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Search Results: 1 - 10 of 7281 matches for " Muhammad Usman "
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Gnomon Assessment for Geographic Coordinate, Solar Horizontal & Equatorial Coordinates, Time of Local Sunrise, Noon, Sunset, Direction of Qibla, Size of Earth & Sun for Lahore Pakistan  [PDF]
Muhammad Usman Saleem
Open Journal of Applied Sciences (OJAppS) , 2016, DOI: 10.4236/ojapps.2016.62011
Abstract: This study was an effort to calculate geographic coordinates, solar horizontal and equatorial cooridnates, direction of Qibla, sizes of Earth and Sun through gnomon (a vertical stick casting shadow). Gnomon has been used in history for remote navigations, astronomical and geodetic measurements. Geographic coordinates were found?out with accuracy of less than 0.19 decimal degree from gnomon. Sun’s horizontal and equatorial coordinates were calculated and analyzed with US Navy Observatory and NOAA Earth System Research Laboratory. These coordinates were less than 0.01 decimal degree different from coordinates provided by US Navy and following same daily trend with time. Through spherical triangle, direction of Qibla was found out. And it came out less than 0.18 decimal degree different from its value, used in Lahore. Radius of the earth measured from gnomon using concepts of Al Buruni was 1.45 km different from its true value. Using geometrical technique of similar and concurrent triangles, size of the sun was calculated and it was 1,651,088.776 km. Investigation of systematic and random errors in each calculation was the part of this research.
Statistical Investigation and Mapping of Monthly Modified Refractivity Gradient over Pakistan at the 700 Hectopascal Level  [PDF]
Muhammad Usman Saleem
Open Journal of Antennas and Propagation (OJAPr) , 2016, DOI: 10.4236/ojapr.2016.42005
Abstract: This study is an effort to investigate the spatial-temporal variability of the modified refractivity gradient at the 700 hPa pressure level over Pakistan and its neighbouring regions of Afghanistan, India, Iran and the Arabian Sea using the remote sensing data of the AQUA (AIRX3STM) satellite from 2008 to 2012. Trapping conditions only found in December were spread over Khyber Pakhtunkhwa (Gilgit-Baltistan, Pakistan) with an average value of -182.042 M/Km and showing Leptokurtic distributions. The lowest monthly average value super-refractive conditions existed in the autumn season with a strong monthly correlation (>0.91 M/Km). A very high monthly correlation (0.9 M/Km) was found for the super-refractive conditions over the whole time period. The largest spatial and temporal normal conditions appeared in January with the average value for normal conditions being 132.72 M/Km (found over Zabul, Afghanistan) with Leptokurtic distributions. During May normal conditions were the smallest in spatial extent over Pakistan, India and Afghanistan, showing Platykurtic distributions. Sub-refractive conditions mostly prevailed at all times. The probability for extreme sub-refractive conditions was very high in 2008-2012. The highest average sub-refractive conditions appeared in the winter and autumn seasons (spread around Quetta and Kalam, Pakistan). The highest monthly average sub-refractive conditions with a value of 1,265,188 M/Km were found in January and spread around the Sarbaz River Iran. Correlations for the existence of sub-refractive conditions varied from 0.8 M/Km (moderate strong) to 0.4 M/Km during the autumn to winter season. Permanent super-refractive conditions existed over Baluchistan from February to September.
Electronic and Optical Properties of [110]-Tilted InAs/GaAs Quantum Dot Stacks
Muhammad Usman
Physics , 2014, DOI: 10.1103/PhysRevB.89.081302
Abstract: Multi-million atom simulations are performed to study stacking-angle ($\theta$) dependent strain profiles, electronic structure, and polarization-resolved optical modes from [110]-tilted quantum dot stacks (QDSs). Our calculations reveal highly asymmetrical biaxial strain distributions for the tilted QDSs that strongly influence the confinements of hole wave functions and thereby control the polarization response. The calculated values of degree of polarizations, in good agreement with the available experimental data, predict a unique property of the tilted QDSs that the isotropic polarization response can be realized from both [110] and [-110] cleaved-edges $-$ a feature inaccessible from the conventional [001]-QDSs. Detailed investigations of polar plots further establish that tilting the QDSs provides an additional knob to fine tune their polarization properties.
