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Vehicular Ad hoc NETworks (VANETs), an emerging technology, use vehicles as nodes to form a mobile ad hoc network for the dissemination of safety and entertainment messages. The thesis provides a scheme for the fast propagation of messages in VANETs and evaluates the impact of vehicles as obstacles on signal propagation. An improved scheme for intermediate node selection in DBA-MAC (Dynamic Backbone Assisted MAC) is proposed, which consists of a CW (Contention Window) constraint scheme and an updated criterion of suitability. A performance comparison shows that messages in the proposed scheme propagate faster than in DBA-MAC. The CW constraint scheme is also applicable in VANET protocols which adapt the CW mechanism to communicate the suitability of vehicles for acting as intermediate nodes. Additionally, the mathematical models for DBAMAC and the proposal are established, which indicate the probability of candidates to be chosen over alternatives in the intermediate node selection. A novel metric - delay taking into account the effect of formation time(DEFT) - is proposed. DEFT combines the network formation time and propagation delay. It shows the impact of network formation on propagation latency. The configuration for optimal performance can be acquired using the proposed DEFT. In order to evaluate the proposals, a novel distribution of vehicle location is proposed. In the proposed distribution, the security distances between adjacent vehicles in the same lane are considered. The estimation of vehicles’ location can be more practical and accurate using the proposed distribution. In the wide body of the VANET literature, it is assumed that all the vehicles within the radio range of a transmitter are able to receive the signal. Yet, in practice, the vehicles as obstacles between the transmitter and the receiver affect the signal propagation significantly. This thesis presents the impact of these obstacles on the network connectivity and system performance of the protocols. The results and the analysis show that neglecting obstacles in practice leads to a significant degree of error in the estimation of system performance. In practice, tall vehicles forward messages in a more efficient way than do lower vehicles since they are free from the obstacle effect. An improved scheme is proposed, in which the height of vehicles is used as a factor to determine their suitability for message forwarding.

The components necessary to propagate a synthetic ionic signal are described, and experiments leading to the required experimental system are the focus of this work. Two thiol-derivatized fluorescent probe molecules were synthesized that balanced both electrochemical and fluorescent properties necessary for trace analysis. Self-assembled monolayers (SAMs) of 11-(1-1’-biphenyl-4-yloxy)-1-undecanethiol were formed on Au/glass slides by open-circuit incubation and potential-assisted adsorption methods. A potentiostat was built capable of producing current responses on the microsecond time-scale. Monolayer integrity was established by two methods: cyclic voltammetry and chronoamperometry. Monolayers formed under potential-assisted adsorption conditions showed attenuation of the peak current due to Fe(CN)6 3−/4− redox probe in cyclic voltammetry, indicating a tightly packed monolayer. Chronoamperometric studies also confirmed the monolayer integrity by fitting the current response of a potential-step to an equivalent circuit. The chronoamperometric study was dependent on solvent and electrolyte. In water, the difference between bare Au and monolayer protected Au was large, whereas in DMF, the difference was negligible. Likewise, the use of tetra-butyl ammonium hexafluorophosphate as the electrolyte showed little difference between bare Au and monolayer protected Au. The electrochemical reduction of the SAMs was done in various solvents and electrolytes and the products were analysed by HPLC with fluorescent detection. Along the series of solvents from water to MeCN to DMF the current efficiencies for release increased but still were very low. In water and MeCN, the thiol was the sole detectable product, while in DMF, the sole detected product was the disulfide. Reproducibility of release was poor in MeCN and water, probably due to the low solubility of the thiol. Single-channel analysis of two acyclic bola-amphiphiles (diester and diamide) was done to establish their feasibility as components of a synthetic signal propagation system. Channels from the diester derivative have a Na+ conductance of 10.2 pS and a Cs+ conductance of 39.3 pS. Channels from the diester have a Cs+/Na+ permeability ratio of 4.7, Cs+/Cl- permeability ratio of 7.5 and a Na+/Cl- permeability ratio of 3.1. Channels of the diester bola-amphiphile have two lifetimes; 117 ms and 842 ms at -100 mV, 1 M CsCl electrolyte and DiPhyPC lipid at 25 °C. Similarly, Channels from the diamide derivative have a Na+ conductance of 10.3 pS and a Cs+ conductance of 38.9 pS. Chaimels of the diamide have a Cs+/Na+ permeability ratio of 5.2, Cs+/Cl- permeability ratio of 7.2 and a Na+/Cl- permeability ratio of 2.1. The diamide bola-amphiphile channels have a lifetime of 277 ms at +100 mV, 1 M CsCl electrolyte and DiPhyPC lipid at 25 °C. Both channels show a regular non-uniform step-conductance pattern. The sublevel openings, when graphically represented with lifetime data, show the trend that the lower conductance states of one-level openings are also the shorter-lived channels. A traceless linker to release alcohols from a gold surface was developed. Thiobutyric acid was found to undergo intramolecular thiolactone formation after electrochemical reduction from an Au-electrode to liberate the alcohol. A thiobutyric ester at the C-terminus of gramicidin was synthesized. This compound released gramicidin by chemical reduction with DTT as seen by HPLC analysis and MALDI TOF MS. The electrochemical release of the Au-immobilized thiobutyric ester of gramicidin adjacent to a lipid bilayer, as monitored by bilayer clamp technique, produced an increase in channel activity that is consistent with incorporation of gramicidin. / Graduate

