Sharif Digital Repository

In this paper, we propose utilizing multiple optical phase conjugate (OPC) modules along fiber spans, in order to improve the performance of coherent optical orthogonal frequency division multiplexing (CO-OFDM) communication systems in long-haul fiber-optic channels. The role of OPC in CO-OFDM systems is investigated by formulating the four-wave mixing (FWM) process generated along a CO-OFDM with two and three OPCs. Analytical results reveal that increasing the number of OPC modules reduces the average FWM power along the transmission link and as a result mitigates the degrading effect of interaction between amplified spontaneous emission noise and FWM. The proposed schemes have also been... 

In this paper, we present a mathematical model for the idea of using more than one optical phase conjugate (OPC) module for coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems in long-haul fiber optic channels. The four-wave mixing (FWM) formulation is derived for three different scenarios in which different numbers of OPC modules are used. The simulation results reveal that using more than one OPC modules leads to a much better BER performance operating on a typical OFDM system  

In this study, the authors evaluate the performance of asynchronous optical multi-carrier code division multiple access (MC-CDMA) for intensity modulated/direct detection systems to be used in next-generation optical access networks. The proposed system employs Gold sequence in frequency-domain to provide asynchronous resource sharing. In long-reach passive optical networks, fibre impairments degradation to the performance of ordinary optical CDMA can be avoided by frequency-domain digital compensation methods in the proposed MC-CDMA. In comparison with orthogonal frequency division multiple access, peak-to-average power ratio in the proposed MC-CDMA is reduced and asynchronous resource... 

In this paper, a new analytical method is presented that can be used to determine the behavior of particular steel beam-to-column extended end plate connection in linear and nonlinear regions. A common means of forming a rigid joint between a universal beam and column is to weld an end plate to the beam end and then to bolt it to the column. A physically based analytical method that can predict the behavior of bolted and extended end plate eave connections, using the connection dimensions as input, is presented. This article demonstrates an analytical procedure for the establishment of elastic and plastic parts of the M-χ curve of this form of connection. An analytical method is proposed for... 

In this paper, we present and discuss the physical implementation and testing of a Spectrally-Encoded Spread-Time Code Devision Multiple Access (SE/ST-CDMA) transceiver. The proposed design is suitable for multiple-access and anti jamming wireless communication systems. The design and implementation takes into account the ability to automatically detect and mitigate tone jammers by altering the corresponding Gold sequence codes assigned uniquely to each user and setting the corresponding code's chip to zero. A simple real-time anti jamming mechanism capable of dealing with frequency hopping jammers is also proposed. The simulation results are provided in terms of Bit Error Rate (BER) versus... 

In this paper, we present a design methodology with implementational insight and evaluate the performance of a multicarrier code-division multiple access (MC-CDMA) system using intensity modulation/direct detection scheme suitable for an indoor visible light communication (VLC) environment. The proposed system is a CDMA system based on an orthogonal frequency division multiplexing (OFDM) platform. To overcome the light-dimming issue, we use a newly proposed methodology, namely, polarity reversed optical OFDM (PRO-OFDM). A unipolar signal is either added to the minimum current or subtracted from the maximum current in the LED linear current range to provide various amounts of dimming. The... 

In Cognitive Radio (CR), in order to avoid interference in the Primary Users (PU), some subcarriers need to be deactivated. In systems based on Multicarrier Code Division Multiple Access (MC-CDMA), this causes losing orthogonality among different spreading codes, leading to poor Bit Error Rate (BER) performance. The performance of such system can be improved by using Generalized Hadamard Codes (GHC) instead of conventional Hadamard codes which avoids the loss of orthogonality. This is due to the fact that unlike conventional Hadamard codes, GHC are conjectured to exist for any arbitrary length. In this paper, we propose using a novel spreading code, namely, GHC for synchronous MC-CDMA in CR... 

In this paper, we propose an iterative block equalizer for long-haul Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) communication systems. The proposed scheme is based on a soft Minimum Mean-Squared Error (MMSE) iterative block equalizer. Two different channel estimation methods are also discubed. The performance and computational complexity of the proposed iterative equalization and channel estimation techniques are evaluated as well. The obtained Bit Error Rate (BER) performance reveal that the iterative equalizer outperforms the linear equalizers by almost two orders of magnitude  

Synchronization of fractional-order hyperchaotic complex systems is an interesting phenomenon since it has several applications in applied sciences. Based on the complexity of hyperchaotic dynamical systems and unpredictability of hidden attractors, which may exist in their phase spaces and could be beneficial in secure communications, a scheme to derive the analytical formula of the control functions is stated to study active control synchronization. Further comparison of this analytical formula with numerical experiments yields an interesting agreement. A novel fractional-order complex Sprott system is proposed to study this kind of synchronization and its dynamics. A secure communication... 

