paper, we consider the information theoretic secure transmission in a classical four node wiretap relay network, consisting of a source node, a destination node, and a relay node, in the presence of a fourth adversarial wiretapper node that can observe all transmissions from the source as well as the relay node. For a MIMO quasi-static fading channel model between every pair of nodes, we explore the importance of degrees of freedom in providing secure transmissions, through secrecy outage analysis in the high signal-to-noise ratio regime. We characterize the achievable diversity versus secure multiplexing gain tradeoff for decode-and-forward as well as compress-and-forward relaying, under the full-duplex constraint.
Wireless networks are susceptible to eavesdropping due to the inherent broadcast nature of the wireless medium. However, it is also this broadcast nature of wireless communications that allows cooperation among multiple users or relay nodes. Thus, it is unclear how cooperation and secrecy interacts, particularly in the scenario when each node in the network is equipped with multiple antennas (MIMO). In this paper, we consider the information theoretic secure transmission in a classical four node wiretap relay network, consisting of a source node, a destination node, and a relay node, in the presence of a fourth adversarial wiretapper node that can observe all transmissions from the source as well as the relay node. For a MIMO quasi-static fading channel model between every pair of nodes, we explore the importance of degrees of freedom in providing secure transmissions, through secrecy outage analysis in the high signal-to-noise ratio regime. We characterize the achievable diversity versus secure multiplexing gain tradeoff for decode-and-forward as well as compress-and-forward relaying, under the full-duplex constraint.
In this paper, we consider a secrecy relaying communication scenario where all nodes are equipped with multiple antennas. An eavesdropper has the access to the global channel state information (CSI), and all the other nodes only know the CSI not associated with the eavesdropper. A new secrecy transmission protocol is proposed, where the concept of interference alignment is combined with cooperative jamming to ensure that artificial noise from transmitters can be aligned at the destination, but not at the eavesdropper due to the randomness of wireless channels. Analytical results, such as ergodic secrecy rate and outage probability, are developed, from which more insightful understanding of the proposed protocol, such as multiplexing and diversity gains, can be obtained. A few special cases, where outage probability cannot be decreased to zero regardless of SNR, are also discussed. Simulation results are provided to demonstrate the performance of the proposed secrecy transmission protocol.
Achieving secure transmission in wireless network is a significantly important research topic. Classic cryptography encrypts a confidential message to an unreadable cipher message. Thus only the authentic receiver with correct secrecy key is able to decrypt the message. However, with the growing computational capability of adversaries, the requirement of secrecy key generation and distribution is getting stringent. Another method to achieve wireless security, which has attracted much attention in recent years, is through physical layer processing, named physical layer security. In a typical wiretap channel model consisting of a transmitter, a legal receiver and an adversary (illegal receiver), early works have proven that it is possible to achieve positive secrecy rate using multiple antenna techniques, such as beamforming, at the transmitter. In this thesis, we focus on two types of malicious behaviors of the adversary: passive eavesdropping and active attack. For the passive eavesdropping, we first consider the multi-input-single-output (MISO) wiretap channel, in which the legitimate transmitter utilizes the artificial noise aided precoding strategy to maximize the achievable secrecy rate of the channel. Both scenarios of known and unknown channel state information (CSI) for the eavesdropper channels are considered. By deriving the closed-form expression of ergodic secrecy rate, we prove that there exists an optimal power ratio between the information signal and the artificial noise. We then extend our study to a four-node fading channel model in a wireless relay network, which includes two single-antenna users, one single-antenna eavesdropper and one relay node equipped with multiple antennas. By studying the decode-and-forward (DF) scheme, cooperative jamming (CJ) scheme and one proposed hybrid relay scheme, relay-and-jamming (RJ), we aims to find the optimal relay scheme with the condition that the transmitter and the relay node are under individual power constraints and the CSI of the illegitimate channel is only statistically known. Our analysis shows that the hybrid RJ scheme is optimal when the transmitter has a relatively small power budget and/or the transmitter is far away from the intended receiver. Besides the passive eavesdropping, the adversary could also choose the active attack. The pilot spoofing attack is one active attack conducted by the adversary during the channel estimation phase of the legitimate transmission. In practical systems, the pilot signals are usually known \textit{a priori} and repeatedly used information. In this attack, an intelligent adversary spoofs the transmitter on the estimation of CSI by sending the identical pilot signal as the legitimate receiver. By doing so, the adversary could obtain a larger channel rate in the data transmission phase, and also drastically weaken the strength of the received signal at the legitimate receiver if the adversary utilizes large enough power. Due to the serious consequences caused by such an attack, we first propose an energy ratio detector (ERD) that explores the asymmetry of received signal power levels at the transmitter and the legitimate receiver when the attack occurs. Our analysis shows that the ERD could provide very high probability of detection with certain requirement on the false alarming probability. Furthermore, we design a two-way training based scheme to provide not only attack detection but also secure transmission. The two-way training based detector (TWTD) exploits the intrusive component created by the adversary, followed by a secure beamforming-assisted data transmission. In addition to the good detection performance, our scheme could estimate both legitimate and illegitimate channels, with which the beamforming could be designed for data transmission.
