Browsing by Author "Dayawansa, IJ"

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item: Conference-Abstract
1 Watt, quasi-cw, diffraction limited, semiconductor double tapered laser oscillator
(Engineering Research Unit, Faculty of Engiennring, University of Moratuwa, 1996-05-31) Dayawansa, IJ
item: Thesis-Abstract
Design modeling and simulation of a repeaterless optical fiber network for Sri Lanka
Kandanearachchi, KP; Dayawansa, IJ
A reliable and wideband telecommunication network is a vital infrastructure development, where wide band services such as ATM, ADSL and IP based services could be supported. In Sri Lanka, the requirement of this kind of an optical network is very significant as other operators also can share the capacity of the network for transporting their traffic. On the other hand the network problems such as excessive BER (Bit Error Rate) etc are experienced after its construction. In order to avoid such limitations in the network, the network needs to be modeled on appropriate software tools and run with designed network parameters, so that the desired BER could be ensured. During the initial phase of the study, the total telephone demand by year 20If was estimated as nearly 2 million subscribers. This was estimated through the world trend for telephone subscribers together with economic indicators such as GNP and GDP. The Nodes of the Network was determined based on the present distribution of customers in the County. In this case all the Tertiary Switching Center areas and the Secondary Center Areas where the customer base is more than 2.5% of total customers were taken as the main nodes of the network. In addition Jaffna and Baticaloa were also taken as nodes considering the potential growth of traffic in northern and eastern parts of the Island. The Gravity model and Earlang's E formula, traffic tables, were used to find the traffic between nodes and the number of circuits between nodes. Based on the traffic distribution between nodes, a part of the network was proposed as a fully reliable Ring Network, while other nodes are connected through extended links. The IP traffic, which is thought to be the major traffic flow in the future, were estimated considering the broadband Internet growth in the country. Also the traffic, which are expected to be migrated from traditional PSTN to IP Network were identified and estimated to find the total bandwidth requirement of the network by year 2015 The number of wavelengths in the proposed Network were decided based on the final bandwidth requirement. This resulted an island wide network consisting of WDM Ring Network having 08 wavelengths that basically covers the southern part of the country and two other extensions having a wavelength each to northern and eastern parts of the country. The Colombo and the Kandy nodes were selected as Full Fiber Terminal Stations as most of the traffic flow between these two nodes. Wavelengths are added and dropped at each branch station based on the traffic volumes between these nodes. The wavelengths were selected such that the. space between adjacent wavelengths is 0.8rm to avoid nonlinear effects and cross talks. The G-655 non-zero dispersion fiber was selected to mange the dispersion and non-linear effects. DFB and APD are the Source and the Detector respectively to suit long I ransmissions having arrow pectra widths and also I o meet better sensitivity at the receiver.
item: Conference-Abstract
Electromagnetic effects on a human brain due to cellular phones
(1998) Dayawansa, IJ
item: Thesis-Abstract
Estimation of propagation characteristics in mobile environment
Vishakha, KSM; Dias, D; Dayawansa, IJ
Propagation in land mobile services including cellular is affected in varying degrees by topography, vegetation, man made structures, ground constants, the troposphere and the ionosphere. Also propagation of energy is strongly influenced by several factors including the natural and artificial relief, propagation frequency, antenna heights and others. The mobile radio signal varies in time and with spatial displacement. Even in the static case where the transmitter and receiver are fixed, the channel can be dynamic, since scatterers and reflectors are likely to be in motion. Thus multi path arises from the fact that through reflection, diffraction and scattering radio waves can travel from a transmitter to a receiver by many paths. Hence a precise estimation of signal characteristics in this environment constitutes a hard task. The thesis describes exercises in the estimation of the characteristics of cellular mobile signals operating in 900 MHz band through the real measurements obtained in different environments in Sri Lanka and the results of these exercises. Even though similar researches have been conducted in places like Europe, North America and Japan to derive empirical formulae such as Okomura- Hata Model, ITU-Rp.529, COST231, no study has been done as yet to check the suitability of these models to the Sri Lankan environment
item: Article-Full-text
Infrared passive counting system for vehicular traffic
Dayawansa, IJ
Vehicular traffic flow is controlled by the use of signal lights. The signal lights change according to a predetermined time sequence, which is estimated by considering traffic flow data. Traffic flow data consists of the rate of flow of vehicular traffic at a particular junction, at a particular time, on a particular day of the year and the width of the road apart from other parameters as considered important by Traffic Engineers. It would be desirable to have a demand controlled traffic signaling system which records the instantaneous traffic flow rate and determines the time sequence for the control system for the signal light although the predetermined times normally used are determined on the basis of demand but on statistical data rather than demand on the spot. The conventional methods of measuring traffic flow rate on the spot are by counting traffic. Methods of the counting vehicular traffic utilizes methods such as the change of magnetic field within a buried current carrying coil under the surface of the road and the break of a circuit which is normally closed by an infrared beam or a microwave beam, when a vehicle passes. Both these methods employ an active circuit whose condition changes when a vehicle passes. This paper presents a vehicular traffic counting system that was designed and implemented using a pyroelectric cell as the sensor. The system has the advantage of not needing an active emitter such as an infrared beam or microwave radiation. The pyroelectric cell responds to a change in the incident infrared radiation. Every hot body emits radiation and a moving vehicle is an efficient emitter of infrared radiation. The radiation emitted by the moving vehicle was allowed to fall on a pyroelectric detector, which was kept fixed by the side of the road. The rate of change of radiation incident on the cell generated a current through the cell, which was converted to a voltage. This detected signal was used to count moving vehicles and the count was displayed on a seven-segment LED display. The system is passive and is able to count moving traffic from a distance of about twenty meters.
