Sharif Digital Repository

Field-effect transistors have been widely used to study electronic transport and doping in colloidal quantum dot solids to great effect. However, the full power of these devices to elucidate the electronic structure of materials has yet to be harnessed. Here, we deploy nanodielectric field-effect transistors to map the energy landscape within the band gap of a colloidal quantum dot solid. We exploit the self-limiting nature of the potentiostatic anodization growth mode to produce the thinnest usable gate dielectric, subject to our voltage breakdown requirements defined by the Fermi sweep range of interest. Lead sulfide colloidal quantum dots are applied as the active region and are treated... 

Topology of a mobile wireless sensor/ad hoc network is prone to link breakages due to node mobility which compels the periodic re-construction of the topology in so called ""hello"" intervals. The problem addressed in this paper is determining the maximum ""hello"" interval preserving the connectivity of a homogeneous topology with high probability. In order to study the optimum ""hello"" interval, statistical topology lifetime (STL) is formally defined as a value of ""hello"" interval that ensures statistical connectivity with a given threshold. Temporal properties of the topology are studied based on two phenomena; one concerning a connectivity phase transition with time and the other one... 

In this paper, we introduce an efficient route discovery mechanism to enhance the performance and multicast efficiency of On-Demand Multicast Routing Protocol (ODMRP). Our framework, called limited flooding ODMRP, improves multicasting mechanism by efficiently managing flooding mechanism based on delay characteristics of the contributing nodes. In our model, only the nodes that satisfy the delay requirements can flood the Join-Query messages. We model the contributing nodes as M/M/1 queuing systems. Our framework considers the significant parameters in delay analysis, including random packet arrival, service process, and random channel access in the relying nodes, and exhibits its best... 

In many wireless sensor network applications, nodes are mobile, while many protocols proposed for these networks assume a static network. Thus, it is important to evaluate if a traditional protocol designed for a static network can tolerate different levels of mobility. This paper provides an analytic model to investigate the effect of mobility on a well-known cluster-based protocol, LEACH. The model evaluates data loss after construction of the clusters due to node mobility, which can be used to estimate a proper update interval to balance the energy and data loss ratio. Thus, the results can help the network designer to adjust the topology update interval given a value of acceptable data... 

One of the intrinsic problems of mobility in wireless networks is the discovery of mobile nodes. A widely used solution for this problem is to use different variations of beacons, such as hello packets. Although a poorly designed beaconing scheme may lead to unnecessary energy usage or poor throughput, a systematic approach to analyze and select beaconing parameters is not provided in the literature. Here, we propose a model to study the beaconing efficiency using some measures such as the link lifetime, the probability of link establishment, and the delay to discover a new neighbor. The model is general and does not adhere to any particular mobility model; the only input from the mobility... 

This paper investigates a class of mobile wireless sensor networks that are unconnected most of the times; we refer to them as delay-tolerant wireless sensor networks (DTWSN). These networks inherit their characteristics from both delay tolerant networks (DTN) and traditional wireless sensor networks. After introducing DTWSNs, three main problems in the design space of these networks are discussed: routing, data gathering, and neighbor discovery. A general protocol is proposed for DTWSNs based on opportunistic broadcasting in delay-tolerant networks with radio device on-off periods. Three performance measures are defined in the study: the energy for sending queries to ask for data from... 

Flocking through leader following structures in mobile networks raises attractive control problems. Due to limited sensing radii, leaders locally influence a network of agents. In this paper, we consider the problem of real-time maximization of flocking velocity. By using local information and a Particle-Swarm-Optimization (PSO) algorithm, a Leader Agent (LA) actively motivates flocking at high speed. The LA manages topology of the network in its neighborhood and increases flocking velocity. PSO output quality and calculation costs show that the proposed optimization algorithm is practically feasible. A case-study is also presented  

Differential Evolution (DE) and Genetic Algorithms (GA) are population based search algorithms that come under the category of evolutionary optimization techniques. In the present study, these evolutionary methods have been utilized to conduct the optimum design of a fuzzy controller for mobile robot trajectory tracking. Comparison between their performances has also been conducted. In this paper, we will present a fuzzy controller to the problem of mobile robot path tracking for a CEDRA rescue robot. After designing the fuzzy tracking controller, the membership functions will be optimized by evolutionary algorithms in order to obtain more acceptable results  

This paper presents an unilateralization technique for distributed amplifiers (DAs) based on the transformer coupling between the gate and drain lines. Theoretical analysis of the DA indicates that the voltage waves in the gate and drain lines can be described by a system of linear partial differential equations. The transformer coupling between the lines allows for cancellation of the reverse transmission coefficient of the system. There is an optimal value for the coupling coefficient between the lines that unilateralizes the DA. This optimal coupling coefficient is derived in terms of the gate-drain capacitance and capacitances of the gate and drain lines. Using the proposed technique,... 

The inflow pattern of liquid into a droplet is studied experimentally using a surface active dye and compared with results of CFD simulations. The results show visual agreement between experiments and simulations. The CFD simulations show also good agreement with the surface tension measured by drop profile analysis tensiometry (PAT). The inflow of the surfactant induces a Marangoni instability caused by the local arrival of the surfactant at the drop surface. The onset of this Marangoni instability observed experimentally has a delay of about 10. s when compared with the simulation results. Different scenarios are discussed, including a boundary layer barrier, a kinetic-controlled... 

