Security is a demanding challenge in wireless systems due to the broadcast nature of the channel. One the other hand security at physical layer can increase overall system's security since it can be combined with other security schemes from higher layers. High throughput required by modern wireless networks can be assured by MIMO (Multiple-input multiple-output), but when high spectral e-ciencies are needed multilevel modulations with high peak-to-average power ratios should be used, which may afiect e-ciency of power ampliflcation. This problem can be avoided by the MISO (Multi input Single Output) transmitter considered here, where transmitted multilevel constellations are the result of the combination of several uncorrelated BPSK (Bi-Phase Shift Keying) components, that are amplifled and transmitted independently by an antenna. The constellation shaping done by this transmitter means directivity in the transmitted constellation that can be used to assure security at physical layer. Security as well complexity are assured since any eavesdropper must know the set of coe-cients associated to each BPSK component as well as the antenna array conflguration. It is shown that the inherent security assured by this transmitter allows secrecy at physical layer. Several examples are analyzed and the corresponding results show the efiectiveness of the proposed approach to implement a security scheme at physical layer level.
Security is a demanding challenge in wireless systems due to the broadcast nature of the channel. One the other hand security at physical layer can increase overall system's security since it can be combined with other security schemes from higher layers. High throughput required by modern wireless networks can be assured by MIMO (Multiple-input multiple-output), but when high spectral e-ciencies are needed multilevel modulations with high peak-to-average power ratios should be used, which may afiect e-ciency of power ampliflcation. This problem can be avoided by the MISO (Multi input Single Output) transmitter considered here, where transmitted multilevel constellations are the result of the combination of several uncorrelated BPSK (Bi-Phase Shift Keying) components, that are amplifled and transmitted independently by an antenna. The constellation shaping done by this transmitter means directivity in the transmitted constellation that can be used to assure security at physical layer. Security as well complexity are assured since any eavesdropper must know the set of coe-cients associated to each BPSK component as well as the antenna array conflguration. It is shown that the inherent security assured by this transmitter allows secrecy at physical layer. Several examples are analyzed and the corresponding results show the efiectiveness of the proposed approach to implement a security scheme at physical layer level.
The use of multiple antennas at both the transmitter and receiver aims to improve performance or to increase symbol rate of systems, but it usually requires higher implementation complexity. Multiple Input Multiple Output (MIMO) architectures can be used for combined transmit and receive diversity, for the parallel transmission of data or spatial multiplexing. When used for spatial multiplexing, MIMO technology promises high bit rates in a narrow bandwidth. Therefore, it is of high significance to spectrum users. In this case, MIMO system considers the transmission of different signals from each transmit element so that the receiving antenna array receives a superposition of all transmitted signals. Mobile communication systems must support multiple users achieving at the same time privacy of users contents. Security common solutions are based on encrypted algorithms from higher layers, such as private and public encrypted keys. Other possibility is to implement physical layer security schemes. One advantage of physical layer security relies on their ability to be combined with other security schemes from higher layers. Due to the broadcast nature of MIMO systems, security is a critical issue. However, the constellation shaping on the desired direction introduced by a new proposed transmitter means that we have directivity at the transmitted constellation that can be employed to assure security at physical layer. Privacy is achieved since each user must know the set of coefficients associated to each BPSK component as well as the array configuration; otherwise receives useless data. Therefore, the inherent security lies on the constellation directivity, i.e., the direction in which the constellation is optimized, which can be improved by changes on coefficients’ phases or using constellations that are decomposed with a higher number of BPSK components. The several cases analyzed in this speech show effectiveness of the proposed approach to implement a security scheme at physical layer level.
The use of physical layer methods for improving the security of wireless links has recently become the focus of a considerable research effort. Such methods can be used in combination with cryptography to enhance the exchange of confidential messages over a wireless medium in the presence of unauthorized eavesdroppers, or they can be used to enable secrecy in the absence of shared secret keys through the use of coding strategies, jamming or beamforming. Indeed, one of the driving forces behind the recent emergence of physical layer techniques for security is the push toward adding extra degrees of freedom in the form of multiple antennas at the transmitter and receiver of the link. Multiple-input multiple-output (MIMO) wireless systems have been extensively studied during the past two decades, and their potential gains in throughput, diversity, and range have been well quantified. MIMO approaches are now an integral part of the WiFi and 4G standards in use today. It is not surprising that MIMO architectures are useful in improving wireless security as well, since they can provide focused transmit selectivity of both information and noise toward desired and undesired receivers. In this chapter, we discuss a number of different ways that physical layer security can be achieved in wireless networks with MIMO links. We will focus primarily on signal processing related issues (e.g., beamforming, power control, resource allocation) that enable reliable reception at intended recipients and minimize data leakage to eavesdroppers, and we will consider a variety of different MIMO settings including point-to-point, broadcast, interference, and multi-hop networks. We cannot offer an exhaustive survey of such methods in just a single chapter; instead, we present a few representative approaches that illustrate ✐ ✐ — — ✐ ✐ ✐ ✐ ✐ ✐ Physical Layer Security in Wireless the gains that multiple antennas provide. Additional and more complete treatments of the physical layer security problem can be found in [1-4] as well as other chapters in this book.
