Cooperative Communications in Resource-Constrained Wireless Networks

Cooperative communications have been proposed to exploit the spatial diversity gains inherent in multiuser wireless systems without the need of multiple antennas at each node. This is achieved by having the users relay each others messages and thus forming multiple transmission paths to the destination. In resource constrained networks, such as wireless sensor networks, the advantages of cooperation can be further exploited by optimally allocating the energy and bandwidth resources among users based on the available channel state information (CSI) at each node. In the first part of this article, we provide a tutorial survey on various power allocation strategies for cooperative networks based on different cooperation strategies, optimizing criteria, and CSI assumptions. In the second part, we identify the similarities between cooperative networks and several sensor network applications that utilize collaboration among distributed sensors to achieve the system goal. These applications include decentralized detection/estimation and data gathering. The techniques developed in cooperative communications can be used to solve many sensor network problems     

An optimal replenishment policy for items with inventory-level-dependent demand and fixed lifetime under the LIFO policy

In a recent paper, Hwang and Hahn considered inventory replenishment problems for an item with an inventory-level-dependent demand rate and a fixed lifetime. They developed an EQQ model under the situation of considering the first-in–first-out (FIFO) issuing policy. First, this paper reconsiders Hwang and Hann's problem by employing the last-in–first-out (LIFO) issuing policy, which is more practical in the retail industry. An inventory model is developed. Secondly, the concavity of the objective function is proved. Thirdly, this paper presents conditions where the present model has a unique optimal solution and a method for finding the global optimal solution. A simple solution procedure and sensitivity analyses of parameters are also provided.     

M1 gamma strength for zirconium nuclei in the photoneutron channel

Photoneutron cross sections were measured for 91Zr, 92Zr, and 94Zr near the neutron separation energy with quasimonochromatic gamma rays. The data exhibit some extra components around the neutron threshold. A coherent analysis of the photoneutron data for 92Zr together with the neutron capture on 91Zr based on the microscopic Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation model for the E1 strength has revealed the presence of an M1 resonance at 9 MeV. The microscopic approach systematically shows the same M1 strength in the photoneutron cross section for 91Zr and 94Zr. The total M1 strength is about 75% larger than the strength predicted by the systematics, being qualitatively consistent with the giant M1 resonance observed in the inelastic proton scattering.     

Supply chain coordination for newsvendor-type products with two ordering opportunities and demand information update

This paper studies a coordination issue with two ordering opportunities in a two-echelon supply chain, where one manufacturer sells a single newsvendor-type product through one buyer. The manufacturer does not hold inventory and activates production or order with an infinite capacity and a fixed setup cost in response to the buyer’s order. The buyer places two orderings during the selling period of the product: one happens at the beginning of the period and the other at some specified time within the selling period. The whole selling period is divided into two stages or sub-periods by the buyer’s second order. The stochastic demands in the two sub-periods are assumed to be auto-correlated. The excess demand before the second order is partially backordered, whereas the excess demand at the end of the selling season is utterly lost. Under both the centralized and decentralized settings, we develop the models of how the buyer determines his two-ordering policies. We analyse the existence and uniqueness of the optimal solutions to the models and present the corresponding analytical solutions. Furthermore, we propose an improved revenue-sharing contract that can realize the perfect coordination of the supply chain and study how the revenue-sharing policies affect the supply chain members’ decisions. Finally, we show the superiority of the presented two-ordering strategy through numerical examples.     

Direct observation of defect levels in InN by soft X-ray absorption spectroscopy

We have used synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to study the electronic structure of nitrogen-related defects in InN(0001). Several defect levels within the band gap or the conduction band of InN were clearly resolved in NEXAFS spectra around the nitrogen K-edge. We attribute the level observed at 0.3 eV below the conduction band minimum (CBM) to interstitial nitrogen, the level at 1.7 eV above the CBM to antisite nitrogen, and a sharp resonance at 3.2 eV above the CBM to molecular nitrogen, in full agreement with theoretical simulations.     

Quantitative Determination of Lignan Constituents from Schisandra chinensis by Liquid Chromatography

The fruits of Schisandra chinensis (Schisandraceae/Magnoliaceae) are a traditional oriental medicine possessing diverse biological activities. A simple and specific analytical method for the quantitative determination of eight lignan constituents from the methanolic extract of the fruits of Schisandra chinensis was developed. The lignan constituents present in the fruits of Schisandra chinensis were separated with an acetonitrile-water-reagent alcohol gradient at a flow rate of 1.0 mL per minute. The HPLC separation was performed on a Phenomenex Luna C18 (2) (150 × 4.6 mm I.D., particle size 5 μm) reversed phase column with detection at 215 nm. The limit of detection was in the range from 0.2 to1.5 μg mL^?1. The relative standard deviation (RSD) values for the determination of lignan constituents in fruits of Schisandra chinensis were less than 2.0%. The method was successfully used to analyze different products available in the market containing Schisandra chinensis and also to study the percentage compositions of various lignans present in Schisandra chinensis procured from different regions in S. Korea.     

Determination of the optimal trade credit policy: a supplier-Stackelberg model

This paper considers a two-echelon supply chain, where one supplier sells through a retailer a product with a stable market demand. We focus on how the supplier induces the retailer through trade credit to order more to reduce his/her own inventory-related cost. Under a ‘supplier-Stackelberg’ setting, we provide the supplier with the method of determining two trade credit scenarios: unconditional and conditional trade credit. We show that the unconditional trade credit scenario is always beneficial to the retailer but harmful to the supplier in most situations, while the conditional trade credit scenario is always beneficial to both parties. In addition, we specify the conditions under which the provision of unconditional trade credit is beneficial to the supplier. The three insights obtained in this paper are the following: (i) When the retailer’s per-unit opportunity cost is less than his/her per-unit opportunity gain, unconditional trade credit can induce the retailer to order less instead of more. (ii) If the supplier offers the retailer unconditional trade credit, the length of trade credit offered will have an upper bound. (iii) A well-designed conditional trade credit policy can realize a win-win outcome but also enables the supplier to occupy all the savings in the channel's cost incurred by trade credit, but any unconditional trade credit policy does not.     

Direct observation of localized defect States in semiconductor nanotube junctions

We report scanning tunneling microscopy of semiconductor-semiconductor carbon nanotube junctions with different band gaps. Characteristic features of the wave functions at different energy levels, such as a localized defect state, are clearly exhibited in the atomically resolved scanning tunneling spectroscopy. The peaks of the Van Hove singularity on each side penetrate and decay into the opposite side across the junction over a distance of approximately 2 nm. These experimental features are accounted for, with the help of tight-binding calculation, by the presence of pentagon-heptagon pair defects at the junction.     

Measurement of scattering rate and minimum conductivity in graphene

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of 1-20x10(3) cm2/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from 2-15x10(11) cm(-2). In the low carrier density limit, the conductivity exhibits values in the range of 2-12e2/h, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short-range disorder whereas low mobility samples are dominated by long-range scatterers.     

A simple technique for improved performance of intracavity Fabry-Perot frequency selectors

By placing an intracavity Fabry-Perot transmission etalon between quarter-wave plates it is possible to operate it at normal incidence without spurious oscillation due to reflections from the etalon. The insertion loss and reduced finesse associated with a tilted etalon are thereby avoided. This technique will allow the use of longer etalons thus allowing much greater mode selectivity.