In this study, an efficient handover algorithm based on the received signal strength (RSS) prediction is presented for two-tier macro-femtocell networks in which, because of the fading effects of channel and short coverage range of femtocells, ping-pong handovers may take place. In the proposed approach, first each mobile station (MS) uses the recursive least square algorithm for predicting the RSS from the candidate base stations (BSs) including both femtocell and macrocell BSs. Then, according to the predicted RSS values, several future values of signal-to-interference plus noise ratio (SINR) are calculated. Afterwards, the candidate list of BSs is pruned according to the estimated future... 

Deployment of low-power low-cost small access points such as femtocell access points (FAPs) constitute an attractive solution for improving the existing macrocell access points' (MAPs) capacity, reliability, and coverage. However, FAP deployment faces challenging problems such as interference management and the lack of flexible frameworks for deploying the FAPs by the service providers (SPs). In this paper, we propose a novel solution that jointly associates the user equipment (UE) to the APs, and allocates the FAPs to the SPs such that the the total satisfaction of the UEs in an uplink OFDMA network is maximized. We model the competitive behaviors among the UEs, FAPs, and SPs as a dynamic... 

In this study, the authors consider the secrecy of a one-receiver, two-eavesdropper broadcast channel (BC) with three degraded message sets. Consider a three-receiver BC with three messages, where the first message is decoded by all the receivers. The second message is decoded by the first and the second receivers and is to be kept secret from the third receiver. The third message is decoded by the first receiver and is to be kept secret from the second and the third receivers. The authors consider the imperfect secrecy condition at the second receiver, that is, it is allowed to partially decode the third message. However, the perfect secrecy condition at the third receiver, does not allow... 

Localization is a crucial problem in wireless sensor networks and most of the localization algorithms given in the literature are non-adaptive and designed for fixed sensor networks. In this paper, we propose a learning based localization algorithm for mobile wireless sensor networks. By this technique, mobility in the network will be discovered by two crucial methods in the beacons: position and distance checks methods. These two methods help to have accurate localization and constrain communication just when it is necessary. The proposed method localizes the nodes based on connectivity information (hop count), which doesn't need extra hardware and is cost efficient. The experimental... 

In this paper, a methodology for transporting objects with a group of wheeled nonholonomic mobile manipulators is presented. A full dynamic model of a mobile manipulator with a three wheeled mobile base and a three DOF manipulator is derived using the Gibbs-Appell method. Since the dynamical equations of a mobile robot are highly nonlinear, an input-output linearization technique is used to control individual robots. Transporting the object is divided into two steps. First, the robots use a decentralized behavior-based method to approach and surround the object. Then, a virtual structure method is used to control the robots to transport the object cooperatively. A numerical simulation study... 

A 37 dBm power amplifier (PA) is designed on a 0.25 μm optical T-gate pseudomorphic high electron mobility transistor (pHEMT) technology. The design of this two-stage PA along with a step-by-step design procedure is presented in this paper. This methodology can be used for design of PA in different technologies and frequencies. The PA delivers 5 W output power over the frequency band of 13-19 GHz. It shows average power-added efficiency of 37% and large signal gain of 15 dB in measurements which is consistent with simulation results. The output power and efficiency of the realised amplifier reach maximums of 37.6 dBm and 45%, respectively. Considering output power, bandwidth, chip area and... 

An anatomically detailed eighteen-rotational-degrees-of-freedom model of the human spine using optimization constrained to equilibrium and stability requirements is developed and used to simulate several symmetric tasks in upright and flexed standing postures. Predictions of this stability and kinematics-driven (S. +. KD) model for trunk muscle forces and spine compressive/shear loads are compared to those of our existing kinematics-driven (KD) model where both translational and rotational degrees-of-freedom are included but redundancy is resolved using equilibrium conditions alone. Unlike the KD model, the S. +. KD model predicted abdominal co-contractions that, in agreement with... 

This paper considers the problem of digital data transmission through the Global System for Mobile communications (GSM). A data modem is presented that utilizes codebooks of Speech-Like (SL) symbols to transmit data through the GSM Adaptive Multi Rate (AMR) voice codec. Using this codebook of finite alphabet, the continuous vocoder channel is modeled by a Discrete Memory less Channel (DMC). A heuristic optimization algorithm is proposed to select codebook symbols from a database of observed human speech such that the capacity of DMC is maximized. Simulation results show that the proposed data modem achieves higher data rates and lower symbol error rates compared to previously reported... 

Mobile ad hoc networks (MANETs) have been turned into very attractive area of research in the duration of recent years, whereas security is the most challenging point that they undergo. Cryptography is an essential solution for providing security within MANETs. However, storing all keys in every node, if practically possible, is inefficient in large scale MANETs due to some limitations such as memory or process capability. This paper extends our previous idea which was a novel probabilistic key management algorithm that stores only a few randomly chosen keys instead of all ones. In this paper, several different scenarios are proposed for key selection in which they are more practical, and... 

This paper considers the problem of secure data communication through the Global System for Mobile communications (GSM). The algebraic codebook method for data transmission through the Adaptive Multi Rate 12.2Kbps voice channel is investigated and its maximum achievable data rate is calculated. Based on the vocoder channel properties, the method's Bit Error Rate (BER) performance is improved by repetition coding and classification methods. Simulation results show that by simultaneous application of repetition coding and clustering methods, the decoder's performance improves about 6.5% compared to the case of no clustering for 1Kbps data communication in AMR 4.75 voice codec