M.S. Theses
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Item Squeeze casting of Zn-Bi monotectic alloy(Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1994., 1994.) Erturan, Hakan.; Savaş, Mahmut A.Zn- 1.9 wt.% Bi monotectic alloy was squeeze cast under pressures of 50, 85 and 120 MPa at a constant die temperature of 250 °C. The same die was used also for the gravity cast specimens of the same alloy. For comparison purposes the pure zinc castings were gravity cast and squeeze cast under 120 MPa at a die temperature of 400 °C. It was found that the porosity level in the castings were decreased with the application of squeezing pressure which resulted in an increase in tensile strength and hadness values. However, the microstructural examination showed that the segregation tendency of bismuth phase along the centerline was increased with increasing pressure and caused an inhomogeneous distribution of bismuth phase in the radial direction. The distribution of bismuth phase was also inhomogenous in the longitudinal direction due to gravitational segregation. In the macrostructural examination of the samples, the average grain size was found to decrease as the squeezing pressure was increased. A variation in grain size along longitudinal axis of the specimens was also observed. The grains were coarser near the bottom end and were finer near the punch end of castings. It appears that the lower part of the castings were solidified before pressure application.Item Handoff decision algorithms for rapidly deployable mobile infrastructure communication networks(Thesis (M.S.) - Bogazici University. Institue for Graduate Studies in Science and Engineering, 2002., 2002.) Önel, Tolga.; Ersoy, Cem.; Çayırcı, E.Mobile communication systems should use limited resources in an efficient and convenient manner. One way of achieving this is to use smaller cells in the expense of corresponding handoff and administration overhead.Several algorithms are proposed to decide whether a handoff is necessary for a mobile or not. These algorithms differ according to the metrics used, where the metrics are monitored, and how these metrics are used.Some applications like military tactical communication and disaster area communication infrastructure deployment require rapid deployment of the mobile infrastructure. To satisfy rapid deployment and survivability requirement, architectures with mobile cell-managing units are employed.For the systems having a mobile infrastructure, handoff requires special treatment. We propose a handoff decision algorithm based on fuzzy inference systems. We use simulation to evaluate the performance of the proposed algorithm. The evaluated performance of the algorithm shows that our algorithm performs the best among the conventional received signal strength based handoff decision algorithms. With slight modifications, our proposed handoff decision algorithm is applicable to the systems having immobile infrastructure.