The next-generation wireless technologies are currently being researched to address the ever-increasing demands for higher data rates, massive connectivity, improved reliability, and extended coverage. Recently, massive multiple-input multiple-output (MIMO) has gained significant attention as a new physical-layer transmission technology that can achieve unprecedented spectral and energy efficiency gains via aggressive spatial multiplexing. Thus, massive MIMO has been one of the key enabling technologies for the fifth-generation and subsequent wireless standards. This dissertation thus focuses on developing a system, channel, and signal models by considering the practical wireless transmission impairments for massive MIMO systems, and ascertaining the viability of massive MIMO in fulfilling massive access, improved spectrum, enhanced security, and energy efficiency requirements. Specifically, new system and channel models, pilot sequence designs and channel estimation techniques, secure transmit/receive beamforming techniques, transmit power allocation schemes with enhanced security provisions, energy efficiency, and user fairness, and comprehensive performance analysis frameworks are developed for massive MIMO-aided non-orthogonal multiple access (NOMA), cognitive spectrum-sharing, and wireless relaying architectures.Our first work focuses on developing physical-layer transmission schemes for NOMA-aided massive MIMO systems. A spatial signature-based user-clustering and pilot allocation scheme is first formulated, and thereby, a hybrid orthogonal multiple access (OMA)/NOMA transmission scheme is proposed to boost the number of simultaneous connections. In our second work, the viability of invoking downlink pilots to boost the achievable rate of NOMA-aided massive MIMO is investigated. The third research contribution investigates the performance of underlay spectrum-sharing massive MIMO systems for reverse time division duplexing based transmission strategies, in which primary and secondary systems concurrently operate in opposite directions. Thereby, we show that the secondary system can be operated with its maximum average transmit power independent of the primary system in the limit of infinity many primary/secondary base-station antennas. In our fourth work, signal processing techniques, power allocation, and relay selection schemes are designed and analyzed for massive MIMO relay networks to optimize the trade-off among the achievable user rates, coverage, and wireless resource usage. Finally, the cooperative jamming and artificial noise-based secure transmission strategies are developed for massive MIMO relay networks with imperfect legitimate user channel information and with no channel knowledge of the eavesdropper. The key design criterion of the aforementioned transmission strategies is to efficiently combine the spatial multiplexing gains and favorable propagation conditions of massive MIMO with properties of NOMA, underlay spectrum-sharing, and wireless relay networks via efficient signal processing.

This paper is persuaded to understand the physical layer security in wireless commu-nications utilizing NOMA (Non Orthogonal Multiple Access) concepts in the presence of an eavesdropper. Physical layer security maintains the confidentiality and secrecyof the system against eavesdroppers. We use the power domain in this paper, where NOMA allows many users to share resources side by side. Power allocation concern-ing channel condition is taken into consideration where user whose channel condition is weak is allocated with eminent power to directly decode the signal, whereas theuser with better channel condition applies successive interference cancellation (SIC)to decode the signal. Here, the base station communicates with the users and sends data signals while the eavesdropper secretly eavesdrops on the confidential informa-tion simultaneously. In this thesis, to improve the physical layer security, jamming method was usedwhere users are assumed to be in full duplex, send jamming signals to degrade the performance of the eavesdropper. Analytic expressions of CDF, PDF, outage proba-bility and secrecy capacity are obtained from analyzing the NOMA jamming scheme. The numerical results are evaluated with the simulations results and analysed theeffect of jamming on improving the performance of the NOMA system in presenceof an eavesdropper.

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This thesis consists of two research projects on wireless communication systems. In the first project, we propose a fast inphase and quadrature (I/Q) imbalance compensation technique for the analog quadrature modulators in direct conversion transmitters. The method needs no training sequence, no extra background data gathering process and no prior perfect knowledge of the envelope detector characteristics. In contrast to previous approaches, it uses points from both the linear and predictable nonlinear regions of the envelope detector to hasten convergence. We provide a least mean square (LMS) version and demonstrate that the quadrature modulator compensator converges. In the second project, we propose a technique to deceive the automatic gain control (AGC) block in an eavesdropper's receiver to increase wireless physical layer data transmission secrecy. By sharing a key with the legitimate receiver and fluctuating the transmitted signal power level in the transmitter side, a positive average secrecy capacity can be achieved even when an eavesdropper has the same or even better additive white gaussian noise (AWGN) channel condition. Then, the possible options that an eavesdropper may choose to fight against our technique are discussed and analyzed, and approaches to eliminate these options are proposed. We demonstrate that a positive average secrecy capacity can still be achieved when an eavesdropper uses these options.

