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In the recent decades, there have been dramatic changes in the ways people create and share information, hence a demand for wireless communication systems with over 100 Gb/s data rate has been envisioned. Promising techniques have been devised by researchers to fulfill this expectation, such as interference alignment (IA) and THz communication and many more. In theory, given ideal assumptions, they show extraordinary increases in the promised data rate relative to current wireless communication systems. Nevertheless, because of the hardware impairments, many practical challenges have to be addressed before bringing these advanced techniques from theory to practice. In this dissertation, a selection of practical issues of interference alignment and THz communication are discussed.First, we consider the synchronization and channel estimation problem associated with enabling IA. Before achieving IA, in view of the uncoordinated interference, a concurrent synchronization and channel estimation mechanism with robustness against co-channel interference is needed. We start by deriving the maximum-likelihood expressions for optimal estimators. Then, it is realized that such estimators are infeasible in practice for requiring a computationally costly 2-D search. To reduce the complexity, we propose a Zadoff-Chu sequence based pilot structure. By utilizing its unique properties, the 2-D search problem can be converted into two 1-D search problems. Experimental results verify the great performance of our proposed method in terms of both the residual frequency offset and the sum rate of an IA network. Second, we focus on the channel reciprocity in time division duplex (TDD) networks. The physical channel is reciprocal, but the channel considered by IA is not, because it also involves the radio frequency (RF) front-ends of the transceiver. The difference between the two individual circuits yields mismatch between the forward and backward channels. We provide the modeling of the non-reciprocity for the TDD MIMO channels, and propose a corresponding calibration method to minimize the mismatch. Experimental validation has been conducted. The results highlight the indispensability of the reciprocity calibration for approaching the network capacity promised by IA. Last, with a view to assessing THz communication system design requirements in practice, we measure the channels for both 350 GHz and 650 GHz carrier frequencies in a typical indoor environment. To obtain the spatial and temporal characteristics of the channel, the power angle profile (PAP) and the power delay profile (PDP) are measured using our proposed measurement systems and methods. From the measurement results, we have observed important facts regarding the THz wave propagation, which significantly affect the requirements for designing an indoor THz communication system.

The cell wall is an essential component of most bacterial cells, and cell wall targeting antibiotics have greatly improved human health and life span. Despite intense research over the last 50 years, we still lack a complete understanding of how bacteria maintain cell wall homeostasis. Here we attempt to integrate new data from the last few years with some established ideas in the field, and we propose a new model of cell wall biogenesis in rod-shaped bacteria (Chapter 1). The lipid II cycle is central to peptidoglycan (PG) synthesis. A common C55 lipid carrier, undecaprenyl-pyrophosphate (UPP) is used for the synthesis of both peptidoglycan and wall teichoic acids to ferry precursors across the cytoplasmic membrane. Here we demonstrate that B. subtilis requires either one of two UPP phosphatases, UppP or BcrC, for the recycling of this essential molecule for continuous synthesis of cell wall (Chapter 2). Peptidoglycan synthesis relies on intermediates derived from central metabolism. We found that an aspB mutant is auxotrophic for aspartate and lyses when grown on Difco sporulation medium due to limitation of the peptidoglycan precursor meso-2,6-diaminopimelate (mDAP), a downstream metabolic intermediate of aspartate. Interestingly, we found that when bacteria experience a shortage of aspartate, the first breakpoint is not protein but peptidoglycan synthesis, which predisposes them to cell wall weakness and sensitizes them to antibiotics targeting late steps of PG synthesis. This work highlights the ability of perturbations of central metabolism to sensitize cells to peptidoglycan synthesis inhibitors (Chapter 3). Bacillus subtilis uses alternative sigma factors to regulate gene transcription upon stresses. Being a powerful double-edged sword, it is essential to keep this regulatory network under check. Here we show that the absence of its anti-sigma factor(s) leads to dysregulation of SigM, which drives a positive feedback loop for its own synthesis and SigM accumulates to a toxic level. High SigM activity overproduces membrane proteins and causes protein secretion stress, which lead to cell morphology and severe growth defects (Chapter 4). Collectively, the work in this dissertation provides additional insights of essential pathways and gene regulation for cell wall homeostasis in Bacillus subtilis.