Browsing by Subject "Wireless Communications"

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    Development of High Efficiency Metamaterial Antenna Structures for Near-Field and Far-Field Applications
    (2022-08) Dave, Aditya
    With the advent of mmwave 5G, and future G technologies, there is a path paved for multitude of applications in the cellular, augmented and virtual reality (AR/VR), internet-of-things (IoT) etc. domains. There is a need for compact, highly directional and low-loss antennas for reduced size, greater coverage, low power consumption. Partially reflective surfaces as superstrates are well known for enhancing antenna radiation. However, in the past, electrically large surfaces were used with little regards to the size and aperture efficiency of the antennas. In this dissertation, compact source antennas are used with smaller 2D metamaterial superstrates acting as partially reflective surfaces (PRS) to form metamaterial antenna (MMA) block. The dissertation is divided into three segments. After going over the theoretical framework for infinite periodic surfaces and development of equivalent circuits in chapter 2, finite PRS surfaces with source antennas are analyzed in chapters 3 and 4. Different types of PRS surfaces and source antennas are changed one at a time to explain the design methodology and arrive at highly aperture efficient MMA blocks. In chapter 5, single MMA block is used to create virtual arrays using beam-splitting PRS designs and analyze its performance with conventional arrays. The single MMA blocks are also showcased in array element reduction applications to reduce feedline complexities associated with conventional arrays. Chapter 6 focuses on formation of passive phased arrays using near field phase manipulation properties of the PRS. This property is used to create dual beam antennas. Next, designs that focus on creating polarization splitters using yet another variation of PRS, called beam and polarization splitting PRS (BPS-PRS) are proposed. The dual beam antennas and polarization splitters can be applied to emerging multiple-input multiple-output (MIMO) communication applications. MMAs are also useful as GNSS positional sensors as seen by their low phase center variation properties which are also showcased. Finally, chapter 7 focuses on near-field applications of the MMA by proposing free space vertical interconnects and power dividers that are useful for high frequency printed circuit board (PCB) integrated chip-to-chip intra-connects and interconnects. Additional loss reduction technologies that are useful for on-chip silicon implementations are also demonstrated by using Copper nanowires on coplanar waveguide transmission lines for frequency ranges up to 180 GHz. Chapter 8 concludes the work and gives directions for the future work.
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    MIMO Wireless Communications with limited feedback
    (2012-09) Ravindran, Niranjay
    In this dissertation, we investigate the effects of having imperfect channel state information due to limited training and feedback resources in multi-user systems with multiple antennas. We find that while the achievable rate is highly sensitive to the quality of channel training and feedback, the rate gap relative to the rate achievable with perfect channel information can be uniformly bounded for all values of signal-to-noise ratio, with proper design of the feedback link to acquire accurate channel information, that is, the multiplexing gain can be preserved. Further, when a large number of users are present in a system, we find the strong requirement for accurate channel information remains, contrary to many transmission strategies that are commonly proposed for this regime. We conclude that given a limited feedback budget, it is desirable to first use resources to acquire highly accurate channel information, and only then allocate resources to exploit multi-user diversity. We also obtain results characterizing the statistics of random subspace quantization, which we use to compute the reduction in feedback overhead possible when this form of quantization is used. Finally, we consider open-loop multi-hop ad hoc networks with multiple antennas and opportunistic routing, and investigate using multiple antennas to optimize the spatial reuse, per-hop length and per-hop rate to maximize end-to-end performance.