Nippon Telegraph and Telephone

It is known that the bending strength of polymer concrete decreases with increasing the water content due to water penetration. Therefore, the structural performance of the polymer concrete product like a manhole under service condition should be evaluated considering time-dependent change in mechanical properties of the polymer concrete according to water ingress. This study experimentally clarifies how the compressive, tensile, bending strengths and stress-strain relationship of the polymer concrete subjected to water penetration changes over time by means of the systematic acceleration laboratory test. It was found that the time-dependent change of the water content and associated strength and stress-strain relationship of the polymer concrete specimens and can be numerically simulated by water penetration analysis based on a diffusion mode and the assumption of the correlation between the local water content and the mechanical properties.

A direct tensile strength test was developed to determine the tensile strength and tensile stress-strain relationship of polymer concrete. Referring to previous studies of direct tensile tests on cement concrete, the test method was developed by focusing on the method of transmitting tensile force from the testing machine to the specimen and the shape of the specimen. As a method of transmitting tensile force, a cylindrical specimen was inserted into a cylindrical jig and fixed by tightening a nut on the tightening ring with a torque wrench. Secondary bending was reduced by placing spherical seats at both ends of all threaded rods in the cylindrical jig. The specimen shape was a dog-bone-shaped cylindrical with a narrower diameter at the test section. It was confirmed that the fracture zone could be guided into the test section and that strain could be measured well in the test section. The tensile strength of the polymer concrete measured by the developed direct tensile test method was almost consistent with the split tensile strength.

The use of the sub‐terahertz (THz) band is a promising candidate to cope with the ever‐increasing traffic in wireless access. Since the transmission range in the sub‐THz band is reduced due to its large propagation loss, received power needs to be enhanced using beamforming technologies. The authors focused on Bessel beams (BBs), which show high received power performance up to a certain distance, and evaluated their performance experimentally. The experiments were carried out at 140 GHz using small (15 cm diameter) dielectric and metasurface lenses. When using these small lenses, received power was increased by up to 15 and 11 dB, respectively, and the received power for distances up to 70 cm was greater than that of a conventional collimated beam.

Distributed acoustic sensing (DAS) using optical fiber cables, widely deployed for communications, can capture various information on the surrounding environment. Phase-sensitive optical time-domain reflectometry (Φ-OTDR) DAS is suitable for sensing deployed cables because Φ-OTDR has long measurement distances. However, a simple Φ-OTDR setup suffers from interference fading, in which sensitivity degradation occurs at many points. Multi-frequency Φ-OTDR is a practical solution that can remove the sensitivity degradation points effectively by averaging the signals of multi-frequency pulses. The method used to average multi-frequency signals affects the total measurement performance, including the sensitivity and the measurable vibration amplitude range. While vector-based averaging on the IQ plane is a powerful method in terms of sensitivity, calculating accurate vibration waveforms becomes difficult when vibrations become strong (> sub-μϵ). Consequently, large-scale vibration patterns are not accurately visualized, limiting potential applications. In this work, we develop a vector-based averaging method that can resolve this issue. We clarify that the known problem of the difference in phase response to strain change between multiplexed frequencies is the key matter causing the issue. To address this discrepancy, we propose a dynamically updated vector-based averaging method, enabling us to monitor strong vibrations. In the proposed method, parameters used in vector-based averaging—rotation angles and reference frequencies—that are fixed in the conventional method are updated over time in accordance with the fiber state. We demonstrate the effectiveness of the proposed method in both laboratory and field environments. We successfully visualize patterns of large-scale vibrations experienced by field-deployed communication cables, such as those caused by vehicle movements and wind blowing.

The use of sub-THz bands makes it possible to take advantage of physical properties of electromagnetic waves that have not been actively exploited to date. Shortened radio wavelengths in sub-THz bands and large-diameter antennas extend the near-field range to hundreds of meters and enable more precise beam manipulation using electromagnetic near-field phenomena. In this work, we investigated the Airy beam, whose main lobe power distribution follows a curved trajectory along its propagation. We first explain the main physical properties of the Airy beam, which include the curved trajectory of the main lobe power distribution, asymmetric sidelobe distribution, and self-healing. Then, we perform experimental verifications to confirm such physical properties. Specifically, we experimentally demonstrate a method to control the curved trajectories, which has not been explicitly demonstrated using sub-THz bands in the literature so far. In addition, we experimentally perform the transmission of four different streams using four Airy beams, taking advantage of the fact that low-interference transmission of multiple streams is possible thanks to the asymmetric sidelobe distribution properties of Airy beams. Our findings show that the total transmission rate exceeds 400 Gb/s, indicating that Airy beams can be utilized for high-capacity wireless transmission in sub-THz bands. Finally, we experimentally validate the self-healing property of the Airy beam through wireless transmission over a wireless link that is partially blocked by an obstacle. This provides new use cases for sub-THz bands, such as obstacle avoidance. Our theoretical and experimental research on Airy beams will leverage the diverse physical properties of electromagnetic waves to provide expanded and versatile uses in wireless communications.

