The proliferation of connected embedded devices has driven wireless communications into commercial, military, industrial, and personal systems. It is unreasonable to expect privacy and security to be inherent in these networks given the spatial density of these devices, limited spectral resources, and the broadcast nature of wireless communications systems. Communications for these systems must have sufficient information capacity and secrecy capacity while typically maintaining small size, light weight, and minimized power consumption. With increasing crowding of the electromagnetic spectrum, interference must be leveraged as an available resource. This work develops a new beamforming method for direction-dependent modulation that provides wireless communications devices with enhanced physical layer security and the ability to simultaneously communicate and harvest energy by exploiting co-channel interference. We propose a method that optimizes a set of time-varying array steering vectors to enable direction-dependent modulation, thus exploiting a new degree of freedom in the space-time-frequency paradigm. We formulate steering vector selection as a convex optimization problem for rapid computation given arbitrarily positioned array antenna elements. We show that this method allows us to spectrally separate co-channel interference from an information-bearing signal in the analog domain, enabling the energy from the interference to be diverted for harvesting during the digitization and decoding of the information-bearing signal. We also show that this method provides wireless communications devices with not only enhanced information capacity, but also enhanced secrecy capacity in a broadcast channel. By using the proposed method, we can increase the overall channel capacity in a broadcast system beyond the current state-of-the-art for wireless broadcast channels, which is based on static coding techniques. Further, we also increase the overall secrecy capacity of the system by enabling secrecy for each user in the system. In practical terms, this results in higher-rate, confidential messages delivered to multiple devices in a broadcast channel for a given power constraint. Finally, we corroborate these claims with simulation and experimental results for the proposed method. / PHD

Doctor of Philosophy / Department of Electrical and Computer Engineering / Balasubramaniam Natarajan / 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.

The implications of non-Gaussian interference for various communication systemsare explored. The focus is on the Kappa distribution, Generalized Gaussian distribu-tions, and the distribution of the interference in molecular communication systems.A review of how dynamic systems that are not in equilibrium are modeled by theKappa distribution and how this distribution models interference in HF communica-tions systems at sunrise is provided. The channel model, bit error rate for single andmultiple antennas, channel capacity, and polar code performance are shown.<div><br><div>Next, a review of the Generalized Gaussian distribution that has been found tomodel the interference resulting from surface activities is provided. This modeling isextended to find the secrecy capacity so that information cannot be obtained by theeavesdropper.</div><div><br></div><div>Finally, future nanomachnines are examined. The vulnerability to a receptorantagonist of a ligand-based molecule receiver is explored. These effects are consideredto be interference as in other wireless systems and the damage to signal reception isquantified.</div></div>

Propagation-based key establishment is a physical layer method for generating encryption keys based on two radios observing a reciprocal propagation channel. This work explores the impact of mutual coupling when communicating nodes are equipped with multiple antennas, multipath richness in the propagation environment, and practical limitations caused by the nonreciprocal nature of RF circuits on key establishment. First, network theory is used to formulate a model of a realistic communication system which incorporates transmit sources and receive loads, impedance matching networks, low-noise amplifiers (LNAs), mutually coupled antenna arrays, and a passive eavesdropper. Afterwards, a detailed analysis is performed to quantify the impact of coupling, type of impedance matching network, and proximity of a multi-antenna eavesdropper on key rate metrics. Next, the degradation on key establishment caused by the radiocircuitry non-reciprocal contributions to the propagation channel is analyzed. A calibration technique based on total least square algorithm is used to overcome the non-reciprocity. Results demonstrate that the method is highly effective in removing the impact of non-reciprocal circuit contributions over a range of operational parameters. Lastly, for key establishment, the propagation conditions can cause the available key rate to be significantly different from the secure key rate which takes into account the presence of a passive eavesdropper. To study this in detail, a realistic multiple-input multiple-output (MIMO) propagation environment is modeled for two communicating radios and an eavesdropper. Afterwards different propagation conditions are assumed and results demonstrate that secure key rate converges to available key rate when K-factor is small and the eavesdropper is not located very close (< 2.5 wavelength) to one of the nodes.

