Theses and Dissertations from UMD
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Item Efficient Media Access Control and Distributed Channel-aware Scheduling for Wireless Ad-Hoc Networks(2013) Chen, Hua; Baras, John S; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We address the problem of channel-aware scheduling for wireless ad-hoc networks, where the channel state information (CSI) are utilized to improve the overall system performance instead of the individual link performance. In our framework, multiple links cooperate to schedule data transmission in a decentralized and opportunistic manner, where channel probing is adopted to resolve collisions in the wireless medium. In the first part of the dissertation, we study this problem under the assumption that we know the channel statistics but not the instant CSI. In this problem, channel probing is followed by a transmission scheduling procedure executed independently within each link in the network. We study this problem for the popular block-fading channel model, where channel dependencies are inevitable between different time instances during the channel probing phase. We use optimal stopping theory to formulate this problem, but at carefully chosen time instances at which effective decisions are made. The problem can then be solved by a new stopping rule problem where the observations are independent between different time instances. We first characterize the system performance assuming the stopping rule problem has infinite stages. We then develop a measure to check how well the problem can be analyzed as an infinite horizon problem, and characterize the achievable system performance if we ignore the finite horizon constraint and design stopping rules based on the infinite horizon analysis. We then analyze the problem using backward induction when the finite horizon constraint cannot be ignored. We develop one recursive approach to solve the problem and show that the computational complexity is linear with respect to network size. We present an improved protocol to reduce the probing costs which requires no additional cost. Based on our analysis on single-channel networks, we extend the problem to ad-hoc networks where the wireless spectrum can be divided into multiple independent sub-channels for better efficiency. We start with a naive multi-channel protocol where the scheduling scheme is working independently within each sub-channel. We show that the naive protocol can only marginally improve the system performance. We then develop a protocol to jointly consider the opportunistic scheduling behavior across multiple sub-channels. We characterize the optimal stopping rule and present several bounds for the network throughputs of the multi-channel protocol. We show that by joint optimization of the scheduling scheme across multiple sub-channels, the proposed protocol improves the system performance considerably in contrast to that of single-channel systems. In the second part of the dissertation, we study this problem under the assumption that neither the instant CSI nor the channel statistics are known. We formulate the channel-aware scheduling problem using multi-armed bandit (MAB). We first present a semi-distributed MAB protocol which serves as the baseline for performance comparison. We then propose two forms of distributed MAB protocols, where each link keeps a local copy of the observations and plays the MAB game independently. In Protocol I the MAB game is only played once within each block, while in Protocol II it can be played multiple times. We show that the proposed distributed protocols can be considered as a generalized MAB procedure and each link is able to update its local copy of the observations for infinitely many times. We analyze the evolution of the local observations and the regrets of the system. For Protocol I, we show by simulation results that the local observations that are held independently at each link converge to the true parameters and the regret is comparable to that of the semi-distributed protocol. For Protocol II, we prove the convergence of the local observations and show an upper bound of the regret.Item ON THE RAPID INTENSIFICATION OF HURRICANE WILMA (2005)(2012) Chen, Hua; Zhang, Da-Lin; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous studies have focused mostly on the roles of environmental factors in the rapid intensification (RI) of tropical cyclones (TCs) due to the lack of high-resolution data in the inner-core regions. In this study, we examine the RI issue by analyzing 72-h cloud-permitting model predictions of Hurricane Wilma (2005) with the Weather and Research Forecast (WRF) model at the finest grid sizes of 1-2 km. The 72-h predictions cover Hurricane Wilma¡¦s initial 18-h spin up, an 18-h RI and the subsequent 36-h weakening stage. The model prediction uses the initial and lateral boundary conditions, including a bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory's then-operational data, except for the time-independent sea surface temperature (SST) field. The model predicts an RI rate of more than 4 hPa h-1 for an 18-h period, with the minimum central pressure of less than 889 hPa. It was found that an upper-level warm core forms in the same layer as the upper outflow, in coincidence with the onset of RI. The warm core results from the subsidence of stratospheric air associated with the detrainment of convective bursts (CBs). The upper divergent outflow appears to play an important role in protecting the warm core from ventilation by environmental flows. Results also show the development of more CBs preceding RI, but most subsidence warming radiates away by internal gravity waves and storm-relative flows. In contrast, many fewer CBs occur during RI, but more subsidence warming contributes to the balanced upper-level cyclonic circulation in the warm core (as intense as 20,,aC) region. Furthermore, considerable CB activity can still take place in the outer eyewall as the storm weakens during its eyewall replacement. Sensitivity simulations reveal that the upper-level warm core and CB activity depend critically on warm SST. We conclude that significant CB activity in the inner-core regions is an important ingredient in generating an upper-level warm core that is hydrostatically more efficient to the RI of TCs, given all the other favorable environmental conditions. The formation of a divergent upper-level outflow that prevents the warm core from ventilation is examined through asymmetric contraction processes associated with new rainbands forming inside the eyewall. The relative vorticity, generated in the downshear region and then advected cyclonically downstream, can induce convergence in the boundary layer. With the aid of high moisture content, the convergence can trigger deep convection and contribute to the formation of the new rainbands. Finally, the importance of a small eye size is demonstrated using three widely accepted approximations: angular momentum conservation, solid body rotation and gradient wind balance. Results show that the storm intensifies much faster for a given contraction speed if the eye size is small.