Sharif Digital Repository

Pulse compression techniques such as linear frequency modulation (LFM) and phase-coded pulse are an active research topic in radar systems, spread spectrum communication applications, etc. Pulse compression technique is employed to achieve high range resolution with enough low average power. A problem in pulse compression is masking of small targets by the range sidelobes of large nearby targets. Furthermore, the range sidelobes may be detected as separate targets and thus increase the false alarm. The most popular waveform among pulse compression techniques is the LFM (also known as chirp). However, the LFM waveform is susceptible to electronic warfare; hence using the biphase coded... 

Taking advantage of aluminum and its alloys in industry is increasing. A common method in metal forming is hot forming. In this condition a precise prediction of flow behaviour of the alloy is necessary to achieve a product of desirable mechanical and physical properties. In this situation, influence of parameters such as chemical composition, temperature, strain, strain rate as well as recovery and recrystallization phenomena should be considered and by using a suitable model, the metal behavior prediction is possible. Distribution of coherent Al3Sc dispersoids in high volume fraction accompany with Al6Mn result in strengthening to this alloy. Because of the high stacking fault energy,... 

Wireless sensor networks (WSNs) are tiny devices with limited computation and power supply. For such devices, data transmission is a very energy-consuming operation. Data aggregation eliminates redundancy and minimizes the number of transmissions in order to save energy. This research explores the efficiency of data aggregation by focusing on different aspects of the problem such as energy efficiency, latency and accuracy. To achieve this goal, we first investigate data aggregation efficiency with constraint on delay, which can be compatible with other important system properties such as energy consumption and accuracy. By simulation, we will show that, depending on the application, we can... 

The fabrication of micron-sized instruments MEMS have been in fast progress in different fields of science and technology such as those of chemical processes in chemical engineering, propulsion in aerospace industries, microchip cooling and inkjet printers in electrical industries, and medical appliances in medical and biomedical science. They have forced the manufacturers and researchers to work and think in fabricating such small-scale sizes. However, the complexity of fabricating MEMS with moving parts has also promoted the manufacturers to think of alternative ways. For example the use of moving parts to pump fluid in such devises causes such difficulties and the researchers could adopt... 

In steel structured buildings the connection of columns to foundations is of practical significance. This connection customarily is performed by a base plate, which is connected to the foundation with bolts. The columns and base plates are usually prepared separately, and later are welded together by contractors. Due to the errors in the process of fabrication, these plates are proposed with larger dimensions than those suggested by the designers. However, it is believed that the utilization of stiffeners, which are welded to the columns in a perpendicular state, reduces the stress field inside the base plates significantly. Therefore, by use of stiffeners the thickness of base plates can be... 

Using clean and renewable energy as driving force for vehicles is becoming more and more important because of rising fuel prices and also environmental concerns. Using solar and wind power for direct use in vehicles seems impractical with today’s technology. Compressed air can serve as an intermediate medium to store energy and utilize it for running vehicles. The main purpose of this research was to conceptually design an air driven vehicle with acceptable performance in a city for everyday life. After a brief history, at first step advantages and disadvantages of using air as driving force of vehicle is studied. This study was carried out in different categories including cost, capability... 

In this research, an anistropic thin-walled beam composite is used to consider the effects of fiber orientation and lay-up configuration on the aeroelastic stability and response of an aircraft wing. The circum ferentially asymmetric stiffness (CAS) model consists of a group of non-classical effects; such as transverse shear, material anisotropic, warping inhibition, etc. The aerodynamic load has been modeled based on the strip theory and compressible unsteady flow in the finite- state form. In addition, to form the mass, stiffness and damping matrices of non-conservative aeroelastic systems the extended Galerkin’s method (EGM) and the method of separation of variables has been used. After... 

- Pulse-compression is one of the spread spectrum techniques that allows a radar to simultaneously achieve the energy of a long pulse and the resolution of a short pulse. The long pulse band-width is increased by phase or frequency modulation, then this modulated long pulse is compressed by matched filter of the receiver. A simple class of pulse-compression waveforms is Binary phase-coded pulse that uses rectangular sub pulses. When the transmitted signal encounters a moving target, the performance of radar degrades due to Doppler frequency shift of received signal. Binary codes have high side lobe levels for long Codes, so to solve this problem the Quadriphase-coded pulse that constructed... 

Collision attack is one of the usual attacks in cryptanalysis of hash functions. One method for applying this attack is linear approximation of equivalent compress function. Finding collisions for linearized compress function, having low Hamming weight, with a large probability could provide collisions for the main compress function. For this purpose, the problem of finding collision for used compress function is modeled by that of the linearized one, having low Hamming weight differences with a large probability. The problem of finding collisions with low Hamming weight differences is equivalent to the problem of finding codewords with low Hamming weights in linear binary code. Canteaut and... 

In this dissertation, different information networks such as Multiple-Access-Relay Network (MARN) and Broadcast-Relay Network (BRN) are introduced. MARN has one receiver, many relays and many senders. An achievable rate region is obtained for the MARN, using Partial Decode-and-Forward (PDF) strategy. In the case of semi-deterministic MARN, an outer bound and an inner bound to the capacity region are derived. BRN is a model with one sender, two relays and two receivers. Two different types of cooperation are considered for BRN: full cooperation and partial cooperation between relays. Inner bounds on the capacity region of BRN are obtained and compared with previous works. The main idea of the... 

