Co-Channel Interference of Spread Spectrum Systems in a Multiple User Environment

This paper considers the mutual interference problem of several users employing the same spread spectrum technique in selected multiple user environments. The spread spectrum techniques consist of pseudo noise (PN), time division multiple access/PN, synchronous and asynchronous frequency hopping (FH). The environment consists of a desired transmitter-receiver pair located in an area where there areMinterfering users distributed in accordance with a specified probability density function. Both coherent phase-shift-keyed-and noncoherent frequency-shift-keyed modulations are considered. The general relationship between the probability of bit error of PN and FH systems is derived which is independent of the signal modulation and distribution of users. The degradation of the communication system performance (average probability of bit error) of the desired link as a function of the total number of interfering users within the considered area is investigated. The analysis shows that the mutual interference problem is less severe with users employing synchronous FH than with the other spread spectrum techniques. The comparison between asynchronous FH and PN is highly dependent on the relative location of interferers to the desired link and the time duty factor of the hopping.     

Investigation of microstructure and wear behaviors of al matrix composites reinforced by carbon nanotube

In this study, the effect of CNT amount in Al-CNT composites produced by adding carbon nanotube (CNT) to 7075 Al alloy in various amounts on microstructure and wear behaviors of aluminum matrix composites was investigated. CNT was added to 7075 Al alloy powder at five different amounts. The powders were mechanically milled for 2 hours. Mechanical milled powders were cold pressed and then pre-shaped by hot pressing. Pre-shaped samples were sintered for 1 hour under 10^?6 millibar in 580°C. Microstructure examinations, hardness measurements, and wear tests were carried out. The results show that CNT's in the microstructure were agglomerated as nanotube amount increases and there was no uniform distribution. The highest hardness value was obtained in AMC reinforced with 1% CNT while it is seen that hardness of the composite decreases and weight loss increases as CNT amount increases.     

Impacts of impregnation with Imersol-Aqua on the compression strength of some solid wood materials

The aim of this study was to investigate the effects of impregnation with Imersol-Aqua on the compression strength of some solid wood materials. For this aim, Oriental beech (Fagus orientalis Lipsky), European oak (Quercus petrea Liebl.), Scotch pine (Pinus sylvestris Lipsky), Uludag fir (Abies Bornmülleriana Mattf.), Oriental spruce (Picea orientalis Lipsky) and Lombardy poplar (Populus nigra Lipsky) wood samples were prepared according to TS 2595 and impregnated with Imersol-Aqua, commonly being used in construction wood materials by the method of short, medium and long-term of dipping according to ASTM D 1413 and producers’ definition. After the impregnation process, compression strength was measured according to TS 2595. Consequently, among the non-impregnated wood materials, the highest compression strength was obtained in beech and pine samples. Compression strength at this situation from the highest to lowest can be enumerated beech, pine, oak, spruce, fir and poplar. With regard to the impregnation period, the sequence form the highest to lowest was as long-term, medium-term and short-term dipping. In the interaction of wood material and impregnation period, the highest compression strength values were obtained in Scotch pine (71.220 N mm⁻²) impregnated with long-term dipping method whereas the lowest in Lombardy poplar (35.710 N mm⁻²) impregnated with short-term dipping method.

In consequence, in the massive constructions and furniture elements that the compression strength after the impregnation is of great concern, long-term impregnation of solid wood material could be recommended.     

Thermoeconomic optimization for a finned‐tube evaporator configuration of a roof‐top bus air‐conditioning system

