Joint Optimization of User Set Selection and Transmit Power Allocation for Orthogonal Random Beamforming in Multiuser MIMO Systems

When the number of users is finite, the performance improvement of the orthogonal random beamforming (ORBF) scheme is limited in high signal‐to‐noise ratio regions. In this paper, to improve the performance of the ORBF scheme, the user set and transmit power allocation are jointly determined to maximize sum rate under the total transmit power constraint. First, the transmit power allocation problem is expressed as a function of a given user set. Based on this expression, the optimal user set with the maximum sum rate is determined. The suboptimal procedure is also presented to reduce the computational complexity, which separates the user set selection procedure and transmit power allocation procedure.     

40 GHz Vertical Transition with a Dual‐Mode Cavity for a Low‐Temperature Co‐fired Ceramic Transceiver Module

A new vertical transition between a substrate integrated waveguide in a low‐temperature co‐fired ceramic substrate and an air‐filled standard waveguide is proposed in this paper. A rectangular cavity resonator with closely spaced metallic vias is designed to connect the substrate integrated waveguide to the standard air‐filled waveguide. Physical characteristics of an air‐filled WR‐22 to WR‐22 transition are compared with those of the proposed transition. Simulation and experiment demonstrate that the proposed transition shows a ?1.3 dB insertion loss and 6.2 GHz bandwidth with a 10 dB return loss for the back‐to‐back module. A 40 GHz low‐temperature co‐fired ceramic module with the proposed vertical transition is also implemented. The implemented module is very compact, measuring 57 mm × 28 mm × 3.3 mm.     

A PIN diode controlled variable attenuator using a 0-dB branch-line coupler

We describe a simple PIN diode controlled variable attenuator that employs a 0-dB branch line directional coupler. The response of the attenuator was measured between 1.3 GHz and 2.6 GHz. At the center frequency, the attenuation monotonically varied from 0.7 dB to 23 dB with the control voltage, and the distributed branch-line coupler structure resulted in low input reflection. Our attenuator is easier to design, smaller in area than a double hybrid coupled attenuator, and has comparable or better reflection and attenuation performance characteristics.     

Effective Polysulfide Rejection by Dipole‐Aligned BaTiO3 Coated Separator in Lithium–Sulfur Batteries

Although the exceptional theoretical specific capacity (1672 mAh g^?1) of elemental sulfur makes lithium–sulfur (Li–S) batteries attractive for upcoming rechargeable battery applications (e.g., electrical vehicles, drones, unmanned aerial vehicles, etc.), insufficient cycle lives of Li–S cells leave a substantial gap before their wide penetration into commercial markets. Among the key features that affect the cyclability, the shuttling process involving polysulfides (PS) dissolution is most fatal. In an effort to suppress this chronic PS shuttling, herein, a separator coated with poled BaTiO₃ or BTO particles is introduced. Permanent dipoles that are formed in the BTO particles upon the application of an electric field can effectively reject PS from passing through the separator via electrostatic repulsion, resulting in significantly improved cyclability, even when a simple mixture of elemental sulfur and conductive carbon is used as a sulfur cathode. The coating of BTO particles also considerably suppresses thermal shrinkage of the poly(ethylene) separator at high temperatures and thus enhances the safety of the cell adopting the given separator. The incorporation of poled particles can be universally applied to a wide range of rechargeable batteries (i.e., metal‐air batteries) that suffer from cross‐contamination of charged species between both electrodes.     

Dynamic Response Modeling of MEMS Micromirror Corner Cube Reflectors With Angular Vertical Combdrives

A model for the dynamic response of microelectromechanical-systems micromirrors with angular vertical combdrive electrostatic actuators has been proposed based on a numerical solution of the nonlinear dynamic equations for torsional oscillation. The torque generated by the electrostatic drive is assumed to depend at least on the second order of the turn angle, and the model shows a shift in the resonant frequency with drive voltage, in agreement with the experimental results obtained from components fabricated by self-assembly of bonded silicon parts. The dynamic response of a corner cube reflector (CCR) based on a pair of scanning mirrors and a fixed mirror is predicted using the model, and the theoretical response agrees well with the experimental results. From the model, a small initial misalignment of the CCR mirrors can be determined     

Doping Effects on Thermoelectric Properties in the Mg2Sn System

A weak point of Mg₂X thermoelectrics is the absence of a p-type composition, which motivates research into the Mg₂Sn system. Mg₂Sn thermoelectrics were fabricated by a vacuum melting method and a spark plasma sintering process. As a result, Mg₂Sn single phases were acquired in a wide range of Mg-to-Sn atomic ratios (67:33 to 71:29), showing slightly different thermoelectric characteristics. However, the thermoelectric properties of the undoped system were not sufficient for application in commercial production. To maximize the p-type characteristics, many atoms [Ni (VIIIA), Cu (IB), Ag (IB), Zn (IIB), and In (IIIB)] were doped into the Mg₂Sn phase. Among them, the power factor values increased only in the Ag-doped case. Ag-doping resulted in a power factor that was more than 10 times larger than the value in the undoped case. This result could be important for developing p-type polycrystalline thermoelectrics in the Mg₂X (X?=?Si, Sn) system. However, other atoms [Ni (VIIIA), Cu (IB), Zn (IIB), and In (IIIB)] were not determined to act as acceptor atoms. The maximum ZT value for the Ag-doped Mg₂Sn thermoelectric was more than 0.18, which is comparable to the value for the n-type Mg₂Si system.     

Design of an MMIC Transversal Filter in MM-Wave Bands

A novel MMIC transversal filter structure using multiple-coupled-line directional couplers and its design method are proposed. There is a critical drawback in fabricating MMIC transversal filters because the length of the conventional transversal filter is much longer than the width. In order to solve this structural problem, a novel transversal filter which can easily be realized in MMIC type is proposed. The usefulness of the proposed transversal filter for MMIC applications is verified by the 3D full-wave analysis. The analysis results show the characteristics of low insertion loss, improved selectivity, and little bandwidth shrinkage.