For a graph G=(V,E), finding a set of disjoint edges that do not share any vertices is called a matching problem, and finding the maximum matching is a fundamental problem in the theory of distributed graph algorithms. Although local algorithms for the approximate maximum matching problem have been widely studied, exact algorithms have not been much studied. In fact, no exact maximum matching algorithm that is faster than the trivial upper bound of O(n2) rounds is known for general instances. In this paper, we propose a randomized $O(s_{max}^{3/2})$-round algorithm in the CONGEST model, where smax is the size of maximum matching. This is the first exact maximum matching algorithm in o(n2) rounds for general instances in the CONGEST model. The key technical ingredient of our result is a distributed algorithms of finding an augmenting path in O(smax) rounds, which is based on a novel technique of constructing a sparse certificate of augmenting paths, which is a subgraph of the input graph preserving at least one augmenting path. To establish a highly parallel construction of sparse certificates, we also propose a new characterization of sparse certificates, which might also be of independent interest.

A propagation experiment on an actual channel is conducted to confirm the effectiveness of the 1-tap time domain beamforming (TDBF) technique we proposed in previous work. This technique offers simple beamforming for the millimeter waveband massive multiple-input multiple-output (MIMO) applied wireless backhaul and so supports the rapid deployment of fifth generation mobile communications (5G) small cells. This paper details propagation experiments in the 75GHz band and the characteristics evaluations of 1-tap TDBF as determined from actual channel measurements. The results show that 1-tap TDBF array gain nearly equals the frequency domain maximal ratio combining (MRC) value, which is ideal processing; the difference is within 0.5dB. In addition, 1-tap TDBF can improve on the signal-to-interference power ratio (SIR) by about 13% when space division multiplexing (SDM) is performed assuming existing levels of channel estimation error.

A polarization switchable slot-coupled microstrip antenna using PIN diodes is proposed and studied. The microstrip feed line installed behind the ground plane is divided into two branches and each tip of the branches is connected to the ground plane through a PIN diode. One of the diodes is oriented from the tip to the ground plane and the other is oriented from the ground to the tip so that a slot in the ground can be selected to feed the patch by switching the dc bias between positive and negative. This selection contributes to switch the polarization between horizontal and vertical. In this paper, the authors investigate the polarization switching antenna theoretically and experimentally and confirmed sufficient differencce of antenna gain between horizontal and vertical polarization.

Path loss in high frequency bands above 6GHz is the most fundamental and significant propagation characteristic of IMT-2020. To develop and evaluate such high frequency bands, ITU-R SG5 WP5D recently released channel models applicable up to 100GHz. The channel models include path loss models applicable to 0.5-100GHz. A path loss model is used for cell design and the evaluation of the radio technologies, which is the main purpose of WP5D. Prediction accuracy in various locations, Tx positions, frequency bands, and other parameters are significant in cell design. This article presents the prediction accuracy of UMa path loss models which are detailed in Report ITU-R M.2412 for IMT-2020. We also propose UMa_A' as an extension model of UMa_A. While UMa_A applies different equations to the bands below and above 6GHz to predict path loss, UMa_A' covers all bands by using the equations of UMa_A below 6GHz. By using the UMa_A' model, we can predict path loss by taking various parameters (such as BS antenna height) into account over a wide frequency range (0.5-100GHz). This is useful for considering the deployment of BS antennas at various positions with a wide frequency band. We verify model accuracy by extensive measurements in the frequency bands from 2 to 66GHz, distances up to 1600 m, and an UMa environment with three Tx antenna heights. The UMa_A' extension model can predict path loss with the low RMSE of about 7dB at 2-26.4GHz, which is more accurate than the UMa_A and UMa_B models. Although the applicability of the UMa_A' model at 66GHz is unclear and needs further verification, the evaluation results for 66GHz demonstrate that the antenna height may affect the prediction accuracy at 66GHz.