item: Conference-Full-text
Microstrip vehicular antenna for mobile-satellite communications
(2001) Senavirathna, HMSB; Dayawansa, IJ
Circularly polarized antennas are used in mobile-satellite or MSAT communication systems. This reduces the polarization mismatch due to the rotational orientation between the transmitter and the receiver and it also minimizes the interference from multipath reflections. Circularly polarized microstrip antennas are commonly used in these systems because of compactness, the light-weight and the low-profile. It eases the installation on a vehicular body or on the surface ofa satellite. This paper presents a circularly polarized microstrip circular patch antenna designed for vehicular use for MSAT communications at a frequency of around I.6GHz. It has right-hand circular polarization (RHCP) and near hemispherical coverage needed by the user. The antenna was first simulated using MatLab software for TM110 resonance mode and fabricated on a substrate of epoxy glass fiber. The antenna was tested on a curved ground plane, which was similar to the surface of a vehicle. This low profile microstrip antenna is suitable for vehicle rooftop mounting. A combination of the resonance cavity model and the transmission-line model was used to analyze the antenna, which consists of a single circular patch as the driven clement. In order to obtain circular polarization, perturbation segments to the perimeter of the patch were inserted with a single probe feed on the 45° axis with respect to the perturbation segments, known as mode-de- tuning. As such no external phasing network is required to obtain the 90° time phase difference required for circular polarization. It was fed with a 50Ω coaxial feed line which was soldered to the inset probe. By adjusting the area of the perturbation segments circular polarization and a good axial ratio was obtained. Half power beam widths of the antenna are 115° in the H plane and 105° in the E plane. The antenna has a gain of 5.7dBi and a bandwidth of 286MHz which is approximately 17% of the centre frequency. The design criteria and performance of the circularly polarized microstrip antenna, which is suitable for MSAT communications, are presented.
item: Thesis-Abstract
Optical properties of likely constituents of interstellar dust
(2014-07-01) Dayawansa, IJ
Optical properties of polyoxymethelene (POM) at room temperature have been measured from the near ultra-violet to infra-red as an initial stage of a line between interstellar dust and organic matter, and we report our results which are particularly relevant to interstellar extinction. There is a strong possibility of a more complex organic component which could significantly contribute to the interstellar extinction. Measurements have also been made of the effect of fast neutron bombardment on the optical properties of quartz (SiO). At a high total flux of neutrons the crystalline quartz will change to its amorphous form which has extinction properties that resemble the interstellar extinction. Extinction due to small particles of several forms of SiO.2 have been measured and among them the hydrated mineral, opal, behaved like an amorphous silica. Neutron irradiated olivine showed a stronger and a broader lOym band in addition to weaker bands towards the longer wavelengths which indicated that atomic damage has been produced. At high fluxes more atomic damage is expected to change the crystalline structure and there by cause changes in the infra-red absorption properties. Extinction measurements were also made for smoke particles of MgO in the infra-red. When the measurements Were made with the particles deposited on substrates, in addition to a very broad surface mode absorption feature around 20ym an extinction maximum was observed typical of the bulk mode at 25ym. Extinction measurements for MgO smoke particles in air also showed similar results. However when the particles were dispersed in a non-absorbing medium, the bulk absorption mode was not observed. This implies that the appearance of the bulk mode is due to clumping. It was also observed that the width of the band reduced significantly with decreasing powder density f in the medium. Hence it is concluded that considerable broadening is due to interactions which is not properly accounted for in the single particle theory.
