Sharif Digital Repository

In general, reliability is the ability of a system to perform and maintain its functions in routine, as well as hostile or unexpected, circumstances. The Redundancy Allocation Problem (RAP) is a combinatorial problem which maximizes system reliability by discrete simultaneous selection from available components. The main purpose of this study is to develop an effective approach to solve RAP, expeditiously. In this study, the basic assumption is considering Erlang distribution density for component failure rates. Another assumption is that each subsystem can have one of cold-standby or active redundancy strategies. The RAP is a NP-Hard problem which cannot be solved in reasonable time using... 

Purpose: Presence of photon attenuation severely challenges quantitative accuracy in single-photon emission computed tomography (SPECT) imaging. Subsequently, various attenuation correction methods have been developed to compensate for this degradation. The present study aims to implement an attenuation correction method and then to evaluate quantification accuracy of attenuation correction in small-animal SPECT imaging. Methods: Images were reconstructed using an iterative reconstruction method based on the maximum-likelihood expectation maximization (MLEM) algorithm including resolution recovery. This was implemented in our designed dedicated small-animal SPECT (HiReSPECT) system. For... 

The modulation transfer function (MTF) is well known as a crucial parameter in quality assurance of computed tomography (CT) scanners, which provides detailed information of both contrast and resolution of CT images. Different methods have been introduced and developed to calculate the MTF of CT scanners. However, a robust methodology which accurately estimates the MTF of CT scanners under the use of every range of object electron density and tube current-time product (mAs) has not been reported so far. To this aim, a new wavelet-based circular edge method for MTF measurement has been presented in this work. Owning to the edge spread function (ESF) susceptibility to noise, the approach was... 

I present an in-depth review of the trends and the directions taken by researchers worldwide in optical code-division multiple-access (OCDMA) systems. I highlight those trends and features that I believe are essential to the successful introduction of various OCDMA techniques in communication systems and data networks in the near future. In particular I begin by giving a comprehensive review of the construction of optical orthogonal codes (OOCs). Specifically I discuss the recently developed algorithms that are based on matrix algebra, which simplify and enhance the efficiencies of algorithms in OOC generation. In communication systems studies I first focus on and discuss various OCDMA... 

Discrimination of chiral targets is generally achieved with chiral nanomaterials. However, the limited number of intrinsic chiral nanostructures as well as their complex synthesis procedure has led to the production of chirality-induced nanomaterials as alternatives. Chirality can be induced in nanomaterials by either chirality transfer or the formation of chiral assemblies. Using the latter approach, in this work, we have provided chiral supramolecular assemblies of CdTe quantum dots (QDs) from achiral starting materials. CTAB-QD assemblies showed chiroptical activities, and their orange emission in combination with the blue emission of carbon dots was utilized as a ratiometric chiral... 

Autocorrelation receiver (AcR) is usually used as the general receiver for transmitted reference (TR) communication systems on a quite intuitive basis. In this paper, we apply the principle of statistical invariance to obtain the optimum TR receiver on a decision-theoretic basis, and it is shown that the autocorrelation receiver is optimal only in a special case. Performance of the optimum receiver and the suboptimum AcR for some important modulation schemes is also evaluated. The results imply that allocating a part of the transmitted energy and bandwidth to reference pulses would not help improve the performance of the system and the optimal performance might be achieved via much simpler... 

In this article we present a new scheme to support multirate and multi-quality-of-service transmission in passive optical networks based on a hybrid wavelength-division multiplexing/optical code-division multiplexing scheme. The idea is to use multilength variable-weight optical orthogonal codes as signature sequences of a hybrid WDM/OCDM system. To provide the requested classes of service, the code weight and code length of MLVW-OOCs are designed based on the characteristics of the requested classes of service. In order to mitigate multiple access interference, we propose to utilize a multilevel signaling technique and interference remover structure based on advanced optical logic gate... 

Discrimination of chiral targets is generally achieved with chiral nanomaterials. However, the limited number of intrinsic chiral nanostructures as well as their complex synthesis procedure has led to the production of chirality-induced nanomaterials as alternatives. Chirality can be induced in nanomaterials by either chirality transfer or the formation of chiral assemblies. Using the latter approach, in this work, we have provided chiral supramolecular assemblies of CdTe quantum dots (QDs) from achiral starting materials. CTAB-QD assemblies showed chiroptical activities, and their orange emission in combination with the blue emission of carbon dots was utilized as a ratiometric chiral... 