We propose two simple weight calculation methods (primary method and enhanced method), that estimate approximated phase plane from a few antenna phase and calculate weights of all antenna elements, for wireless backhaul systems that utilize millimeter wave band massive antenna arrays. Such systems are expected to be used instead of optical fiber for connecting many small cell base stations (SCBSs) to the core network, and supporting the rapid deployment of SCBSs. However, beamforming with massive antenna arrays requires many analog-digital converters (ADCs) and incurs the issue of implementation complexity. The proposed methods overcome the problem by reducing the number of ADCs. Computer simulations clarify the appropriate layout and the number of ADCs connected to antenna elements; the effectiveness of the proposed methods is confirmed by evaluations with measured channel state information (CSI) in propagation experiments on a wireless backhaul system. Experimental verifications on the case of calculating the weight of 200 elements from the phases of just 9 elements show that the array gain degradation from ideal (the case in which the phases of all elements are used estimation) with both methods is less than 0.4 dB in the direct wave dominant environment. In addition, the enhanced method holds the array gain degradation to under 0.8dB in an environment existing reflected waves. These results show that the proposed methods can attain high accuracy beamforming while reducing ADC number.

A waveguide 2-plane hybrid coupler with two operating bands is proposed. The cross-sectional shape of the coupled region inside the proposed coupler is designed with a two-dimensional arbitrary geometry sorting method. Simulations of the proposed hybrid coupler has a fractional bandwidth (FBW) of 2.17% at the center of 24.99GHz, and at the center of 28.28GHz an FBW of 6.13%. The proposed coupler is analyzed by the mode-matching finite-element hybrid method, and the final result is obtained using a genetic algorithm. The analyzed result of the coupling for the main modes in the coupled region is presented. The design result is confirmed by measurements.

This paper presents the characteristics of path loss produced by traffic sign blockage. Multi frequency bands including high frequency bands up to 40 GHz are analyzed on the basis of measurement results in urban microcell environments. It is shown that the measured path loss increases compared to free space path loss even on a straight line-of-sight road, and that the excess attenuation is caused by the blockage effects of traffic signs. It is also shown that the measurement area affected by the blockage becomes small as frequency increases. The blocking object occupies the same area for all frequencies, but it takes up a larger portion of the Fresnel Zone as frequency increases. Therefore, if blockage occurs, the excess loss in high frequency bands becomes larger than in low frequency bands. In addition, the validity of two blockage path loss models is verified on the basis of measurement results. The first is the 3GPP blockage model and the second is the proposed blockage model, which is an expanded version of the basic diffraction model in ITU-R P.526. It is shown that these blockage models can predict the path loss increased by the traffic sign blockage and that their root mean square error can be improved compared to that of the 3GPP two slope model and a free space path loss model. The 3GPP blockage model is found to be more accurate for 26.4 and 37.1GHz, while the proposed model is more accurate for 0.8, 2.2, and 4.7GHz. The results show the blockage path loss due to traffic signs is clarified in a wide frequency range, and it is verified that the 3GPP blockage model and the proposed blockage model can accurately predict the blockage path loss.

This paper proposes a path loss model for crowded outdoor environments that can consider the density of people. Measurement results in an anechoic chamber with three blocking persons showed that multiple human body shadowing can be calculated by using finite width screens. As a result, path loss in crowded environments can be calculated by using the path losses of the multipath and the multiple human body shadowing on those paths. The path losses of the multipath are derived from a ray tracing simulation, and the simulation results are then used to predict the path loss in crowded environments. The predicted path loss of the proposed model was examined through measurements in the crowded outdoor station square in front of Shibuya Station in Tokyo, and results showed that it can accurately predict the path loss in crowded environments at the frequencies of 4.7GHz and 26.4GHz under two different conditions of antenna height and density of people. The RMS error of the proposed model was less than 4dB.

A new path loss model of interference between mobile terminals in a residential area is proposed. The model uses invertible formulas and considers the effects on path loss characteristics produced by paths having many corners or corners with various angles. Angular profile and height pattern measurements clarify three paths that are dominant in terms of their effect on the accurate modeling of path loss characteristics in residential areas: paths along a road, paths between houses, and over-roof propagation paths. Measurements taken in a residential area to verify the model's validity show that the model is able to predict path loss with greater accuracy than conventional models.

