Jun-ichi Takada

Bio: Jun-ichi Takada is an academic researcher from Tokyo Institute of Technology. The author has contributed to research in topics: Antenna (radio) & Communication channel. The author has an hindex of 27, co-authored 272 publications receiving 2629 citations. Previous affiliations of Jun-ichi Takada include Amrita Vishwa Vidyapeetham & Fujitsu.

A slot design for uniform aperture field distribution in single-layered radial line slot antennas

Abstract: A single-layered radial line slot antenna (RLSA) is proposed for a direct broadcast from a satellite (DBS) subscriber antenna. It utilizes a radially outward-traveling wave and the aperture field is intolerably tapered if the uniform slots are used. Two novel techniques are applied. First, the slot design for uniform aperture field distribution is introduced. Second, the radial line is terminated by a matching spiral, in place of a dummy load. It suppresses reflection and radiates all the residual power in circular polarization. For a model antenna of 600-mm diameter, the efficiency of 65% is observed. >

A linearly polarized radial line slot antenna

Abstract: A design for linearly polarized radial line slot antenna (RLSA) is proposed. A novel slot arrangement is presented which couples to the rotationally symmetric guided waves to produce a uniform aperture distribution with linear polarization. Basic characteristics were measured in the 12-GHz band on a model antenna. The radiation patterns indicate a reasonable cross-polarization level more than 15 dB below the copolarization. The poor VSWR response is typical for waveguide broadside arrays. A technique to suppress this reflection is to tilt the beam from the boresight. >

Polarization behavior of discrete multipath and diffuse scattering in urban environments at 4.5 GHz

Abstract: The polarization behavior of the mobile MIMO radio channel is analyzed from polarimetric double-directional channel measurements, which were performed in a macrocell rural environment in Tokyo. The recorded data comprise non-line-of-sight, obstructed line-of-sight, and line-of-sight conditions. The gradient-based maximum-likelihood estimation framework RIMAX was used to estimate both specular and dense multipath components. Joint angular-delay results are gained only for the specular components. The dense multipath components, which may be attributed to diffuse scattering, can be characterized only in delay domain. Different characteristics describing the polarization behavior and power-weighted cross- and copolarization ratios for both types of components are introduced. Statistical analysis of long measurement track segments indicates global trends, whereas local analysis emphasizes specific behavior such as polarization dependency on angle of incidence in streets and under shadowing conditions. The results also underline the importance of modeling changing and transient propagation scenarios which are currently not common in available MIMO channel models.

A slot design for uniform aperture field distribution in single-layered radial line slot antennas

Abstract: The slot design for aperture field synthesis in a single-layered RLSA (radial line slot antenna) is summarized. In order to realize a desired slot coupling, the slot length and the spacing are varied on the basis of a full-wave analysis. An example is presented concerning the design of the uniform aperture field distribution. >

Channel models for wireless body area networks

Abstract: Wireless patient monitoring using wearable sensors is a promising application. This paper provides stochastic channel models for wireless body area network (WBAN) on the human body. Parameters of the channel models are extracted from measured channel transfer functions (CTFs) in a hospital room. Measured frequency bands are selected so as to include permissible bands for WBAN; ultra wideband (UWB), the industry, science and medical (ISM) bands, and wireless medical telemetry system (WMTS) bands. As channel models, both a path loss model and a power delay profile (PDP) model are considered. But, even though path loss models are derived for the all frequency bands, PDP model is only for the UWB band due to the highly frequency selectiveness of UWB channels. The parameters extracted from the measurement results are summarized for each channel model.

Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond

Abstract: Frequencies from 100 GHz to 3 THz are promising bands for the next generation of wireless communication systems because of the wide swaths of unused and unexplored spectrum. These frequencies also offer the potential for revolutionary applications that will be made possible by new thinking, and advances in devices, circuits, software, signal processing, and systems. This paper describes many of the technical challenges and opportunities for wireless communication and sensing applications above 100 GHz, and presents a number of promising discoveries, novel approaches, and recent results that will aid in the development and implementation of the sixth generation (6G) of wireless networks, and beyond. This paper shows recent regulatory and standard body rulings that are anticipating wireless products and services above 100 GHz and illustrates the viability of wireless cognition, hyper-accurate position location, sensing, and imaging. This paper also presents approaches and results that show how long distance mobile communications will be supported to above 800 GHz since the antenna gains are able to overcome air-induced attenuation, and present methods that reduce the computational complexity and simplify the signal processing used in adaptive antenna arrays, by exploiting the Special Theory of Relativity to create a cone of silence in over-sampled antenna arrays that improve performance for digital phased array antennas. Also, new results that give insights into power efficient beam steering algorithms, and new propagation and partition loss models above 100 GHz are given, and promising imaging, array processing, and position location results are presented. The implementation of spatial consistency at THz frequencies, an important component of channel modeling that considers minute changes and correlations over space, is also discussed. This paper offers the first in-depth look at the vast applications of THz wireless products and applications and provides approaches for how to reduce power and increase performance across several problem domains, giving early evidence that THz techniques are compelling and available for future wireless communications.

A review of antennas and propagation for MIMO wireless communications

Abstract: Multiple-input-multiple-output (MIMO) wireless systems use multiple antenna elements at transmit and receive to offer improved capacity over single antenna topologies in multipath channels In such systems, the antenna properties as well as the multipath channel characteristics play a key role in determining communication performance This paper reviews recent research findings concerning antennas and propagation in MIMO systems Issues considered include channel capacity computation, channel measurement and modeling approaches, and the impact of antenna element properties and array configuration on system performance Throughout the discussion, outstanding research questions in these areas are highlighted

Ultrawideband propagation channels-theory, measurement, and modeling

Abstract: This paper presents an overview of ultrawideband (UWB) propagation channels. It first demonstrates how the frequency selectivity of propagation processes causes fundamental differences between UWB channels and "conventional" (narrowband) channels. The concept of pathloss has to be modified, and the well-known WSSUS model is not applicable anymore. The paper also describes deterministic and stochastic models for UWB channels, identifies the key parameters for the description of delay dispersion, attenuation, and directional characterization, and surveys the typical parameter values that have been measured. Measurement techniques and methods for extracting model parameters are also different in UWB channels; for example, the concepts of narrowband channel parameter estimation (e.g., maximum-likelihood estimation) have to be modified. Finally, channel models also have an important impact on the performance evaluation of various UWB systems.

MIMO WLAN system

Abstract: A multiple-access MIMO WLAN system that employs MIMO, OFDM, and TDD. The system (1) uses a channel structure with a number of configurable transport channels, (2) supports multiple rates and transmission modes, which are configurable based on channel conditions and user terminal capabilities, (3) employs a pilot structure with several types of pilot (e.g., beacon, MIMO, steered reference, and carrier pilots) for different functions, (4) implements rate, timing, and power control loops for proper system operation, and (5) employs random access for system access by the user terminals, fast acknowledgment, and quick resource assignments. Calibration may be performed to account for differences in the frequency responses of transmit/receive chains at the access point and user terminals. The spatial processing may then be simplified by taking advantage of the reciprocal nature of the downlink and uplink and the calibration.

Online and Remote Learning in Higher Education Institutes: A Necessity in light of COVID-19 Pandemic

Abstract: In light of the rising concerns about the spread of COVID-19 and calls to contain the Corona Virus, a growing number of tertiary institutions have shut down in regards to face-to-face classes globally. The Corona virus has revealed emerging vulnerabilities in education systems around the world. It is now clear that society needs flexible and resilient education systems as we face unpredictable futures. A meta-analysis methodology was adopted for this study and pertinent literature was visited to capture the essence of continued learning during these unprecedented times. Findings reveal that universities worldwide are moving more and more towards online learning or E- Learning. Findings also reveal that apart from resources, staff readiness, confidence, student accessibility and motivation play important function in ICT integrated learning. This exploratory paper proposes that staff members should use technology and technological gadgets to enhance learning especially during these exceptional times. Findings also propose online and remote learning as a necessity in times of lock downs and social distancing due to COVID-19 pandemic. It also provides a strong platform for further research.