Within this thesis, we investigate the possibilities of physical layer secrecy for two special system models. In detail, we study beamforming and protection strategies in the Multiple-Input Single-Output (MISO) Gaussian Wiretap Channel (WTC) and the Gaussian two-hop relay WTC with multiple antennas at transmitter and receiver. In both system models, we examine the influence of partial Channel State Information (CSI) on the link to the eavesdropper and compare the achievable secrecy rates with the case of full CSI. We show for the MISO WTC that in the fast fading scenario the Beamforming Vector (BV) can be optimized such that the ergodic secrecy rate is maximized with regard to the degree of channel knowledge. Further we show that the ergodic secrecy rate can be significantly increased by usage of Artificial Noise (AN), if applied in a smart way. This means that the degree of channel knowledge on the link to the eavesdropper influences the portion of power that is spent for AN at the transmitter as well as the direction, in which the AN signal is sent. In addition, we apply the same beamforming and protection strategies to the slow fading scenario and find that these techniques also reduce the secrecy outage probability. For the two-hop relay WTC, we introduce Information Leakage Neutralization (IN) as a new protection strategy. If applied to a system model, where the transmitter has full CSI, the instantaneous secrecy rate performs almost as well as the instantaneous capacity of the peaceful system without an eavesdropper. The IN protected scheme outperforms the AN protected approach and performs much better than any beamforming scheme without additional protection mechanism. Another positive aspect of the IN protected scheme in the case of full CSI is that conventional channel codes can be applied instead of wiretap codes. For the case of partial CSI, where the transmitter has only an outdated estimate on the channel between relay and the eavesdropper, we show that the IN protected scheme can also be applied. Here, it strongly depends on the channel realizations and the delay of the estimate, whether the IN or the AN protection scheme should be applied.
Within this thesis, we investigate the possibilities of physical layer secrecy for two special system models. In detail, we study beamforming and protection strategies in the Multiple-Input Single-Output (MISO) Gaussian Wiretap Channel (WTC) and the Gaussian two-hop relay WTC with multiple antennas at transmitter and receiver. In both system models, we examine the influence of partial Channel State Information (CSI) on the link to the eavesdropper and compare the achievable secrecy rates with the case of full CSI. We show for the MISO WTC that in the fast fading scenario the Beamforming Vector (BV) can be optimized such that the ergodic secrecy rate is maximized with regard to the degree of channel knowledge. Further we show that the ergodic secrecy rate can be significantly increased by usage of Artificial Noise (AN), if applied in a smart way. This means that the degree of channel knowledge on the link to the eavesdropper influences the portion of power that is spent for AN at the transmitter as well as the direction, in which the AN signal is sent. In addition, we apply the same beamforming and protection strategies to the slow fading scenario and find that these techniques also reduce the secrecy outage probability. For the two-hop relay WTC, we introduce Information Leakage Neutralization (IN) as a new protection strategy. If applied to a system model, where the transmitter has full CSI, the instantaneous secrecy rate performs almost as well as the instantaneous capacity of the peaceful system without an eavesdropper. The IN protected scheme outperforms the AN protected approach and performs much better than any beamforming scheme without additional protection mechanism. Another positive aspect of the IN protected scheme in the case of full CSI is that conventional channel codes can be applied instead of wiretap codes. For the case of partial CSI, where the transmitter has only an outdated estimate on the channel between relay and the eavesdropper, we show that the IN protected scheme can also be applied. Here, it strongly depends on the channel realizations and the delay of the estimate, whether the IN or the AN protection scheme should be applied.
Secrecy on the physical layer is a promising technique to simplify the overall cross-layer secrecy concept. In many recent works on the multiple antenna wiretap channel, perfect channel state information to the intended receiver as well as the passive eavesdropper are assumed. In this paper, the transmitter has only partial information about the channel to the eavesdropper, but full information on the main channel to the intended receiver. The applied channel model is the flat-fading multiple-input single-output wiretap channel. We minimize the outage probability of secure transmission under single-stream beamforming and the use of artificial noise in the null space of the main channel. Furthermore, we derive a suboptimal beamforming scheme based on a Markov bound, which performs reasonably well. The results generalize the cases with perfect as well as without channel state information of the eavesdropper channel. Numerical simulations illustrate the secrecy outage probability over the degree of channel knowledge and confirm the theoretical results.
In this paper, we analyze the four-node relay wiretap channel, where the relay performs amplify-and-forward. There is no direct link between transmitter and receiver available. The transmitter has multiple antennas, which assist in securing the transmission over both phases. In case of full channel state information (CSI), the transmitter can apply information leakage neutralization in order to prevent the eavesdropper from obtaining any information about the signal sent. This gets more challenging, if the transmitter has only an outdated estimate of the channel from the relay to the eavesdropper. For this case, we optimize the worst case secrecy rate by choosing intelligently the beamforming vectors and the power allocation at the transmitter and the relay.
Secrecy on the physical layer is a promising technique to simplify the overall cross-layer secrecy concept. In many recent works on the multiple antenna wiretap channel, perfect channel state information to the intended receiver as well as the passive eavesdropper are assumed. In this work, the transmitter has only partial information about the channel to the eavesdropper, but full information on the main channel to the intended receiver. The applied channel model is the flat-fading multiple-input single-output wiretap channel. We minimize the outage probability of secure transmission under single-stream beamforming. Additionally, we derive a suboptimal beamforming scheme based on a Markov bound, which performs reasonably well. The results generalize the cases with perfect as well as without channel state information of the eavesdropper channel. Numerical simulations illustrate the secrecy outage probability over the degree of channel knowledge and confirm the theoretical results.
Secrecy on the physical layer receives increased research interest. Especially in the wireless communication scenarios, the interest in confidentiality of messages increases. In this paper, we discuss the effectiveness of the beamforming strategies maximum ratio transmission (MRT), zero forcing (ZF), and an optimized beamforming (BF) strategy for the transmission under secrecy constraints. The applied channel model is the flat-fading MISO (multiple-input single-output) wiretap channel, where the channel to the eavesdropper is only partly known at the transmitter. The scenario with perfect and no channel information to the eavesdropper are included as special cases. We show that under certain conditions the achievable secrecy rates for the transmission with optimized beam-forming can be increased by transmitting artificial noise (AN) in the null space of the channel to the intended receiver, which produces additional interference only at the eavesdropper. We provide a characterization of the optimal beamforming and artificial noise beamforming and analyze the optimal power allocation for asymptotic cases (low and high SNR as well as no and perfect channel information). Numerical simulations illustrate the results.
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.
In this paper, we consider the secure transmission in ergodic fast fading multiple-input single-output single-antenna-eavesdropper (MISOSE) wiretap channels with only the statistics of eavesdropper's channel state information at the transmitter (CSIT). Two kinds of the legitimate CSIT are assumed, that is, full and statistical legitimate CSIT. With full legitimate CSIT, we generalize and optimize the previously proposed artificial noise (AN) aided secure beamforming to improve its secrecy rate performance. The AN covariance matrix in our scheme is more flexible than previous scheme and the region of non-zero secrecy rate is enlarged significantly according to our simulations. For the case with statistical legitimate CSIT, we further prove that the secure beamforming is secrecy capacity achieving for the Rayleigh faded channels. In this case, the AN is not necessary. Extensions to cases where legitimate receiver and eavesdropper have multiple antennas will also be discussed.