Sharif Digital Repository

One technique to reduce variance of output in comparing two or more systems via simulation is random number synchronization. the primary technique for achieving synchronization is to assign an independent seed to each random process and then use the same collection of seeds across all systems . Another way of synchronization in systems with limited customer population is to assign different seed to each customer (transaction) and then use the same collection of seeds across all systems. The main Question to be considered here is which of these methods of synchronization can better reduce variance of output in a queue with finite customer population  

Chaotic behavior takes place in different physical systems such as chemical reactors. In this thesis, dynamical behavior of chemical reactors in which polymerization or series reactions take place has been considered. It has been shown that these reactors have chaotic dynamics under special condition. Two reactors of the second type (with series reactions) for cases of known and unknown parameters have been synchronized by a control method and stability of the proposed scheme has been established by the Lyapunov stability theorem. Simulation results show the effectiveness of proposed method. Since concentration measurement is difficult and expensive, an observer has been used for... 

Time offset (TO) at an orthogonal frequency division multiplexing (OFDM) system destroys orthogonality of subcarriers and causes interchannel interference (ICI) and inter symbol interference (ISI). This problem is more challenging in multi-user multi-input multi-output (MU-MIMO) OFDM systems such as coordinated multi-point (CoMP) uplink transmission where joint processing is performed at base stations (BSs) and different delays form users to BSs cannot be compensated by timing strategies. In this thesis, we use delay-amplify and forward relays and post processing at receivers to decrease signal to interference plus noise ratio (SINR) degradation due to large TOs at insufficient cyclic prefix... 

In recent years, there have been increasing attempts to study brain connectivity. Among a number of brain mapping techniques, Electroencephalography is an easy to use and cheap method that can be used in the study of brain function. One way of understanding the intricate wiring pattern and functions of brain is to consider it as a complex network. In this approach, a graph of brain functions, based on the functional relation of recorded electric signals, is constructed and then the network is evaluated with a number of network metrics that measure its different aspect of structure. Different neurological and psychological diseases can affect the connectivity power within the brain; as a... 

The long term evolution (LTE) standard is introduced and developed by the 3rd generation partnership project (3GPP) based on orthogonal frequency division multiplexing (OFDM). OFDM systems, in spite of having many advantages such high performance in bandwidth usage, are very sensitive to inter carrier interference (ICI) as a drawback. In order to prevent ICI, the frequency offset, mainly caused by the miss-match between oscillators' frequency and also between the sampling frequency of the transmitter and the receiver, should be estimated and compensated. Frequency synchronization is a part of the tasks of a synchronizer. In this thesis, main tasks of a synchronizer are illustrated and... 

Ultra-wideband (UWB) receivers have long been utilizied as a low-power solution for many applications, namely positioning, sensor network, and radar systems. As their name suggests, owing to the ultra-wide bandwidth nature of these receivers, which cover frequencies from 3.1GHz all the way to 10GHz, they entail many challenges at all levels of abstraction. Two of these most important challenges are in-band interferers, e.g., IEEE802.11a, and jitter. Because of the low-power operation of UWB receivers, with PSD of <-41.3dBm/Hz, the majority of the UWB RX’s function is based on the correlation principle, making it vulnerable to jitter. In recenet years, a new attention has been paid toward the... 

It has been shown that there is a relationship between the topological and spectral properties of a graph and its dynamics. However, most of the studies have not considered the direction of the links, even though most of the real networks are directed. In this thesis, we study the intrinsic relationship between the graphs’ structures, the spectral characteristics of their associated matrices, and the synchronization implications for different directed random graphs by changing their link directionality. To this end we make use of Kuramoto model for modeling the synchronization dynamics of the network. We show that, in contrast to the results of studies which suggest the λ2/λN as a measure of... 

In this thesis, the problem of locating a source based on the time difference of arrival (TDOA) has been studied and three different methods have been presented to improve the precision of the positioning. The first method is a solution to the least square error (LSE) by utilizing a convex problem. Generally, the mentioned problem is a non-convex problem; however, it is possible to convert it to a convex one by some relaxation in the constraints. By some justifiable and insightful relaxation, not only we obtain a convex problem, but also the exact solution of the main problem is derived. The second method is using high perturbed TDOA measurements which severely affected the localization.... 

In this thesis, Model reference adaptive control in fractional order systems was discussed. Various types of this problem can be introduced based on fractional order dynamics in system equations, reference model equations or adaptation laws, which some of them play more important role in real-world applications, also some of them have more complexity in governing mathematical equations and in stability analysis. Fractional order dynamics were recently used in many real-world applications like model reference control, Chaos synchronization and Chaos control, so studying adaptive control in this fields, could be important. Nowadays, Model reference adaptive control was used in many industrial... 

Ultra-wideband (UWB) frequencies with low level of power spectral density, which is set by FCC standard, have long been utilized in many short range applications, namely sensor networks, indoor positioning and radar systems. As their name suggests, the ultra-wide bandwidth of this standard, covering frequencies from 3.1 to 10.6GHz, provides high data rate communications. Two of the most critical challenges toward UWB receiver design is narrow-band interference coexistence, including IEEE802.11a and jitter due to the very narrow width of the UWB pulse in time domain. General previously proposed solutions to receive these narrow pulses is based on correlation principle, which is very sensitive... 

Neural systems are the threshold ones. It means if the electrical potential passed through a specific amount then they could spike and cause the activity of other cells. The models that really mimic the action of neurons are usually complicated and are not suitable when you put them on a network to study the collective behavior of the neurons. Simple threshold models have been designed for such purposes. One on most studied ones is the ’integrate and fire’ model, in which cells integrate the inputs until the threshold potential and then spike. usually, a network of these objects are simulated and different properties of such network are investigated.However, this model has some shortcomings... 

Excitability is one of the most important characteristics of a neuron. In 1948, Hodgkin identified three different types of excitability of neurons. Excitability an all of its types can be observed in Hodgkin-Huxley model of neuronal dynamics (H-H model) as a four-dimensional system of differential equations and in at least two dimensional reductions of H-H type models. By applying the theory of dynamical systems (e.g. bifurcation theory), one can give a mathematical definition of excitability.Excitability of the neuron is equivalent to that the neuronal model is near a bifurcation through which the state of the system approaches to a stable limit cycle. In this thesis, a two-dimensional... 

Centuries passed and the human knew himself as the protagonist who searches around nature and discovers the phenomena. But after the birth of ``neuroscience", his wisdom and the process of reasoning were also added to the list of uncovered subjects. Since its arrival, many scientists started investigating ``reasoning", "sleep", ``memory disorders" etc. with a such framework. One of the main branches of this stream is the ``Synchronization" problem when the neurons get synced in the matter of spiking likelihood. ``Synchronization" means a lot to the community, because it is said that it is one major symptom of Epilepsy. With that said, we need to get to the root of this effect. It seems...