item: Thesis-Abstract
Smart base station antenna
Punchihewa, SAS; Dayawansa, IJ
Telecommunications incur a strong impact on the society. Out of its many sectors, mobile communications experienced an unprecedented growth around the globe in recent times .Service providers will have to satisfy this increased customer need using a spectrum, which does not grow proportionately. Several multiple access systems such as frequency division multiple access, time division multiple access and code division multiple access are used at present to increase the efficiency of spectrum utilization. The smart antenna, consisting of an array of elements, monitors its signal environment and forms a beam towards the wanted signal. Thus, on top of the existing access methods it provides an additional multiple access method namely space division multiple access in which several users access portions of space simultaneously. There exist different methods or algorithms for formation of the beam towards the desired signal. Some of them form a beam and rotate while monitoring the satisfaction of certain conditions, which indicate the correct formation of the beam. Some others find the directions of arrival of signals (DOA) and then form the beam towards the desired direction of which the resolution is higher. In spite of high-resolution capability, these algorithms demand knowledge of the propagation characteristics of the mobile channel. This necessitates modeling of the channel after theoretical or empirical considerations. This dissertation presents the work carried out to determine the DOA of a desired signal which is to be used in an adaptive antenna in a variety of propagation channels. The suitability of MUSIC (Multiple Signal Classification) algorithm was investigated. It was necessary to find the ability of the algorithm to estimate the DOA of impinging signals. However, the channel modeling was also a necessity. To determine the accuracy of the estimation, the error between the actual and estimated DOA was determined and analyzed. MATLAB was used for simulations because of its capabilities to handle large amount of matrix related computational activities efficiently. An artificial channel with free space conditions was initially used to test the method of estimating the DOA, and to check the suitability of error analysis as a method of determining the accuracy. In this artificial channel, estimation of several DOA was performed for different conditions of environment monitoring and different antenna array geometry. Different number of signals was used with different angles of arrival. Hence, the dependence of errors on the above different conditions was determined and there by the suitability of the error analysis to determine the accuracy was examined. by European Union were used and the performance of the MUSIC algorithm in different channel conditions was analyzed. Using the measured signal value data in the Colombo Fort area, the channel was mathematically modeled and MUSIC algorithm was tested for Colombo Fort. MUSIC algorithm was found to be suitable for use in adaptive cellular base station antenna.
item: Thesis-Abstract
Tropospheric range error corrections for the global positioning system in Sri Lanka
Seneviratne, A; Dayawansa, IJ; Dias, D
The Global Positioning System is the most accurate positioning and navigation system in use today. It uses the time of arrival of radio signals transmitted from satellites placed in high altitude orbits around the globe. The ideal GPS theory assumes free space radio propagation whereas in reality, the signals have to propagate through the atmosphere. When propagating through the atmosphere the finite refractive index of the various layers of the atmosphere causes the electromagnetic waves to travel distances that are longer than the corresponding free space distances. This causes an error in the observed time of arrival which is carried on to the positional computation. The error due to refraction in the troposphere is of particular interest. Unlike the ionosphere, the troposphere is non-dispersive. Its refractive properties depend more on physical parameters such as pressure and temperature. Due to this reason, the refractivity tends to depend on the location as well. It has been shown that a good correlation exists between the refractivity at the surface of the earth and the range error. Hence this error may be determined using the refractivity at the surface of the earth. In this dissertation, the effect of the troposphere on GPS observations made within the geographical extent of Sri Lanka is studied. The range errors at reference locations within Sri Lanka are determined for different parts of the year. The results are compared with established results for the region and reasons for discrepancies are briefly discussed. The design of a conceptual GPS receiver processor that can self-correct tropospheric range errors at the surface of the earth by sensing the pressure and the temperature is proposed. A computer program is used to simulate the operation of this receiver.
item: Article-Full-text
Water quasi-cw, diffraction limited semiconductor double tapered laser oscillator
Dayawansa, IJ
Semiconductor laser sources are compact and convenient. They can be easily integrated with other semiconductor devices. They are useful as the source for free space communications between satellites; for optical radar and for pumping fibre amplifiers in optical fibre communication systems. They are also useful for many other applications. For free space communications, a single frequency high power light source of narrow line width and large band-width is required. Coherent communication systems require a high power source, with a single transverse mode and single longitudinal mode