Sharif Digital Repository

Using a simple nonlinear model based on rate equations, and by employing a harmonic balance method, we develop a theory of optimal mixing in directly modulated semiconductor laser diodes. We perform a consistent numerical solution to the mixing in laser diodes to the arbitrary accuracy and intermodulation index (m, n). Through numerical computations we demonstrate that there is an optimal bias in mixing, corresponding to a relaxation frequency, fr, coinciding with the subcarrier frequency, f1, at which the mixing power is maximized nearly simultaneously for all intermodulation products, fmn. In terms of increasing the signal's current amplitude, it will be shown that it would result in a... 

In this paper, we introduce a technique for improving power distribution characteristics of space time block codes (STBCs) which include peak to average power ratio (PAPR), average to minimum power ratio (Ave/min), and probability of transmitting “zero” by antenna. We consider STBCs in multiple input multiple output (MIMO) and multiple input single output (MISO) systems that achieve full diversity for linear or maximum likelihood (ML) receivers. It is proved that by multiplying a specific non-singular square matrix by the code matrix, a new code is obtained that achieves full diversity and the power distribution characteristics are improved. The proposed technique is general can be applied... 

In this paper, a modified frequency control model is proposed, where the demand response (DR) control loop is added to the traditional load frequency control (LFC) model to improve the frequency regulation of the power system. One of the main obstacles for using DR in the frequency regulation is communication delay which exists in transferring data from control center to appliances. To overcome this issue, an adaptive delay compensator (ADC) is used in order to compensate the communication delay in the control loop. In this regard, a weighted combination of several vertex compensators, whose weights are updated according to the measured delay, is employed. Generating the phase lead is the... 

A mobile ad hoc network is a collection of mobile hosts forming a temporary network on the fly, without using any fixed infrastructure. Characteristics of mobile ad hoc networks such as lack of central coordination, mobility of hosts, dynamically varying network topology, and limited availability of resources make QoS provisioning very challenging in such networks. In this paper, we introduce a fuzzy QoS traffic conditioner for mobile ad hoc networks. The proposed traffic conditioner consists of fuzzy admission control (FAC), fuzzy traffic rate controller (FTRC), and fuzzy scheduler (FS). The proposed FAC monitors the delay and available bandwidth and decides whether to accept or reject the... 

In this paper, we evaluate the capacity of Multiple-Input Multiple-Output (MIMO) systems using directional antennas in non-isotropic scattering environments. Our analytical analysis approach results in closed-form approximate formulas in low Signal to Noise Ratio (SNR) regime (in contrast to previous research works which have investigated the same problem by employing experimental or simulation frameworks). We propose an upper bound for the introduced approximate error. Our results show that MIMO systems equipped with directional antennas may have much higher capacity in nonisotropic scattering environments compared to MIMO systems equipped with omni-directional antennas. We also evaluate... 

In this paper, using the so-called Parallel Space-Time Markov Chain (PSTMC) framework we analyze the IEEE 802.11 Distributed Coordination Function (DCF) frame service time, jitter, and queuing delay in a single-hop non-saturated wireless network. PSTMC framework provides the possibility of simultaneous modeling of backoff and post-backoff procedures, in addition to the transmission queue status of a non-saturated 802.11 station. To the best of our knowledge, the presented contribution is the first analysis of service time, i.e., access delay and retransmission delay, plus queuing delay at the same time, when the precise modeling of binary exponential backoff (BEB) scheme in medium access... 

With the introduction of restructuring concepts to traditional power systems, a great deal of attention is given to the utilization of distributed generation. Since the integration of DG units has been known as an alternative for main grid as a resource for energy supply, the determination of optimal sizing and sitting is an important issue in the planning procedure of DG. This work presents a comprehensive multi-objective model for integration of distributed generations into a distribution network, regarding various technical, economical and environmental issues such as reduction of carbon dioxide emissions and investment & running costs while the bus voltages shall be kept within... 

In this paper, an iterative signal reconstruction method is extended to cancel multi-user clipping noise in a uplink MC-CDMA systems. Clipping is the simplest method to overcome high peak-to-average power ratio of multi-carrier signals but it makes the multi-carrier signals distorted. Reconstruction methods use non-distorted samples to reconstruct distorted samples in the receiver but multi-user interference causes the methods do not work properly because all the received samples are distorted by clipping and interference and so there is no undistorted samples to be used in recovering clipped samples. On the other hand, multi-user interference cancellation methods do not work properly... 

The critical issue in designing correlated data networks like Wireless Sensor Networks is to minimize the total cost of data transmission in the network, and decrease the amount of data flow. The problem of finding optimal aggregation tree for correlated data gathering in single sink network is considered as an NP-Complete problem and hence heuristic methods are usually applied to solve it[1]. In this paper, we apply genetic algorithm (GA) to solve the problem. In our method, we improve the performance of genetic search by selecting proper initial population. This initial population is determined in two ways, by using Prime's algorithm, and shortest path tree. The main issue is to regard... 

In this paper we will propose a novel distributed architecture for overlay tree construction in collaborative virtual environments that is unlimitedly scalable due to its fully decentralized nature. The routing issue over this architecture will later be discussed and several forwarding schemes based on distributed hash tables (DHT) will be proposed. One of the main concerns for a routing algorithm is to keep a trade-off between the algorithm locality and the algorithm performance. In a global algorithm where nodes make the forwarding decision based on the information gathered from all other nodes in the network, the routing performance may obviously be better. However such a large group... 

