66,722 research outputs found
On f(R) theories in two-dimensional spacetime
In recent years, theories in which the Einstien-Hilbert lagrangian is
replaced by a function f(R) of the Ricci Scalar have been extensively studied
in four-dimensional spacetime. In this work we carry out an analysis of such
theories in two-dimensional spacetime with focus on cosmological implications.
Solutions to the cosmological field equations are obtained and their properties
are analysed. Inflationary solutions are also obtained and discussed.
Quantization is then carried out, the Wheeler-DeWitt equation is set up and its
exact solutions obtained
The Riddle of Gravitation
There is no doubt that both the special and general theories of relativity
capture the imagination. The anti-intuitive properties of the special theory of
relativity and its deep philosophical implications, the bizzare and dazzling
predictions of the general theory of relativity: the curvature of spacetime,
the exotic characteristics of black holes, the bewildering prospects of
gravitational waves, the discovery of astronomical objects as quasers and
pulsers, the expansion and the (possible) recontraction of the universe..., are
all breathtaking phenomena. In this paper, we give a philosophical
non-technical treatment of both the special and the general theory of
relativity together with an exposition of some of the latest physical theories.
We then give an outline of an axiomatic approach to relativity theories due to
Andreka and Nemeti that throws light on the logical structure of both theories.
This is followed by an exposition of some of the bewildering results
established by Andreka and Nemeti concerning the foundations of mathematics
using the notion of relativistic computers. We next give a survey on the
meaning and philosophical implications of the the quantum theory and end the
paper by an imaginary debate between Einstein and Neils Bohr reflecting both
Einstein's and Bohr's philosophical views on the quantum world.
The paper is written in a somewhat untraditional manner; there are too many
footnotes. In order not to burden the reader with all the details, we have
collected the more advanced material the footnotes. We think that this makes
the paper easier to read and simpler to follow. The paper in full is adressed
more to experts.Comment: 40 pages, LaTeX-fil
Analysis of AC link topologies in non-isolated DC/DC triple active bridge converter for current stress minimization
This paper presents analysis of the non-isolated DC/DC triple active bridge (TAB) converter under various purely inductor-based AC link topologies. The objective of the analysis is to find the topology that incorporates the least value of the AC link inductors which leads to reduced converter footprint in addition to minimum internal current stresses. Modelling of the TAB under each of the different topologies is presented in per unit expressions of power transfer and reactive power assuming fundamental harmonic analysis. The power expressions are used to calculate the inductor values necessary to achieve same rated power transfer of Dual Active Bridge (DAB) converter for the sake of standardizing comparison. On this basis, the topology requiring the least value of interface inductors, hence lowest footprint, is identified. Furthermore, based on phase shift control, particle swarm optimization (PSO) is used to calculate optimal phase shift ratios in each of the proposed topologies to minimize reactive power loss (hence current stress). The topology with minimum stresses is therefore identified and the results are substantiated using a Matlab-Simulink model to verify the theoretical analysis
Generic closed loop controller for power regulation in dual active bridge DC-DC converter with current stress minimization
This paper presents a comprehensive and generalized analysis of the bidirectional dual active bridge (DAB) DC/DC converter using triple phase shift (TPS) control to enable closed loop power regulation while minimizing current stress. The key new achievements are: a generic analysis in terms of possible conversion ratios/converter voltage gains (i.e. Buck/Boost/Unity), per unit based equations regardless of DAB ratings, and a new simple closed loop controller implementable in real time to meet desired power transfer regulation at minimum current stress. Per unit based analytical expressions are derived for converter AC RMS current as well as power transferred. An offline particle swarm optimization (PSO) method is used to obtain an extensive set of TPS ratios for minimizing the RMS current in the entire bidirectional power range of - 1 to 1 per unit. The extensive set of results achieved from PSO presents a generic data pool which is carefully analyzed to derive simple useful relations. Such relations enabled a generic closed loop controller design that can be implemented in real time avoiding the extensive computational capacity that iterative optimization techniques require. A detailed Simulink DAB switching model is used to validate precision of the proposed closed loop controller under various operating conditions. An experimental prototype also substantiates the results achieved
Full-Duplex Systems Using Multi-Reconfigurable Antennas
Full-duplex systems are expected to achieve 100% rate improvement over
half-duplex systems if the self-interference signal can be significantly
mitigated. In this paper, we propose the first full-duplex system utilizing
Multi-Reconfigurable Antenna (MRA) with ?90% rate improvement compared to
half-duplex systems. MRA is a dynamically reconfigurable antenna structure,
that is capable of changing its properties according to certain input
configurations. A comprehensive experimental analysis is conducted to
characterize the system performance in typical indoor environments. The
experiments are performed using a fabricated MRA that has 4096 configurable
radiation patterns. The achieved MRA-based passive self-interference
suppression is investigated, with detailed analysis for the MRA training
overhead. In addition, a heuristic-based approach is proposed to reduce the MRA
training overhead. The results show that at 1% training overhead, a total of
95dB self-interference cancellation is achieved in typical indoor environments.
The 95dB self-interference cancellation is experimentally shown to be
sufficient for 90% full-duplex rate improvement compared to half-duplex
systems.Comment: Submitted to IEEE Transactions on Wireless Communication
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