We consider a confidential communication system in which a source sends a confidential information to the destination in the presence of an eavesdropper. Multiple relays are used to provide cooperative diversity to the destination. The destination and the eavesdropper are equipped with multiple antennas while each relay and source are equipped with single antenna. We are interested to protect the transmitted information from eavesdropping and to find the effect of receive diversity on the secrecy capacity of the proposed model. We consider the maximal ratio combining (MRC) technique at the destination and derive the expression of combiner output signal-to-noise ratio (SNR) using optimum weighting vector so that the eavesdropper is unable to decode a single bit from the original massage. Finally, we derive the expression of optimum secrecy capacity in terms of the number of relays and the number of antennas at the destination. Our results show that the optimization of weighting vector at the receiver enhances secrecy capacity of a wiretap channel.
This paper investigates the secrecy outage performance of buffer-aided multirelay multiple-input multiple-output cooperative systems in the presence of a passive eavesdropper. Due to the unavailability of the channel state information of eavesdropper's channel, a buffer-aided joint transmit antenna and relay selection scheme based on the main channel is proposed to enhance the secrecy performance. Specifically, we model the evolution of the relay buffers as a Markov chain and derive new exact and asymptotic closed-form expressions for the secrecy outage probability, which provides an efficient way to assess the effect of system parameters on the secrecy outage probability. Moreover, simple asymptotic results are further exploited under two special scenarios, i.e., L → ∞ and L → ∞ (where L denotes the size of the relay buffers), for characterizing the achievable secrecy diversity gain, the secrecy coding gain, and the secrecy diversity-multiplexing tradeoff. Our results reveal that: 1) a secrecy diversity gain of NRMmin(NS, ND) is achieved when L → ∞, however, when L → ∞, the secrecy diversity gain increases to NRM(NS+ ND), where NS, NR, and ND represent the number of antennas at the source, each of M relays and the destination, respectively. 2) The eavesdropper's channel does not affect the secrecy diversity gain but only the secrecy coding gain in both the two scenarios.
Secure communication over a half-duplex relay system in the presence of an external wiretapper is investigated. In such a scenario, conventional relaying protocols generally provide secrecy rates that converge to a finite constant in the high signal-to-noise ratio (SNR) regime. It is shown in this work that strictly superior performance can be achieved by allowing the destination to transfer random keys in the form of known interference to the relay insecurely over the wireless medium. When there is no direct link between the source and the destination, an amplify-and-forward (AF) protocol is shown to achieve a secrecy rate that scales as 1/2 log SNR, if each of the legitimate nodes has a single antenna only. However, in the presence of a direct link between source and destination, this scaling rule no longer applies. The bottleneck of the AF scheme in the presence of a direct source-destination link is identified, motivating the use of multiple antennas at the destination to almost surely guarantee a secure degree of freedom of one half over the channel. The results hold even if the eavesdropper has more antennas than the source and the relay combined.
This chapter considers securing wireless communications at the physical layer using multiple antennas. In particular, the multiple antenna (MIMO) wiretap channel is presented and its secrecy capacity is provided. Further investigation of the high-SNR characterization of the secrecy capacity, i.e., secure degrees of freedom, reveals that the secrecy capacity of the MIMO wiretap channel does not scale with the transmit power when the eavesdropper has an advantage over the legitimate transmitter in the number of antennas. An external multi-antenna cooperative jammer terminal is introduced to the channel model in order to improve its secrecy capacity scaling with power. The secure degrees of freedom for this new multi-terminal multi-antenna channel is characterized. While for these models the eavesdropper channel state information is known at the legitimate terminals, the chapter next removes this assumption and presents results that utilize multiple antennas in order to provide secure communications irrespective of the eavesdropper channel state. An achievable strong secrecy rate and the secure degrees of freedom characterization for the MIMO wiretap channel when the eavesdropper channel is arbitrary, varying, and unknown at the legitimate terminals are presented. Finally, the extension of this model to the two-user multiple access MIMO wiretap channel is considered and its secure degrees of freedom region is characterized. Introduction The wiretap channel and the notion of secrecy capacity, introduced in [1] and generalized by [2], have provided the framework for physical layer design for wireless channels with information theoretic guarantees against eavesdropping. A natural model in wireless communications is one which utilizes multiple antennas for transmission and reception at each node. The improvement in secrecy rate that can be obtained using multiple antennas in a point-to-point channel with an eavesdropper has been studied extensively in several references, including [3, 4]. Specifically, the secrecy capacity of the Gaussian model consisting of a multiple antenna legitimate transmitter, a multiple antenna legitimate receiver, and a multiple antenna eavesdropper, termed the multiple-input multiple-output (MIMO) wiretap channel, has been identified in full generality in [5, 6].