Dependence of diffusivity on density and solute diameter in liquid phase: a molecular dynamics study of Lennard-Jones system

Investigations into the variation of self-diffusivity with solute radius, density, and degree of disorder of the host medium is explored. The system consists of a binary mixture of a relatively smaller sized solute, whose size is varied and a larger sized solvent interacting via Lennard-Jones potential. Calculations have been performed at three different reduced densities of 0.7, 0.8, and 0.933. These simulations show that diffusivity exhibits a maximum for some intermediate size of the solute when the solute diameter is varied. The maximum is found at the same size of the solute at all densities which is at variance with the prediction of the levitation effect. In order to understand this anomaly, additional simulations were carried out in which the degree of disorder has been varied while keeping the density constant. The results show that the diffusivity maximum gradually disappears with increase in disorder. Disorder has been characterized by means of the minimal spanning tree. Simulations have also been carried out in which the degree of disorder is constant and only the density is altered. The results from these simulations show that the maximum in diffusivity now shifts to larger distances with decrease in density. This is in agreement with the changes in void and neck distribution with density of the host medium. These results are in excellent agreement with the predictions of the levitation effect. They suggest that the effect of disorder is to shift the maximum in diffusivity towards smaller solute radius while that of the decrease in density is to shift it towards larger solute radius. Thus, in real systems where the degree of disorder is lower at higher density and vice versa, the effect due to density and disorder have opposing influences. These are confirmed by the changes seen in the velocity autocorrelation function, self part of the intermediate scattering function and activation energy.     

Group-metric multiuser decoding

We propose the new group metric (GM) soft-decision decoder for convolutionally coded synchronous multiple-access channels. The GM decoder exploits the independently operating encoders of the multiuser channel by making decoding decisions for a subset of the users, but incorporating all the multiuser information in its metrics. For a single user, this decoder will have a reduced complexity that is exponential in the sum of encoder memory and the number of users. The soft-decision maximum-likelihood (ML) joint decoder is well known. This optimal decoder suffers from a high complexity requirement that is exponential in the product of encoder memory and the number of users. The size of the decoded subset is a design parameter which allows a tradeoff between complexity and performance. The performance of the GM decoder, once properly characterized, can be analyzed using standard techniques. In addition, a new analysis technique is presented which considers decomposable sequences for the fading channel. With this analysis, we have a new tool for bounding error probabilities for multiuser decoders. Applying this technique to the GM decoder, we can directly identify sequences that are decomposable some fraction of the time, and obtain a new upper bound. Further, this improved bound can be expressed in closed form. Numerical results show that the actual performance gap between the GM and ML decoders can be quite small     

Beamforming, diversity, and interference rejection for multiuser communication over fading channels with a receive antenna array

We consider M-ary communication with K users over a space diversity channel, consisting of a single transmit antenna for each user and multiple receive antennas. We examine two different flat fading models, namely, phase coherent wavefront fading and noncoherent element-to-element fading. In the case of wavefront fading, the fade is constant across the face of the receive antenna and we can associate an angle of arrival to the signal. We present a variation of the MUSIC algorithm for estimating this parameter and use it to form a spatial beam. In the case of noncoherent element-to-element fading, the fading path to each sensor is different (although possibly correlated) and no angle of arrival can be exploited for conventional beamforming. For each channel model, we develop several detection strategies which assume various amounts of prior information about the fading. We then consider blind extensions of these detectors based on subspace tracking, which do not require a prior model for the interfering users' signals.     

Short‐Fluorinated iCVD Coatings for Nonwetting Fabrics

Water repellency is often generated by taking advantage of surface textures and low surface energy coatings such as the one afforded by long perfluorinated side‐chains polymers. However, new regulations are phasing out these polymers because of their related health and safety hazard concerns. This is a particular challenge for water‐repellent fabrics as consumers expect safer products with stable performance and new functionalities. In this work, an approach is developed that allows for iCVD deposition of durable, conformal short fluorinated polymers stabilized with a crosslinking agent. As a result, high hydrophobicity and low liquid adhesion are achieved simultaneously while maintaining initial substrate breathability. It is explained why this polymeric coating—1H,1H‐perfluorooctyl methacrylate co divinylbenzene—exhibits remarkable hydrophobic properties amidst a wide range of other possible candidates. In order to further enhance the dynamic water repellency performance, the chemical treatment is combined with physical texturing—obtained through microsandblasting, a process particularly suitable for fabrics—thus making this combined approach a suitable candidate to meet the industrial needs. This work paves the way for the development of environmentally friendly, highly repellent coatings for large volume production and the application of roll‐to‐roll coating techniques, and multifunctionalization of fabrics and wearable devices.     