This paper presents a methodology of a design optimization technique that can be useful in assessing the best configuration of a finned‐tube evaporator, using a thermoeconomic approach. The assessment has been carried out on a direct expansion finned‐tube evaporator of a vapor compression cycle for a roof‐top bus air‐conditioning (AC) system at a specified cooling capacity. The methodology has been conducted by studying the effect of some operational and geometrical design parameters for the evaporator on the entire cycle exergy destruction or irreversibility, AC system coefficient of performance (COP), and total annual cost. The heat exchangers for the bus AC system are featured by a very compact frontal area due to the stringent space limitations and structure standard for the system installation. Therefore, the current study also takes in its account the effect of the variation of the design parameters on the evaporator frontal area. The irreversibility due to heat transfer across the stream‐to‐stream temperature difference and due to frictional pressure drops is calculated as a function of the design parameters. A cost function is introduced, defined as the sum of two contributions, the investment expense of the evaporator material and the system compressor, and the operational expense of AC system that is usually driven by an auxiliary engine or coupled with the main bus engine. The optimal trade‐off between investment and operating cost is, therefore, investigated. A numerical example is discussed, in which a comparison between the commercial evaporator design and optimal design configuration has been presented in terms of the system COP and evaporator material cost. The results show that a significant improvement can be obtained for the optimal evaporator design compared with that of the commercial finned‐tube evaporator that is designed based on the conventional values of the design parameters. Copyright © 2007 John Wiley & Sons, Ltd.     

The electronic band structure of InN,InAs and InSb compounds

The electronic band structure of InN, InAs and InSb has been investigated by ETB. The ETB method has been formulated for sp³d² basis and nearest neighbor interactions of the compounds and its energy parameters have been derived from the results of the present first principles calculations carried on InN, InAs and InSb. It has been found that the present ETB parameters can produce the band structure of the compounds successfully.     

Impacts of impregnation with Timbercare Aqua on the red and yellow color tone of some woods and varnishes

This study has been performed for determining the effects of impregnation with Timbercare Aqua (Tc) on the red and yellow color tone of some woods and varnishes. For this purpose, the test samples prepared from Oriental beech, European oak, Scotch pine, Oriental spruce, and Uludag fir woods, which met the requirements of ASTM D 358, was impregnated with Tc according to ASTM D 1413 and producer's definition. After impregnation, wood surfaces have been coated by Sayerlack parquet varnish (Sp), Sayerlack interior varnish (Si), and Sayerlack exterior varnish (Se) varnishes in accordance with the ASTM D 3023 standards. According to ASTM D 2244, the red and yellow color tone of samples after varnishing process was determined. As a result, the value of red color tone was the highest in Oriental beech (15.21) and the lowest in Uludag fir (5.38). For the wood material, impregnation material, and varnish interaction, the red color tone value was the highest in Oriental beech + Tc + Sp (18.43) and the lowest in Uludag fir + Tc + Si (3.92). The value of the yellow color tone was the highest in pine (34.45) and the lowest in Oriental beech (26.50). For the wood material, impregnation material, and varnish interaction, the yellow color tone value was the highest in Oriental spruce + Tc + Sp (42.12) and the lowest in Oriental beech + Tc + Si (21.47). This effect may be due to the impacts of impregnation chemical on wood extractives and color pigments in varnish. Accordingly, it should be taken into care for applications where the red and yellow color tone value is important. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Software-reliability engineering: technology for the 1990s

It argued that software engineering is about to reach a new stage, the reliability stage, that stresses customers' operational needs and that software-reliability engineering will make this stage possible. Software-reliability engineering is defined as the applied science of predicting, measuring, and managing the reliability of software-based systems to maximize customer satisfaction. The basic concepts of software reliability-engineering and the reasons why it is important are examined. The application of software-reliability engineering at each stage of the life cycle is described     

Effect of collection time on chemical composition of the essential oil of Foeniculum vulgare subsp. piperitum growing wild in Turkey

The essential oils of Foeniculum vulgare subsp. piperitum were subjected to detailed GC-MS analysis in order to determine possible similarities and differences in their chemical compositions, depending on growth years (2001 and 2002). In both oils, methyl chavicol (40.29 and 21.69%), limonene (17.66 and 22.24%), fenchone (16.90 and 12.98%), α-pinene (1.86 and 34%) and α-phellandrene (2.30 and 2.74%) were the main components, respectively. A total of 32 components were identified accounting for 85.01 and 67.14% of the oils of F. vulgare, respectively. The main characteristic of the oils is the high content of the limonene, fenchone and estragol. Our findings indicated that the oil of Turkish bitter fennel belonged to methyl chavicol rich type.