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This thesis discusses the design of two Physical Layer Security (PLS) techniques on Software Defined Radios (SDRs). PLS is a classification of security methods that take advantage of physical properties in the waveform or channel to secure communication. These schemes can be used to directly obfuscate the signal from eavesdroppers, or even generate secret keys for traditional encryption methods. Over the past decade, advancements in Multiple-Input Multiple-Output systems have expanded the potential capabilities of PLS while the development of technologies such as the Internet of Things has provided new applications. As a result, this field has become heavily researched, but is still lacking implementations. The design work in this thesis attempts to alleviate this problem by establishing SDR designs geared towards Over-the-Air experimentation. The first design involves a 2x1 Multiple-Input Single-Output system where the transmitter uses Channel State Information from the intended receiver to inject Artificial Noise (AN) into the receiver's nullspace. The AN is consequently not seen by the intended receiver, however, it will interfere with eavesdroppers experiencing independent channel fading. The second design involves a single-carrier Alamouti coding system with pseudo-random phase shifts applied to each transmit antenna, referred to as Phase-Enciphered Alamouti Coding (PEAC). The intended receiver has knowledge of the pseudo-random sequence and can undo these phase shifts when performing the Alamouti equalization, while an eavesdropper without knowledge of the sequence will be unable to decode the signal. / Master of Science

This dissertation addresses physical layer security concerns, resiliency of the Orthogonal Frequency Division Multiplexing (OFDM) and the Multiple Input Multiple Output (MIMO) systems; the `de-facto' air-interface of most wireless broadband standards including LTE and WiMAX. The major contributions of this dissertation are: 1) developing jamming taxonomy, 2) proposing OFDM and MIMO equalization jamming attacks and countermeasures, 3) developing antijam (AJ) MIMO systems, and 4) designing null space projected overlapped-MIMO radar waveform for spectrum sharing between radar and communications system. First, we consider OFDM systems under various jamming attacks. Previous research is focused on jamming OFDM data transmissions. We focus on energy efficient attacks that can disrupt communication severely by exploiting the knowledge of target waveform. Specifically, these attacks seek to manipulate information used by the equalization algorithm to cause errors to a significant number of symbols, i.e., pilot tones jamming and nulling. Potential countermeasures are presented in an attempt to make OFDM waveform robust and resilient. The threats were mitigated by randomizing the location and value of pilot tones, causing the optimal attack to devolve into barrage jamming. We also address the security aspects of MIMO systems in this dissertation. All MIMO systems need a method to estimate and equalize channel, whether through channel reciprocity or sounding. Most OFDM-based MIMO systems use sounding via pilot tones. Like OFDM attacks, this research introduces MIMO channel sounding attack, which attempts to manipulate pilot tones to skew the channel state information (CSI) at the receiver. We describe methods of designing AJ MIMO system. The key insight is that many of the theoretical concepts learned from transmit beamforming and interference alignment (IA) in MIMO systems can be applied to the field of AJ and robust communications in the presence of jammers. We consider a realistic jamming scenario and provide a `receiver-only' and a transmitter `precoding' technique that allow a pair of two-antenna transceivers to communicate while being jammed by a malicious non-cooperative single-antenna adversary. Finally, we consider designing a collocated MIMO radar waveform, which employs a new MIMO architecture where antenna arrays are allowed to overlap. This overlapped-MIMO radar poses many advantages including superior beampattern and improvement in SNR gain. We combine this radar architecture with a projection-based algorithm that allows the radar waveform to project onto the null space of the interference channel of MIMO communications system, thus enabling the coexistence of radar and communications system. / Ph. D.