Levodopa [L-3, 4-dihydroxyphenylalanine, or L-DOPA] is an important neurotransmitter used for the treatment of neural disorders such as Parkinson's disease. Abnormal L-Dopa concentrations in biological fluids can be used for the evaluation of such diseases. In this work, a rapid and sensitive method for L-DOPA detection has been reported which is based on in situ formation of polylevodopa nanoparticles. Under alkaline conditions, L-DOPA is spontaneously oxidized to its quinone derivative and shows fluorescence properties. The fluorescence signal of the oxidation product has been monitored and used for the determination of L-DOPA in the presence of dopamine, uric acid, ascorbic acid and other... 

In this study the authors present an in-depth study, analysis and discussion on maximum likelihood (ML)-based receiver for a typical spectrally encoded spread-time CDMA in the presence of multiple narrowband interference (NBI) signals in an additive white Gaussian noise channel. Furthermore, the authors demonstrate that by combining useful properties of ML-based receiver and three-level codes, that is, codes with values of -1, 0, +1, the authors can introduce a new strategy in which superior performance with respect to previous receiver structure based on two-level codes, that is, codes with values of -1, +1, can be attained. With the help of an example, the authors drive, first, the... 

Despite the huge potential, value predictors have not been used in modern processors. This is partially due to the complex structures associated with such predictors. In this paper we study value predictors and investigate solutions to reduce storage requirements while imposing negligible coverage cost. Our solutions build on the observation that conventional value predictors do not utilize storage efficiently as they allocate too much space for small and frequently appearing values. We measure data width requirement and entropy in a subset of predictor resources and show that values stored in predictors show limited sizes and very small entropy. We exploit this behavior and suggest... 

Embedded systems are usually energy constrained. Moreover, in these systems, increased productivity and reduced time to market are essential for product success. To design complex embedded systems while reducing the development time and cost, there is a great tendency to use commercial off-the-shelf ("COTS") devices. At system level, dynamic voltage and frequency scaling (DVFS) is one of the most effective techniques for energy reduction. Nonetheless, many widely used COTS processors either do not have DVFS or apply DVFS only to processor cores. In this paper, an easy-to-implement COTS-based evaluation platform for low-energy embedded systems is presented. To achieve energy saving, DVFS is... 

Azerbaijani and Turkish are two closely-related languages from Oguz branch of Turkic languages, which are said to be mutually intelligible. Regarding this background, we designed an experiment within the framework of receptive multilingualism to investigate the role of linguistic factors in intelligibility of the Turkish language to Iranian Azerbaijanis. The study was carried out by controlling extra-linguistic factors such as language contact and negative attitude in order to make the findings attributable to only linguistic factors as the main focus of the present study. In the study, 40 native speakers of Azerbaijani in Iran were exposed to the Turkish language through short videos... 

Given the significant number of the studies conducted to explore construct validity, adopting various approaches, and the paucity of the studies that employed the experimental approach to validation, the current study was aimed at exploring the adequacy of the experimental approach to construct validation, drawing upon advertising literacy as the construct of the researchers’ concern. To this end, the researchers pursued the experimentation, test administration, and validation phases, respectively. To identify the effect of the experimentation, an independent samples t-test was run; there was a significant difference between the experimental and control groups. The closing phase was devoted... 

A novel packet-based photonic label switching router for a multirate all-optical switch using generalized multiprotocol label switching is proposed. The idea is based on using optical code-division multiple access (OCDMA) as multiplexing technique and treating OCDMA codes as labels. The system can coexist with current wavelength division multiplexing systems on the same infrastructure. The concept of switch fabric is introduced. Label processing and label swapping functionalities of the switch are discussed. In-depth analyses are made for spectrally phase-encoded OCDMA (SPE-OCDMA) due to its capabilities of supporting high data rates, large code cardinality, and its secure transmission.... 

In this paper we examine the transmission of two types of ultra-wideband (UWB) signals, multiband orthogonal frequency division multiplexing (MB-OFDM) and impulse radio ultra-wideband (IR-UWB), over single mode fiber at 1550 nm. In order to investigate the impact of optical components such as laser diode, external modulator and single mode fiber on UWB signals, we develop mathematical models for these components. These models are experimentally verified and corresponding numerical parameter values are obtained by experiment. Using these models we discuss the transmission of two types of UWB signals over single mode fiber. A new figure of merit namely distortion factor is defined. Using... 

In this paper we study and evaluate the performance of a spectral-phase encoded optical CDMA (SPE-OCDMA) system using advanced receiver structures based on second harmonic generation (SHG) effect imposed in a Thick or Thin crystal. This receiver structure is employed as the nonlinear pre-processor prior to the conventional low-speed photodetector. In our performance evaluation amplified spontaneous emission (ASE) of the optical amplifiers, receiver front end thermal noise and photodetector shot noise which are effective in low power conditions, and multiple access interference (MAI) noise which is the dominant source of noise in high power conditions in any coherent OCDMA communications...