Naoki Honma

This paper proposes a third-order Butler matrix to extend orbital angular momentum (OAM) multiplexing to orders that are multiples of three for short-range wireless communications in fog/edge computing applications. The proposed network employs a single-layer microstripline structure, facilitating simplified fabrication and miniaturization. To validate its performance, we fabricated a prototype and conducted network evaluations and OAM multiplexing transmission experiments at a center frequency of 2.5 GHz. At this frequency, the prototype exhibited key characteristics required for OAM mode generation: low insertion loss ( $\lt {\mathrm {0.44~dB}}$ ), well-balanced amplitude (error $\lt {\mathrm {0.41~dB}}$ ) and precise phase characteristics among the ports (error $\lt {1.7}^{\circ}$ ). Furthermore, transmission experiments were conducted using four array configurations under various conditions to evaluate system performance. In particular, the circularly polarized array demonstrated notable robustness compared to a co-polarized array, maintaining high mode orthogonality and a high achievable rate even under axial rotation between the transmitting and receiving arrays. This work contributes not only to the simplification, miniaturization, and low-power consumption of networks for edge devices but also to enhancing the scalability of seamless communication for fog/edge computing by enabling flexible multiplexing tailored to adaptive use cases.

This article proposes a novel human-aware energy beamforming (BF) method for safe and secure microwave wireless power transfer (MWPT). The proposed method first estimates an equivalent channel between a transmitting array antenna and a human body from continuous-wave pilot signals Doppler-shifted by human vital activities by utilizing the MWPT system as a quasi-monostatic multiple-input multiple-output (or single-output) radar. This allows the online estimation of the radio frequency (RF) exposure power without prior human geometry information. Then, the closed-form optimum BF weight can be derived to maximize the figure of merit (FoM), defined as the ratio of the received power to exposure power in the form of a generalized Rayleigh quotient. This simultaneously facilitates the suppression of RF exposure and enhancement of transfer efficiency. Comprehensive experiments were conducted to evaluate the proposed BF performance using our multi-antenna testbed with proven applicability to various human-sensing experiments. The indoor experiment proved that the proposed method performs null steering auto-tuned to human positions within a distance of up to 3 m and to the physical characteristics of six subjects, even under realistic conditions with multipath fading and array error. It achieved the highest FoM of 21.65dB among conventional BF methods for a typical case. This was an improvement of 7.10 dB when compared with the result of maximum ratio transmission BF, which maximizes the transfer efficiency. This study demonstrates the possibility of the coexistence of humanity and microwave wireless power transfer (MWPT) technology for its future social implementation.

In this letter, we propose a hybrid indoor positioning method that integrates propagation distance estimation using a single ultra-wideband (UWB) anchor and pedestrian dead reckoning (PDR) on commercial smartphones. The proposed method estimates the smartphone's coordinates by comparing two distances: one derived from time-of-flight information provided by the UWB system, and the other obtained through smartphone-based PDR. Since the terminal position is obtained directly in a closed form, the computational complexity is reduced, facilitating integration with PDR on portable devices. Experiments were conducted in an indoor environment using a single UWB anchor capable of both transmission and reception, along with a commercial smartphone. The effectiveness of the proposed method was validated by comparing its positioning accuracy and computational complexity with brute-force search. The proposed approach achieved an average positioning error of 1.7 m and reduced the computational complexity to approximately 1/1000 of that required by the brute-force search, thereby validating its effectiveness.

In noncontact heart rate estimation using microwaves, the accuracy of heart rate estimation decreases due to interference from respiratory harmonics. Various methods have been proposed to mitigate the influence of respiration; however, most of these methods evaluate the average heart rate over an observation period of 10 s or longer, making it impossible to observe heart rate fluctuations within that interval. In this letter, we propose a technique called time–frequency ridge-based heart-rate estimation yielded through mode matching, which is suitable for radar-based systems. Experimental results from 14 participants demonstrated that a heart rate estimation accuracy of 0.8 bpm (50th percentile) was achieved with an observation time of 6 s.

This paper evaluates multiple target localization accuracy when employing multistatic Multiple-Input Multiple-Output (MIMO) radar. We adopt a spectrum fusion technique that improves the target localization accuracy when there are multiple targets, by which the MUltiple SIgnal Classification (MUSIC) spectrum, calculated from all combinations of transmitting and receiving stations, is multiplied. The simulation and experiments are performed for up to eight targets. Our results show that localization accuracy can be improved by increasing the number of transmitting and receiving stations. The distance error for eight targets was 1.6 m in the simulation and 2.4 m in the experiment. This method can be used to estimate the number of targets with high accuracy.