Including energy harvesting capability in a wireless network is attractive for multiple reasons. First and foremost, powering base stations with renewable resources could significantly reduce their reliance on the traditional energy sources, thus helping in curtailing the carbon footprint. Second, including this capability in wireless devices may help in increasing their lifetime, which is especially critical for devices for which it may not be easy to charge or replace batteries. This will often be the case for a large fraction of sensors that will form the {em digital skin} of an Internet of Things (IoT) ecosystem. Motivated by these factors, this work studies fundamental performance limitations that appear due to the inherent unreliability of energy harvesting when it is used as a primary or secondary source of energy by different elements of the wireless network, such as mobile users, IoT sensors, and/or base stations. The first step taken towards this objective is studying the joint uplink and downlink coverage of radio-frequency (RF) powered cellular-based IoT. Modeling the locations of the IoT devices and the base stations (BSs) using two independent Poisson point processes (PPPs), the joint uplink/downlink coverage probability is derived. The resulting expressions characterize how different system parameters impact coverage performance. Both mathematical expressions and simulation results show how these system parameters should be tuned in order to achieve the performance of the regularly powered IoT (IoT devices are powered by regular batteries). The placement of RF-powered devices close to the RF sources, to harvest more energy, imposes some concerns on the security of the signals transmitted by these RF sources to their intended receivers. Studying this problem is the second step taken in this dissertation towards better understanding of energy harvesting wireless networks. While these secrecy concerns have been recently addressed for the point-to-point link, it received less attention for the more general networks with randomly located transmitters (RF sources) and RF-powered devices, which is the main contribution in the second part of this dissertation. In the last part of this dissertation, we study the stability of solar-powered cellular networks. We use tools from percolation theory to study percolation probability of energy-drained BSs. We study the effect of two system parameters on that metric, namely, the energy arrival rate and the user density. Our results show the existence of a critical value for the ratio of the energy arrival rate to the user density, above which the percolation probability is zero. The next step to further improve the accuracy of the stability analysis is to study the effect of correlation between the battery levels at neighboring BSs. We provide an initial study that captures this correlation. The main insight drawn from our analysis is the existence of an optimal overlapping coverage area for neighboring BSs to serve each other's users when they are energy-drained. / Ph. D. / Renewable energy is a strong potential candidate for powering wireless networks, in order to ensure green, environment-friendly, and self-perpetual wireless networks. In particular, renewable energy gains its importance when cellular coverage is required in off-grid areas where there is no stable resource of energy. In that case, it makes sense to use solar-powered base stations to provide cellular coverage. In fact, solar-powered base stations are deployed already in multiple locations around the globe. However, in order to extend this to a large scale deployment, many fundamental aspects of the performance of such networks needs to be studied. One of these aspects is the stability of solar-powered cellular networks. In this dissertation, we study the stability of such networks by applying probabilistic analysis that leads to a set of useful system-level insights. In particular, we show the existence of a critical value for the energy intensity, above which the system stability is ensured. Another type of wireless networks that will greatly benefit from renewable energy is internet of things (IoT). IoT devices usually require several orders of magnitude lower power compared to the base stations. In addition, they are expected to be massively deployed, often in hard-to-reach locations. This makes it impractical or at least cost inefficient to rely on replacing or recharging batteries in these devices. Among many possible resources of renewable energy, radio frequency (RF) energy harvesting is the strongest candidate for powering IoT devices, due to ubiquity of RF signals even at hard-to-reach places. However, relying on RF signals as the sole resource of energy may affect the overall reliability of the IoT. Hence, rigorous performance analysis of RF-powered IoT networks is required. In this dissertation, we study multiple aspects of the performance of such networks, using tools from probability theory and stochastic geometry. In particular, we provide concrete mathematical expressions that can be used to determine the performance drop resulting from using renewable energy as the sole source of power. One more aspect of the performance of RF-powered IoT is the secrecy of the RF signals used by the IoT devices to harvest energy. The placement of RF-powered devices close to the RF sources, to harvest more energy, imposes some concerns on the security of the signals transmitted by these RF sources to their intended receivers. We study the effect of using secrecy enhancing techniques by the RF sources on the amount of energy harvested by the RF-powered devices. We provide performance comparison of three popular secrecy-enhancing techniques. In particular, we study the scenarios under which each of these techniques outperforms the others in terms of secrecy performance and energy harvesting probability. This material is based upon work supported by the U.S. National Science Foundation (Grant CCF1464293). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the NSF.

The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.