In a MIMO (Multiple Input Multiple Output) radar system, proper design of transmit signal and receive filter is an advantageous tool to improve the detection performance. Thus, in this paper, we consider the problem of transmit code and receive filter design, in order to maximize the Signal to Noise Ratio (SNR) at the receiver while enforcing a similarity constraint between the transmit space-time code (STC) and a reference STC which has a desirable ambiguity function. We will show that by solving such constrained optimization problem through successive iterations, our proposed method leads to satisfactory results  

Weighting elements to achieve radiation patterns with desired characteristics is a classical work in phased array antennas. These characteristics can be low sidelobe level, narrow beamwidth, high directivity, pattern nulling in special angle and etc. For each of these characteristics, different methods have been introduced. Most of methods have been presented for uniform arrays, however there are lots of methods to obtain a desired pattern for antennas with non-uniform element distances. The problem with these methods is complexity or not very good results. In this paper, fast and easy methods based on Least Square Error that leads to good results are presented. In addition, weighting of... 

Distributed generation connected with AC, DC, or hybrid loads and energy storage systems is known as a microgrid. Campus microgrids are an important load type. A university campus microgrids, usually, contains distributed generation resources, energy storage, and electric vehicles. The main aim of the microgrid is to provide sustainable, economical energy, and a reliable system. The advanced energy management system (AEMS) provides a smooth energy flow to the microgrid. Over the last few years, many studies were carried out to review various aspects such as energy sustainability, demand response strategies, control systems, energy management systems with different types of optimization... 

This paper resolve the salinity-dependent interactions of polar components of crude oil at calcite-brine interface in atomic resolution. Molecular dynamics simulations carried out on the present study showed that ordered water monolayers develop immediate to a calcite substrate in contact with a saline solution. Carboxylic compounds, herein represented by benzoic acid (BA), penetrate into those hydration layers and directly linking to the calcite surface. Through a mechanism termed screening effect, development of hydrogen bonding between –COOH functional groups of BA and carbonate groups is inhibited by formation of a positively-charged Na+ layer over CaCO3 surface. Contrary to the common... 

An important tool for assessing a radar's performance is the ambiguity function. On the other hand, many different schemes have been designed for the transmit waveforms of a MIMO radar. For each one, a criterion has been chosen to improve the performace, e.g. achieving better missed detection. In this paper, considering the ambiguity function as the benchmark, we propose a waveform design scheme, which optimizes the ambiguity function of the MIMO radar system  

Flow hydrodynamics of laminar falling film of aqueous Li Br solution (Li Br - H2O) on a horizontal elliptical tube has been investigated in this research. The film velocity distribution and film thickness, namely, the flow characteristics are determined by solving analytically simultaneous simplified Navier - Stokes equations and continuity equation in polar and Cartesian coordinates. The analysis is based on steady state laminar flow of falling liquid film of Li Br - H2O on a horizontal elliptical tube in polar model and Cartesian model (CM), for cases in which traction on the film surface is considered negligible. Models are compared with each other in three cases of aspect ratios (Ar),... 

Velocity distributions in (r,θ) directions are evaluated by solving simultaneous simplified Navier-Stokes equations (NSE) and continuity equation (CE) in polar coordinate. The analysis is based on steady state laminar flow of thin falling liquid film on a horizontal circular tube, for cases in which traction on the film surface is considered negligible. It is a common geometry for part of engineering problems such as evaporator, condenser, absorber, generator of absorption chillers and other similar units in mechanical and chemical engineering. Knowledge of the velocity profiles is usually needed for: 1 - solving governing energy and species equations 2- estimating the average and film... 

Velocity distributions in (r,θ) directions are evaluated by solving simultaneous simplified Navier-Stokes equations (NSE) and continuity equation (CE) in polar coordinate. The analysis is based on steady state laminar flow of thin falling liquid film on a horizontal circular tube, for cases in which traction on the film surface is considered negligible. It is a common geometry for part of engineering problems such as evaporator, condenser, absorber, generator of absorption chillers and other similar units in mechanical and chemical engineering. Knowledge of the velocity profiles is usually needed for: 1- solving governing energy and species equations 2- estimating the average and film... 

This work concerns the experimental investigation of surfactant alternating CO 2 injection in carbonate rocks. The core samples provided from a low-temperature fractured light oil reservoir, located in southwest Iran. The experiments were designed to observe the effect of CO 2-foam injection on gas mobility and oil recovery at different surfactant concentrations. The core samples were initially saturated with synthetic/field brine, 5,000 ppm, and then flooded with live oil to reach connate water saturation at reservoir condition, 115F and 1,700 psia. The commercial surfactant used was sodium lauryl sulfate as an anionic surfactant. The results of this work, along with field-scale simulation... 

Knowledge of Air-water two phase flows is significant to different engineering systems such as chemical reactors and power plant and petrochemical and petroleum industry. One of the most industrial cases of two phase flow is two phase flow in vertical large pipes. In this paper in order to find two phase flow behavior along vertical large diameter pipes we simulate air inlets with different number of holes and different hole diameters in the same flow rate of air. In addition, flow characteristics such as cross-sectional void fraction and velocity and pressure were considered. To achieve this aim, main equations of flow have been developed for investigation of flow behavior in air-water two...