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In-band full duplex techniques have been shown practical in many recent works, empowering two wireless radios to simultaneously transmit and receive in the same frequency band. With the advances in full duplex techniques, full duplex capable relays have been shown feasible. Instead of generating data itself, a full duplex relay can forward the received signal while receiving it. However, most of these designs can only utilize one relay at the same time due to the potential inter-relay interferencewhen multiple relays coexist.In this thesis, BiPass is first presented to enable end-to-end full duplex with the help of full duplex relay clusters. In previous works, full duplex has been shown to potentially double the channel capacity and achieve lower delays. However, full duplex is only available between two adjacent nodes within the communication range. BiPass to proposed to break this limitation. With the help of full duplex relays, simultaneous bidirectional in-band cut-through transmissions between two far apart nodes are enabled with BiPass, so they can do full duplex communications as if they were within each other’s transmission range.Then AllPass is presented, a system that can employ multiple full duplex relays at the same time, while introducing only the same amount of extra delay as a one-relay system. This constant extra delay, regardless of number of relays being employed, is especially useful in reality to work with existing systems. AllPass is transparent to existing systems as it does not need modifications or cooperation of the transmitter or receiver. Many previous works utilizing only one relay can potentially be extended to multiple relays case with AllPass, which may further improve the performance.At last, we discuss whether constructive and destructive full duplex relays are possible. A full duplex has the opportunity to properly modify the signal before forwarding it, which has been explored to enable constructive and destructive full duplex relaying recently. With these designs, the signal through the full duplex relay could constructively or destructively add up with the signal from the direct link at the receiver side. We find that for OFDM signals, such relays can not be achieved if the additional latency of the relayed path is more than one sample time than the direct link. Our analysis and results challenge the assumptions on the relay latency requirements of previous works to realize such systems.Throughout the thesis, extensive experiments are performed with software defined radios including NI PXIe, WARP and USRP. Our results show that full duplex relay cluster can improve the system performance in various scenarios. The last part of this thesis gives the conclusion and outlines the directions of future work. We believe that the proposed full duplex relay cluster designs will inspire more researches and act as an enabling technique for many more exciting applications in the future.

Interference in front end transceivers directly impact system performance by saturating receiver circuitry and generating non-linearities. While proper shielding can effectively eliminate external interference, self-interference continues to limit the performance of sensing and communication systems today. Self-interference exists when the transmitted signal interferes with the received signal through parasitic signal paths and from spurious reflections. This dissertation explores the use of novel transducers, front-end designs, and active cancellers to eliminate self-interference in multiple sensing and communication modalities: acoustic, optical, and electronics. First, we demonstrate integration of electroadhesion with a piezoelectric multimaterial fiber transducer to reduce acoustic signal reflection at the interface with the target substrate. The large acoustic impedance of air introduces a 10⁵ mismatch with the acoustic transducer and the substrate. By monolithically integrating electroadhesive and acoustic functions into a single fiber device, the fiber transducer can controllably adhere to the target and subsequently eliminate the interfacial air microvoids during operation. Second, the limited directivity of optical circulators give rise to parasitic signal paths in optical Mach-Zehdner based laser Doppler vibrometers. Parasitic signal paths are signal paths in an interferometer which are unexpected and unbalanced with respect to the signal and reference arms. The path length imbalance demodulates laser phase noise as amplitude noise at the receiver which degrades sensitivity due to the decrease in signal to noise ratio. We derive an expression for this this parasitic phase induced intensity noise (P-PIIN) and develop a new interferometer design which enables mutually balanced signal, reference, and dominant parasitic paths to achieve 90 dB of signal to noise ratio and femtometer vibration sensitivity. Third, self-interference in modern electronic communication systems limit the maximum achievable throughput in a given bandwidth. Self-interference from both parasitic signal paths and spurious reflections overwhelms the significantly weaker receive signal when both are operating simultaneously. As such, communication systems such as cable only operate either in time domain duplexing (alternate between transmitting and receiving) or frequency domain duplexing (transmit and receive on different frequencies) modes. By actively eliminating self-interference, communication systems are able to send and receive information at the same time on the same frequency: full duplex operation.

Over the past decade, wireless data communication has experienced a phenomenal growth, which is driven by the popularity of wireless devices and the growing number of bandwidth hungry applications. During the same period, various advanced communication technologies have emerged to improve network throughput. Some examples include multi-input multi-output (MIMO), full duplex, cognitive radio, mmWave, among others. An important research direction is to understand the impacts of these new technologies on network throughput performance. Such investigation is critical not only for theoretical understanding, but also can be used as a guideline to design algorithms and network protocols in the field. The goal of this dissertation is to understand the impact of some advanced technologies on network throughput performance. More specifically, we investigate the following three technologies: MIMO, full duplex, and mmWave communication. For each technology, we explore the performance envelope of wireless networks by studying a throughput maximization problem.