Communication networks are an integral part of the Internet of Things (IoT) era. They enable endless opportunities for connectivity in a wide range of applications, leading to advances in efficiency of day-to-day life. While creating opportunities, they also incur several new challenges. In general, we wish to design a system that performs optimally well in all aspects. However, there usually exist competing objectives which lead to tradeoffs. In this thesis, driven by several applications, new features and objectives are included into the system model, making it closer to reality and needs. The results presented in this thesis aim at providing insight into the fundamental tradeoff of the system performance which can serve as a guideline for the optimal design of real-world communication systems. The thesis is divided into two parts. The first part considers the aspect of signal reconstruction requirement as a new objective in the source and channel coding problems. In this part, we consider the framework where the quality and/or availability of the side information can be influenced by a cost-constrained action sequence. In the source coding problem, we impose a constraint on the reconstruction sequence at the receiver that it should be reproduced at the sender, and characterize the fundamental tradeoff in the form of the rate-distortion-cost region, revealing the optimal relation between compression rate, distortion, and action cost. The channel coding counterpart is then studied where a reconstruction constraint is imposed on the channel input sequence such that it should be reconstructed at the receiver. An extension to the multi-stage channel coding problem is also considered where inner and outer bounds to the capacity region are given. The result on the channel capacity reveals interesting consequence of imposing an additional reconstruction requirement on the system model which has a causal processing structure. In the second part, we consider the aspect of information security and privacy in lossy source coding problems. The sender wishes to compress the source sequence in order to satisfy a distortion criterion at the receiver, while revealing only limited knowledge about the source to an unintended user. We consider three different aspects of information privacy. First, we consider privacy of the source sequence against the eavesdropper in the problem of source coding with action-dependent side information. Next, we study privacy of the source sequence due to the presence of a public helper in distributed lossy source coding problems. The public helper is assumed to be either a user who provides side information over a public link which can be eavesdropped, or a legitimate user in the network who helps to relay information to the receiver, but may not ignore the information that is not intended for it. Lastly, we take on a new perspective of information privacy in the source coding problem. That is, instead of protecting the source sequence, we are interested in the privacy of the reconstruction sequence with respect to a user in the system. For above settings, we provide the complete characterization of the rate-distortion(-cost)-leakage/equivocation region or corresponding inner and outer bounds for discrete memoryless systems. / <p>QC 20140514</p>

Recently, massive multiple-input multiple-output (MIMO) systems have gained significant attention as a new network architecture to not only achieving unprecedented spectral and energy efficiencies, but also to alleviating propagation losses and inter-user/inter-cell interference. Therefore, massive MIMO has been identified as one of the key candidate technologies for the 5th generation wireless standard. This dissertation thus focuses on (1) developing a performance analysis framework for cognitive massive MIMO systems by investigating the uplink transmissions of multi-cell multi-user massive MIMO secondary systems, which are underlaid in multi-cell multi-user primary massive MIMO systems, with taking into consideration the detrimental effects of practical transmission impairments, (2) proposing a new wireless-powered underlay cognitive massive MIMO system model, as the secondary user nodes is empowered by the ability to efficiently harvest energy from the primary user transmissions, and then access and utilize the primary network spectrum for information transmission, and (3) developing a secure communication strategy for cognitive multi-user massive MIMO systems, where physical layer secure transmissions are provisioned for both primary and secondary systems by exploiting linear precoders and artificial noise (AN) generation in order to degrade the signal decodability at eavesdropper. The key design feature of the proposed cognitive systems is to leverage the spatial multiplexing strategies to serve a large number of spatially distributed user nodes by using very large numbers of antennas at the base-stations. Moreover, the fundamental performance metrics, the secondary transmit power constraints, which constitute the underlay secondary transmissions subject to a predefined primary interference temperature, and the achievable sum rates of the primary and secondary systems, are characterized under different antenna array configurations. Additionally, the detrimental impact of practical wireless transmission impairments on the performance of the aforementioned systems are quantified. The important insights obtained throughout these analyses can be used as benchmarks for designing practical cognitive spectrum sharing networks.

The broadcast nature of wireless medium has made information security as one of the most important and critical issues in wireless systems. Physical layer security, which is based on information-theoretic secrecy concepts, can be used to secure the wireless channels by exploiting the noisiness and imperfections of the channels. Massive multiple-input multiple-output (MIMO) systems, which are equipped with very large antenna arrays at the base stations, have a great potential to boost the physical layer security by generating the artificial noise (AN) with the exploitation of excess degrees-of-freedom available at the base stations. In this thesis, we investigate physical layer security provisions in the presence of passive/active eavesdroppers for single-hop massive MIMO, dual-hop relay-assisted massive MIMO and underlay spectrum-sharing massive MIMO systems. The performance of the proposed security provisions is investigated by deriving the achievable rates at the user nodes, the information rate leaked into the eavesdroppers, and the achievable secrecy rates. Moreover, the effects of active pilot contamination attacks, imperfect channel state information (CSI) acquisition at the base-stations, and the availability of statistical CSI at the user nodes are quantified. The secrecy rate/performance gap between two AN precoders, namely the random AN precoder and the null-space based AN precoder, is investigated. The performance of hybrid analog/digital precoding is compared with the full-dimensional digital precoding. Furthermore, the physical layer security breaches in underlay spectrum-sharing massive MIMO systems are investigated, and thereby, security provisions are designed/analyzed against active pilot contamination attacks during the channel estimation phase. A power-ratio based active pilot attack detection scheme is investigated, and thereby, the probability of detection is derived. Thereby, the vulnerability of uplink channel estimation based on the pilots transmitted by the user nodes in time division duplexing based massive MIMO systems is revealed, and the fundamental trade-offs among physical layer security provisions, implementation complexity and performance gains are discussed.

