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Computing devices are commonly equipped with multiple transport technologies such as IrDA, Bluetooth and WiFi. Transport switching technologies, such as Quality of Transport (QoT), take advantage of this heterogeneity to keep network sessions active as users move in and out of range of various transports or as the networking environment changes. During an active session, the goal is to keep the device connected over the best transport currently available. To accomplish that, this thesis introduces a two-phase decision making protocol. In phase one, intra-device prioritization, users indicate the relative importance of criteria such as speed, power, service charge, or signal range through a comprehensive user interface. QoT-enabled devices process this information with the prioritized soft constraint satisfaction (PSCS) scoring function to ascertain the transport that best meets the user's needs. The second phase, inter-device negotiation, facilitates two QoT-enabled devices in agreeing to a unified selection of the best transport. This phase uses a modified version of the PSCS scoring function based on the preferences of both users. Additionally, devices may utilize multiple transports simultaneously to more accurately meet user demands. The PSCS scoring function considers pairs of transports and calculates the ratio that will yield the desired performance. Another set of functions, also presented in this thesis, is then used to accomplish the desired performance level despite the potential introduction of additional overhead.

Single-mode fiber transmission can no longer satisfy exponentially growing capacity demand. Space-division multiplexing (SDM) appears to be the only way able to dramatically improve the transmission capacity, for which, novel optical fiber is one of the key technologies. Such fibers must possess the following characteristics: 1) high mode density per cross-sectional area and 2) low crosstalk or low modal differential group delay (DMGD) to reduce complexity of digital signal processing. In this dissertation, we explore the design and characterization of three kinds of fibers for SDM: few-mode fiber (FMF), few-mode multi-core fiber (FM-MCF) and coupled multi-core fiber (CMCF) as well as their applications in transmission and networking. For the ultra-high density need of SDM, we have proposed the FMMCF. It combines advantages of both the FMF and MCF. The challenge is the inter-core crosstalk of the high-order modes. By applying a hole-assisted structure and careful fiber design, the LP11 crosstalk has been suppressed down to -40dB per km. This allows separate transmission on LP01 and LP11 modes without penalty. In fact, a robust SDM transmission up to 200Tb/s has been achieved using this fiber. To overcome distributed modal crosstalk in conjunction with DMGD, supermodes in CMCFs have been proposed. The properties of supermodes were investigated using the coupled-mode theory. The immediate benefits include high mode density and large effective area. In supermode structures, core-to-core coupling is exploited to reduce modal crosstalk or minimize DMGD. In addition, higher-order supermodes have been discovered in CMCFs with few-mode cores. We show that higher-order supermodes in different waveguide array configurations can be strongly affected by angle-dependent couplings, leading to different modal fields. Analytical solutions are provided for linear, rectangular and ring arrays. Higher-order modes have been observed for the first time using S2 imaging method. Finally, we introduce FMF to gigabit-capable passive optical networks (GPON). By replacing the conventional splitter with a photonic lantern, upstream combining loss can be eliminated. Low crosstalk has been achieved by a customized mode-selective photonic lantern carefully coupled to the FMF. We have demonstrated the first few-mode GPON system with error-free performance over 20-km 3-mode transmission using a commercial GPON system carrying live Ethernet traffic. We then scale the 3-mode GPON system to 5-mode, which resulted in a 4dB net gain in power budget in comparison with current commercial single-mode GPON systems.

Wavelength division multiplexed (WDM) optical networks are rapidly becoming the technology of choice in next-generation Internet architectures. This dissertation addresses the important issues of improving four aspects of optical networks, namely, routing efficiency, fairness, differentiated quality of service (QoS) and throughput. A new approach for implementing efficient routing and wavelength assignment in WDM networks is proposed and evaluated. In this approach, the state of a multiple-fiber link is represented by a compact bitmap computed as the logical union of the bitmaps of the free wavelengths in the fibers of this link. A modified Dijkstra's shortest path algorithm and a wavelength assignment algorithm are developed using fast logical operations on the bitmap representation. In optical burst switched (OBS) networks, the burst dropping probability increases as the number of hops in the lightpath of the burst increases. Two schemes are proposed and evaluated to alleviate this unfairness. The two schemes have simple logic, and alleviate the beat-down unfairness problem without negatively impacting the overall throughput of the system. Two similar schemes to provide differentiated services in OBS networks are introduced. A new scheme to improve the fairness of OBS networks based on burst preemption is presented. The scheme uses carefully designed constraints to avoid excessive wasted channel reservations, reduce cascaded useless preemptions, and maintain healthy throughput levels. A new scheme to improve the throughput of OBS networks based on burst preemption is presented. An analytical model is developed to compute the throughput of the network for the special case when the network has a ring topology and the preemption weight is based solely on burst size. The analytical model is quite accurate and gives results close to those obtained by simulation. Finally, a preemption-based scheme for the concurrent improvement of throughput and burst fairness in OBS networks is proposed and evaluated. The scheme uses a preemption weight consisting of two terms: the first term is a function of the size of the burst and the second term is the product of the hop count times the length of the lightpath of the burst.

