166 research outputs found
Slotted ALOHA in Intelligent Transport Systems
No full textThe performance of a mobile communication system on motorways, using the slotted ALOHA multiple access protocol, has been investigated. The system has been modelled by assuming a capture receiver at the base station, a channel with a variable Rice factor, path loss and oxygen absorption. The system was assumed to operate solely in one cell, containing a strip of 200m. of a three lane motorway. The distribution of the vehicle positions on the motorway-strip has been taken into account. Performance is expressed in throughput and delay. The performance of the system has been investigated for three traffic types: light traffic, heavy traffic and queuing traffic. The effect of power diversity, a performance enhancing technique, has been investigated. The conclusions are that the slotted ALOHA protocol can function properly when the system is not overloaded. Multipath fading and oxygen absorption have no effect on the system performance. The performance enhancing effect of power diversity is small.Electrical Engineering, Mathematics and Computer ScienceTelecommunicatie- en Verkeersbegeleidingssysteme
Utilidad de la resonancia magnetica para el diagnostico de cardiopatia isquemica, en presencia de bloqueo completo de rama izquierda
No full text
Aplicación del método de resonancia magnética cardiovascular en el estudio de las cardiopatías congénitas
No full textLa resonancia magnética cardiovascular (RMCV) es el método de imagen no invasivo ideal para el diagnóstico de las cardiopatías congénitas (CC) ya que ofrece importante información anatómica y funcional en el mismo estudio. Este método es útil para llevar un seguimiento a largo plazo del paciente con cardiopatía congénita. Las principales ventajas de la RMCV consisten en que es un estudio de imagen no invasivo y en que está exento de radiación ionizante. El objetivo de este artículo es el demostrar que las cardiopatías congénitas pueden ser diagnosticadas mediante la resonancia magnética con una mayor precisión comparada con otros métodos de imagen cardiovascular
Capture effects in spread-aloha packet protocols.
No full textThesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.Research in the field of random access protocols for narrow-band systems started as early
as the 1970s with the introduction of the ALOHA protocol. From the research done in
slotted narrow-band systems, it is well known that contention results in all the packets
involved in the contention being unsuccessful. However, it has been shown that in the
presence of unequal power levels, ore of the contending packets may be successful. Ibis
is a phenomenon called capture. Packet capture has been shown to improve the
performance of slotted narrow-band systems.
Recently, much work has been done in the analysis of spread-spectrum ALOHA type
code-division multiple access (CDMA) protocols. The issue of designing power control
techniques to improve the performance of CDMA systems by reducing multiple access
interference (MAl) has been a subject of much research. It has been shown that in the
presence of power control schemes, the performance of spread-ALOHA CDMA systems
is improved. However, it is also widely documented that the design of power control
schemes capable of the ideal of compensation of radio propagation techniques is not
possible for various reasons, and hence the imperfections in power control.
None of the research known to the author has looked at capture in spread-ALOHA
systems, and to a greater extent, looked at expressions for the performance of spreadALOHA
systems in the presence of capture. In this thesis we introduce spread-ALOHA systems with capture as a manifestation of the
imperfections in power control. We propose novel expressions for the computation of the
perfonnance ofspread-ALOHA systems with capture
A simulation of mobile slotted-aloha networks with correlated shadow attenuation during retransmission
No full textThe performance of a slotted-ALOHA network is investigated for correlated shadow attenuation during retransmission by means of software simulation. If terminals are stationary rather than mobile, the path attenuation during each transmission of a terminal is identical. The major part of all retransmissions is thus received with relatively weak power. This effect is shown to degrade the network performance significantly.Applied SciencesElectrical EngineeringTelecommunications and Traffic Control Systems Grou
Effects of error correction and error detection coding in a narrowband fast fading slotted-Aloha network with BPSK modulation
No full textIn this thesis, the effects of generic codes on throughput and packet error probabilities in a mobile radio channel with multiple users will be examined. Data packets are transmitted in a random-access mobile radio channel, assuming Rayleigh fading, interference, and Gaussian noise. We address the slotted-Aloha protocol...Applied SciencesElectrical EngineeringTelecommunications and Traffic Control Systems Grou
Stability of mobile slotted ALOHA networks
No full textIt is well known that, on a mobile data communication link, the received power fluctuates due to various propagation effects. The main goal of this report is to investigate the effects of these signal fluctuations on the stability and performance of slotted ALOHA networks. For this purpose, a discrete Markov model is used. This network model provides insight on queueing in mobile communications. In contrast to previously developed models, both dynamic behaviour and detailed propagation models are examined simultaneously. The propagation aspects considered include multipath fading, shadowing and power attenuation due to the distance between transmitter and receiver. A simulation program is written to examine the influence of the spatial distribution of traffic on the stability and performance of the network. This simulation is also used to examine a control mechanism to prevent instability of the network. It is confirmed that capture enhances throughput and stability. In the domain of traffic parameters, areas are found where a typical network is unstable. It is concluded that the assumption of a uniform distribution of offered traffic leads to optimistic estimates for network performance. Further, it is concluded that control procedures can prevent networks from becoming unstable. Regrettably, the performance of stable and unsaturated networks is degraded by a control mechanism.Electrical Engineering, Mathematics and Computer ScienceTelecommunicatie- en Verkeersbegeleidingssysteme
Optimal controlled ALOHA for two-way data communication in a cable television network
No full textBased on a unified analysis of both pure and slotted ALOHA systems, a new control algorithm for ALOHA systems is proposed. A feature of great practical importance is the algorithm's automatic adjustment to changes in average traffic intensity or the (finite or infinite) number of active stations in the system. In addition, the algorithm has a low-complexity implementation. Computer simulations, concentrating on the use for two-way data communication in a cable television network, have demonstrated that the practical performance of the algorithm closely approximates the theoretical optimum, even under extremely heavy traffic load conditions. Furthermore, dynamic performance simulations have shown that the algorithm assures swift recovery from overload situation
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