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No description available.

Η παρούσα εργασία περιλαμβάνει την μελέτη και τον χαρακτηρισμό του καναλιού ενός δορυφορικού καναλιού καθώς και την επίγεια διάδοση Αρχικά γίνεται μια εισαγωγή στα ασύρματα συστήματα τηλεπικοινωνιών . Ειδικά παρουσιάζεται το δορυφορικό σύστημα και αναλύονται τα μέρη αυτού. Γίνεται παρουσίαση των δορυφορικών ζεύξεων και μελετάται το περιβάλλον μέσα στο οποίο λαμβάνει χώρα μια σύνδεση. Αρχικά αναλύονται οι μέθοδοι διάδοσης της ηλεκτρομαγνητικής ακτινοβολίας και οι διάφορες εξασθενήσεις που υφίσταται το σήμα λόγω φαινομένων της ατμόσφαιρας καθώς και άλλων επιδράσεων. Γίνεται έπειτα μια εκτενής αναφορά στις διαλείψεις και στην ταξινόμησή τους. Στη συνέχεια γίνεται μια κατηγοριοποίηση των διαλείψεων αυτών με βάση τα χαρακτηριστικά τους. Ακολουθεί η στατιστική αναπαράσταση των καναλιών με διαλείψεις χρησιμοποιώντας τις βασικότερες κατανομές που συναντώνται στα τηλεπικοινωνιακά συστήματα. / This work includes the study and channel characterization for a satellite signal and also its terrestrial propagation. First we introduce the wireless telecommunications systems. We focus on the satellite system and its components are analyzed. A presentation of the satellite links follows and the propagation environment is studied. We analyze the propagation methods of electromagnetic radiation and the various losses that the signal endures due to atmospheric phenomena and other effects. There is then a reference to the fading channel and and its analysis. Then there is categorization of the fading according to its characteristics. The statistical representation of the fading channel follows, with the use of the key distributions found in telecommunication systems. Lastly, the satellite transmission is simulated for various orbit heights and conditions