As a promising candidate to break the single-mode fiber capacity limit, mode-division multiplexing (MDM) explores the spatial dimension to increase transmission capacity in fiberoptic communication. Two linear impairments, namely loss and multimode interference, present fundamental challenges to implementing MDM. In this dissertation, techniques to resolve these two issues are presented. To de-multiplex signals subject to multimode interference in MDM, Multiple-InputMultiple-Output (MIMO) processing using adaptive frequency-domain equalization (FDE) is proposed and investigated. Both simulations and experiments validate that FDE can reduce the algorithmic complexity significantly in comparison with the conventional time-domain equalization (TDE) while achieving similar performance as TDE. To further improve the performance of FDE, two modifications on traditional FDE algorithm are demonstrated. i) normalized adaptive FDE is applied to increase the convergence speed by 5 times; ii) masterslave carrier recovery is proposed to reduce the algorithmic complexity of phase estimation by number of modes. Although FDE can reduce the computational complexity of the MIMO processing, due to large mode group delay (MGD) of FMF link and block processing, the algorithm still requires enormous memory and high hardware complexity. In order to reduce the required tap length (RTL) of the equalizer, differential mode group delay compensated fiber (DMGDC) has been proposed. In this dissertation, the analytical expression for RTL is derived for DMGDC systems under the weak mode coupling assumption. Instead of depending on the overall MGD of the link iii in DMGD uncompensated (DMGDUC) systems, the RTL of DMGDC systems depend on the MGD of a single DMGDC fiber section. The theoretical and numerical results suggest that by using small compensation step-size, the RTL of DMGDC link can be reduced by 2 orders of magnitude compared to DMGDUC link. To compensate the loss of different modes, multimode EDFAs are presented with reconfigurable multimode pumps. By tuning the mode content of the multimode pump, modedependent gain (MDG) can be controlled and equalized. A proto-type FM-EDFA which could support 2 LP modes was constructed. The experimental results show that by using high order mode pumps, the modal gain difference can be reduced. By applying both multimode EDFA and equalization techniques, 26.4Tb/s MDM-WDM transmission was successfully demonstrated. A brief summary and several possible future research directions conclude this dissertation.

Underlay signaling is a robust physical layer technique, allowing for transmitting a very low power signal in conjunction with the primary signals across the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of the cellular (primary) network, where the underlay is an auxiliary channel made available to the primary users and the network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. Hence, this thesis delves into the study of underlay coexistence with OFDM, specifically 5G, by performing extensive simulations and analytical analysis and investigating the impact of underlay signaling on the throughput performance of 5G networks. We develop the underlay signaling based on the frequency-domain spread spectrum and add the underlay signal prior to the Inverse Fast Fourier Transform (IFFT) operation of OFDM. Furthermore, we present a real 5G setup built on the srsRAN project, where we showcase a proof-of-concept demonstration of underlay coexistence with the 5G over the air, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through our research, we conclusively demonstrate that a low-data rate underlay signal can be successfully transmitted alongside the existing 5G signal. Our study concludes by carefully selecting the appropriate design parameters, such as the signal-to-interference power level (5G power in relation to underlay), spreading factor, and coding gain at which we can reliably detect and decode underlay signals having no impact on the 5G performance. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, with the potential to provide Low Probability of Interception (LPI) and Low Probability of Detection (LPD) attributes and heightened reliability while concurrently transmitting with the 5G NR, bolstering the overall effectiveness of the communication. / Master of Science / Underlay signaling is a technique that allows for transmitting a low-power signal alongside the primary signals, occupying the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of cellular (primary) networks where the underlay is an auxiliary channel made available to the primary users and network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. OFDM is a multicarrier modulation scheme that divides the available frequency band into multiple narrow subcarriers, each carrying a portion of the data. The key advantage of OFDM is its ability to combat frequency-selective fading, where different frequencies experience different levels of fading and interference. By using many closely spaced sub-carriers, OFDM can effectively mitigate the impact of fading, allowing for robust and reliable communication even in challenging channel conditions. Thus, this thesis investigates the co-existence of underlay signaling and OFDM in 5G. We develop the underlay signaling based on the frequency-domain spread spectrum. Extensive simulations and analytical analysis are performed to understand the impact of underlay signaling on OFDM performance in terms of bit error rates and throughput. Additionally, a real 5G setup is presented, demonstrating a proof-of-concept of underlay co-existence with 5G NR, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through the research, it is conclusively demonstrated that a low-data rate error-free underlay signal can be successfully transmitted alongside the existing 5G signal. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, characterized by its low interception and low detection attributes, enhancing reliability while concurrently transmitting with 5G NR, bolstering the overall effectiveness of the communication.