Nonreciprocal components like circulators are often used to separate transmitting and receiving signals sharing a common antenna such as those in the front-end of a monostatic continuous-wave radar or a full-duplex radio that performs simultaneous transmit and receive (STAR) over the same frequency band. In those systems, the isolation of the circulator is critical in preventing the receiver from being interfered or jammed by the transmitted signal. Classical ferrite circulators are essentially cavities supporting non-reciprocal resonant modes. They are intrinsically narrowband, bulky, and hard to be integrated on modern integrated circuits (ICs). In comparison, active circulators have been developed based on the nonreciprocal transfer characteristics of transistors. They offer small physical dimensions, versatile functionalities, and compatibility with IC technology. However, active circulators have generally limited noise and power performances which prevent them from being widely deployed in systems requiring wide dynamic ranges.Time-varying transmission lines (TVTL) such as Distributedly Modulated Capacitors (DMC) are transmission lines loaded with varactors whose capacitances are modulated by a unidirectionally propagating wave. They translate the direction difference of waves propagating on it into the difference in frequency based on its unidirectional frequency conversion property. In such way, TVTL can behave as a circulator and exhibit broadband isolation due to the non-resonant nature of the traveling wave structure. Positive-dB parametric gain may be realized during the frequency conversion process which helps to compensate for the circuit loss.In this work, a complete theory, based on (1) the distributed parametric effect on a TVTL and (2) the distributed capacitive mixers, is presented with emphasis on the theoretical bounds of the isolation and gain performances of the DMC. The DMCs implemented on Rogers PCB and on GaN MMIC are both presented with the measured results agreeing well with the theoretical derivations and the simulation results. Especially, the MMIC DMC has demonstrated experimentally verified positive-dB receiving (RX) gain and > 10 dB transmitting-versus-receiving (TX/RX) isolation over 0.7 - 2.5 GHz. Furthermore, a circulator prototype combining a pair of MMIC DMC units in a balanced architecture is assembled and tested. The balanced architecture offers a new level of isolation in addition to what is offered by a single DMC. More than 25 dB TX/RX isolation and less than 2 dB RX loss have been experimentally observed for almost one octave at 0.95 - 1.8 GHz.

Demand for mobile data traffic is projected to exceed 30 exabytes per month in 2020, representing an over 100x increase since 2010. Prior generations of cellular deployments have serviced increased demand largely through use of more bandwidth - from 200KHz in GSM, to now 100MHz in CA-LTE. This method of scaling is closed, as low frequency spectrum has crowded and saturated. A proposed technique to enhance spectrum access in 5G deployments is agile full-duplex (FD) transceivers, which can transmit and receive at overlapped frequencies, or tune to arbitrarily spaced transmit/receive(TX/RX) frequency division duplexed (FDD) channels, to make use of available spectrum. The key problem in such a system is mitigating the interference the system's own transmitter creates for its receiver during simultaneous operation. Current implementations mitigate TX to RX interference at the antenna interface using off-chip, fixed-frequency duplexers, limiting a device's spectrum access to a handful of pre-defined, widely separated TX/RX band combinations. Accordingly, a universal mobile device tunable across global carrier band combinations does not exist. This work develops a transceiver architecture enabling simultaneous transmission and reception on a single single shared antenna, over a wide frequency tuning range, for FD/FDD systems. The architecture is enabled by an active TX replica which cancels interference at the RX input, a highly linear passive-mixer first receiver design based on class-AB transconductors which operates linearly in the presence of residual TX interference, and digital adaptation techniques which match the interference over time-varying operating conditions. Analysis is presented for the system's fundamental performance bounds in power and sensitivity, leading to noise mitigation techniques which minimize receiver degradation in the presence of the cancellation circuits. The analysis is validated by two chip prototypes, which demonstrate over $>$50dB cancellation of a +16dBm peak 20MHz TX signal, from 1GHz to 2GHz, up to an antenna VSWR of 5:1. This work demonstrates the potential for a fully integrated, frequency-tunable FD/FDD transceiver system, which could ultimately double existing mobile network capacity, and enable a universal duplexer-less radio.