Cell assemblies are considered to be physiological as well as functional units in the brain. A repetitive and stereotypical sequential activation of many neurons was observed, but the mechanisms underlying it are not well understood. Feedforward networks, such as synfire chains, with the pools of excitatory neurons unidirectionally connected and facilitating signal transmission in a cascade-like fashion were proposed to model such sequential activity. When embedded in a recurrent network, these were shown to destabilise the whole network’s activity, challenging the suitability of the model. Here, we investigate a feedforward chain of excitatory pools enriched by inhibitory pools that provide disynaptic feedforward inhibition. We show that when embedded in a recurrent network of spiking neurons, such an augmented chain is capable of robust signal propagation. We then investigate the influence of overlapping two chains on the signal transmission as well as the stability of the host network. While shared excitatory pools turn out to be detrimental to global stability, inhibitory overlap implicitly realises the motif of lateral inhibition, which, if moderate, maintains the stability but if substantial, it silences the whole network activity including the signal. Addition of a disinhibitory pathway along the chain proves to rescue the signal transmission by transforming a strong inhibitory wave into a disinhibitory one, which specifically guards the excitatory pools from receiving excessive inhibition and thereby allowing them to remain responsive to the forthcoming activation. Disinhibitory circuits not only improve the signal transmission, but can also control it via a gating mechanism. We demonstrate that by manipulating a firing threshold of the disinhibitory neurons, the signal transmission can be enabled or completely blocked. This mechanism corresponds to cholinergic modulation, which was shown to be signalled by volume as well as phasic transmission and variably target classes of neurons. Furthermore, we show that modulation of the feedforward inhibition circuit can promote generating spontaneous replay at the absence of external inputs. This mechanism, however, tends to also cause global instabilities. Overall, these results underscore the importance of inhibitory neuron populations in controlling signal propagation in cell assemblies as well as global stability. Specific inhibitory circuits, when controlled by neuromodulatory systems, can robustly guide or block the signals and invoke replay. This mounts to evidence that the population of interneurons is diverse and can be best categorised by neurons’ specific circuit functions as well as their responsiveness to neuromodulators.

This research solves current RF propagation modeling gaps by modifying a single-frequency electromagnetic propagation analysis technique for use on communication signals and propagation channels. This research extended the Methods of Ordered Multiple Interactions (MOMI) algorithm to communication signal propagation studies through the use of Fourier decomposition thereby allowing the analysis and prediction of communication signals propagating over rough surfaces. Current methods of predicting and analyzing communication signal propagation rely on either using only a single frequency instead of a band of frequencies, stochastic techniques that model the environmental effect on the propagated signal, or on empirical models based of large amounts of measured situational data. None of these methods fully capture the actual effect that an environment imparts on a communication signal as it propagates. This research also modifies the Physical Optics (PO) algorithm utilizing Fourier decomposition to compare the Extended MOMI algorithm to. Both algorithms are applied to propagation scenarios utilizing frequencies in the 1-GHz and 5-GHz bands against a series of signal bandwidths and surface roughnesses. The results are analyzed singularly for Extended-MOMI and against Extended-Physical Optics to better understand the benefits associated with using the Extended-MOMI, the limits of the narrowband approximation, the errors incurred when utilizing a simpler or faster propagation algorithm, and to generally characterize these rough surface propagation channels. This research also defines and explores which metrics provide the best characterization and utility for communication signal propagation with the additional insights of amplitude-frequency-phase relationships the new algorithm provides. / Doctor of Philosophy / Communication signal propagation, defined as the propagation of signals that have non-zero bandwidths from one point to another, has significant importance in communication signal design, system design, and deployment as well as in spectrum planning applications. Current methods of predicting and analyzing communication signal propagation rely on either using only a single frequency instead of a band of frequencies, stochastic techniques that model the environmental effect on the propagated signal, or on empirical models based of large amounts of measured situational data. None of these methods fully capture the actual effect that an environment imparts on a communication signal as it propagates. A technique that accurately models the environmental effect on propagating communication signals would result in knowledge about a communication signal strength and shape as it passes through the propagation space. Analyzing communication signals with single frequency propagation algorithms requires assuming all the frequencies that make up the communication signal propagate exactly the same way, an assumption known as the narrowband approximation. It is not known when the narrowband approximation breaks down in various circumstances. Consequently a more rigorous approach needed to be identified to enable a more accurate and complete analysis of communication signals, which is the objective of the research. This research solves these modeling gaps by modifying a single-frequency electromagnetic propagation analysis technique, the Method of Ordered Multiple Interactions, for use on communication signals and propagation channels. The new algorithm, Extended-MOMI, allows for an examination of communication signal propagation over rough surfaces. This new algorithm incorporates all of the information needed for communication signal propagation analysis; something that is missing from current methods. This technique enables tailored communication signal propagation studies as well as an investigations into when the narrowband assumption is valid and when simpler and faster algorithms could be utilized for a now known increase in error. This research also explores which metrics are best utilized with the additional signal information the new algorithm enables.

