505 research outputs found
A Cross-Layer Approach for Minimizing Interference and Latency of Medium Access in Wireless Sensor Networks
In low power wireless sensor networks, MAC protocols usually employ periodic
sleep/wake schedule to reduce idle listening time. Even though this mechanism
is simple and efficient, it results in high end-to-end latency and low
throughput. On the other hand, the previously proposed CSMA/CA-based MAC
protocols have tried to reduce inter-node interference at the cost of increased
latency and lower network capacity. In this paper we propose IAMAC, a CSMA/CA
sleep/wake MAC protocol that minimizes inter-node interference, while also
reduces per-hop delay through cross-layer interactions with the network layer.
Furthermore, we show that IAMAC can be integrated into the SP architecture to
perform its inter-layer interactions. Through simulation, we have extensively
evaluated the performance of IAMAC in terms of different performance metrics.
Simulation results confirm that IAMAC reduces energy consumption per node and
leads to higher network lifetime compared to S-MAC and Adaptive S-MAC, while it
also provides lower latency than S-MAC. Throughout our evaluations we have
considered IAMAC in conjunction with two error recovery methods, i.e., ARQ and
Seda. It is shown that using Seda as the error recovery mechanism of IAMAC
results in higher throughput and lifetime compared to ARQ.Comment: 17 pages, 16 figure
Optimizing the depth and the direction of prospective planning using information values
Evaluating the future consequences of actions is achievable by simulating a mental search tree into the future. Expanding deep trees, however, is computationally taxing. Therefore, machines and humans use a plan-until-habit scheme that simulates the environment up to a limited depth and then exploits habitual values as proxies for consequences that may arise in the future. Two outstanding questions in this scheme are “in which directions the search tree should be expanded?”, and “when should the expansion stop?”. Here we propose a principled solution to these questions based on a speed/accuracy tradeoff: deeper expansion in the appropriate directions leads to more accurate planning, but at the cost of slower decision-making. Our simulation results show how this algorithm expands the search tree effectively and efficiently in a grid-world environment. We further show that our algorithm can explain several behavioral patterns in animals and humans, namely the effect of time-pressure on the depth of planning, the effect of reward magnitudes on the direction of planning, and the gradual shift from goal-directed to habitual behavior over the course of training. The algorithm also provides several predictions testable in animal/human experiments
Hierarchical models of goal-directed and automatic actions
Decision-making processes behind instrumental actions can be divided into two categories: goal-directed actions, and automatic actions. The structure of automatic actions, their interaction with goal-directed actions, and their behavioral and computational properties are the topics of the current thesis. We conceptualize the structure of automatic actions as sequences of actions that form a single response unit and are integrated within goal-directed processes in a hierarchical manner. We represent this hypothesis using the computational framework of reinforcement learning and develop a new normative computational model for the acquisition of action sequences, and their hierarchical interaction with goal-directed processes. We develop a neurally plausible hypothesis for the role of neuromodulator dopamine as a teaching signal for the acquisition of action sequences. We further explore the predictions of the proposed model in a two-stage decision-making task in humans and we show that the proposed model has higher explanatory power than its alternatives. Finally, we translate the two-stage decision-making task to an experimental protocol in rats and show that, similar to humans, rats also use action sequences and engage in hierarchical decision-making. The results provide a new theoretical and experimental paradigm for conceptualizing and measuring the operation and interaction of goal-directed and automatic actions
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Manipulation of crowding stress in corn.