Due to the tremendous increase in wireless data traffic, one of the major challenges for future wireless systems is the utilization of the available spectrum to achieve better data rates over limited spectrum. Currently, systems operate in what is termed "Half Duplex Mode," where they are either transmitting or receiving, but never both using the same temporal and spectral resources. Full-duplex transmission promises to double the spectral efficiency where bidirectional communications is carried out over the same temporal and spectral resources. The main limitation impacting full-duplex transmission is managing the strong self-interference signal imposed by the transmit antenna on the receive antenna within the same transceiver. Several recent publications have demonstrated that the key challenge in practical full-duplex systems is un-cancelled self-interference power caused by a combination of hardware imperfections, especially Radio Frequency (RF) circuits' impairments. In this thesis, we consider the problem of self-interference cancellation in full-duplex systems. The ultimate goal of this work is to design and build a complete, real-time, full-duplex system that is capable of achieving wireless full-duplex transmission using practical hardware platforms. Since RF circuits' impairments are shown to have significant impact on the self-interference cancellation performance, first, we present a thorough analysis of the effect of RF impairments on the cancellation performance, with the aim of identifying the main performance limiting factors and bottlenecks. Second, the thesis proposes several impairments mitigation techniques to improve the overall self-interference cancellation capability by mitigating most of the transceiver RF impairments. In addition to impairments mitigation, two novel full-duplex transceiver architectures that achieve significant self-interference cancellation performance are proposed. The performance of the proposed techniques is analytically and experimentally investigated in practical wireless environments. Finally, the proposed self-interference cancellation techniques are used to build a complete full-duplex system with a 90\% experimentally proven full-duplex rate improvement compared to half-duplex systems.

For both the conventional radio frequency and the comparably recent optical wireless communication systems, extensive effort from the academia had been made in improving the network spectrum efficiency and/or reducing the error rate. To achieve these goals, many fundamental challenges such as power efficient constellation design, nonlinear distortion mitigation, channel training design, network scheduling and etc. need to be properly addressed. In this dissertation, novel schemes are proposed accordingly to deal with specific problems falling in category of these challenges. Rigorous proofs and analyses are provided for each of our work to make a fair comparison with the corresponding peer works to clearly demonstrate the advantages.The first part of this dissertation considers a multi-carrier optical wireless system employing intensity modulation (IM) and direct detection (DD). A block-wise constellation design is presented, which treats the DC-bias that conventionally used solely for biasing purpose as an information basis. Our scheme, we term it MSM-JDCM, takes advantage of the compactness of sphere packing in a higher dimensional space, and in turn power efficient constellations are obtained by solving an advanced convex optimization problem. Besides the significant power gains, the MSM-JDCM has many other merits such as being capable of mitigating nonlinear distortion by including a peak-to-power ratio (PAPR) constraint, minimizing inter-symbol-interference (ISI) caused by frequency-selective fading with a novel precoder designed and embedded, and further reducing the bit-error-rate (BER) by combining with an optimized labeling scheme.The second part addresses several optimization problems in a multi-color visible light communication system, including power efficient constellation design, joint pre-equalizer and constellation design, and modeling of different structured channels with cross-talks. Our novel constellation design scheme, termed CSK-Advanced, is compared with the conventional decoupled system with the same spectrum efficiency to demonstrate the power efficiency. Crucial lighting requirements are included as optimization constraints. To control non-linear distortion, the optical peak-to-average-power ratio (PAPR) of LEDs can be individually constrained. With a SVD-based pre-equalizer designed and employed, our scheme can achieve lower BER than counterparts applying zero-forcing (ZF) or linear minimum-mean-squared-error (LMMSE) based post-equalizers. Besides, a binary switching algorithm (BSA) is applied to improve BER performance.The third part looks into a problem of two-phase channel estimation in a relayed wireless network. The channel estimates in every phase are obtained by the linear minimum mean squared error (LMMSE) method. Inaccurate estimate of the relay to destination (RtD) channel in phase 1 could affect estimate of the source to relay (StR) channel in phase 2, which is made erroneous. We first derive a close-form expression for the averaged Bayesian mean-square estimation error (ABMSE) for both phase estimates in terms of the length of source and relay training slots, based on which an iterative searching algorithm is then proposed that optimally allocates training slots to the two phases such that estimation errors are balanced. Analysis shows how the ABMSE of the StD channel estimation varies with the lengths of relay training and source training slots, the relay amplification gain, and the channel prior information respectively.The last part deals with a transmission scheduling problem in a uplink multiple-input-multiple-output (MIMO) wireless network. Code division multiple access (CDMA) is assumed as a multiple access scheme and pseudo-random codes are employed for different users. We consider a heavy traffic scenario, in which each user always has packets to transmit in the scheduled time slots. If the relay is scheduled for transmission together with users, then it operates in a full-duplex mode, where the packets previously collected from users are transmitted to the destination while new packets are being collected from users. A novel expression of throughput is first derived and then used to develop a scheduling algorithm to maximize the throughput. Our full-duplex scheduling is compared with a half-duplex scheduling, random access, and time division multiple access (TDMA), and simulation results illustrate its superiority. Throughput gains due to employment of both MIMO and CDMA are observed.