Yukihiro Tadokoro

Advanced transmission cycle control scheme for autonomous decentralized TDMA protocol in safe driving support systems

When the volume of traffic on a roadway is so high such as traffic jam, medium access control (MAC) protocols in inter-vehicle communication (IVC) systems do not work well. The primary reason is the huge amount of communication traffic, which could be resolved by conventional transmission cycle control (TCC) schemes. However, when using TCC schemes, the failure of packet transmission extends the communication interval between vehicles, which can cause driving support systems that use IVC to malfunction. This paper proposes an advanced TCC scheme for realizing a safe driving support system. The proposed scheme exploits the frame-information exchange in the autonomous decentralized TDMA protocol, so that subsequent packets are transmitted rapidly even when failure occurs. Such a scheme is shown to be effective in terms of both communication quality and driving support systems.

Header reduction to increase the throughput in decentralized TDMA-based vehicular networks

In the decentralized-TDMA (D-TDMA) protocol the terminals select a free slot based on the frame information (FI) included in each packet to be transmitted. However, the FI constitutes a large portion of the packet which seriously compromised the throughput of the D-TDMA. We are proposing a technique for increasing the throughput by reducing the length of the FI without jeopardizing the packet success rate. Our technique, which is based on FI redundancies, lets the terminals transmit the relevant information when necessary. A computer simulation was conducted for an urban environment in which vehicles are moving. The simulation results show that the proposed technique significantly increases the throughput without degrading the quality of the D-TDMA protocol.

A new packet detection scheme in CDMA unslotted ALOHA system with successive interference cancellation

Packet detection is one of the most important problems in packet communication systems. In a CDMA Unslotted ALOHA system, which is one of the wireless packet communication systems, multiple access interference (MAI) makes the performance of the packet detection worse. To reduce the effect of MAI, we propose a new packet detection scheme in this paper. In this scheme, Successive Interference Cancellation(SIC) is applied. The packets signal is detected after the cancellation of MAI using SIC. This proposed scheme gives good performance of the packet detection.

SNT-index: Spatio-temporal index for vehicular trajectories on a road network based on substring matching

In the present study, we propose a novel spatio-temporal index structure for network-constrained trajectories, SNT-index --- Suffix-array-based Network-constrained Trajectory Index. The proposed method fast finds trajectories that follow a given route pattern within a certain time interval. The proposed method is based on two key concepts. The first is the introduction of the FM-index, which is often discussed in the field of compressed full-text substring matching. The FM-index enables fast pattern matching but does not support temporal filtering. The second concept is employing an inverse suffix array, which realizes the integration of the FM-index and the temporal information stored in a forest of B+trees. Unlike the previous methods, SNT-index can guarantee the exact results by traversing B+trees for only one or two network edges in the route pattern query. This property has not been realized in the past and contributes to extremely fast retrieval, especially when the route pattern query is long. We clarify the advantage of the proposed method by comparing the theoretical time complexity and the results of numerical experiments obtained using real trajectory data collected from taxi cabs in Rome, Italy. The results of the comparison indicate that the performance of the proposed method exceeds that of the existing method not only theoretically but also practically.

Concept, analysis, and demonstration of a novel delay network exhibiting stochastic resonance induced by external noise

Abstract Stochastic resonance offers the possibility of signal amplification by the addition of noise. This curious, interesting phenomenon has received considerable attention since the 1990s. Since such effect has the potential to improve the signal processing performance, intensive works have been done about this topic. One of the most effective implementations of stochastic resonance is in the Collins network, which can provide outstanding performance in that the network output consists of an amplified version of a weak, sub-threshold signal. In practical situations, the sub-threshold signal is easily buried in external noise from the environment. The present paper focuses on the discrete-time system (plus continuous-time system) and analyzes this situation to clarify the performance degradation of the amplification effect. As a countermeasure, we herein propose a novel delay network. The present analysis indicates that the proposed scheme produces an amplification effect in the presence of external noise. The results of the analysis are used to determine the condition for which the delay network is effective, and the results of an experimental evaluation verifies the validity of the analysis.

Singular probability distribution of shot-noise driven systems

We study the stationary probability distribution of a system driven by shot noise. We find that both in the overdamped and underdamped regime, the coordinate distribution displays power-law singularities in its central part. For sufficiently low rate of noise pulses they correspond to distribution peaks. We find the positions of the peaks and the corresponding exponents. In the underdamped regime the peak positions are given by a geometric progression. The energy distribution in this case also displays multiple peaks with positions given by a geometric progression. Such structure is a signature of the shot-noise induced fluctuations. The analytical results are in excellent agreement with numerical simulations.

Linear response analysis of vibrational resonance in over-damped systems

The phenomenon whereby the response to a weak low-frequency input signal is maximized in an oscillatory system with additional high-frequency input signals of appropriate amplitude is known as vibrational resonance (VR). In this study, the occurrence of VR in over-damped systems is analytically and numerically explained in the framework of linear response theory. The origin of the large response of VR is shown to be the dynamical bifurcation caused by tuning the input signals.

Angular Sensitivity of VHF-Band CNT Antenna

A carbon nanotube (CNT)-based antenna detects incoming electromagnetic waves based on electrically induced mechanical vibration. In this study, we perform an analysis of the angular sensitivity of a CNT-based antenna. To analyze the mechanical vibration, we introduce a simple model using the image charge method. The analysis shows that the angular sensitivity depends on |sin2θ|, where θ is the angle of arrival. This is clearly different from the traditional Hertzian dipole detector, whose sensitivity varies with |sinθ|. In addition, the full width at half maximum of the angular sensitivity distribution for the CNT-based antenna is 0.36 π, which is smaller than the value of 0.5 π for a Hertzian dipole detector.

A simple optimum nonlinear filter for stochastic-resonance-based signal detection

Stochastic resonance (SR) is a physical phenomenon through which system performance is enhanced by noise. Applications of SR in signal processing are expected to realize the detection of a weak signal buried in strong noise. Extraction of the effect of SR requires the design of an effective nonlinear system. Although a number of studies have investigated SR, most have employed conventional nonlinear filters. The present study proposes simple optimum nonlinear characteristics that maximize the performance enhancement, which is measured by the signal-to-noise ratio. The mathematical expression is simple, and the obtained performance is beyond that of linear systems. Surprisingly, the proposed nonlinear method can obtain the Cramér-Rao bounds and is equivalent to the maximum likelihood estimator. In addition, such optimization demonstrates systematically that the applications of SR to signal detection is effective only in non-Gaussian noise environments.

On linear solutions to a class of risk sensitive control for linear systems with stochastic parameters

In this paper, we propose a new risk sensitive (RS) control law of finite time for linear systems with stochastic system parameters. Stochastic parameters entail a nonlinear RS controller that is not compatible with linear systems, and can be thus unreliable. Moreover, it is difficult to obtain a globally defined exact solution due to the nonlinearity. To solve these problems, we derive a novel problem setting based on standard RS control. The standard RS type cost function is converted to a weighted sum of quadratic functions such that resulting feedback is linear in the state variable. How to convert the cost function without losing the characteristic of RS control is the main topic of this paper. We derive a novel weight independently of the state and input to obtain a linear RS control law. This allows one to derive the exact solution that is globally defined similarly to the well-known linear quadratic control problem. Such a linear controller offers high reliability and safe implementation without restricting the control region of the state. A numerical simulation demonstrates the effectiveness of the proposed method.