Atomistic theoretical study of electronic and polarization properties of elliptical, single and vertically stacked InAs quantum dots
Muhammad Usman
Physics , 2012,
Abstract: The demonstration of isotropic polarization response from semiconductor quantum dots (QDs) is a crucial step towards the design of several optoelectronic technologies. Among many parameters that impact the degree of polarization (DOP) of a QD system, the shape asymmetry is a critical factor. We perform multi-million-atom simulations to study the impact of the elliptical shapes on the electronic and polarization properties of single and vertically stacked InAs QDs. The comparison between a low aspect ratio (AR) and a high AR QD reveals that the electronic and the polarization properties strongly depend on the AR of the QD; the elongation of a tall QD allows tuning of the DOP over a much wider range. We then extend our analysis to an experimentally reported vertical stack of nine QDs (9-VSQDs) that has shown significant potential to achieve isotropic polarization properties. We analyse the contribution from the shape asymmetry in the large, experimentally measured, in-plane polarization anisotropy. Our analysis shows that the orientation of the base elongation controls the sign of the DOP; however the magnitude of the base elongation has only a very little impact on the magnitude of the DOP. We further predict that the elliptical shape of the 9-VSQDs can only tune either DOP[110] or DOP[-110] for an isotropic response. Our model results, in agreement with the TEM findings, suggest that the experimentally grown 9-VSQDs has either a circular-base or a slightly [-110] elongated base. Overall the detailed investigation of the DOP as a function of the QD shape asymmetry provides a theoretical guidance for the continuing experimental efforts to achieve tailored polarization properties from the QD nano-structures for the design of optical devices.
Dark matter in $f(\mathcal{R})$ gravity
Muhammad Usman
Physics , 2015,
Abstract: We discuss here the possibility of explaining the rotational velocity curves of the galaxies in $f(\mathcal{R})$ gravity by solving field equations numerically. In a given constant rotational velocity region, we prove that all values of rotational velocities does not lead to an analytic solution of the field equations in vacuum. We then obtain the numerical solutions of the field equations which suggests that a very slightly modification from linear relations of $\mathcal{R}$ can be an alternative to the so called dark matter.
Understanding electric field control of electronic and optical properties of strongly-coupled multi-layer quantum dot molecules
Muhammad Usman
Physics , 2015, DOI: 10.1039/c5nr04710b
Abstract: Strongly-coupled quantum dot molecules (QDMs) are widely deployed in the design of a variety of optoelectronic, photovoltaic, and quantum information devices. An efficient and optimized performance of these devices demands engineering of the electronic and optical properties of the underlying QDMs. The application of electric fields offers a knob to realise such control over the QDM characteristics for a desired device operation. We perform multi-million-atom atomistic tight-binding calculations to study the influence of electric fields on the electron and hole wave function confinements and symmetries, the ground-state transition energies, the band-gap wavelengths, and the optical transition modes. The electrical fields both parallel ($\vec{E_p}$) and anti-parallel ($\vec{E_a}$) to the growth direction are investigated to provide a comprehensive guide on the understanding of the electric field effects. The strain-induced asymmetry of the hybridized electron states is found to be weak and can be balanced by applying a small $\vec{E_a}$ electric field, of the order of 1 KV/cm. The strong interdot couplings completely break down at large electric fields, leading to single QD states confined at the opposite edges of the QDM. This mimics a transformation from a type-I band structure to a type-II band structure for the QDMs, which is a critical requirement for the design of intermediate-band solar cells (IBSC). The analysis of the field-dependent ground-state transition energies reveal that the QDM can be operated both as a high dipole moment device by applying large electric fields and as a high polarizibility device under the application of small electric field magnitudes. [abstract is truncated to fit the character count of arXiv]
Missing Data Imputations for Upper Air Temperature at 24 Standard Pressure Levels over Pakistan Collected from Aqua Satellite  [PDF]
Muhammad Usman Saleem, Sajid Rashid Ahmed
Journal of Data Analysis and Information Processing (JDAIP) , 2016, DOI: 10.4236/jdaip.2016.43012
Abstract: This research was an effort to select best imputation method for missing upper air temperature data over 24 standard pressure levels. We have implemented four imputation techniques like inverse distance weighting, Bilinear, Natural and Nearest interpolation for missing data imputations. Performance indicators for these techniques were the root mean square error (RMSE), absolute mean error (AME), correlation coefficient and coefficient of determination ( R2 ) adopted in this research. We randomly make 30% of total samples (total samples was 324) predictable from 70% remaining data. Although four interpolation methods seem good (producing <1 RMSE, AME) for imputations of air temperature data, but bilinear method was the most accurate with least errors for missing data imputations. RMSE for bilinear method remains <0.01 on all pressure levels except 1000 hPa where this value was 0.6. The low value of AME (<0.1) came at all pressure levels through bilinear imputations. Very strong correlation (>0.99) found between actual and predicted air temperature data through this method. The high value of the coefficient of determination (0.99) through bilinear interpolation method, tells us best fit to the surface. We have also found similar results for imputation with natural interpolation method in this research, but after investigating scatter plots over each month, imputations with this method seem to little obtuse in certain months than bilinear method.
