Sharif Digital Repository

Ultimate state boundedness for underactuated spacecraft subject to large non-matched disturbances is attained. First, non-smooth time-invariant state feedback control laws that make the origin asymptotically stable are obtained. Then, the controller is extended to make the closed-loop system globally uniformly ultimately bounded under the following conditions: 1) the disturbances acting on the directly actuated states are known and 2) the disturbance acting on the unactuated state is bounded and its profile need not be known. Finally, numerical simulations are presented to verify the analytical results. A large step disturbance is considered, and it is shown that the proposed controller... 

This study presents an online motion planning algorithm for generating reference trajectories during flight phases of a planar monopedal robot to transfer the configuration of the mechanical system from a specified initial pose to a specified final one. The algorithm developed in this research is based on the reachability and optimal control formulations of a time-varying linear system with input and state constraints. A two-level control scheme is developed for asymptotic stabilisation of a desired period-one orbit during running of the robot. Within-stride controllers, including stance and flight phase controllers, are employed at the first level. The flight phase controller is a feedback... 

This letter discusses the surface-reconstruction-induced self-twisting behavior of Si100 nanobelts and nanowires (NWs) with rectangular cross section. Giving a thorough physical interpretation, we explain the reason behind this phenomenon and present a continuum-based model. It is revealed that these structures can self-assemble into both right- and left-handed helicoids depending on their crystal arrangements. More specifically, for NWs with the same number of layers in each of their cross sections directions, two distinct values of torsion angle are possible for each of right- and left-handed twisted morphologies. In conclusion, four modes of torsion can be observed in Si100 NWs.... 

In this paper, a confined infill panel previously experimented under out-of-plane loading is modeled and analyzed by the dynamic explicit method (DEM). This method is shown to have a good estimation of the experimental results, and is thus adopted to investigate the out-of-plane behavior of an infill model. Based on the analytical results, the stress distribution of an infill panel prior to and after cracking, and the effect of an arching mechanism on post-cracking behavior, is studied. A semi-empirical relationship is accordingly developed for predicting the transverse resistance of masonry infills due to one-way arching action, which accounts for the effects of the boundary frame... 

In this paper, static behavior of nano/microm1irrors under Casimir force is studied. At the first, the equilibrium equation governing the statical behavior of nano/micromirrors is obtained. Then energy method is employed to investigate statical stability of nano/micromirrors equilibrium points and a useful equation is suggested for successful and stable design of nano/micromirrors under Casimir force. Then, equilibrium angle of nano/micromirrors is calculated both numerically and analytically using the homotopy perturbation method (HPM). It is observed that with increasing the instability number defined in the paper, the rotation angle of the mirror is increased and suddenly, pull-in occurs.... 

In this paper, we propose a new scenario in order to increase the total throughput of a WLAN. In this scenario, some secondary users (SUs) are considered in a primary WLAN such that SUs cooperate with primary users (PUs) in transmitting the primary packets. Each SU sends its packets as well as the primary packets collided at the access point, but overheard successfully at the SU, by random access the primary WLAN. We show that SUs cooperation is able to compensate their adverse effect in the WLAN, so total throughput in the WLAN increases without a significant degrading effect for the stability region of PUs. To this end, we use an analytical model based on open queueing networks. By solving... 

The first objective of this brief is to discover the solution existence conditions in the methods recently proposed for tuning fractional-order [proportional-integral] (FO-[PI]) controllers. The FO-[PI] controller tuned by these methods can simultaneously ensure the desired phase margin, the desired gain crossover frequency, and the flatness of the phase Bode plot at such a frequency. In this brief, the achievable performance region of these tuning methods is also found in the gain crossover frequency-phase margin plane. Moreover, some results on the shape of this region and the uniqueness of the controller resulting from the considered tuning methods are given. By combining the presented... 

A quantum wire of uniform cross section (but with eventual disorder) with three regions: dot, left lead, and right lead, is considered. Assuming that the same unitary transformation diagonalizes all unit cells of this wire, we propose a new method for the calculation of the Green's function (GF) and the coherent transmission coefficient. In particular, the problem of a uniform dot (simple cubic uniform dot attached to the simple cubic wire), with all onsites equal and all hoppings equal is solved fully analytically. The energy and dot-length dependence of the GF, local density of states (LDOS), the transmission coefficient and bound state energies are also derived. © 2004 Elsevier B.V. All... 

A numerical model based on the boundary element method is proposed for the sloshing of a flowing liquid in a three-dimensional tank. Assuming a mean flow in the tank in addition to a perturbation flow, the nonlinear boundary conditions of the liquid free-surface are linearized. Using the boundary element method along with the modal analysis technique a reduced order model is obtained which is used to calculate the fundamental sloshing frequencies and modes in the tank with an inlet and outlet. The obtained results for a test case are compared with the literature data to validate the proposed model. The results are in a very good agreement with analytical results and show an acceptable... 

A single storey full-scale unreinforced brick masonry building with confined walls was designed based on Iranian building code and constructed on the shaking table facility of EERC of Sharif University of Technology. The model consisted of plain brick masonry walls confined with concrete tie columns and steel bond beams and had a typical jack-arch roof system. Wall panels included openings of different dimensions making it possible to investigate the effects of shape, geometry and location of openings. The model was subjected to several seismic excitations, consisting of scaled records of Bam 2003, Tabas 1978 and Elcentro 1940 earthquakes as well as a harmonic acceleration with a gradually... 