The growth of data capacity in optical communications links, which form the critical backbone of the modern internet, is facing a slowdown due to fundamental nonlinear limitations, leading to an impending "capacity crunch" on the horizon. Current technology has already exhausted degrees of freedom such as wavelength, amplitude, phase and polarization, leaving spatial multiplexing as the last available dimension to be efficiently exploited. To minimize the significant energy requirements associated with digital signal processing, it is critical to explore the upper limit of unmixed spatial channels in an optical fiber, which necessitates ideally packing spatial channels either in real space or in momentum space. The former strategy is realized by uncoupled multi-core fibers whose channel count has already saturated due to reliability constraint limiting fiber sizes. The later strategy is realized by the unmixed multimode fiber whose high spatial efficiency suggest the possibility of high channel-count scalability but the right subset of mode ought to be selected in order to mitigate mode coupling that is ever-present due to the plethora of perturbations a fiber normally experiences. The azimuthal modes in ring-core fibers turn out to be one of the most spatially efficient in this regard, by exploiting light’s orbital angular momentum (OAM). Unmixed mode counts have reached 12 in a ~1km fiber and 24 in a ~10m fiber. However, there is a fundamental bottleneck for scalability of conventionally bound modes and their relatively high crosstalks restricts their utility to device length applications. In this thesis, we provide a fundamental solution to further fuel the unmixed-channel count in an MMF. We utilize the phenomenon of topological confinement, which is a regime of light guidance beyond conventional cutoff that has, to the best of our knowledge, never been demonstrated till publications based on the subject matter of this thesis. In this regime, light is guided by the centrifugal barrier created by light’s OAM itself rather than conventional total internal reflection arising from the index inhomogeneity of the fiber. The loss of these topologically confined modes (TCMs) decreases down to negligible levels by increasing the OAM of fiber modes, because the centrifugal barrier that keeps photons confined to a fiber core increases with the OAM value of the mode. This leads to low-loss transmission in a km-scale fiber of these cutoff modes. Crucially, the mode-dependent confinement loss of TCMs further lifts the degeneracy of wavevectors in the complex space, leading to frustration of phase-matched coupling. This thus allows further scaling the mode count that was previously hindered by degenerate mode coupling in conventionally bound fiber modes. The frustrated coupling of TCMs thus enables a record amount of unmixed OAM modes in any type of fiber that features a high index contrast, whether specially structured as a ring-core, or simply constructed as a step-index fiber. Using all these favorable attributes, we achieve up to 50 low-loss modes with record low crosstalk (approaching -45 dB/km) over a 130-nm bandwidth in a ~1km-long ring-core fiber. The TCM effect promises to be inherently scalable, suggesting that even higher modes counts can be obtained in the future using this design methodology. Hence, the use of TCMs promises breaking the record spectral efficiency, potentially making it the choice for transmission links in future Space-Division-Multiplexing systems. Apart from their chief attribute of significantly increasing the information content per photon for quantum or classical networks, we expect that this new light guidance may find other applications such as in nonlinear signal processing and light-matter interactions.

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