In this thesis, we investigate the problem of OFDM channel estimation exploiting channel sparsity. Due to the sparse nature of the scattering objects, most of wireless multipath channel have a sparse impulse response. Therefore it is possible to use the methods in the field of sparse signal processing for the purpose of estimating such channels for better accuracy, and efficient spectrum usage.
First of all the problem of OFDM channel estimation using scattered pilots is stated. Then the so-called IMAT methods together with the other sparse signal processing methods are used for the purpose of estimating channel nonzero taps. Furthermore, since efficient use of spectrum in... 

Some researches have been carried out in the last 20 years in order to increase the present IC engines thermal efficiency and their optimum performance. (HCCI) engine is viewed as a combination of spark-ignition (SI) and compression-ignition (CI) engines. This is because HCCI engines use premixed fuel/air mixture like SI engines and have auto ignition combustion after the mixture is compressed like CI engines. Control of combustion and ignition timing are the main challenges of these engines. Ignition delay, compression ratio, fuel air equivalence ratio and intake temperature and pressure are considered to be the most effective parameters on HCCI combustion.HCCI engines have great potentials... 

Due to the wide application of Mass Concrete in the body and the foundation of dams, foundations of tall buildings, industrial machinery, and power plants, foundation and piers of bridges, etc.,it is necessary to devise methods in order to prevent the adverse effects of the high volume of such concretes. Since the major negative outcome of high volume is the decrease of the strength of concrete, which in turn is the direct consequence of the cracks resulted from high increase in the temperature within the volume of the concrete as a result of Hydration process and the produced heat which remains inside the bulk of concrete, any solution should involve some form of transferring the heat to... 

In the present work, the numerical simulation of 2D inviscid compressible flows by using the spectral difference (SD) method on quadrilateral meshes is performed. The SD method combines the most desirable features of structured and unstructured grid methods to attain computational efficiency and geometric flexibility. Similar to the discontinuous Galerkin (DG) and spectral volume (SV) methods, the SD scheme utilizes the concept of discontinuous and high-order local representations to achieve conservation and high accuracy. The SD method is based on the finite-difference formulation and thus its formulation is simpler than the DG and SV methods ... 

Interference management is one of the key issues in wireless networks wherein multiple source-destination pairs share the same medium and frequency band and interfere with each other. The exponentially growth in the number of wireless systems and services results in the spectrum shortage which has emerged the technologies that increase the spectral efficiency. Providing the users with cognition equipments in these networks enables them to sense the medium and use the obtained data to adapt their transmissions to cooperate with other users and improve their own rates as well as the others which results in the cognitive radio technology. In order to access the full potential of this technology,... 

In this study, the solution of compressible flows is performed using finite volume lattice Boltzmann method (FVLBM). A model associated with 13 discrete velocity vectors and 2 energy levels is used and the Boltzmann transport equation is solved using a cell-centered finite volume on structured meshes. The values of distribution functions on each cell faceare determined by averaging from their values at the two control points located on the center of two neighboring cells. The fourth-order Runge-Kutta time-stepping scheme is applied to discretize temporal derivative term. The second- and fourth-order numerical dissipation termsareadded to the algorithm to stabilize the solution when solving... 

In this study, the numerical simulation of cavitating flows with compressibility effects is performed. The algorithm employs the multiphase Euler equations with homogeneous equilibrium model. The baseline differential equations system is similar to the one-phase system of equations and comprised of the mixture density, mixture momentums and mixture energy equations. Thephases considered for cavitating flows is liquid-vapor and liquid-gas fields. The system of governing equations is discretized using a cell-centered finite volume AUSM’s upwind scheme. The computations are presented for steady noncavitating/cavitating flows around 1D/2Dproblems for different conditions. A sensitivity study is... 

Articular cartilage Function is vital to joint tasks and its motion. The main biomechanical tasks of articular cartilage are a) the distribution of loads occurred in joints; b) providing joints with frictionless surface. The particular mechanical properties of articular cartilage make it possible for the Cartilage tissue to do its duty successfully. The biological factor that causes articular cartilage to damage is still unknown. It is believed that mechanical factors are the main reason for the cartilage destruction in osteoarthritis. Factors like age, heavy exercises and impact can begin tissue destructions and result in change in mechanical properties. To study the process of the... 

The coding and compression of information sources with special probability distribution is the center of concentration of researchers in this field in the last decades. In this dissertation, after a concise introduction to source coding, we focus on the universal source coding and data compression from an algorithmic and information theoretic point of view. First we introduce the fundamental concepts of source coding and data compression. Then, we analysis some algorithms on integer coding and verify their universality and asymptotically optimality. Usually statistics of information sources is not available in communication systems and storage devices, so that applying universal source... 

Sparse signal processing (SSP), as a powerful tool and an efficient alternative to traditional complete transforms, has become a focus of attention during the last decade. In this ap-proach, we want to approximate a given signal as a linear combination of as few as possible basis signals. Each basis signal is called an atom and their collection is called a dictionary. This problem is generally difficult and belongs to the NP-hard problems; since it requires a combinatorial search. In recent years however, it has been shown both theoretically and experimentally that the sparset possible representation of a signal in an overcomplete dictio-nary is unique under some conditions and can be found in...