Widely recognized security vulnerabilities in current wireless radio access technologies undermine the benefits of ubiquitous mobile connectivity.Security strategies typically rely on bit-level cryptographic techniques and associated protocols at various levels of the data processing stack.These solutions have drawbacks that have slowed down the progress of new wireless services.Physical layer security approaches derived from an information theoretic framework have been recently proposed with secret key generation being the primary focus of this dissertation.Previous studies of physical layer secret key generation (PHY-SKG) indicate that a low secret key generation rate (SKGR) is the primary limitation of this approach.To overcome this drawback, we propose novel SKG schemes to increase the SKGR as well as improve the security strength of generated secret keys by exploiting multiple input and multiple output (MIMO), cooperative MIMO (co-op MIMO) networks.Both theoretical and numerical results indicate that relay-based co-op MIMO schemes, traditionally used to enhance LTE-A network throughput and coverage, can also increase SKGR.Based on the proposed SKG schemes, we introduce innovative power allocation strategies to further enhance SKGR.Results indicate that the proposed power allocation scheme can offer 15% to 30% increase in SKGR relative to MIMO/co-op MIMO networks with equal power allocation at low-power region, thereby improving network security.Although co-op MIMO architecture can offer significant improvements in both performance and security, the concept of joint transmission and reception with relay nodes introduce new vulnerabilities.For example, even if the transmitted information is secured, it is difficult but essential to monitor the behavior of relay nodes.Selfish or malicious intentions of relay nodes may manifest as non-cooperation.Therefore, we propose relay node reliability evaluation schemes to measure and monitor the misbehavior of relay nodes.Using a power-sensing based reliability evaluation scheme, we attempt to detect selfish nodes thereby measuring the level of non-cooperation.An overall node reliability evaluation, which can be used as a guide for mobile users interested in collaboration with relay nodes, is performed at the basestation.For malicious behavior, we propose a network tomography technique to arrive at node reliability metrics.We estimate the delay distribution of each internal link within a co-op MIMO framework and use this estimate as an indicator of reliability.The effectiveness of the proposed node reliability evaluations are demonstrated via both theoretical analysis and simulations results.The proposed PHY-SKG strategies used in conjunction with node reliability evaluation schemes represent a novel cross-layer approach to enhance security of cooperative networks.
Widely recognized security vulnerabilities in current wireless radio access technologies undermine the benefits of ubiquitous mobile connectivity.Security strategies typically rely on bit-level cryptographic techniques and associated protocols at various levels of the data processing stack.These solutions have drawbacks that have slowed down the progress of new wireless services.Physical layer security approaches derived from an information theoretic framework have been recently proposed with secret key generation being the primary focus of this dissertation.Previous studies of physical layer secret key generation (PHY-SKG) indicate that a low secret key generation rate (SKGR) is the primary limitation of this approach.To overcome this drawback, we propose novel SKG schemes to increase the SKGR as well as improve the security strength of generated secret keys by exploiting multiple input and multiple output (MIMO), cooperative MIMO (co-op MIMO) networks.Both theoretical and numerical results indicate that relay-based co-op MIMO schemes, traditionally used to enhance LTE-A network throughput and coverage, can also increase SKGR.Based on the proposed SKG schemes, we introduce innovative power allocation strategies to further enhance SKGR.Results indicate that the proposed power allocation scheme can offer 15% to 30% increase in SKGR relative to MIMO/co-op MIMO networks with equal power allocation at low-power region, thereby improving network security.Although co-op MIMO architecture can offer significant improvements in both performance and security, the concept of joint transmission and reception with relay nodes introduce new vulnerabilities.For example, even if the transmitted information is secured, it is difficult but essential to monitor the behavior of relay nodes.Selfish or malicious intentions of relay nodes may manifest as non-cooperation.Therefore, we propose relay node reliability evaluation schemes to measure and monitor the misbehavior of relay nodes.Using a power-sensing based reliability evaluation scheme, we attempt to detect selfish nodes thereby measuring the level of non-cooperation.An overall node reliability evaluation, which can be used as a guide for mobile users interested in collaboration with relay nodes, is performed at the basestation.For malicious behavior, we propose a network tomography technique to arrive at node reliability metrics.We estimate the delay distribution of each internal link within a co-op MIMO framework and use this estimate as an indicator of reliability.The effectiveness of the proposed node reliability evaluations are demonstrated via both theoretical analysis and simulations results.The proposed PHY-SKG strategies used in conjunction with node reliability evaluation schemes represent a novel cross-layer approach to enhance security of cooperative networks.
Relay nodes (RNs), which are low-power nodes that provide enhanced coverage and capacity at low cost are an integral part of the Long Term Evolution Advanced (LTE-A) standard. In order to boost throughput, cooperative multiple input multiple output (co-op MIMO) techniques are proposed wherein RNs close to end users are recruited to operate as virtual antenna arrays (VAA). In this paper, we exploit the co-op MIMO structure to design and implement a physical layer (PHY-layer) security scheme for LTE-A networks. Specifically, we consider two relay-based co-op MIMO architectures and propose novel secret key generation (SKG) schemes for those cases. Information theoretic results regarding secret key generation rate (SKGR) are presented. Results indicate that relay-based co-op MIMO schemes, traditionally used to enhance LTE-A network throughput and coverage, can also increase SKGR. Our work demonstrates the viability of SKG technique as a potential PHY-layer security scheme for LTE-A networks.
