Sharif Digital Repository

In this paper, we study the downlink of cooperative Direct-Sequence Code Division Multiple Access (DS-CDMA) systems, where a set of Mobile Terminals (MT) cooperate with the purpose of improving their downlink communications reliability with the Base Station (BS). For combating Multiple-Access Interference (MAI) as well as transferring complexity and spatial processing from the mobile terminals to the transmitter of BS, multiuser transmitter precoding is considered. In addition to multiuser precoding based on Minimum Mean-Square Error (MMSE) criterion for mitigating MAI, the proper transmission power control strategy under a fixed total power constraint is proposed and analyzed. Compared with... 

We study the problem of power efficient tracking interval management for distributed target tracking wireless sensor networks (WSNs). We first analyze the performance of a distributed target tracking network with one moving object, using a quantitative mathematical analysis. We show that previously proposed algorithms are efficient only for constant average velocity objects; however, they do not ensure an optimal performance for moving objects with acceleration. Towards an optimal performance, first, we derive a closed-form mathematical expression for the estimation of the minimal achievable power consumption by an optimal adaptive tracking interval management algorithm. This can be used as... 

We study the problem of power optimization for object tracking using distributed Wireless Sensor Networks (WSNs). The accuracy of the object tracking is dependent on the tracking time interval. Smaller tracking time interval increases the accuracy of tracking a moving object. However, this increases the power consumption significantly. This paper proposes a modified adaptive sleep time management scheme called Modified Predict and Mesh (MPaM) to adapt tracking time interval such that it minimizes power consumption while keeping an acceptable tracking accuracy. Also a quantitative analysis to compare the performances of the conventional PaM and proposed Modified PaM (MPaM) schemes is... 

We consider the minimization of power consumption in target tracking wireless sensor networks (WSNs) using dynamic modification of tracking interval. In this context, we first analyze the performance of such networks, using a quantitative mathematical analysis. Then we calculate an upper bound for the achievable improvement in total power consumption, when using an adaptive time interval modification algorithm for tracking moving objects with acceleration. Towards this optimum functionality, we propose a novel adaptive algorithm (AHC) to adapt the tracking interval such that it minimizes power consumption while keeping an acceptable accuracy. Simulation results show that using the proposed... 

The accuracy of the object tracking is dependent on the tracking time interval. Smaller tracking time interval increases the accuracy of tracking a moving object. However, this increases the power consumption significantly. This paper proposes a new adaptive algorithm (AEC) to adapt tracking time interval such that it minimizes power consumption while keeping the required accuracy. Simulation results show that using the proposed algorithm, the tracking network has a good performance with the added advantage of reducing the power consumption significantly when compared with existing nonadaptive methods (like PATES). Moreover, simulation results show that the performance of the proposed... 

We investigate the use of simple nontoxic Cu2SnS3 (CTS) nanoparticles (NPs) as low-cost dopant-free hole-transport materials (HTMs) a substitute for spiro-OMeTAD in an n-i-p mesoscopic architecture of perovskite solar cells (PSCs). Besides, this work confirms the critical role of the crystalline phase of CTS NPs on the performance of the device. Using a facile one-pot heating-up procedure, pure zincblende and wurtzite structures of CTS NPs were obtained by sulfur element and thiourea as the sulfur source, respectively, and were dispersed in chloroform to make very stable nonpolar ink that is compatible with the perovskite. Nanoparticles with the wurtzite crystal phase showed much better... 

The present research sheds new light on the development of a triple-cation quasi-two-dimensional (2D) perovskite family with the general formula of (S1−xS′x)2[Cs0.05(FA1−xMAx)0.95]3Pb4(I1−xBrx)13, in which two spacers, namely 5-ammonium valeric acid iodide (S) and tetra-n-octylammonium bromide (S′) were simultaneously incorporated. Morphology, crystal structure, optical properties, photovoltaic performance, and internal resistances of such compound were systemically studied in comparison with an analogous single-cation 2D counterpart (i.e. (S)2(FA)3Pb4I13) as a reference. X-ray diffraction set forth that the films deposited based upon these compounds had a 2D perovskite crystal structure... 

Antisolvent crystallization is known as an effective approach for the deposition of pinhole-free solution-processed perovskite layers for high-performance solar cells. Here, we introduce a modified antisolvent dripping method by adding tetra ethyl orthosilicate (TEOS) into chlorobenzene as a conventional antisolvent. Through TEOS modification, perovskite solar cells show efficiencies as high as 16% with more than 85% retention after 290 h storage at ambient conditions in comparison to 20% in pristine cells. This significant enhancement in efficiency and stability mainly related to the decrement of the density of surface defects, which is confirmed by considerably enhanced photoluminescence... 

We study the problem of optimizing the symbol error probability (SEP) performance of cluster-based cooperative wireless sensor networks (WSNs). It is shown in the literature that an efficient relay selection protocol based on simple geographical information of the nodes to execute the cooperative diversity transmission, can significantly improve the SEP performance at the destination of such networks. Also, a similar analysis on the optimal power allocation for the source and the relay exists in the literature. However, we propose that to achieve the minimum SEP at the destination, a joint optimization of power allocation and relay selection should be accomplished. To this aim, we... 

