Consolidation of high melting SiO2-Al2O3-La2o3 glass powders with gas pressure vacuum viscous sintering technology

Rare earth (RE) doped silica glasses are widely used as gain medium in optical fibers, providing high efficiency, broad working bandwidth, and high mechanical strength for high power fiber laser applications. However, some of the compositions are limited to the melting temperature of Pt crucibles at about 1700 °C. Therefore, an alternative method is proposed to prepare bulk glasses from high melting crystalline powders by two steps: the vitrification of the crystalline powder and a subsequent consolidation process. Consequently, the GPWS procedure was developed for the consolidation/densification step: to consolidate the already vitrified amorphous powders. In this work, one high melting SAL glass composition has been taken as the model glass. With the here developed viscous stretching measurement as well as the FEM Simulation results, the viscosity and surface tension of the SAL glass at high temperature have been determined. With these parameters, the range of working parameters for this glass composition has been determined using the viscous sintering theory. Finally the GPWS SAL glasses have been characterized in five aspects of inhomogeneities: crystallization, knots, striae, absorption groups and bubbles. Compared to normal vacuum sintering, the GPWS method sinters in higher rate and is able to compensate the loss of sintering rate by using large particles. It provides the ability to suppress bubble formation at high temperature or to close bubbles when the entrapped gases are diffusive. Compared to the SAL glasses prepared from conventional melting, the GPWS SAL glasses are free of OH absorption groups as well as bubbles

Channel Estimation for Reconfigurable Intelligent Surface-Aided Multiuser Communication Systems Exploiting Statistical CSI of Correlated RIS-User Channels

Reconfigurable intelligent surface (RIS) is a promising candidate technology for the upcoming Sixth Generation (6G) communication system for its ability to manipulate the wireless communication environment by controlling the coefficients of reflection elements (REs). However, since the RIS usually consists of a large number of passive REs, the pilot overhead for channel estimation in the RIS-aided system is prohibitively high. In this paper, the channel estimation problem for a RIS-aided multi-user multiple-input-single-output (MISO) communication system with clustered users is investigated. First, to describe the correlated feature for RIS-user channels, a beam domain channel model is developed for RIS-user channels. Then, a pilot reuse strategy is put forward to reduce the pilot overhead and decompose the channel estimation problem into several subproblems. Finally, by leveraging the correlated nature of RIS-user channels, an eigenspace projection (EP) algorithm is proposed to solve each subproblem respectively. Simulation results show that the proposed EP channel estimation scheme can achieve accurate channel estimation with lower pilot overhead than existing schemes.Comment: 10 pages, 11 figure

Near-Field Integrated Sensing, Positioning, and Communication: A Downlink and Uplink Framework

A near-field integrated sensing, positioning, and communication (ISPAC) framework is proposed, where a base station (BS) simultaneously serves multiple communication users and carries out target sensing and positioning. A novel double-array structure is proposed to enable the near-field ISPAC at the BS. Specifically, a small-scale assisting transceiver (AT) is attached to the large-scale main transceiver (MT) to empower the communication system with the ability of sensing and positioning. Based on the proposed framework, the joint angle and distance Cram\'er-Rao bound (CRB) is first derived. Then, the CRB is minimized subject to the minimum communication rate requirement in both downlink and uplink ISPAC scenarios: 1) For downlink ISPAC, a downlink target positioning algorithm is proposed and a penalty dual decomposition (PDD)-based double-loop algorithm is developed to tackle the non-convex optimization problem. 2) For uplink ISPAC, an uplink target positioning algorithm is proposed and an efficient alternating optimization algorithm is conceived to solve the non-convex CRB minimization problem with coupled user communication and target probing design. Both proposed optimization algorithms can converge to a stationary point of the CRB minimization problem. Numerical results show that: 1) The proposed ISPAC system can locate the target in both angle and distance domains merely relying on single BS and limited bandwidths; and 2) the positioning performance achieved by the hybrid-analog-and-digital ISPAC approaches that achieved by fully digital ISPAC when the communication rate requirement is not stringent.Comment: 13 pages, 6 figure

Statistical CSI Based Beamforming for Reconfigurable Intelligent Surface Aided MISO Systems with Channel Correlation

