Toshio Ihara

Bio: Toshio Ihara is an academic researcher from Ministry of Posts and Telecommunications. The author has contributed to research in topic(s): Radar & Attenuation. The author has an hindex of 13, co-authored 40 publication(s) receiving 1012 citation(s). Previous affiliations of Toshio Ihara include National Institute of Information and Communications Technology.

Effects of antenna directivity and polarization on indoor multipath propagation characteristics at 60 GHz

Abstract: In millimeter-wave indoor communications systems, the radiation patterns and polarizations of the antennas at base stations and remote terminals have a significant influence on channel characteristics. The work reported in this paper investigated the effects of the radiation patterns of the antennas at remote terminals on multipath propagation characteristics. These effects were investigated by indoor propagation measurements at 60 GHz conducted in a modern office room and by ray-tracing simulations based on geometrical optics. Multipath channel characteristics are compared in terms of impulse responses and their root-mean-square (rms) delay spreads for an omnidirectional antenna and for three directive antennas with different beam widths. From the results of measurements and ray-tracing simulations, the use of a directive antenna at the remote terminal is demonstrated to be an effective method of reducing the effects of multipath propagation. Further reduction in the multipath effects is found to be achieved by the use of circular polarization instead of linear polarization with the directive antennas.

Polarization dependence of multipath propagation and high-speed transmission characteristics of indoor millimeter-wave channel at 60 GHz

Abstract: Measurements of polarization dependence of multipath-propagation characteristics were conducted at 60 GHz in a room within a modern office building. Delay profiles were measured using a swept frequency method with a high time-delay resolution of 2 ns. Directions of arrival of the multipath components reflected from walls were analyzed by rotating a narrow-beam receiving antenna with an angular resolution of 5 degrees. Results of this experiment showed that multipath delayed waves due to wall reflection can be effectively suppressed by employing circular polarization instead of linear polarization. Based on this experimental result, a computer simulation was made to clarify the polarization dependence of the high-speed transmission characteristics of an indoor communications system operating at 60 GHz. >

Measurements of reflection and transmission characteristics of interior structures of office building in the 60-GHz band

Abstract: We have measured (at 57.5 GHz) the reflection and transmission coefficients of typical walls, floor, ceiling, window, and partitions and have compared the measured reflection characteristics with the reflection characteristics estimated by multilayer dielectric models. Although the positional inhomogeneity of structures gives rise to some variation from one measured position to another, these positional variabilities are not large. The overall trend of reflection coefficients calculated by multilayer dielectric models thus agrees fairly well with the trend of the measurements. We have shown that the use of circular polarization can reduce the reflection even if the interior parts of the structure have complicated structures. In addition, the shadowing effects of several types of partitions were measured. The transmission coefficients of partitions depend on the materials. The transmission losses of plywood-made partitions are found to be fairly low.

Measurements of reflection characteristics and refractive indices of interior construction materials in millimeter-wave bands

Abstract: The reflection characteristics and refractive indices of construction materials in the millimeter wave bands are needed for development of millimeter-wave application systems such as indoor communication systems and short-range sensing systems. Because so little experimental data of construction materials at millimeter-wave bands has been available, the authors have measured the reflection and transmission coefficients of a concrete plate, plasterboard, rock wool, a floorboard, a carpet tile, at 57.5, 78.5, and 95.9 GHz. The refractive indices of the materials that can be modeled as a homogeneous dielectric plate with a smooth surface were estimated by the combined use of reflection and transmission data. The refractive indices of a concrete plate, plasterboard, a floorboard, and rock wool at 57.5 GHz were found to be 2.55-j0.084, 1.50-j0.01, 1.98-j0.083, and 1.26-j0.005, respectively. These indices showed little frequency dependence in the measured frequency range, except for that of the floorboard, which varied about 10%. The authors also measured the reflection characteristics of materials with inhomogeneous structures or rough surfaces, and found that the specular reflection from a concrete plate and a floorboard can be reduced significantly by covering them with carpet tiles with rough surfaces.

Measurement of the complex refractive index of concrete at 57.5 GHz

Abstract: The complex refractive index of concrete is measured at 57.5 GHz by the combined use of free-space reflection and transmission methods. The measured refractive index of the concrete 14 months after concreting was 2.55-j0.084. This index, in particular the imaginary part, is sensitive to the curing time after concreting and the water content. The transmission coefficient of a concrete plate, mainly determined from the imaginary part of the complex refractive index, is less than -20 dB/cm when the concrete is fresh and moist and reaches -9 dB/cm after aging for fourteen months. In contrast, the reflection coefficient of a concrete plate is less dependent on water content.

Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges

Abstract: Millimeter-wave (mmW) frequencies between 30 and 300 GHz are a new frontier for cellular communication that offers the promise of orders of magnitude greater bandwidths combined with further gains via beamforming and spatial multiplexing from multielement antenna arrays. This paper surveys measurements and capacity studies to assess this technology with a focus on small cell deployments in urban environments. The conclusions are extremely encouraging; measurements in New York City at 28 and 73 GHz demonstrate that, even in an urban canyon environment, significant non-line-of-sight (NLOS) outdoor, street-level coverage is possible up to approximately 200 m from a potential low-power microcell or picocell base station. In addition, based on statistical channel models from these measurements, it is shown that mmW systems can offer more than an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks at current cell densities. Cellular systems, however, will need to be significantly redesigned to fully achieve these gains. Specifically, the requirement of highly directional and adaptive transmissions, directional isolation between links, and significant possibilities of outage have strong implications on multiple access, channel structure, synchronization, and receiver design. To address these challenges, the paper discusses how various technologies including adaptive beamforming, multihop relaying, heterogeneous network architectures, and carrier aggregation can be leveraged in the mmW context.

Millimeter Wave Channel Modeling and Cellular Capacity Evaluation

Abstract: With the severe spectrum shortage in conventional cellular bands, millimeter wave (mmW) frequencies between 30 and 300 GHz have been attracting growing attention as a possible candidate for next-generation micro- and picocellular wireless networks. The mmW bands offer orders of magnitude greater spectrum than current cellular allocations and enable very high-dimensional antenna arrays for further gains via beamforming and spatial multiplexing. This paper uses recent real-world measurements at 28 and 73 GHz in New York, NY, USA, to derive detailed spatial statistical models of the channels and uses these models to provide a realistic assessment of mmW micro- and picocellular networks in a dense urban deployment. Statistical models are derived for key channel parameters, including the path loss, number of spatial clusters, angular dispersion, and outage. It is found that, even in highly non-line-of-sight environments, strong signals can be detected 100-200 m from potential cell sites, potentially with multiple clusters to support spatial multiplexing. Moreover, a system simulation based on the models predicts that mmW systems can offer an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks with no increase in cell density from current urban deployments.

An Overview of Signal Processing Techniques for Millimeter Wave MIMO Systems

Abstract: Communication at millimeter wave (mmWave) frequencies is defining a new era of wireless communication. The mmWave band offers higher bandwidth communication channels versus those presently used in commercial wireless systems. The applications of mmWave are immense: wireless local and personal area networks in the unlicensed band, 5G cellular systems, not to mention vehicular area networks, ad hoc networks, and wearables. Signal processing is critical for enabling the next generation of mmWave communication. Due to the use of large antenna arrays at the transmitter and receiver, combined with radio frequency and mixed signal power constraints, new multiple-input multiple-output (MIMO) communication signal processing techniques are needed. Because of the wide bandwidths, low complexity transceiver algorithms become important. There are opportunities to exploit techniques like compressed sensing for channel estimation and beamforming. This article provides an overview of signal processing challenges in mmWave wireless systems, with an emphasis on those faced by using MIMO communication at higher carrier frequencies.

An empirical model and an inversion technique for radar scattering from bare soil surfaces

Abstract: Polarimetric radar measurements were conducted for bare soil surfaces under a variety of roughness and moisture conditions at L-, C-, and X-band frequencies at incidence angles ranging from 10 degrees to 70 degrees . Using a laser profiler and dielectric probes, a complete and accurate set of ground truth data was collected for each surface condition, from which accurate measurements were made of the rms height, correlation length, and dielectric constant. Based on knowledge of the scattering behavior in limiting cases and the experimental observations, an empirical model was developed for sigma degrees /sub hh/, sigma degrees /sub vv/, and sigma degrees /sub hv/ in terms of ks (where k=2 pi / lambda is the wave number and s is the rms height) and the relative dielectric constant of the soil surface. The model, which was found to yield very good agreement with the backscattering measurements of the present study as well as with measurements reported in other investigations, was used to develop an inversion technique for predicting the rms height of the surface and its moisture content from multipolarized radar observations. >

State of the Art in 60-GHz Integrated Circuits and Systems for Wireless Communications

Abstract: This tutorial presents an overview of the technological advances in millimeter-wave (mm-wave) circuit components, antennas, and propagation that will soon allow 60-GHz transceivers to provide multigigabit per second (multi-Gb/s) wireless communication data transfers in the consumer marketplace. Our goal is to help engineers understand the convergence of communications, circuits, and antennas, as the emerging world of subterahertz and terahertz wireless communications will require understanding at the intersections of these areas. This paper covers trends and recent accomplishments in a wide range of circuits and systems topics that must be understood to create massively broadband wireless communication systems of the future. In this paper, we present some evolving applications of massively broadband wireless communications, and use tables and graphs to show research progress from the literature on various radio system components, including on-chip and in-package antennas, radio-frequency (RF) power amplifiers (PAs), low-noise amplifiers (LNAs), voltage-controlled oscillators (VCOs), mixers, and analog-to-digital converters (ADCs). We focus primarily on silicon-based technologies, as these provide the best means of implementing very low-cost, highly integrated 60-GHz mm-wave circuits. In addition, the paper illuminates characterization techniques that are required to competently design and fabricate mm-wave devices in silicon, and illustrates effects of the 60-GHz RF propagation channel for both in-building and outdoor use. The paper concludes with an overview of the standardization and commercialization efforts for 60-GHz multi-Gb/s devices, and presents a novel way to compare the data rate versus power efficiency for future broadband devices.