68 research outputs found
Optimal power allocation for MIMO-OFDM based Cognitive Radio systems with arbitrary input distributions
In Cognitive Radio (CR) systems, the data rate of the Secondary User (SU) can be maximized by optimizing the transmit power, given a threshold for the interference caused to the Primary User (PU). In conventional power optimization algorithms, the Gaussian input distribution is assumed, which is unrealistic, whereas the Finite Symbol Alphabet (FSA) input distribution, (i.e., M-QAM) is more applicable to practical systems. In this paper, we consider the power optimization problem in multiple input multiple output orthogonal frequency division multiplexing based CR systems given FSA inputs, and derive an optimal power allocation scheme by capitalizing on the relationship between mutual information and minimum mean square error. The proposed scheme is shown to save transmit power compared to its conventional counterpart. Furthermore, our proposed scheme achieves higher data rate compared to the Gaussian optimized power due to fewer number of subcarriers being nulled. The proposed optimal power algorithm is evaluated and compared with the conventional power allocation algorithms using Monte Carlo simulations. Numerical results reveal that, for distances between the SU transmitter and the PU receiver ranging between 50m to 85m, the transmit power saving with the proposed algorithm is in the range 13-90%, whereas the rate gain is in the range 5-31% depending on the modulation scheme (i.e., BPSK, QPSK and 16-QAM) used
Reducing the Peak to Average Power Ratio of LDS-OFDM signals
Low Density Signature-Orthogonal Frequency Division Multiplexing (LDS-OFDM) has been introduced recently as an efficient multiple access technique. High Peak to Average Power Ratio (PAPR) is an important obstacle to multicarrier communication systems. This paper concentrates on the PAPR investigation and its reduction for LDS-OFDM signals. Specifically, we will investigate the impact of subcarrier allocation schemes and the phases of the signatures on the PAPR of LDS-OFDM signals. Firstly, the PAPR of LDS-OFDM with conventional signatures is investigated. Then we propose two methods for PAPR reduction; Newman phases and DFT pre-coding. The former method is simple and doesn't imply changes in the system structure while the DFT pre-coding implies a modification in the system. Simulation results show that using Newman phases considerably reduces the PAPR of LDS-OFDM. Further PAPR reduction is achieved using DFT pre-coded LDS-OFDM on the cost of higher complexity
Optimum Power Allocation for OFDM Based Cognitive Radio Systems with Arbitrary Input Distributions
In the literature, optimal power assuming Gaussian input has been evaluated in OFDM based Cognitive Radio (CR) systems to maximize the capacity of the secondary user while keeping the interference introduced to the primary user band within tolerable range. However, the Gaussian input assumption is not practical and Finite Symbol Alphabet (FSA) input distributions, i.e., M-QAM are used in practical systems. In this paper, we consider the power optimization problem under the condition of FSA inputs as used in practical systems, and derive an optimal power allocation strategy by capitalizing on the relationship between mutual information and minimum mean square error. The proposed scheme is shown to save transmit power in a CR system compared to its conventional counterpart, that assumes Gaussian input. In addition to extra allocated power, i.e., power wastage, the conventional power allocation scheme also causes nulling of more subcarriers, leading to reduced transmission rate, compared to the proposed scheme. The proposed optimal power algorithm is evaluated and compared with the conventional algorithm assuming Gaussian input through simulations. Numerical results reveal that for interference threshold values ranging between 1 mW to 3 mW, the transmit power saving with the proposed algorithm is in the range between 55-75%, 42-62% and 12-28% whereas the rate gain is in the range between 16.8-12.4%, 13-11.8% and 3-5.8% for BPSK, QPSK and 16-QAM inputs, respectively
An Extended Model for the Evolution of Prebiotic Homochirality: A Bottom-Up Approach to the Origin of Life
A generalized autocatalytic model for chiral polymerization is investigated
in detail. Apart from enantiomeric cross-inhibition, the model allows for the
autogenic (non-catalytic) formation of left and right-handed monomers from a
substrate with reaction rates and , respectively. The
spatiotemporal evolution of the net chiral asymmetry is studied for models with
several values of the maximum polymer length, N. For N=2, we study the validity
of the adiabatic approximation often cited in the literature. We show that the
approximation obtains the correct equilibrium values of the net chirality, but
fails to reproduce the short time behavior. We show also that the autogenic
term in the full N=2 model behaves as a control parameter in a chiral symmetry-
breaking phase transition leading to full homochirality from racemic initial
conditions. We study the dynamics of the N -> infinity model with symmetric
() autogenic formation, showing that it only achieves
homochirality for , where is an N-dependent
critical value. For we investigate the behavior of
models with several values of N, showing that the net chiral asymmetry grows as
tanh(N). We show that for a given symmetric autogenic reaction rate, the net
chirality and the concentrations of chirally pure polymers increase with the
maximum polymer length in the model. We briefly discuss the consequences of our
results for the development of homochirality in prebiotic Earth and possible
experimental verification of our findings
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Joint user selection and multi-mode scheduling in multicell MIMO cellular networks
This paper considers multicell scheduling for coordinated
MIMO cellular networks. In a multicell scenario, intercell
interference, which is the major challenge degrading system
performance, can be suppressed via multicell coordination. However,
when the number of users is larger than that can be served,
user/multi-mode selection is necessary, and can significantly enhance
the performance in such interference-limited environment.
