Fabrication of High Performance PM Nanocrystalline Bulk AA2124

AA2124 nanopowders <100 nm in particle size and 20 nm internal structure produced by high energy ball milling of gas-atomized micronpowders ~45 μm in particle size and 700 nm internal structure were processed in to bulk rods. The micro- and nanopowders were hot compacted using uniaxial pressing for preliminary densification at 0.7T _m of the alloy. Selected intact hot compacts (HCs) were promoted for warm severe plastic deformation via equal channel angular pressing (ECAP) at the minimum possible deforming temperature for final densification. Effect of the fabrication method of the consolidated powders was investigated. A combined processing via HC/ECAP produced bulk nanostructured rods 2.5 μm and 50-60 nm in grain size for the micro- and nanopowder consolidates, respectively. The powder properties controlled the degree of densification and mechanical behavior during the hot compaction stage, which influenced strongly the deformation behavior during subsequent ECAP. At the end of HC/ECAP one pass, the HC stage was responsible for about 83 and 95% of the total grain coarsening encountered for the micro- and nanopowder HCs, respectively. Throughout the various consolidation stages employed, the ball-milled (BM) nanopowder consolidates exhibited 2/3 the grain growth and displayed almost twice the hardness and compressive strength values of the gas-atomized micronpowder ones. Influence of BM and HC on the Al₂O₃ layer formed around the individual powder particles was also investigated.     

Diversity analysis for frequency-selective MIMO-OFDM systems with general spatial and temporal correlation model

In this paper, the effect of a general spatial and temporal fading correlation structure on the performance of coded multiple-input multiple-output (MIMO)-orthogonal frequency-division multiplexing (OFDM) systems is studied. The analysis handles an arbitrary joint transmit-receive spatial correlation model, including the non-Kronecker model. An upper bound on the maximum achievable diversity order for frequency-selective MIMO-OFDM systems with general temporal and spatial correlation is derived. Furthermore, a space-time-frequency code design that can achieve the upper bound for any arbitrarily correlated channel scenario is provided. The general framework of the analysis includes space-frequency (SF)-coded systems as a special case. For the SF-coded MIMO-OFDM system, it is shown that any SF code designed to achieve full diversity in the independent fading channel can achieve full diversity in an arbitrary spatially correlated channel. The derived analytical results are consistent with those in the existing literature for special correlation structures. Extensive simulation results are provided to confirm the theoretical analysis.     

A Leakage-Based Precoding Scheme for Downlink Multi-User MIMO Channels

In multiuser MIMO downlink communications, it is necessary to design precoding schemes that are able to suppress co-channel interference. This paper proposes designing precoders by maximizing the so-called signal-to-leakage-and-noise ratio (SLNR) for all users simultaneously. The presentation considers communications with both single- and multi-stream cases, as well as MIMO systems that employ Alamouti coding. The effect of channel estimation errors on system performance is also studied. Compared with zero-forcing solutions, the proposed method does not impose a condition on the relation between the number of transmit and receive antennas, and it also avoids noise enhancement. Simulations illustrate the performance of the scheme     

Transmit beamforming for space-frequency coded MIMO-OFDM systems with spatial correlation feedback

This paper addresses the problem of joint optimization of transmit beamforming and space-frequency (SF) coding for MIMO-OFDM systems with spatial correlation feedback in broadband communications. This problem is challenging in the sense that the transmitter should be designed to beamform across multiple eigenspaces associated with the multipath environment simultaneously. With arbitrary transmit spatial correlation, the performance analysis for SF-coded MIMO-OFDM systems with beamforming is provided, and a general optimization problem for the beamforming design is formulated. Three suboptimal approaches to design the beamformer based on the derived design criteria are proposed: i) eigenvalue selection scheme; ii) eigenspace selection scheme; and iii) per-subcarrier approach based on decoding at each subcarrier. The proposed schemes take into account the multiple eigenspace information associated with the multipath-delay channel. Improvement in the performance over SF coding without beamforming is shown through simulations in terms of bit error rate. The eigenvalue selection scheme provides the best performance among the proposed algorithms. This scheme locates the subspace associated with the largest eigenvalues in the eigenspace of the covariance matrices. With the eigenvalue selection scheme, the performance improvement is about 3 dB over the SF coding without beamforming for highly correlated channels as shown in our simulations.     

Cryptanalysis of Merkle-Hellman Cipher Using Parallel Genetic Algorithm

Mobile Networks and Applications - In 1976, Whitfield Diffie and Martin Hellman introduced the public key cryptography or asymmetric cryptography standards. Two years later, an asymmetric...     

Design of Si/SiGe heterojunction complementarymetal-oxide-semiconductor transistors

An optimized Si/SiGe heterostructure for complementary metal-oxide semiconductor (CMOS) transistor operation is presented. Unlike previous proposals, the design is planar and avoids inversion of the Si layer at the oxide interface. The design consists of a relaxed Si_0.7Ge _0.3 buffer, a strained Si quantum well (the electron channel), and a strained S_1-xGe_x (0.7>x>0.5) quantum well (the hole channel). The channel charge distribution is predicted using a 1-D analytical model and quantum mechanical solutions. Transport is modeled using 2-D drift-diffusion and hydrodynamic numerical simulations. An almost symmetric performance of p- and n-transistors with good short-channel behavior is predicted. Simulated ring oscillators show a 4- to 6-fold reduction in power-delay product compared to bulk Si CMOS at the 0.2-μm channel length generation     

Attraction Modulated by Spacing of Pheromone Components and Anti-attractants in a Bark Beetle and a Moth

Orientation for insects in olfactory landscapes with high semiochemical diversity may be a challenging task. The partitioning of odor plumes into filaments that are interspersed with pockets of ‘clean air’ may help filament discrimination and upwind flight to attractive sources in the face of inhibitory signals. We studied the effect of distance between odor sources on trap catches of the beetle, Ips typographus, and the moth, Spodoptera littoralis. Insects were tested both to spatially separated pheromone components [cis-verbenol and 2-methyl-3-buten-2-ol for Ips; (Z,E)-9,11-tetradecadienyl acetate and (Z,E)-9,12-tetradecadienyl acetate for Spodoptera], and to separated pheromone and anti-attractant sources [non-host volatile (NHV) blend for Ips; (Z)-9-tetradecenyl acetate for Spodoptera]. Trap catch data were complemented with simulations of plume structure and plume overlap from two separated sources using a photo ionization detector and soap bubble generators. Trap catches of the beetle and the moth were both affected when odor sources in the respective traps were increasingly separated. However, this effect on trap catch occurred at smaller (roughly by an order of magnitude) odor source separation distances for the moth than for the beetle. This may reflect differences between the respective olfactory systems and central processing. For both species, the changes in trap catches in response to separation of pheromone components occurred at similar spacing distances as for separation of pheromone and anti-attractant sources. Overlap between two simulated plumes depended on distance between the two sources. In addition, the number of detected filaments and their concentration decreased with downwind distance. This implies that the response to separated odor sources in the two species might take place under different olfactory conditions. Deploying multiple sources of anti-attractant around a pheromone trap indicated long-distance (meter scale) effects of NHV on the beetle and a potential use for NHV in forest protection.