This paper proposes a priority control method utilizing channel reservation for high-priority access points (APs) in IEEE 802.11 wireless LANs. The proposed algorithms are based on a network-controlled channel allocation scheme called RATOP. Computer simulation results demonstrated the positive effect of the proposed scheme on average and minimum throughput in a large area. This paper also proposes an enhanced algorithm that releases reserved channels to improve throughput. An estimation method based on theoretical analysis provides the number of required reserved channels for a given throughput ratio of high-priority APs to low-priority APs.

Digital coherent optical fiber communication systems adopt forward error correction (FEC) and multilevel modulation in accordance with transmission distance, data rate, and power consumption requirements. Bit-interleaved coded modulation (BICM) is practical coded modulation because of the simplicity of applying a binary code to converted binary-input memoryless channels (B-MCs) under a random bit-interleaver. The power consumption of soft-decision (SD) FEC has recently become a challenge due to transceiver power constraints. The multilevel coding (MLC) scheme efficiently reduces the decoding complexity, while the code design and complexity reduction depend on the modulation order. In this paper, we propose new binary codes for channel-polarized multilevel coding (CP-MLC) that convert multiple B-MCs to unreliable and reliable bit-channels and apply only SD-FEC to unreliable bit-channel to reduce the decoding complexity without depending on the modulation order. Systematic CP-MLC can also be used in probabilistically -shaped signals by using it in the component code of a probabilistic amplitude shaping scheme.

We propose a side-polished fiber coupler in which part of the core of one fiber is removed to branch the target fibers. With this coupler, we demonstrate variable branching ratio of any single-mode fiber that complies with G.652.D and G.657.A1 commonly used in optical access networks. We also present design and fabrication methodologies of the proposed fiber coupler to achieve a stable branching ratio regardless of the effective refractive indices of target fibers. Simulation and experimental results reveal that the proposed coupler greatly expands the range in which high branching ratios can be obtained compared with the conventional directional coupler; it achieves high branching ratios of more than 50% over the target range of the effective refractive index that covers G.652.D and G.657.A1 fibers. Experiments also show that the proposed coupler ensures a variable branching ratio independent of the effective refractive indices of the target fibers while keeping the excess loss as low as that of the conventional coupler.

Future advancements in data centers are anticipated to incorporate advanced circuit switching technologies, especially optical switching, which achieve high transmission capacity and energy efficiency. Previous studies addressed a Clos-network design problem to guarantee an admissible blocking probability to maximize the switching capacity, which is defined by the number of terminals connected to the network. However, as the number of available N×N switches increases, the switching capacity no longer increases due to the switch port limitation. This paper proposes a design of a multiple-plane twisted-folded (TF) Clos network, named MP-TF, to enhance the switching capacity, which is limited by the original TF-Clos, by guaranteeing an admissible blocking probability. MP-TF consists of identical M TF-Clos planes and pairs of a 1×M selector and an M×1 selector, each pair of which is associated with a transmitter and receiver pair. We formulate a design model of MP-TF as an optimization problem to maximize the switching capacity. We introduce connection admission control in MP-TF, named MP-CAC. We derive the theorem that the MP-TF design model using MP-CAC guarantees the admissible blocking probability. Numerical results observe that MP-TF increases the switching capacity as the number of TF-Clos planes when available N×N switches are sufficient; for example, with seven planes, the switching capacity is 1.97 times larger than that of one plane, given a request active probability of 0.6 and an admissible blocking probability of 0.01. We find that the computation time for MP-TF diminishes with an increase in the number of TF-Clos planes. Designing MP-TF is similar to designing a single TF-Clos plane, differing mainly in the handling of connection admission control. With a larger number of N×N switches, MP-TF enables the design of a smaller TF-Clos plane. We provide the analyses of optical power management and network cost of MP-TF.

We investigate 100-Tb/s-class C+L+U-band 14.85-THz bandwidth inline-amplified transmission over non-zero-dispersion-shifted fiber (NZ-DSF). The WDM signal bandwidth is extended to the U-band away from the zero-dispersion wavelength to boost the total throughput. The bandwidth extension to the U-band is enabled by a U-band repeater composing periodically poled lithium-niobate-based optical parametric amplifiers (PPLN-OPAs) and erbium-doped fiber amplifiers (EDFAs). The launch power of the C+L+U-band WDM signal is optimized by calculation accounting for stimulated Raman scattering and the wavelength-dependent accumulation of amplified spontaneous emission noise from PPLN-OPAs and EDFAs and nonlinear interference from NZ-DSF transmission to maximize the total throughput. To select an appropriate calculation method for launch power optimization, we analyze the impact of multi-channel interference, which is ignored in most closed-form expressions of the Gaussian noise (GN) model. We experimentally demonstrate net bitrates of 115.6 and 101.4 Tb/s after 240- and 400-km NZ-DSF transmission with bandwidth extension to the U-band and launch power optimization using a closed-form GN model.

