In this paper, we propose a cost-efficient beamforming optimization algorithm for multi-user wireless communication systems associated with the intelligent reflecting surface (IRS). From the baseline ...successive refinement algorithm, which gives a sub-optimal solution for the power minimization problem under the signal-to-interference-plus-noise-ratio (SINR) constraint at each user, several optimization techniques are proposed to reduce the computation complexity while maintaining the algorithm-level performance. To reduce the number of required multiply-accumulate (MAC) operations, we first simplify the complicated matrix inversion by utilizing the channel hardening effect. We also present the two-phase refinement process for the group-level optimization of phase-shift elements, further relaxing the computation complexity as well as the processing latency. Applying the proposed optimization techniques, as a result, numerical results show that the fully-optimized algorithm can reduce the computational costs by up to 89.4% while showing less than 1 dB power loss, leading to the practical solution for the next-generation IRS-aided communication.
•3D statistic of steel fiber dispersion of UHPFRC has been investigated.•Fibers tend to move along the major upward flow the composite.•Different flow patterns and orientations observed due to the ...wall effect.•Fiber content is the dominant factor affecting the 3D dispersion of fibers.
In this study, influence of flow distance and casting direction to properties of steel fibers in ultra-high-performance fiber-reinforced concrete (UHPFRC) was investigated using 3D data on the fibers that accurately obtained through multiple microcomputed tomography (micro-CT) scans and image processing. 3D images of bundled fibers were separated to individual strands by means of a 3D watershed algorithm. The variation in fiber properties according to distance from the casting point and the casting direction was quantitatively analyzed. The results indicated that many fibers were distributed at the casting position for the horizontally casted specimen. The fibers tended to move along the major upward flow of the UHPFRC mixture, which in turn, gradually inclined upward and oriented horizontally in concentric circles. However, different major flow patterns and fiber orientations appeared depending on the casting direction due to the wall effects induced by formwork boundaries. It was also found that fiber content is the dominant factor affecting the 3D dispersion of fibers.
This study aims to investigate the relationship between the steel fibers and the electromagnetic wave shielding effectiveness of a high-performance fiber-reinforced cementitious composite (HPFRCC). ...The distribution characteristics of the steel fibers and the variation of the electrical conductivity of HPFRCC as a function of the fiber content were quantified based on micro computed tomography (CT) and impedance measurements to determine their correlations with the electromagnetic shielding effectiveness. The impedance results showed that no electrical network was formed in the composite by the steel fibers and it is difficult to manufacture HPFRCC with high-electrical conductivity using steel fibers alone without CNTs or other carbon-based materials. For the steel fiber content of greater than 0.5%, the number of contact points between the steel fibers increased significantly, and the relationship between the fiber content and the number of contact points was observed. Despite the improvement of the electrical conductivity owing to the presence of the steel fibers and to the increase in the contact points between the steel fibers, the shielding effectiveness did not increase further for the steel fiber contents equal or above 1.5%. Consequently, it was found that the factor that controls the shielding effectiveness of HPFRCC is not the electrical network of the steel fibers, but the degree of the dispersion of the individual steel fibers.
It has been a challenge for structural concrete to satisfy both high strength and lightweight, since general lightweight properties have been achieved through the use of lightweight aggregates with ...inherently weak strength. In this study, a cement composite with high strength and lightweight was developed using very low-specific gravity material of hollow particles. A combination of a polycarboxylate-ether superplasticizer, filler materials, and a low water-to-cement ratio was selected to increase the viscosity of the cement matrix for minimizing floating of the hollow particles. As a result, the developed cementitious composites substantially outperformed the material performance of previously reported lightweight concretes. Compressive strength of 69 MPa and 97 MPa at 28 days were achieved with the density less than 1.5 g/cm3 from ambient and heat-treated curing, respectively. From a reaction perspective, the used hollow particles did not actively participate in the cement-based hydration reaction, which indicated that their role is limited to being very light inert fillers in the designed cementitious matrix.
In this paper, practical utilization of multiple distributed reconfigurable intelligent surfaces (RISs), which are able to conduct group-specific operations, for multi-group multicasting systems is ...investigated. To tackle the inter-group interference issue in the multi-group multicasting systems, the block diagonalization (BD)-based beamforming is considered first. Without any inter-group interference after the BD operation, the multiple distributed RISs are operated to maximize the minimum rate for each group. Since the computational complexity of the BD-based beamforming can be too high, a multicasting tailored zero-forcing (MTZF) beamforming technique is proposed to efficiently suppress the inter-group interference, and the novel design for the multiple RISs that makes up for the inevitable loss of MTZF beamforming is also described. Effective closed-form solutions for the loss minimizing RIS operations are obtained with basic linear operations, making the proposed MTZF beamforming-based RIS design highly practical. Numerical results show that the BD-based approach has ability to achieve high sum-rate, but it is useful only when the base station deploys large antenna arrays. Even with the small number of antennas, the MTZF beamforming-based approach outperforms the other schemes in terms of the sum-rate while the technique requires low computational complexity. The results also prove that the proposed techniques can work with the minimum rate requirement for each group.