The suitability of a complex MIMO channel matrix for spatial multiplexing is verified experimentally in terms of the Demmel condition number. The instantaneous 2 2 MIMO-OFDM channel measurements in several indoor environments indicate the location dependency of the condition number. Wideband frequency characteristics are also analyzed to evaluate the applicability of spatial multiplexing.

High gain antennas with narrow-beamforming are required to compensate for the high propagation loss expected in high frequency bands such as the millimeter wave and sub-terahertz wave bands, which are promising for achieving extremely high speeds and capacity. However using narrow-beamforming for initial access (IA) beam search in all directions incurs an excessive overhead. Using wide-beamforming can reduce the overhead for IA but it also shrinks the coverage area due to the lower beamforming gain. Here, it is assumed that there are some situations in which the required coverage distance differs depending on the direction from the antenna. For example, the distance to an floor for a ceiling-mounted antenna varies depending on the direction, and the distance to the obstruction becomes the required coverage distance for an antenna installation design that assumes line-of-sight. In this paper, we propose a novel IA beam search scheme with adaptive beam width control based on the distance to shield obstacles in each direction. Simulations and experiments show that the proposed method reduces the overhead by 20%-50% without shrinking the coverage area in shield environments compared to exhaustive beam search with narrow-beamforming.

High-frequency wireless communication is drawing attention because of its potential to actualize huge transmission capacity in the next generation wireless system. The use of high-frequency bands requires dense deployment of access points to compensate for significant distance attenuation and diffraction loss. Dense deployment of access points in a mobility environment triggers an increase in the frequency of handover because the number of candidate access points increases. Therefore, simple handover schemes are needed. High-frequency wireless systems enable station position to be determined using their wideband and highly directional communication signals. Thus, simple handover based on position information estimated using the communication signal is possible. Interruptions caused by handover are also a huge barrier to actualizing stable high-frequency wireless communications. This paper proposes a seamless handover scheme using multiple radio units. This paper evaluates the combination of simple handover and the proposed scheme based on experiments using a formula racing car representing the fastest high-speed mobility environment. Experimental results show that seamless handover and high-speed wireless transmission over 200Mbps are achieved over a 400-m area even at station velocities of greater than 200km/h.

The maximal independent set (MIS) problem is one of the most fundamental problems in the field of distributed computing. This paper focuses on the MIS problem with unreliable communication between processes in the system. We propose a relaxed notion of MIS, named almost MIS (ALMIS), and show that the loosely-stabilizing algorithm proposed in our previous work can achieve exponentially long holding time with logarithmic convergence time and space complexity regarding ALMIS, which cannot be achieved at the same time regarding MIS in our previous work.

This paper proposes new techniques to simulate a MIMO propagation channel using the ray-tracing method for the purpose of decreasing the computational complexity. These techniques simulate a MIMO propagation channel by substituting the propagation path between a particular combination of transmitter and receiver antennas for all combinations of transmitter and receiver antennas. The estimation accuracy calculated using the proposed techniques is evaluated based on comparison to the results calculated using imaging algorithms. The results show that the proposed techniques simulate a MIMO propagation channel with low computational complexity, and a high level of estimation accuracy is achieved using the proposed Vector-Rotation Approximation technique compared to that for the imaging algorithm.

In a phase-locked-loop (PLL) frequency synthesizer with binary phase comparison, jitter is hard to suppress. In this paper, we propose a PLL frequency synthesizer with an improved binary phase comparison which can solve the above problem. The effectiveness of the proposed method is confirmed by PSpice simulation results.

This paper presents a modulation scheme for impulse radio that uses the first sidelobe for transmitting a non-return-to-zero baseband signal and the implementation of a dual frequency conversion demodulator. The proposed modulation technique realizes two times higher frequency efficiency than that realized by binary phase-shift keying modulation and does not require an up-converter in the transmitter. The dual frequency conversion demodulator compensates for the spectrum distortion caused by the frequency response of the circuits and channel. The effect of frequency compensation is analytically studied. The fabricated demodulator test chip of 65 nm CMOS achieves clock and data recovery at 5.7 Gbps with a power consumption of 24 mW.