Predicting wireless link quality (LQ) is a promising approach to enhancing Quality of Experience (QoE). This paper examines the impact of physical space on LQ; it uses machine learning to predict RSSI and throughput from physical space information. Experiments show that the position/orientation of user equipment (UE) are crucial- with orientation affecting RSSI- and velocity significantly impacting throughput. It is revealed that these LQ parameters have different spatial factor dependencies. An experiment is conducted in a static indoor environment with clear line of sight to simplify the problem. We focus on deriving LQ from current physical space information rather than forecasting future quality.

We demonstrate a transmitter that extends optical signal bandwidths to nearly four times beyond those intrinsically supported by individual digital-to-analog converters (DACs). We employ electronic intermediate-frequency-involved multiplexing (IFI-MUX) to bridge the bandwidth gap between CMOS DACs and optical modulators, while utilizing optical IFI-MUX to extend the final signal bandwidth beyond the modulators' limitations. In an experiment, we generated 224-GBaud single-carrier signals using CMOS DACs with 30-GHz analog bandwidth and an InPbased integrated modulator with 70-GHz electro-optic bandwidth. Net bit rates of up to 2.36 Tbps/ $\lambda$ at back to back and 2.32 Tbps/ $\lambda$ after 80-km standard single-mode fiber transmission were successfully demonstrated.

First‐principles calculations based on density functional theory are performed on silicon (Si) quantum slabs to investigate the valley splitting of the lowest sub‐bands at the Γ point in the conduction band. The calculations show that strong strain in the [110] direction of the slab largely enhances the valley splitting, which may be related to the giant valley splitting observed in a device consisting of silicon‐on‐insulator and buried oxide layers. The relation between the valley splitting and the atomic configuration of the slabs is also examined.

We investigate the estimation of inter-channel stimulated Raman scattering (ISRS), signal-to-noise ratio (SNR), and generalized mutual information (GMI) throughput in 14.1-THz signal bandwidth (S+C+L-bands) inline-amplified transmission over up to 400-km G.652.D single-mode-fiber (SMF). The bandwidth across spectral edges (i.e., including the guard band) reached 15.2 THz. ISRS noticeably transfers signal power from shorter-wavelength channels to longer-wavelength channels in ultra-wideband wavelength division multiplexing (WDM) systems, changing the wavelength dependence of signal impairments. The accuracy of ISRS estimations, which are a key step in the performance estimation or design of ultra-wideband WDM systems, is analyzed for different Raman gain coefficients by comparing estimated and experimentally measured ISRS. By taking into account the wavelength dependence of Raman gain coefficients, an accurate ISRS estimation with less than 0.55-dB peak-to-peak errors is demonstrated. Then, we combine the accurate ISRS estimation with a closed-form Gaussian noise model and evaluate the estimation accuracy of SNR and GMI throughput in the 14.1-THz bandwidth system for different transmission distances. We experimentally demonstrate less than 0.92 dB mean-absolute SNR estimation errors and less than 2.19% absolute total GMI throughput estimation errors after up to 400-km transmission. Finally, using the validated estimation model, we investigate a better launched WDM signal spectrum to enhance the total GMI throughput beyond that achieved in the experiment. Our calculation shows that modifying the launched spectrum can increase the total GMI throughput by ∼3.4% for 160-km transmission and by ∼2.9% for 400-km transmission.

We investigate the stimulated Raman scattering (SRS) threshold P_th for two-mode propagation state in two-mode fiber (TMF). We evaluated a model that can approximately derive the P_th for the two-mode propagation state by the product of the effective area A_eff and intensity ratio of each mode. We experimentally confirm that P_th is increased with increasing 2nd mode (i.e., 1st higher-order mode) intensity ratio, and the derived values from our model agree well with the experimental results. From our proposed model, we showed that P_th degradation due to modal crosstalk at the splice point is sufficiently small under conventional splicing conditions in the transmission links. Additionally, we confirmed that there is no mode dependence in the absorption spectrum due to SRS, and even if 2nd and 1st mode (i.e., fundamental mode) are utilized as the feed light and signal light, respectively, the signal light needs to be set to a shorter wavelength than the feed light by at least 0.2 μm.