Long-term evolution-advanced (LTE-A) networks exploit low-power relay nodes, picocells and femtocells to boost throughput, enhance coverage, decrease latency, and reduce cost. End users in a relay-based LTE-A network can recruit relay nodes to cooperate as virtual antenna arrays, thereby reaping the benefits offered by multiple input single output (MIMO) techniques. Although the relay-based cooperative MIMO (coop MIMO) implementation in LTE-A networks improves performance, security issues are often overlooked. This paper introduces a physical layer security scheme for point-to-point networks, and extends this scheme to MIMO networks. Two practical relay-based coop MIMO architectures and corresponding secret key generation (SKG) schemes are presented. For both the MIMO and coop MIMO networks, the impact of proposed power allocation on SKG rate (SKGR) is quantified via the theoretical and numerical analysis. Results indicate that the proposed power allocation scheme can offer 15%-30% increase in SKGR relative to MIMO/coop MIMO networks with equal power allocation at low-power region, thereby improving network security.
Over the last decade, physical layer secret key generation (PHY-SKG) techniques that exploit reciprocity of wireless channels have attracted considerable interest among researchers in the field of wireless communication. Compared to traditional cryptographic methods, PHY-SKG techniques offer the following advantages: a computationally bounded adversary does not need to be assumed; PHY-SKG avoids the requirement of key management, and secret keys can be dynamically replenished. Additionally, PHY-SKG can enhance existing security schemes because it operates independently of higher layer security schemes. However, a key drawback of PHY-SKG is low secret key generation rate (SKGR), a critical performance metric. Therefore, the role of advanced network technologies (e.g., multiple input multiple output (MIMO) and cooperative MIMO) must be explored to enhance SKGR. This paper describes how MIMO and cooperative MIMO techniques can enhance SKGR.
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.
The impact of relay nodes on the secret key generation via the physical layer resources is investigated. A novel relay-assisted strategy is proposed to improve the generated secret key rate. The main idea is to exploit the random channels associated with relay nodes in the network as additional random sources for the key generation. This approach is particularly useful when the channels between legitimate nodes change slowly. Three increasingly sophisticated yet more practical scenarios are studied, for which relay-assisted key generation protocols are proposed and are shown to be optimal or order-optimal in terms of the key rate. It is also shown that the presence of relay node increases the multiplexing gain. This is in sharp contrast to scenarios with relay helping information transmission, in which the relay node does not increase the multiplexing. Furthermore, a cooperative scheme is also proposed in which relays help key generation but the generated keys are kept secure from these relays.
Motivated by recent developments in wireless communication, this thesis aims to characterize the secrecy performance in several types of typical wireless networks. Advanced techniques are designed and evaluated to enhance physical layer security in these networks with realistic assumptions, such as signal propagation loss, random node distribution and non-instantaneous channel state information (CSI). The first part of the thesis investigates secret communication through relay-assisted cognitive interference channel. The primary and secondary base stations (PBS and SBS) communicate with the primary and secondary receivers (PR and SR) respectively in the presence of multiple eavesdroppers. The SBS is allowed to transmit simultaneously with the PBS over the same spectrum instead of waiting for an idle channel. To improve security, cognitive relays transmit cooperative jamming (CJ) signals to create additional interferences in the direction of the eavesdroppers. Two CJ schemes are proposed to improve the secrecy rate of cognitive interference channels depending on the structure of cooperative relays. In the scheme where the multiple-antenna relay transmits weighted jamming signals, the combined approach of CJ and beamforming is investigated. In the scheme with multiple relays transmitting weighted jamming signals, the combined approach of CJ and relay selection is analyzed. Numerical results show that both these two schemes are effective in improving physical layer security of cognitive interference channel. In the second part, the focus is shifted to physical layer security in a random wireless network where both legitimate and eavesdropping nodes are randomly distributed. Three scenarios are analyzed to investigate the impact of various factors on security. In scenario one, the basic scheme is studied without a protected zone and interference. The probability distribution function (PDF) of channel gain with both fading and path loss has been derived and further applied to derive secrecy connectivity and ergodic secrecy capacity. In the second scenario, we studied using a protected zone surrounding the source node to enhance security where interference is absent. Both the cases that eavesdroppers are aware and unaware of the protected zone boundary are investigated. Based on the above scenarios, further deployment of the protected zones at legitimate receivers is designed to convert detrimental interference into a beneficial factor. Numerical results are investigated to check the reliability of the PDF for reciprocal of channel gain and to analyze the impact of protected zones on secrecy performance. In the third part, physical layer security in the downlink transmission of cellular network is studied. To model the repulsive property of the cellular network planning, we assume that the base stations (BSs) follow the Mat´ern hard-core point process (HCPP), while the eavesdroppers are deployed as an independent Poisson point process (PPP). The distribution function of the distances from a typical point to the nodes of the HCPP is derived. The noise-limited and interference-limited cellular networks are investigated by applying the fractional frequency reuse (FFR) in the system. For the noise-limited network, we derive the secrecy outage probability with two different strategies, i.e. the best BS serve and the nearest BS serve, by analyzing the statistics of channel gains. For the interference-limited network with the nearest BS serve, two transmission schemes are analyzed, i.e., transmission with and without the FFR. Numerical results reveal that both the schemes of transmitting with the best BS and the application of the FFR are beneficial for physical layer security in the downlink cellular networks, while the improvement due