Multiple-Level Power Allocation Strategy for Secondary Users in Cognitive Radio Networks

In this paper, we propose a multiple-level power allocation strategy for the secondary user (SU) in cognitive radio (CR) networks. Different from the conventional strategies, where SU either stays silent or transmit with a constant/binary power depending on the busy/idle status of the primary user (PU), the proposed strategy allows SU to choose different power levels according to a carefully designed function of the receiving energy. The way of the power level selection is optimized to maximize the achievable rate of SU under the constraints of average transmit power at SU and average interference power at PU. Simulation results demonstrate that the proposed strategy can significantly improve the performance of SU compared to the conventional strategies.Comment: 12 page

The F-Landscape: Dynamically Determining the Multiverse

We evolve our Multiverse Blueprints to characterize our local neighborhood of the String Landscape and the Multiverse of plausible string, M- and F-theory vacua. Building upon the tripodal foundations of i) the Flipped SU(5) Grand Unified Theory (GUT), ii) extra TeV-Scale vector-like multiplets derived out of F-theory, and iii) the dynamics of No-Scale Supergravity, together dubbed No-Scale F-SU(5), we demonstrate the existence of a continuous family of solutions which might adeptly describe the dynamics of distinctive universes. This Multiverse landscape of F-SU(5) solutions, which we shall refer to as the F-Landscape, accommodates a subset of universes compatible with the presently known experimental uncertainties of our own universe. We show that by secondarily minimizing the minimum of the scalar Higgs potential of each solution within the F-Landscape, a continuous hypervolume of distinct minimum minimorum can be engineered which comprise a regional dominion of universes, with our own universe cast as the bellwether. We conjecture that an experimental signal at the LHC of the No-Scale F-SU(5) framework's applicability to our own universe might sensibly be extrapolated as corroborating evidence for the role of string, M- and F-theory as a master theory of the Multiverse, with No-Scale supergravity as a crucial and pervasive reinforcing structure.Comment: 15 Pages, 7 Figures, 1 Tabl

Chemistry and radiative shielding in star forming galactic disks

To understand the conditions under which dense, molecular gas is able to form within a galaxy, we post-process a series of three-dimensional galactic-disk-scale simulations with ray-tracing based radiative transfer and chemical network integration to compute the equilibrium chemical and thermal state of the gas. In performing these simulations we vary a number of parameters, such as the ISRF strength, vertical scale height of stellar sources, cosmic ray flux, to gauge the sensitivity of our results to these variations. Self-shielding permits significant molecular hydrogen (H2) abundances in dense filaments around the disk midplane, accounting for approximately ~10-15% of the total gas mass. Significant CO fractions only form in the densest, n>~10^3 cm^-3, gas where a combination of dust, H2, and self-shielding attenuate the FUV background. We additionally compare these ray-tracing based solutions to photochemistry with complementary models where photo-shielding is accounted for with locally computed prescriptions. With some exceptions, these local models for the radiative shielding length perform reasonably well at reproducing the distribution and amount of molecular gas as compared with a detailed, global ray tracing calculation. Specifically, an approach based on the Jeans Length with a T=40K temperature cap performs the best in regards to a number of different quantitative measures based on the H2 and CO abundances.Comment: 21 Pages, 15 figures. Submitted to MNRAS. Comments welcom

Outage minimisation in wireless relay networks with delay constraints and causal channel feedback

Motivated by delay‐sensitive information transmission applications, we solve an optimal power allocation problem with a K‐block delay constraint on data transmission using a cooperative relay network assuming a block fading channel model. Channel information is fed back to the transmitter only in a causal fashion, so that the optimal power allocation strategy is only based on the current and past channel gains. We consider the two simplest schemes for information transmission using a three node (a source, a relay and a destination) relay network, namely the amplify and forward (AF) and decode and forward (DF) protocols. We use a dynamic programming (DP) based methodology to solve a (K‐block delay constrained) general expected cost optimisation problem with a short term (over K blocks) sum power (total transmission power of the source and the relay) constraint. By specialising the cost function appropriately, we solve the delay constrained outage minimisation problem in this paper. We also propose a simple but sub‐optimal power allocation scheme based on a high signal to noise ratio (SNR) approximation, which is computationally much less demanding than the DP‐based optimal method. Extensive numerical results are presented for Rayleigh and Rician fading channels, including results demonstrating the performance gain obtained by optimally allocating the (sum of source and relay) power to the different blocks as opposed to equally distributing the total power across all blocks. The accuracy of the high SNR approximation based power allocation scheme is also illustrated