Microplastics have been used as lightweight aggregates in ultrahigh-performance concretes (UHPCs). The compressive and flexural strengths of UHPCs cured at room and high temperatures were measured, ...and their relationships with the density of the specimens were analyzed. The hardened specimens were exposed to elevated temperatures from 200 to 1000 °C and a computed tomography scan was performed to analyze the microstructures of these specimens. A lightweight UHPC with a density of 2000 kg/m3 and compressive and flexural strengths of 80–90 and 30–50 MPa, respectively, was fabricated. Autoignition of microplastics might lead to deterioration of UHPC, but the spalling could be controlled by steel fibers.
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•Steel fiber statistics have been investigated using micro-computed tomography.•MLE method accurately determined a thresholding value to segment fibers.•Fiber bundles were segmented ...into individual fibers by 3D watershed algorithm.•3D contact points of fibers have been analyzed for the first time.•Orientation of individual fibers largely depend on flow direction and fiber content.
Ultra-high performance fiber-reinforced concrete (UHPFRC) is a construction material with outstanding mechanical properties and durability. The degrees of dispersion and orientation of included steel fibers are known to have a decisive effect on UHPFRC’s material properties. In this study, UHPFRC specimens with different volumes of fibers were prepared to investigate the variation of spatial information of steel fibers in UHPFRC. Each sample was scanned individually through micro-computed tomography (micro-CT) to analyze the three-dimensional fiber statistics. To enhance the accuracy of the analysis, bundled fibers were digitally segmented through reliability-based maximum likelihood estimation (MLE) and a watershed algorithm. It was quantitatively found that the number of contact areas of bundled fibers decreased and fibers tended to be aligned more horizontally as flow distance increased. However, at the casting position and the opposite region, there was irregular distribution, which has the potential to affect the designed material properties of UHPFRC.
Recycling waste fishing nets (WFNs) is essential to reduce plastic litter production in the ocean. This study proposed the application of WFNs as fiber reinforcement for developing high performance ...cementitious composites (HPCC). Micro-computed tomography (micro-CT) and optimized image processing were utilized to characterize the microstructure of the WFN-reinforced HPCC. WFN-HPCC specimens were prepared with two types of WFN fiber (ground waste fishing net GN and ground waste rope GR) and different fiber contents (1, 2, and 3%), and morphological and statistical analyses were performed to evaluate the microstructure of the WFN-HPCC quantitatively—particularly focusing on three dimensional (3D) properties of fiber dispersion, fiber orientation, and pore structure. Furthermore, the mechanical properties of WFN-HPCC were surveyed to investigate the impact of the microstructural characteristics on these properties. The degree of 3D dispersion of WFN fibers increased with increased fiber content, and the fiber orientation was significantly different depending on the fiber type and content. The degree of fiber orientation in horizontal direction was highest in WFN-HPCC with 1% of the GN fibers. Its average polar angle was 72.07° in which fiber clumping was not observed. It was confirmed that the WFN fibers allowed the formation of more pores with diameters ranging from 30 to 700 μm. Along with the observation that the micro-tomographically detected porosity (i.e., quantitatively measured pore via micro-tomography) played a decisive role in the compressive strength of the WFN-HPCC, the addition of the WFN fibers, especially the GN fibers, effectively enhanced the flexural strength of HPCC up to 19.7%.
•Microtomography was applied to investigate fiber and pore statistics.•Addition of WFN reduced air voids due to the early absorption of water on the surface of fibers.•Orientation and dispersion analysis on the fibers have been conducted.•GN fibers were more effective in enhancing flexural strength than GR fibers.
This study investigated the effects of seawater exposure on the mechanical, durability, and microstructural properties of cement mortars, which contained silica fume, metakaolin, or glass powder at ...two different replacement ratios. Test variables included the exposure conditions, exposure periods, supplementary cementitious material types, and replacement ratios. Compressive, flexural, and pull-off strengths were measured periodically. Mass change, water absorption, and rapid chloride permeability tests were conducted to evaluate the durability performance of Portland cement mortars (PCMs). Microcomputed tomography was utilized to obtain the pore information from representative PCMs. Test results revealed that seawater exposure yielded positive effects, including flexural strength and durability improvements during short-term exposure periods while those effects might not be last for long period. Metakaolin was the most effective supplementary cementitious material to minimize the reduction in compressive strength, while silica fume contributed to durability improvements. On the other hand, PCM containing GP did not show significant changes in strength and durability. Strong correlations between the flexural strength and durability were observed, and it was explained by additional hydration (i.e., matrix densification) of PCMs (except for the GP) due to the seawater curing which filled specific size of pores ranging 50–200 µm.
In massive multiple-input multiple-output (MIMO) systems using a large number of antennas, it would be difficult to connect high-resolution analog-to-digital converters (ADCs) to each antenna ...component due to high cost and energy consumption problems. To resolve these issues, there has been much work on implementing symbol detectors and channel estimators using low-resolution ADCs for massive MIMO systems. Although it is intuitively true that using low-resolution ADCs makes it possible to save a large amount of energy consumption in massive MIMO systems, the relationship between energy consumption using low-resolution ADCs and detection performance has not been properly analyzed yet. In this paper, the tradeoff between different detectors and total baseband energy consumption including flexible ADCs is thoroughly analyzed taking the optimal fixed-point operations performed during the detection processes into account. In order to minimize the energy consumption for the given channel condition, the proposed scheme selects the best mode among various processing options while supporting the target frame error rate. The numerous case studies reveal that the proposed work remarkably saves the energy consumption of the massive MIMO processing compared with the existing schemes.
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