Reconfigurable intelligent surface (RIS) is a promising candidate technology of the upcoming Sixth Generation (6G) communication system for its ability to provide unprecedented spectral and energy efficiency increment through passive beamforming. However, it is challenging to obtain instantaneous channel state information (I-CSI) for RIS, which obliges us to use statistical channel state information (S-CSI) to achieve passive beamforming. In this paper, RIS-aided multiple-input single-output (MISO) multi-user downlink communication system with correlated channels is investigated. Then, we formulate the problem of joint beamforming design at the AP and RIS to maximize the sum ergodic spectral efficiency (ESE) of all users to improve the network capacity. Since it is too hard to compute sum ESE, an ESE approximation is adopted to reformulate the problem into a more tractable form. Then, we present two joint beamforming algorithms, namely the singular value decomposition-gradient descent (SVD-GD) algorithm and the fractional programming-gradient descent (FP-GD) algorithm. Simulation results show the effectiveness of our proposed algorithms and validate that 2-bits quantizer is enough for RIS phase shifts implementation.Comment: 10 pages, 9 figures

Learning to Estimate 6DoF Pose from Limited Data: A Few-Shot, Generalizable Approach using RGB Images

The accurate estimation of six degrees-of-freedom (6DoF) object poses is essential for many applications in robotics and augmented reality. However, existing methods for 6DoF pose estimation often depend on CAD templates or dense support views, restricting their usefulness in realworld situations. In this study, we present a new cascade framework named Cas6D for few-shot 6DoF pose estimation that is generalizable and uses only RGB images. To address the false positives of target object detection in the extreme few-shot setting, our framework utilizes a selfsupervised pre-trained ViT to learn robust feature representations. Then, we initialize the nearest top-K pose candidates based on similarity score and refine the initial poses using feature pyramids to formulate and update the cascade warped feature volume, which encodes context at increasingly finer scales. By discretizing the pose search range using multiple pose bins and progressively narrowing the pose search range in each stage using predictions from the previous stage, Cas6D can overcome the large gap between pose candidates and ground truth poses, which is a common failure mode in sparse-view scenarios. Experimental results on the LINEMOD and GenMOP datasets demonstrate that Cas6D outperforms state-of-the-art methods by 9.2% and 3.8% accuracy (Proj-5) under the 32-shot setting compared to OnePose++ and Gen6D

A novel dual-rotor ultrasonic motor for underwater propulsion

Micro underwater vehicles (MUVs) have been highlighted recently for underwater explorations because of their high maneuverability, low price, great flexibility, etc. The thrusters of most conventional MUVs are driven by electromagnetic motors, which need big mechanical transmission parts and are prone to being interrupted by the variance of ambient electromagnetic fields. In this paper, a novel dual-rotor ultrasonic motor with double output shafts, compact size, and no electromagnetic interference is presented, characterized, and applied for actuating underwater robots. This motor was composed of a spindle-shaped stator, pre-pressure modulation unit, and dual rotors, which can output two simultaneous rotations to increase the propulsion force of the MUV. The pre-pressure modulation unit utilized a torsion spring to adjust the preload at the contact faces between the stator and rotor. The working principle of the ultrasonic motor was developed and the vibration mode of the stator was analyzed by the finite element method. Experimental results show that the no-load rotary speed and stalling torque of the prototype ultrasonic motor were 110 r/min and 3 mN m, respectively, with 150 V peak-to-peak driving voltage at resonance. One underwater robot model equipped with the proposed ultrasonic motor-powered thruster could move at 33 mm/s immersed in water. The dual-rotor ultrasonic motor proposed here provides another alternative for driving MUVs and is appropriate for developing specific MUVs when the electromagnetic interference issue needs to be considered. © 2019 by the authors

Vibrational disorder and densification-induced homogenization of local elasticity in silicate glasses

We report the effect of structural compaction on the statistics of elastic disorder in a silicate glass, using heterogeneous elasticity theory with the coherent potential approximation (HET-CPA) and a log-normal distribution of the spatial fluctuations of the shear modulus. The object of our study, a soda lime magnesia silicate glass, is compacted by hot-compression up to 2 GPa (corresponding to a permanent densification of ~ 5%). Using THz vibrational spectroscopic data and bulk mechanical properties as inputs, HET-CPA evaluates the degree of disorder in terms of the length-scale of elastic fluctuations and the non-affine part of the shear modulus. Permanent densification decreases the extent of non-affine elasticity, resulting in a more homogeneous distribution of strain energy, while also decreasing the correlation length of elastic heterogeneity. Complementary (29)Si magic angle spinning NMR spectroscopic data provide a short-range rationale for the effect of compression on glass structure in terms of a narrowing of the Si–O–Si bond-angle and the Si–Si distance