A path loss model for low antenna heights below surrounding buildings in residential areas is presented to contribute to the construction of VHF band wireless systems. The model is constructed on the basis of measurement results at 167.65MHz, near center frequency at VHF band. Path loss characteristics in the middle VHF band are compared to those in bands above UHF. The dominant paths in bands above UHF include propagation paths below surrounding buildings, such as paths along roads. However, in the middle VHF band, these paths are instantly attenuated because their 1st Fresnel zone radius is larger than the average building height or road width. The dominant path in the middle VHF band is the over-roof propagation path, and the 1st Fresnel zone of the path is shielded by the buildings and the ground surface. The proposed path loss model has two features. First, it derives the effective height of the ground surface from the terrain profile of the buildings and the ground surface. Second, it uses formulas of a two-path model to take the shielding of the 1st Fresnel zone into account. Finally, it is shown that the proposed model is able to predict the path loss measurement results more accurately than the conventional model.

In the phase locked loop (PLL) frequency synthesizer which is used in a higher frequency region, the prescaler method is employed in order to increase the operating frequency of the programmable divider. However, since the fixed divider whose division ratio is same as the prescaler is installed at the following stage of the reference divider, the reference frequency is decreased and the performance of the PLL frequency synthesizer is degraded. The prescaler PLL frequency synthesizer using multi-programmable divider is one of the counter measures answering the request. In this paper we propose the reduction of the number of programmable dividers by using the (N+1/2) programmable divider. The effectiveness of the proposed method is confirmed by experimental results.

We target the estimation of antenna patterns of distributed array antenna (DAA) systems for satellite communications. Measuring DAA patterns is very difficult because of the large antenna separations involved, more than several tens of wavelengths. Our goal is to elucidate the accuracy of the DAA pattern estimation method whose inputs are actual antenna pattern data and array factors by evaluating their similarity to actually measured DAA radiation patterns. Experiments on two Ku band parabolic antennas show that their patterns can be accurately estimated even if we change the conditions such as frequency, antenna arrangement and polarization. Evaluations reveal that the method has high estimation accuracy since its errors are better than 1dB. We conclude the method is useful for the accurate estimation of DAA patterns.

In the rendezvous problem, two computing entities (called agents) located at different vertices in a graph have to meet at the same vertex. In this paper, we consider the synchronous neighborhood rendezvous problem, where the agents are initially located at two adjacent vertices. While this problem can be trivially solved in O(Δ) rounds (Δ is the maximum degree of the graph), it is highly challenging to reveal whether that problem can be solved in o(Δ) rounds, even assuming the rich computational capability of agents. The only known result is that the time complexity of O($O(sqrt{n})$) rounds is achievable if the graph is complete and agents are probabilistic, asymmetric, and can use whiteboards placed at vertices. Our main contribution is to clarify the situation (with respect to computational models and graph classes) admitting such a sublinear-time rendezvous algorithm. More precisely, we present two algorithms achieving fast rendezvous additionally assuming bounded minimum degree, unique vertex identifier, accessibility to neighborhood IDs, and randomization. The first algorithm runs within $ ilde{O}(sqrt{nDelta/delta} + n/delta)$ rounds for graphs of the minimum degree larger than $sqrt{n}$, where n is the number of vertices in the graph, and δ is the minimum degree of the graph. The second algorithm assumes that the largest vertex ID is O(n), and achieves $ ilde{O}left( rac{n}{sqrt{delta}} ight)$-round time complexity without using whiteboards. These algorithms attain o(Δ)-round complexity in the case of $delta = {omega}(sqrt{n} log n)$ and δ=ω(n2/3log4/3n) respectively. We also prove that four unconventional assumptions of our algorithm, bounded minimum degree, accessibility to neighborhood IDs, initial distance one, and randomization are all inherently necessary for attaining fast rendezvous. That is, one can obtain the Ω(n)-round lower bound if either one of them is removed.