Secure computation has obtained significant attention in the literature recently. Classic architectures usually use either the Garbled Circuit (GC) or the Goldreich-Micali-Wigderson (GMW) protocols. So far, to reduce the complexity of communications in these protocols, various methods have been proposed. The best known work in both methods reduces the communication up to almost 2k-bits (k is the symmetric security parameter) for each AND gate, and using XOR gate is free. In this paper, by combining GC and GMW, we propose a scheme in the semi-honest adversary model. This scheme requires an Oblivious Transfer (OT) and a 2-bit data transfer for each AND gate, keeping XOR gates free. The... 

In non-orthogonal multiple access (NOMA) systems, serving multiple users in shared resource blocks can allow untrusted users to overhear the messages of other users. In this context, we study a network consisting of a base station (BS), a near user and a far user, where the latter attempts to overhear the message of the former. The near user is a full-duplex (FD) node that can also act as a relay. Two operating scenarios are considered: 1) friendly jammer (FJ), where the FD node broadcasts noise for degrading the channel between the BS and the far user, while receiving data from the BS; and 2) friendly jammer relay (FJR), where, in addition to degrading the channel between the BS and the far... 

In this paper, we focus on a cognitive network scenario, comprised of a WLAN overlaid on a network with time-scheduled primary users. WiMAX is an example of such primary networks. For time-scheduled primary users simple On-Off traffic model with exponential durations is not valid anymore. In this scenario, the cognitive nodes (CNs) hear downlink map (DL-MAP) and thus, know the frequency and time locations of all allocated slots at each frame. Then, cognitive nodes contend with each other in order to transmit their fixed size packets based on IEEE 802.11 MAC protocol. Since the number of empty slots at each frame is variable each packet transmission in cognitive network prolongs a random... 

This paper presents a fragmented-spectrum synchronous OFDM-CDMA modulation and utilize it as secondary users modulation in a cognitive radio-based network to provide high data rate by efficiently exploiting available spectrum bands in a target spectral range while simultaneously offering multiple-access capability. In particular, given preexisting communications in the spectrum where the system is operating, a channel sensing and estimation method is used to obtain information of subcarrier availability. Given this information, some three-level codes are provided for emerging new cognitive radio users. Furthermore, analytical results of the system performance in a typical cognitive radio... 

This paper presents the application of reduced order modeling technique for investigation of liquid sloshing in three-dimensional tanks. The governing equations of sloshing are written using a boundary element formulation for incompressible potential flow. Then, the governing equations are reduced to a more efficient form that is represented only in terms of the velocity potential on the liquid free surface. This particular form is employed for eigen-analysis of fluid motion and the sloshing frequencies and mode shapes are determined. Then, the sloshing frequencies and the corresponding right- and left-eigenvectors are used along the modal analysis technique to find a reduced order model... 

This article presents a new model for pricing a new product considering skimming pricing strategy in the presence of the competition. We consider two periods for price setting including skimming and economy period. The problem is deciding on a skimming price as well as an economy price in order to maximize the total profit. The derived model is a non-linear programming model and we analyzed the structure and properties of optimal solution to develop a solution method. Analytical results as well as managerial insights are presented by mathematical analysis and numerical analysis  

We propose an exactly solvable waveguide lattice incorporating an inhomogeneous coupling coefficient. This structure provides classical analogs to the squeezed number and squeezed coherent intensity distribution in quantum optics where the propagation length plays the role of a squeezed amplitude. The intensity pattern is obtained in a closed form for an arbitrary distribution of the initial beam profile. We have also investigated the phase transition to the spatial Bloch-like oscillations by adding a linear gradient to the propagation constant of each waveguide (α). Our analytical results show that the Bloch-like oscillations appear above a critical value for the linear gradient of the... 

The authors propose a new analytical model based on BCMP closed queueing networks in order to evaluate the performance of IEEE 802.11 DCF MAC protocol when all nodes are in the transmission range of each other, that is, a single hop wireless ad hoc network. By the proposed model, some performance metrics such as saturation and non-saturation throughput, distributions of channel access delay and the number of packets in the MAC buffer are derived. An extension of the proposed model is used for the analysis of IEEE 802.11e EDCA and the same performance metrics are evaluated for this protocol. Analytical results on IEEE 802.11e prove that differentiation in service is possible and channel share... 

The main objective of this paper is to investigate the stability and stabilization problem of fuzzy large-scale systems in which the system is composed of a number of Takagi-Sugeno fuzzy subsystems with interconnection. Instead of fuzzy parallel distributed compensation (PDC) design, nonlinear state feedback controllers are used in stabilization of the overall large-scale system. Based on Lyapunov stability theory, linear matrix inequalities (LMIs) conditions are derived for asymptotic and exponential stability. Two numerical examples are given to confirm the analytical results and illustrate the effectiveness of the proposed strategy  

In this paper, we investigate the performance of dense Poisson-point-process-based cellular networks using finite length codewords. Taking the properties of the power amplifiers (PAs) into account, we derive the outage probability, the per-user throughput and the area spectral efficiency in different conditions. Our analysis is based on some recent results on the achievable rates of finite-length codes and we investigate the effect of the codeword length/PAs properties on the system performance. Our numerical and analytical results indicate that the inefficiency of the PAs affects the performance of dense networks substantially. Also, for a given number of information nats per codeword,...