On the limitation of generalized Welch-bound equality signals

This correspondence characterizes the performance limitation of the well-acclaimed generalized Welch-bound equality (WBE) signals under linear minimum mean-squared error (MMSE) detection. It is analytically proven, and experimentally verified, that when users in an overloaded but finite-size code-division multiple-access (CDMA) system are allocated such signals, the error rate of at least one user "floors" (i.e., it cannot be driven to zero even in the absence of additive noise), independently of the symbol energies. If, in addition, all users are received with equal energy, then the error rate of every user "floors".     

Signal design for bandwidth-efficient multiple-accesscommunications based on eigenvalue optimization

Bandwidth-efficient multiple access (BEMA) is a strategy where transmitter pulses are continually designed at the base station and are dynamically allocated to the transmitters via a feedback channel. Such pulses (or “signature waveforms”) are designed to conserve bandwidth while simultaneously enabling the receiver at the base station to meet a quality-of-service (QoS) specification for each transmitter. The key technical problem in BEMA communication is therefore the design of the transmitter pulses for the base station receiver. In an earlier paper, we presented solutions to this problem that were shown to be superior (in terms of strict bandwidth) to common signaling schemes such as time-, frequency-, and code-division multiple access (TDMA, FDMA, and CDMA). This paper uses the framework developed earlier, but considers strictly time-limited transmitter pulses and the root-mean squared (RMS) bandwidth measure. As in the earlier paper, significant bandwidth savings over the traditional multiple-access strategies are obtained. However, in contrast to the rank-conserving approach, the bandwidth gains of this paper are realized by tailoring the signature waveform design to conserve RMS bandwidth via eigenvalue optimization problems     

Lead-free 0201 manufacturing, assembly and reliability test results

The trend towards smaller, faster and cheaper electronic devices has led to an increase in the use of 0201 (L  0.02 in.; W  0.01 in.) and even smaller sized passive components. The size advantages of the 0201 component make it a popular choice among design engineers but not among manufacturing engineers. From a manufacturing perspective, the size of the 0201 package poses significant challenges to the printed circuit board (PCB) assembly process. The many challenges with 0201 assembly can be attributed to the solder paste volume, pad design, aperture design, board finish, type of solder paste, pick-and-place and reflow profile. If these factors are not optimized, they will introduce undesirable manufacturing defects. The small size of 0201 packages and undetected manufacturing defects will also raise concerns about their second level interconnect reliability, especially for lead-free solder alloys and surface finishes, with new processes and higher reflow requirements. To determine the optimum conditions, a design-of-experiment (DOE) study was carried out to investigate the effects of these parameters on assembly defects and solder joint reliability.This paper presents the test results and comparative literature data on the influence of a few key manufacturing parameters and defects associated with the 0201 component using lead-free and tin–lead solder alloys. Data pertaining to component shear strength before and after isothermal aging at 150 °C and intermetallic growth up to 500 h of aging are presented. A number of test vehicles were also subjected to thermal cycling (1500 cycles) in the range of −55/100 °C to determine the solder fatigue behavior. Shear test results for test vehicles subjected to thermal cycling is also presented. In addition, optical microscopy analysis of solder joint behavior during thermal cycling showing the progress of the solder damage and cross-sectional photos taken at 1500 cycles is included.     

SIGMA: a VLSI systolic array implementation of a Galois field GF(2m) based multiplication and division algorithm

Finite or Galois fields are used in numerous applications like error correcting codes, digital signal processing and cryptography. The design of efficient methods for Galois field arithmetic such as multiplication and division is critical for these applications. A new algorithm based on a pattern matching technique for computing multiplication and division in GF(2^m) is presented. An efficient systolic architecture is described for implementing the algorithm which can produce a new result every clock cycle and the multiplication and division operations can be interleaved. The architecture has been implemented using 2-μm CMOS technology. The chip yields a computational rate of 33.3 million multiplications/divisions per second     

Multiuser detection for overloaded CDMA systems

Multiuser detection for overloaded code-division multiple-access (CDMA) systems, in which the number of users is larger than the dimension of the signal space, is of particular interest when bandwidth is at a premium. In this paper, certain fundamental questions are answered regarding the asymptotic forms and performance of suboptimum multiuser detectors for cases where the desired and/or interfering signal subspaces are of reduced rank and/or have a nontrivial intersection. In the process, two new suboptimum detectors are proposed that are especially well suited to overloaded systems, namely, the group pseudo-decorrelator and the group minimum mean-squared error (MMSE) detector. The former is seen to be the correct extension of the group decorrelator in the sense that it is the limiting form (in the low-noise regime) of the group MMSE detector. Pseudo-decorrelation is also used as a feedforward filter in a new decision feedback scheme. For the particular case of real-valued modulation, it is shown that the proposals of the so-called "improved" linear (also known as "linear-conjugate" or "widely linear") detectors were more simply derived earlier using the idea of minimal sufficiency, which we also apply to the new detectors of this paper.