Lifetime Optimization for Wireless Sensor Networks with Outage Probability Constraints

Due to the limited energy resources in a wireless sensor network (WSN), lifetime of a WSN is a key parameter. In this paper, we address a lifetime optimization problem of a wireless TDMA/CDMA sensor network for joint transmit power and rate allocations in a time-varying fast Rayleigh fading environment. The effect of fast fading is captured by including rate outage and link outage constraints on each link and a given time-slot. A resulting nonconvex problem is then reduced to an approximate convex optimization problem using an appropriate functional approximation and variable substitutions. This centralized problem is then solved by standard barrier-method based optimization algorithms. A partially distributed algorithm is also provided to illustrate how most of the computations can be done locally at each node in a decentralized manner. The novelty of the paper lies in considering fast fading channels via outage probability constraints for the first time in lifetime maximization problems and obtaining a better convex approximation than previously used approximations in the literature. Simulation results illustrate that our centralized algorithm results in optimal power and rate allocations that result in a substantially extended lifetime of the WSN compared to previously published algorithms. The convergence of the partially distributed algorithms to the optimal power and rate solutions is also illustrated

Lifetime Optimization for Wireless Sensor Networks with Outage Probability Constraints

Due to the limited energy resources in a wireless sensor network (WSN), lifetime of a WSN is a key parameter. In this paper, we address a lifetime optimization problem of a wireless TDMA/CDMA sensor network for joint transmit power and rate allocations in a time-varying fast Rayleigh fading environment. The effect of fast fading is captured by including rate outage and link outage constraints on each link and a given time-slot. A resulting nonconvex problem is then reduced to an approximate convex optimization problem using an appropriate functional approximation and variable substitutions. This centralized problem is then solved by standard barrier-method based optimization algorithms. A partially distributed algorithm is also provided to illustrate how most of the computations can be done locally at each node in a decentralized manner. The novelty of the paper lies in considering fast fading channels via outage probability constraints for the first time in lifetime maximization problems and obtaining a better convex approximation than previously used approximations in the literature. Simulation results illustrate that our centralized algorithm results in optimal power and rate allocations that result in a substantially extended lifetime of the WSN compared to previously published algorithms. The convergence of the partially distributed algorithms to the optimal power and rate solutions is also illustrated

Infall, Fragmentation and Outflow in Sgr B2

Observations of H$_{2}$CO lines and continuum at 1.3 mm towards Sgr B2(N) and Sgr B2(M) cores were carried out with the SMA. We imaged H$_{2}$CO line absorption against the continuum cores and the surrounding line emission clumps. The results show that the majority of the dense gas is falling into the major cores where massive stars have been formed. The filaments and clumps of the continuum and gas are detected outside of Sgr B2(N) and Sgr B2(M) cores. Both the spectra and moment analysis show the presence of outflows from Sgr B2(M) cores. The H$_{2}$CO gas in the red-shifted outflow of Sgr B2(M) appears to be excited by a non-LTE process which might be related to the shocks in the outflow.Comment: 5 pages, 3 figures, Published in J. Physics Conference Serie

New stopping criteria for segmenting DNA sequences

We propose a solution on the stopping criterion in segmenting inhomogeneous DNA sequences with complex statistical patterns. This new stopping criterion is based on Bayesian Information Criterion (BIC) in the model selection framework. When this stopping criterion is applied to a left telomere sequence of yeast Saccharomyces cerevisiae and the complete genome sequence of bacterium Escherichia coli, borders of biologically meaningful units were identified (e.g. subtelomeric units, replication origin, and replication terminus), and a more reasonable number of domains was obtained. We also introduce a measure called segmentation strength which can be used to control the delineation of large domains. The relationship between the average domain size and the threshold of segmentation strength is determined for several genome sequences.Comment: 4 pages, 4 figures, Physical Review Letters, to appea