In this paper, we study the downlink of cooperative Direct-Sequence Code Division Multiple Access (DS-CDMA) systems, where a set of Mobile Terminals (MT) cooperate with the purpose of improving their downlink communications reliability with Base Station (BS). For combating Multiple-Access Interference (MAI) as well as transferring complexity and spatial processing from the mobile terminals to the transmitter of BS, multiuser transmitter precoding is considered. In addition to multiuser precoding based on Minimum Mean-Square Error (MMSE) criterion for mitigating MAI, the proper transmission power control strategy under a fixed total power constraint is proposed and analyzed. Compared with... 

In this paper, we study the downlink of cooperative Direct-Sequence Code Division Multiple Access (DS-CDMA), where a set of Mobile Terminals (MT) are cooperating with the purpose of improving their downlink communications reliability with the Base Station (BS). In order to achieve the spatial diversity, a new cooperation scenario is proposed. Afterward, in addition to multiuser precoding, Maximum Ratio Combining (MRC) according to the Maximum Signal to Noise Ratio (MSNR) criterion at the destination is investigated. Then, the optimal transmission power control strategy under a fixed total power constraint is illustrated and analyzed. Compared with equal transmission power assignment where BS... 

The focus of this research is on the fabrication of aluminium-B4C and aluminium-ZrSiO4 composites by vortex method as a liquid phase process of metal matrix composites. SEM observations show that reinforcement particles are dispersed homogeneously throughout the aluminium matrix. Density, mechanical properties such as hardness and tensile strength of Al base MMCs containing different volume fractions (5, 10 and 15 %) of micro-reinforcements are examined separately and compared with those of monolithic aluminium. Results show that mechanical properties of samples are noticeably higher than those of the monolithic aluminium. The optimum amount of reinforcement in this... 

In this research, the effects of nano-sized MgO in Al2O 3-SiC composites were investigated. The overall changes in the density and mechanical properties of sintered samples (hardness, bending strength and toughness) were evaluated. After mixing, drying and uniaxial compaction of the powders, they were first heat-treated at low temperature in an electric furnace to remove any residuals. They were then heat-treated at high temperature (1700 °C) inside a graphite furnace in argon atmosphere for sintering (at normal and high pressure). The content of MgO in the Al 2O3-10 vol% SiC composite was 0, 500, 1000, and 1500 ppm. Thefracture toughness(KIC)of sintered composite with 10... 

In this study Al2O3-SiC micro/nanocomposites have been fabricated by mixing alumina nanopowders and silicon carbide micro/nanopowders, followed by hot pressing at 1550, 1600, 1650 and 1700°C. The density, mechanical properties and fracture mode of Al2O3-SiC composites containing different volume fractions (2.5%, 5%, 7.5%, 10% and 15%) of micro/nanoscale SiC particles were investigated and compared with those of alumina. The relative density of composites could reach values very close to theoretical density, especially after sintering at 1700°C. However, relative density declined by increasing the SiC fraction at the same sintering temperature. The flexural strength of composites was best for... 

Using multiple antennas at the transmit and receive sides of a passive radar brings both the benefits of MIMO radar and passive radar. However one of the obstacles arisen in such configuration is the receive antennas placement in proper positions so that the radar performance is improved. Here we just consider the case of positioning one receiver among multiple illuminators of opportunity. Indeed it is a start for the solution of optimizing the geometry of the multiple receivers in a passive radar  

SbCl3 supported on montmorillonite K-10 is an efficient catalyst for the ring opening of epoxides with aromatic amines under solvent-free conditions and microwave irradiation to give the corresponding b-amino alcohols in high yields with high regioselectivity  

In this paper, we consider an integrated sensing and communication (ISAC) system with wireless power transfer (WPT) where an unmanned aerial vehicle (UAV)-based radar serves a group of energy-limited communication users in addition to its sensing functionality. In this architecture, the radar senses the environment in phase 1 (namely sensing phase) and mean-while, the communications users (nodes) harvest and store the energy from the radar transmit signal. The stored energy is then used for information transmission from the nodes to UAV in phase 2, i.e., uplink phase. Performance of the radar system depends on the transmit signal as well as the receive filter; the energy of the transmit... 

One of the important obstacles in MIMO (Multiple Input Multiple Output) radars is the issue of designing proper transmit signals. Indeed, the capability of signal design is a significant advantage in MIMO radars, through which, the system can achieve much better performance. Many different aspects of this performance improvement have been considered yet, and the transmit signals have been designed to attain such goal, e.g., getting higher SNR or better detector's performance at the receiver. However, an important tool for evaluating the radar's performance is its ambiguity function. In this paper, we consider the problem of transmit signal design, in order to optimize the ambiguity function... 

Two gains play key roles in recently developed MIMO wireless communication systems: "spatial diversity" gain and "spatial multiplexing" gain. The diversity gain refers to the capability to decrease the error rate of the MIMO channel, while the multiplexing gain implicitly refers to the amount of increase in the capacity of the MIMO channel. It has been shown that there is a fundamental tradeoff between these two types of gains, meaning interplay between increasing reliability (via an increase in the diversity gain) and increasing data rate (via an increase in the multiplexing gain). On the other hand, recently, MIMO radars have attracted much attention for their superior ability to enhance...