Competition in corn crop was studied through manipulation of crowding (density and spacing) and assimilate supply in several experiments conducted in 1986-1988. Growth and yield responses of 2 hybrids differing in maturity period and prolificacy were studied in a wide range of densities. Under favorable condition, Agway 584S (single-ear, late maturity) out yielded Cornell 281 (multiple-ear, early maturity). Higher yield in Agway 584S was attributed to heavier kernels compared to Cornell 281, which resulted from a longer grain filling duration. Growth analysis indicated that crop growth rate as well as net assimilation rate were greater in Cornell 281 than in Agway 584S. Higher crop growth rate in Cornell 281, however, was directed toward tiller production with no fertile ears. Agway 584S maturing later had longer growth duration contributing to its higher yield than Cornell 281. Intensity of competition was quantified by comparing the grain yield and its components to isolated plants. Yield per plant decreased 75% and 80% compared to isolated plants in Agway 584S and Cornell 281 respectively, as density increased from 3 to 12 plants m\sp{-2}. Number of kernels per row in Agway 584S and number of productive ears per plant in Cornell 281 were found to be the most sensitive yield components. The timing of competitive stress was investigated by removal of alternate plants at different stages of growth. Most of the competition within the corn canopy occurred during the period between vegetative stage and anthesis. Adjustment in grain yield in response to releasing the competition pressure occurred primarily through increase in kernel number in Agway 584S and number of productive ears per plant in Cornell 281. Artificial shading (50% light reduction) showed that reduction in yield in high densities is due to reduction in photosynthesis rate caused by mutual shading and also by lengthening of the intervals between anthesis and silking. Density-light relationships in six corn hybrids having different leaf orientation indicated that hybrids with upright leaves had some yield advantages over horizontal leaf hybrids in favorable climatological conditions when high densities are used. Results supported the model proposed by Duncan (Crop Sci., 1984) that a linear relationship exists between the logarithm of yield per plant and crowding
Characterization and Evaluation of Ground Glass Fiber as a Cementitious Component in Portland Cement and Geopolymer Concrete Mixtures
A large amount of glass fiber is commercially produced for use in various applications. However, this process generates millions of tons of waste glass fiber annually around the world. This material has an amorphous structure that is rich in silica, alumina and calcium oxides, and if milled into a fine powder, it could potentially be used a supplementary cementitious material (SCM) in portland cement mixtures; or as a source material for production of geopolymer. So, the first objective of this research work, is to evaluate the utilization of ground glass fiber (GGF) as a SCM in portland cement mixtures, and the second objective is to study the mechanical and durability properties of GGF-based geopolymers. To fulfill the first objective, concrete and mortar mixtures containing different dosage of GGF (i.e. 10, 20 and 30% by mass) were prepared. Fresh and hardened properties of these mixtures were tested and compared with two control mixtures, including: (i) a mixture made from 100% portland cement, and (ii) a mixture having 75% portland cement and 25% class F fly ash (by mass). It was observed that utilization of GGF up to 30% (as a cement replacement) did not influence the mechanical properties of the concrete and mortar mixtures significantly compared to control mixtures; however, the use of GGF as SCM resulted in a remarkable improvement in the durability of the mixtures. It was also seen that the utilization of GGF at the 30% replacement level, successfully mitigated the ASR-related expansion of mortar and concrete mixtures containing the crushed glass aggregate. For the second objective, the possibility of producing geopolymer from GGF was investigated. To activate GGF, different dosage and combinations of sodium hydroxide solution (NaOH) and sodium silicate solution were used, and specimens were cured at 60oC for 24 h. Fresh and hardened properties of geopolymer mixtures made from GGF as the precursor, were studied and compared to glass-powder (GLP) and fly ash-based geopolymer mixtures. The effect of change in the Na2O-to-binder ratio (alkali content of the activator solution) and the SiO2/Na2O (silica content of the solution) ratio on the workability of and compressive strength of the mortar mixtures was monitored and compared to the GLP and fly ash-based geopolymers. It was seen that the strength gain in GGF-based geopolymers does not depend on the presence of sodium silicate in the activator solution; and a high compressive strength (as high as 80 MPa) can be achieved in three days, only by using sodium hydroxide solution alone. Furthermore, to better understand parameters affecting the activation of GGF-based geopolymers, effect of temperature (from ambient to 110oC) and duration of heat-curing on the compressive strength and micro-structure of GGF-based geopolymers was studied. The temperature of heat curing was seen to affect the early-age (i.e. 3 to 7 days) compressive strength of the GGF-based samples but had no significant effect on the later-age (i.e. 28 to 56 days) strength. Finally, it was concluded that GGF has a good potential to be used as a precursor to produce high strength geopolymers even at ambient temperature (23oC). Based on the results obtained from the compressive strength experiments, mixtures with the highest compressive strength were selected from each precursor to be used for the durability experiments. Durability aspects of GGF-based geopolymer such as resistance against sodium sulfate solution and magnesium sulfate solution, alkali silica reaction, drying shrinkage and corrosion of steel rebar were investigated and were compared to fly ash and GLP-based geopolymer, and an ordinary portland cement mixture (OPC). Based on this investigation it was found that GGF and fly ash-based geopolymers showed superior performance against ASR-related deterioration in comparison to GLP-based geopolymer and the OPC mixture. It was also observed that despite the fluctuation in properties at early ages, immersion in the sodium sulfate (Na2SO4) solution and magnesium sulfate (MgSO4) solution did not lead to a significant mass or strength loss of GGF-based geopolymer at the later ages. In conclusion, it can be stated that a high compressive strength GGF-based geopolymers could be produced by using an activator solution that is comprised of only NaOH. Durability experiments conducted on GGF-based geopolymer mixtures showed good performance in resisting ASR and sulfate solution exposure. Based on preliminary results it was observed that drying shrinkage of GGF and fly ash-based geopolymer was similar to the OPC mixture while the drying shrinkage of GLP-based geopolymer was significantly higher. Findings from basic experiments conducted in this study showed that factors such as: (i) the low amount of CH in the structure, (ii) low porosity, and (iii) the durable structure of the geopolymer gel in the GGF-based geopolymers, which remains stable under the aggressive conditions such as, exposure to sulfate solutions, are responsible for the superior durability performance of GGF-based geopolymer
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