Both secure and error control coding are very extensive subjects, each with a variety of sub-disciplines. In this paper, we develop a secure channel coding (joint encryption-channel coding) scheme based on concatenated turbo codes for more efficient and secure transmission of the LEO satellite data. Reliability and security are achieved by adapting the pseudorandom puncturing strategy with the change of distance between satellite and ground station in the communication séance, to issue further burdened by reducing energy consumption or increasing bit rate of data transmission. Simulation results show the relevance and superior performance of the proposed scheme compared with the traditional... 

In recent years, cooperative diversity techniques have gained increased attention as a new way to combat degrading effects of Fading. Cooperation among two or more single antenna users generates a virtual diverse multiple-antenna transmission system in the wireless network. In this paper, we investigate the application of adaptive modulation concept to the performance of cooperative techniques. We consider the adaptation of rate and power allocation with M-QAM modulation, and study the performance of Amplify & Forward scenarios in Rayleigh fading channels. We compare these systems with their comparable adaptive non-cooperative scenario and evaluate the energy saving achieved through... 

Convolutional neural networks (CNNs) are at the heart of several machine learning applications, while they suffer from computational complexity due to their large number of parameters and operations. Recently, all-optical implementation of the CNNs has achieved many attentions, however, the recently proposed optical architectures for CNNs cannot fully utilize the tremendous capabilities of optical processing, due to the required electro-optical conversions in-between successive layers. To implement an all-optical multi-layer CNN, it is essential to optically implement all required operations, namely convolution, summation of channels' output for each convolutional kernel feeding the... 

Network-on-Chips are now the popular communication medium to support inter-IP communications in complex on-chip systems with tens to hundreds IP cores. Higher scalability (compared to the traditional shared bus and point-to-point interconnects), throughput, and reliability are among the most important advantages of NoCs. Moreover, NoCs can well match current CAD methodologies mainly relying on modular and reusable structures with regularity of structural pattern. However, since NoCs are resource-limited, determining how to distribute application load over limited on-chip resources (e.g. switches, buffers, virtual channels, and wires) in order to improve the metrics of interest and satisfy... 

Objectives. Stressors have a serious role in precipitating mental and somatic disorders and are an interesting subject for many clinical and community-based studies. Hence, the proper and accurate measurement of them is very important. We revised the stressful life event (SLE) questionnaire by adding weights to the events in order to measure and determine a cut point. Methods. A total of 4569 adults aged between 18 and 85 years completed the SLE questionnaire and the general health questionnaire-12 (GHQ-12). A hybrid model of genetic algorithm (GA) and artificial neural networks (ANNs) was applied to extract the relation between the stressful life events (evaluated by a 6-point Likert scale)... 

This paper presents a modified astrocyte model that allows a convenient digital implementation. This model is aimed at reproducing relevant biological astrocyte behaviors, which provide appropriate feedback control in regulating neuronal activities in the central nervous system. Accordingly, we investigate the feasibility of a digital implementation for a single astrocyte and a biological neuronal network model constructed by connecting two limit-cycle Hopf oscillators to an implementation of the proposed astrocyte model using oscillator-astrocyte interactions with weak coupling. Hardware synthesis, physical implementation on field-programmable gate array, and theoretical analysis confirm... 

In this paper, an iterative method is proposed to jointly cancel multi-user interference and clipping noise in uplink Multi-Carrier Code Division Multiple Access (MC-CDMA) systems. Clipping is the simplest method to overcome high peak-to-average power ratio of multi-carrier signals but it makes the signals distorted. Reconstruction methods use non-distorted samples to reconstruct distorted samples in the receiver but multi-user interference causes the methods do not work properly because all the received samples are distorted due to clipping and interference and so there is no undistorted samples to be used in recovering clipped samples. On the other hand, multi-user interference... 

In the present study, we investigate the effect of the hemodynamic factors of the blood flow on the cerebral aneurysms. To this end, a hypothetical geometry of the aneurysm in the circle of Willis, located in the bifurcation point of the anterior cerebral artery (ACA) and anterior communicating artery (ACoA) is modeled in a three-dimensional manner. Three cases are chosen in the current study: an untreated thin wall (first case), untreated thick wall (second case), and a treated aneurysm (third case). The effect of increasing the aneurysm wall thickness on the deformation and stress distribution of the walls are studied. The obtained results showed that in the second case, a reduction in the... 

In this paper, we propose a new scheme to provide multiservice transmission on generalized multiprotocol label switching (GMPLS) networks using optical code-division multiplexing (OCDM) in each wavelength of a wavelength-division multiplexing system. To provide multiservice transmission in a typical GMPLS network, we propose to classify the fiber bandwidth into a number of waveband and consider the wavelengths of each waveband for a specific class-of-service such that the corresponding codes to be used are designed based on the characteristics of the class-of-service. The traffic behavior of the network is examined using Erlang's model, and the probability density functions (PDF) of the... 

Background: Remote cardiac monitoring and screening have already become an integral telemedicine component. The wide usage of several different wireless electrocardiography (ECG) devices warrants a validation study on their accuracy and reliability. Methods: Totally, 300 inpatients with the Nabz Hooshmand-1 handheld ECG device and the GE MAC 1200 ECG system (as the reference) were studied to check the accuracy of the devices in 1 and 6-limb lead performance. Simultaneous 10-second resting ECGs were assessed for the most common ECG parameters in lead I. Afterward, 6-lead ECGs (limb leads), were performed immediately and studied for their morphologies. Results: Of the 300 patients, 297 had...