Este trabalho trata do problema de alocação de estações rádio base (ERBs) para o sistema de telefonia celular de 4G, que no Brasil utiliza o protocolo LTE (Long Term Evolution). Tal problema consiste em dada uma determinada região geográfica, onde se encontram os possíveis clientes, dispor antenas de modo a cobrir a maior área possível da região em estudo, levando em consideração a capacidade de cada antena em atender os clientes com qualidade de serviço. O algoritmo apresentado calcula o raio de alcance da ERB, a quantidade mínima de ERBs necessárias para cobrir a região em estudo e a localização de cada ERB. Para que o algoritmo pudesse ser desenvolvido foi investigado o sistema de comunicação LTE, modelos de propagação de sinal além do algoritmo memético, visto que a alocação de ERBs é um problema NP-difícil. Para o raio de ação da célula foi considerado, além do modelo de propagação, o calculo de link budget, throughput e relação sinal ruído. Por fim, uma comparação entre o LTE operando nas faixas de frequências de 700 MHz e 2,5 GHz foi realizado. O algoritmo de alocação de ERBs se mostrou eficiente cobrindo mais de 80% da área de estudo em 29 dos 30 casos analisados. Com relação a frequência, o LTE se mostrou mais adequado operando em 700 MHz pois a quantidade de ERBs para cobertura da área de estudo é menor se comparado a frequências de 2,5 GHz. / This work deals with the issue of radio base stations (RBSs) allocation for the 4G cell phone system, which in Brazil uses the LTE (Long Term Evolution) protocol. Such problem consists in a certain geographical region, where potential customers might be found, having antennas to cover the largest possible area of the region under study, taking into account the capacity of each antenna to serve customers with quality of service. The presented algorithm calculates the range of the RBS station, the minimum amount of necessary RBS to cover the area under study and the location of each RBS. In order to the algorithm to be developed the LTE communication system was investigated, signal propagation models beyond memetic algorithm, since the RBS allocation is a NP-hard problem. For the cell’s range of action it was considered, besides the model of propagation, the link budget calculation, throughput and noise signal relation. Therefore, a comparison between LTE operating on 700 MHz and 2,5 GHz frequencies was made. The RBS allocation algorithm was efficient covering more than 80% of the study area in 29 from the 30 analyzed cases. In relation to the frequency, LTE was considered more adequate operating on 700 MHz, for the quantity of RBS to cover the study area is smaller, if compared to 2,5 GHz frequencies.

Recentemente observou-se experimentalmente, Johnson et al. (2010), que fatias organotípicas corticais de rato são capazes de completar padrões espaço-temporais, após serem treinadas. Embora se especule que mecanismos de plasticidade sináptica e homeostática estejam por trás do fenômeno, ainda não existe nenhuma explicação detalhada sobre o assunto. Com o intuito de propor uma explicação clara e consistente para os mecanismos que permeiam a resposta da rede aos estímulos como um todo, nos propomos a estudar este fenômeno por meio de uma rede de neurônios de integração-e-disparo dotada de mecanismos de homeostase intrínseca e de plasticidade sináptica dependente de disparos. O sistema construído foi explorado, de modo a determinar em que condições a rede poderia comportar-se como o sistema real, e treinado de forma similar `a realizada experimentalmente por Johnson et al. (2010). / Recently it has been observed experimentally, Johnson et al. (2010), that organotypic cortical slices of rat are capable of completing spatio-temporal patterns after training. Although it is speculated that synaptic and homeostatic plasticity may have an important role in this phenomenon, there is still no detailed explanation about this subject. In order to propose a clear and consistent explanation for the mechanisms that underlie the network response to stimuli as a whole, we propose to study this phenomenon through a network of integrate-and-fire neurons endowed with intrinsic homeostasis and spike-timing dependent plasticity mechanisms. The constructed system was explored, aiming to determine in which conditions the network could behave as the real system, and trained in a way similar as the experimental one done by Johnson et al. (2010).