Information security has been a very important issue in wireless networks. With the ever-increasing amount of data being exchanged over wireless networks, the confidentiality of information needs to be protected from unauthorized users called eavesdropper. Due to the broadcast nature of the wireless medium, the transmissions between legitimate users maybe overheard and intercepted by the unauthorized parties, which makes wireless transmission vulnerable to potential eavesdropping attacks. The security of wireless communications plays an increasingly important role in the cybercrime defense against unauthorized activities.     We consider the wireless physical layer security which has been explored for the sake of enhancing the protection of wireless communications against eavesdroppers. We consider the problem of secret communication through Rayleigh fading channel in the presence of an eavesdropper in which the transmitter knows the channel state information of both the main and eavesdropper channel. Then, we analyze the average capacity of the main channel and eavesdropper channel from which an expression of secrecy capacity is derived based on the cumulative distribution function and probability density function of the signal to noise ratio. We also analyze an expression for the symbol error rate of the main channel, and the outage probability is obtained for the main transmission. Finally, we perform the numerical results in MATLAB.

The current and future demand for wireless technology is increasing rapidlyin the era of Internet-of-things, information-shower, auto-drive vehicles andthe fifth generation of mobile communications. The flourishing in wirelesstechnologies comes from their advantages such as mobility, flexibility, easy toextend, easy to install and easy to do the maintenance. However, the wirelesstechnology is suffering from many problems such as; lack of security andthe shortage of spectrum bands. The security remains a major challenge forthe wireless communications because of the broadcasting nature of wirelesssignals and massive researches have been conducted to deal with it. Beamformingis one of the physical layer security solutions that is proposed toimprove the security by focusing the majority of the transmitted power towardthe legitimate destination. The main concern about the beamformingtechnique is the relatively small amount of power that escapes from the sidelobes where any illegal user equipped with a sufficiently sensitive receivercan detect its information. The literature has been received many differentsolutions to secure the side lobes emissions. These solutions suffer from fourcommon limitations; 1) the need to modulate the signal at the antenna level,2) the data rates are restricted by the switching speed, 3) they can not easily beintegrated with the current infrastructure, and 4) they work only with phasemodulation. In this thesis, a new, simple, economic, easy to get integratedwith current phased array systems and effective solution has been proposedand analytically analysed under different circumstances, including noiseless,noisy and Rician fading channels and the effect of phase shift errors. The secondproblem addressed in this thesis is the poor spectral efficiency of spaceshift keying modulation. This thesis proposes a new physical layer directcode to improve the spectral efficiency of space shift keying modulation byexploiting the indices of both active and inactive transmitting antennas.

This dissertation presents several novel approaches to enhance security, and increase the throughput, and decrease the delay synchronization in 5G networks. First, a new physical layer paradigm was proposed for secure key exchange between the legitimate communication parties in the presence of a passive eavesdropper was presented. The proposed method ensures secrecy via pre-equalization and guarantees reliable communications using Low-Density Parity Check (LDPC) codes. One of the main findings of this research is to demonstrate through simulations that the diversity order of the eavesdropper will be zero unless the main and eavesdropping channels are almost correlated, while the probability of a key mismatch between the legitimate transmitter and receiver will be low. Simulation results demonstrate that the proposed approach achieves very low secret key mismatch between the legitimate users while ensuring very high error probability at the eavesdropper. Next, a novel medium access control (MAC) protocol Slotted Aloha-NOMA (SAN), directed to Machine to Machine (M2M) communication applications in the 5G Internet of Things (IoT) networks was proposed. SAN is matched to the low-complexity implementation and sporadic traffic requirements of M2M applications. Substantial throughput gains are achieved by enhancing Slotted Aloha with non-orthogonal multiple access (NOMA) and a Successive Interference Cancellation (SIC) receiver that can simultaneously detect multiple transmitted signals using power domain multiplexing. The gateway SAN receiver adaptively learns the number of active devices using a form of multi-hypothesis testing and a novel procedure enables the transmitters to independently select distinct power levels. Simulation results show that the throughput of SAN exceeds that of conventional Slotted Aloha by 80% and that of CSMA/CA by 20% with a probability of transmission of 0.03, with a slightly increased average delay owing to the novel power level selection mechanism. Finally, beam sweeping pattern prediction, based on the dynamic distribution of user traffic, using a form of recurrent neural networks (RNNs) called Gated Recurrent Unit (GRU) is proposed. The spatial distribution of users is inferred from data in call detail records (CDRs) of the cellular network. Results show that the user's spatial distribution and their approximate location (direction) can be accurately predicted based on CDRs data using GRU, which is then used to calculate the sweeping pattern in the angular domain during cell search. Furthermore, the data-driven proposed beam sweeping pattern prediction was compared to random starting point sweeping (RSP) to measure the synchronization delay distribution. Results demonstrate the data- drive beam sweeping pattern prediction enables the UE to initially assess the gNB in approximately 0.41 of a complete scanning cycle that is required by the RSP scheme with probability 0.9 in a sparsely distributed UE scenario.