Upper and Lower Bounding Techniques for Frequency Assignment Problems

We consider two variants of the radio link frequency assignment problem. These problems arise in practice when a network of radio links has to be established. Each radio link has to be assigned a particular frequency. The interference level between the frequencies assigned to the different links has to be acceptable, since otherwise communication will be distorted. The frequency assignments have to comply with certain regulations and physical characteristics of the transmitters. Moreover, the number of frequencies is to be minimized, because each frequency used in the network has to be reserved at a certain cost. We develop several approximation algorithms for the problems, which are based on local search, and we compare their performance on some practical instances. Lower bounding techniques based on nonlinear programming and the chromatic number of a graph are used to estimate the quality of the approximate solutions for these instances. Partially supported by the EUCLID project RT..

Shape optimization in lipid nanotube networks

Starting from a high surface free-energy state, lipid nanotube networks are capable to self-organize into tree-like structures with particular geometrical features. In this work we analyze the process of self-organization in such networks, and report a strong similarity to the Euclidian Steiner Tree Problem (ESTP). ESTP is a well-known NP-hard optimization problem of finding a network connecting a given set of terminal points on a plane, allowing addition of auxiliary points, with the overall objective to minimize the total network length. The present study shows that aggregate lipid structures self-organize into geometries that correspond to locally optimal solutions to such problems

Algorithms for frequency assignment problems

Frequency assignment problems occur when a network of radio links has to be established. Each link has to be assigned an operating frequency from a set of available frequencies. The assignment has to satisfy certain interference limiting restrictions de ned on pairs of links. Moreover, the number of frequencies used is to be minimized. These problems have been investigated by aconsortium consisting of research groups from Delft, Eindhoven, London, Maastricht, Norwich, and Toulouse. The participants developed optimization algorithms based on branch-and-cut and constraint satisfaction, as well as approximation techniques such as simulated annealing, taboo search, variable-depth search, genetic algorithms, and potential reduction methods. These algorithms were tested and compared on a set of real-life instances

Annealing study of oxygenated and non-oxygenated float zone silicon irradiated with 15 MeV protons

Introducing oxygen into the silicon material is believed to improve the radiation hardness of silicon detectors. In this study, oxygenated and non-oxygenated silicon samples were processed and irradiated with 15 MeV protons. In order to speed up the defect reactions after the exposure to particle radiation, the samples were heat treated at elevated temperatures. In this way, the long-term stability of silicon detectors in hostile radiation environment could be estimated. Current-voltage measurements and Surface Photovoltage (SPV) method were used to characterize the samples

The effect of oxygenation on the radiation hardness of silicon studied by surface photovoltage method

The effect of oxygenation on the radiation hardness of silicon detectors was studied. Oxygen-enriched and standard float-zone silicon pin-diodes and oxidized samples were processed and irradiated with 15-MeV protons. After the irradiations, the surface photovoltage (SPV) method was applied to extract minority carrier diffusion lengths of the silicon samples. Adding oxygen to silicon was found to improve the radiation hardness of silicon. The effect was visible in minority carrier diffusion lengths as well as in reverse bias leakage currents. The suitability of SPV method for characterizing irradiated silicon samples was proved. 14 Refs