Synchronization in Wireless Networks for Practical MIMO-OFDM Systems  [PDF]
Muhammad Khurram Kiyani, Muhammad Usman Ahmed, Asim Loan
Int'l J. of Communications, Network and System Sciences (IJCNS) , 2010, DOI: 10.4236/ijcns.2010.35065
Abstract: In this paper a frequency offset estimation technique for Wireless Local Area and Wireless Metropolitan Ar- ea Networks is presented. For frequency offset estimation, we have applied a low-complexity frequency offset estimator for simple AWGN channels to fading channels for MIMO-OFDM systems. Simulation results have shown that the performance of the proposed estimator is better than the low complexity frequency offset estimator designed for AWGN channels.
The Influence of Rain Gauge Network Density on the Performance of a Hydrological Model  [PDF]
George Andiego, Muhammad Waseem, Muhammad Usman, Nithish Mani
Computational Water, Energy, and Environmental Engineering (CWEEE) , 2018, DOI: 10.4236/cweee.2018.71002
Abstract: Rain gauge data suffers from spatial errors because of precipitation variability within short distances and due to sparse or irregular network. Use of interpolation is often unreliable to evaluate due to the aforementioned irregular sparse networks. This study is carried out in the Nette River catchment of Lower Saxony to alleviate the problem of using gauge data to measure the performance of interpolation. Radar precipitation data was extracted in the positions of 53 rain gauge stations, which are distributed throughout the range of the weather surveillance radar (WSR). Since radar data traditionally suffers from temporal errors, it was corrected using the Mean Field Bias (MFB) method by utilizing the rain gauge data and then further used as the reference precipitation in the study. The performances of Inverse Distance Weighting (IDW) and Ordinary Kriging (OK) interpolation methods by means of cross validation were assessed. Evaluation of the effect of the gauge densities on HBV-IWW hydrological model was achieved by comparing the simulated discharges for the two interpolation methods and corresponding densities against the simulated discharge of the reference precipitation data. Interpolation performance in winter was much better than summer for both interpolation methods. Furthermore, Ordinary Kriging performed marginally better than Inverse Distance Weighting in both seasons. In case of areal precipitation, progressive improvement in performance with increase in gauge density for both interpolation methods was observed, but Inverse Distance Weighting was found more consistent up to higher densities. Comparison showed that Ordinary Kriging outperformed Inverse Distance Weighting only up to 70% density, beyond which the performance is equal. The hydrological modelling results are similar to that of areal precipitation except that for both methods, there was no improvement in performance beyond the 50% gauge density.
Demand and Response in Smart Grids for Modern Power System  [PDF]
Muhammad Qamar Raza, Muhammad Usman Haider, S. Muhammad Ali, Muhammad Zeeshan Rashid, Farooq Sharif
Smart Grid and Renewable Energy (SGRE) , 2013, DOI: 10.4236/sgre.2013.42016
Abstract:

Micro-grid plays a vital role in fulfilling the increasing demand by using distributed renewable energy resources. Demand and response technique can be broadly classified under the setup DR deployed (e.g. ISO’s/RTO’s). Demand response program can be implemented to improve power system quality, reliability and increasing demand. In modern power industry, strategic player can take more benefit from more emphasized DR study in terms of social benefit (uninterrupted power supply to consumers) and economy. This paper proposes the distributed micro-grid control and implemented control setup implemented demand response algorithm, which provides better power system reliability. This paper presents contingencies control demand and response for micro-grid. The main advantage of implementation of demand and response algorithms in Micro-grids provides reliable power supplies to consumers. The proposed micro-grid TCP/IP setup provides a chance to respond the contingencies to recover the shed to active condition. Micro-grid controller implements demand and response algorithm reasonable for managing the demand of the load and intelligent load scheme in case of blackout.

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