Nevertheless, user selection in a multicell scenario incurs high
computational complexity and overhead signalling. To this end,
in this paper, low complexity greedy algorithms are proposed for
user selection. The algorithms are based on the concept of angle
between subspaces and intersection of null spaces, and work in
partially-distributed fashion with limited information exchange.
By these algorithms, channel feedback can be greatly reduced
with reasonable computational complexity. In addition, multimode
scheduling schemes are proposed for this multicell set up,
where the spatial streams are adaptively selected. The proposed
algorithms are analysed and compared with the optimal solution
obtained by exhaustive search. Simulation results show that the
algorithms achieve most of the throughput achievable by the
optimal solution
Chiral Polymerization in Open Systems From Chiral-Selective Reaction Rates
We investigate the possibility that prebiotic homochirality can be achieved
exclusively through chiral-selective reaction rate parameters without any other
explicit mechanism for chiral bias. Specifically, we examine an open network of
polymerization reactions, where the reaction rates can have chiral-selective
values. The reactions are neither autocatalytic nor do they contain explicit
enantiomeric cross-inhibition terms. We are thus investigating how rare a set
of chiral-selective reaction rates needs to be in order to generate a
reasonable amount of chiral bias. We quantify our results adopting a
statistical approach: varying both the mean value and the rms dispersion of the
relevant reaction rates, we show that moderate to high levels of chiral excess
can be achieved with fairly small chiral bias, below 10%. Considering the
various unknowns related to prebiotic chemical networks in early Earth and the
dependence of reaction rates to environmental properties such as temperature
and pressure variations, we argue that homochirality could have been achieved
from moderate amounts of chiral selectivity in the reaction rates.Comment: 15 pages, 6 figures, accepted for publication in Origins of Life and
Evolution of Biosphere
Toward homochiral protocells in noncatalytic peptide systems
The activation-polymerization-epimerization-depolymerization (APED) model of
Plasson et al. has recently been proposed as a mechanism for the evolution of
homochirality on prebiotic Earth. The dynamics of the APED model in
two-dimensional spatially-extended systems is investigated for various
realistic reaction parameters. It is found that the APED system allows for the
formation of isolated homochiral proto-domains surrounded by a racemate. A
diffusive slowdown of the APED network such as induced through tidal motion or
evaporating pools and lagoons leads to the stabilization of homochiral bounded
structures as expected in the first self-assembled protocells.Comment: 10 pages, 5 figure
Punctuated Chirality
Most biomolecules occur in mirror, or chiral, images of each other. However,
life is homochiral: proteins contain almost exclusively levorotatory (L) amino
acids, while only dextrorotatory (R) sugars appear in RNA and DNA. The
mechanism behind this fundamental asymmetry of life remains an open problem.
Coupling the spatiotemporal evolution of a general autocatalytic polymerization
reaction network to external environmental effects, we show through a detailed
statistical analysis that high intensity and long duration events may drive
achiral initial conditions towards chirality. We argue that life's
homochirality resulted from sequential chiral symmetry breaking triggered by
environmental events, thus extending the theory of punctuated equilibrium to
the prebiotic realm. Applying our arguments to other potentially life-bearing
planetary platforms, we predict that a statistically representative sampling
will be racemic on average.Comment: 13 pages, 4 color figures. Final version published in Origins of Life
and Evolution of Biospheres. Typos corrected, figures improved, and a few
definitions and word usage clarifie
Causes of Mortality for Indonesian Hajj Pilgrims: Comparison between Routine Death Certificate and Verbal Autopsy Findings
10.1371/journal.pone.0073243PLoS ONE88-POLN
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