Security has become an increasingly significant and urgent issue in wireless networks. The growing computational capability of the eavesdropper (Eve) and more and more complicated secure key management in large scale and heterogeneous networks, pose more and more stringent requirement to the cryptographic protocols. As a new design paradigm, physical layer security has been recognised as a promising solution to enhance the security of wireless links relying on the channel properties and advanced signal processing, and can act as either an alternative or a complementary solution to the conventional cryptographic methods. Without complicated secure key generation and management, physical layer security is quite suitable for the large scale distributed networks. In this thesis, we aim to improve the secrecy performance in emerging wireless communication systems relying on different physical layer security techniques. Firstly, the security of an amplify-and-forward successive relaying network with multiple untrusted relay nodes is investigated, where the conventional detrimental inter-relay interference is exploited to jam the untrusted nodes without assistance of external helpers. Considering different complexity requirements, several relay selection schemes are proposed, and the closed-form expressions of the lower bound of the secrecy outage probability and the maximum secrecy diversity order are derived accordingly. It is shown that the maximum secrecy diversity order of N-1 can be achieved for an N relay nodes network. Secondly, the artificial noise (AN) aided secure transmission strategy for a multiple-input single-output multiple-Eve (MISOME) system with a secure user (Bob) and a normal user (NU) is investigated. The power allocations among Bob, NU and AN, as well as the wiretap code rates, are jointly optimized to maximize the effective secrecy throughput (EST), under the average throughput constraint of NU. An alternative optimization algorithm with guaranteed convergence is proposed to obtain the optimal parameters. It is shown that the EST increases with the transmitting power and the number of transmit antennas, and decreases with the throughput constraint of NU, and the EST can be improved through injecting AN and concurrent transmission of Bob and NU. Thirdly, physical layer security in a multi-antenna small-cell network is investigated, where the multi-antenna base stations (BSs), cellular users, and Eves are all randomly distributed according to independent Poisson point processes. Stochastic geometry is applied to analyze the connection and secrecy outage probabilities and the average achievable secrecy rate. The impact of different parameters, including power allocation, BS and Eve density, and the adaptive eavesdropping or jamming on the secrecy performance is analyzed. It is shown that the average secrecy rate is a quasi-concave function of the power allocation factor and monotonically decreases with the ratio of the Eve-BS density. Finally, we study the physical layer security in a large-scale heterogeneous network consisting of both sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot spoofing attacks from the Eves, the coverage and secrecy probabilities are derived using stochastic geometry and the conditions under which the millimeter wave tier outperforms the sub-6 GHz counterpart are discussed in terms of both coverage and secrecy.It is shown that the mmWave small cell can provide better coverage performance in the high transmission code-rate region, and the secrecy performance of the mmWave system outperforms the sub-6 GHz counterpart in the low redundant rate region, which reveals the advantage of using mmWave for secure communication.
Recently, MIMO (multiple-input-multiple-output) physical layer secure transmission has attracted great attentions. However, current schemes cannot defend against the passive eavesdroppers with arbitrary number of antennas. To address this problem, in this work, we propose a practical physical layer MIMO secure communication scheme (PLSC) to defend against such an eavesdropper with arbitrary number of antennas. In the proposed scheme, the transmitter first independently generates a random binary sequence as the “key bits (KB)” to “encrypt” (XOR) the confidential information. After that, the transmitter sends the “encrypted information” over the wireless channel, along with mapping key bits to the legitimate receiver simultaneously. The key principle lies in that the KB information is coded in the indexes of the activated/non-activated antennas combination of the legitimate user. Then, the legitimate receiver first observes his/her activated antenna indexes to obtain the corresponding key bits. After that, he/she demodulates the “encrypted information” at the activated antennas, and finally “decrypts” (XOR) the confidential information by using the observed key bits. However, due to the uniqueness and independence of MIMO wireless channel, for any other eavesdroppers who suffer an independent channel from legitimate users, we prove that it cannot observe any information about KB from the received signals, regardless of how many antennas it has used. Consequently, without knowledge of KB, it cannot decrypt any information about the confidential information, too. The reliability and security of PLSC are theoretically demonstrated. The simulation and numerical results fully verified the validity and effectiveness of the proposed scheme.
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.
The present work investigates the physical layer security of wireless communication systems over non-homogeneous fading environments, which are typically encountered in realistic wireless transmission scenarios in the context of conventional and emerging communication systems. This is carried out in the context of a single-input multiple-output system that consists of a single-antenna transmitter, a multi-antenna legitimate receiver, and a multi-antenna active eavesdropper. To this end, novel exact analytic expressions are derived for the corresponding average secrecy capacity, which are corroborated by respective results from computer simulations that verify their validity. Capitalizing on the offered results, the physical layer security is quantified in terms of different parameters. This leads to the development of useful insights on the impact of non-homogeneous fading environment and the number of employed antennas on the achieved physical layer security levels of the considered set up.