This paper proposes a modified constant modulus algorithm (CMA)-based beamforming method for a multiple-input multiple-output radar to track and detect vital signs via blind processing. Our method consists of two key functions: (1) the beamforming weight and DC offset are jointly updated to remove the semi-constant clutter and maintain the constant amplitude, which is the original function of CMA, and (2) the weight is updated by considering the phase variance owing to the typical respiratory response to make the weight converge to the vital signs. The proposed method was experimentally evaluated by comparing its heart rate estimation accuracy with that of the conventional CMA-based and maximal-ratio combining (MRC)-based beamforming methods. The experimental results showed that the proposed beamforming method with and without the phase variance constraint achieved the median errors of 3.45 and 4.06 bpm from the reference electrocardiogram, respectively, whereas the conventional CMA-based and MRC-based beamforming methods achieved the median errors of 21.1 and 9.79 bpm, thereby demonstrating the effectiveness of the proposed method.

This paper presents and experimentally evaluates a frequency error elimination technique suitable for unsynchronized bistatic Multiple-Input Multiple-Output (MIMO) radar for human-body detection. First, a mathematical expression of human-body localization using bistatic MIMO radar is presented. Then the direct path is used to eliminate the phase error created by the frequency difference between the transmitter and receiver. A new Doppler-shifted component of the MIMO channel without phase error is derived, and the locations of the multiple targets are calculated by the 2-dimensional MUltiple SIgnal Classification (MUSIC) method. Next, the results of simulations that examine frequency error versus power ratios are discussed to illustrate the effectiveness of the proposed method. An experiment is carried out in an indoor multipath-rich environment. To emulate the unsynchronized condition, the transmitter and receiver use independent Signal Generators (SGs). One to six targets are tested. The experiments demonstrate that our unsynchronized radar system can identify the locations of multiple targets with high accuracy.

This study proposed an implicit CRWPT method to improve the transfer efficiency at blind spots caused by standing-wave nodes. The method uses the diversity effect of dual Pxs. Based on the fluctuation of the reflected signal observed at a transmitting port during load modulation of the dual Pxs (minimum seven load combinations), the reflection characteristics for arbitrary load conditions of the Pxs were predicted through the in situ S -parameter estimation among the antennas. Then, considering the principle of “transmission maximization via reflection minimization” established in an electromagnetically shielded space, the load condition maximizing transfer efficiency was implicitly identified in the absence of feedback from the receiver by identifying the minimum point of the estimated reflection surface. Further, through an experiment conducted in a 200 mm metal cavity at a 2.4 GHz frequency, it was confirmed that the proposed method improved the transfer efficiency by up to 14.4 times compared with the use of a single Px; the method achieved spatial uniformity with a transfer efficiency of approximately 50% or more at most measurement points.

This letter proposes a novel cavity resonant wireless power transfer that can optimize the load condition of a parasitic antenna without receiver feedback or the need to know the S-parameter a priori . In this letter, it is hypothesized that the transmission efficiency to a receiver can be indirectly maximized by minimizing reflection observed at the signal source, and the proposed method is based on this. Furthermore, an in situ S-parameter estimation based on load modulation is proposed. This estimation provides a closed-form optimum load condition for minimizing reflection, and avoids the need for a brute force search with load sweeping. Experiments carried out in a 200 mm conductive cube cavity successfully demonstrate the effectiveness of the proposed method, which obtains a high transmission efficiency of 42.2% that is comparable to the achievable maximum.

In this letter, a self-interference cancellation (SIC) method is presented for short-range full-duplex multiple-input multiple-output (FD-MIMO) communication. The proposed SIC consists of the following steps. First, each peer has multiple transmitting (Tx) and receiving (Rx) antennas optimally arranged to form a one-rank self-interference (SI) matrix between Tx and Rx. The optimal antenna arrangement is derived using a geometric model. Second, null steering is performed by eigenmode beamforming excluding the primary eigenmode of the SI matrix. This realizes powerful SIC at the cost of processing just a single degree of freedom (DoF). The effectiveness of the proposed SIC is evaluated by electromagnetic (EM) simulations. The results confirm that it reduces the SI level by 22.76 dB. In addition, it is shown the proposal offers achievable rates 1.50-times those of the half-duplex (HD) scheme in line-of-sight (LOS).