Tatsuaki Kimura

Understanding the impacts and patterns of network events such as link flaps or hardware errors is crucial for diagnosing network anomalies. In large production networks, analyzing the log messages that record network events has become a challenging task due to the following two reasons. First, the log messages are composed of unstructured text messages generated by vendor-specific rules. Second, network equipment such as routers, switches, and RADIUS severs generate various log messages induced

With the growth of services in IP networks, network operators are required to perform proactive operation that quickly detects the signs of critical failures and prevents future problems. Network log data, including router syslog, are rich sources for such operations. However, it has become impossible to find genuinely important logs that lead to serious problems due to the large volume and complexity of log data. We propose a log analysis system for proactive detection of failures. Our key obse

Deployment of unmanned aerial vehicles (UAVs) performing as flying aerial base stations (BSs) has a great potential of adaptively serving ground users during temporary events, such as major disasters and massive events. However, planning an efficient, dynamic, and 3D deployment of UAVs in adaptation to dynamically and spatially varying ground users is a highly complicated problem due to the complexity in air-to-ground channels and interference among UAVs. In this paper, we propose a novel distri

Recent carrier-grade networks use many network elements (switches, routers) and servers for various network-based services (e.g., video on demand, online gaming) that demand higher quality and better reliability. Network log data generated from these elements, such as router syslogs, are rich sources for quickly detecting the signs of critical failures to maintain service quality. However, log data contain a large number of text messages written in an unstructured format and contain various type

Vehicle-to-vehicle (V2V) communication-based cooperative vehicular networks are a key technology for future smarter transportation systems as they provide various applications for addressing road safety and traffic congestion. To alleviate the performance limitations of existing dedicated short-range communication (DSRC) protocols, cellular-assisted V2V communications have been proposed recently, wherein cellular base stations (BSs) relay the transmission from one vehicle to another. Such cellul

An aerial base station (ABS), i.e., unmanned aerial vehicle-mounted base station, has a significant potential to effectively boost the coverage of next-generation wireless networks, having capability of adaptively serving traffic increase in temporary events (i.e., hotspots). However, designing an efficient 3D deployment of ABSs is a considerably complicated problem due to its high degree of freedom and inter-cell interference among ABSs. In this paper, we propose a novel distributed 3D ABS depl

This paper studies the light-tailed asymptotics of the stationary tail probability vectors of a Markov chain of M/G/1 type. Almost all related studies have focused on the typical case, where the transition block matrices in the non-boundary levels have a dominant impact on the decay rate of the stationary tail probability vectors and their decay is aperiodic. In this paper, we study not only the typical case but also atypical cases such that the stationary tail probability vectors decay periodic

Abstract This article considers the subexponential asymptotics of the stationary distributions of GI/G/1-type Markov chains in two cases: (i) the phase transition matrix in non-boundary levels is stochastic; and (ii) it is strictly substochastic. For case (i), we present a weaker sufficient condition for the subexponential asymptotics than those given in the literature. As for case (ii), the subexponential asymptotics has not been studied, as far as we know. We show that the subexponential asymp

In this paper, we propose a mathematical model for intelligent transportation systems (ITS) in an urban environment, which takes into account both the urban structure and vehicles. Using the stochastic geometry approach, we model the locations of vehicles with a Poisson point process on roads whose interval is a fixed value. We consider typical vehicle-to-infrastructure and vehicle-to-vehicle communication scenarios and derive theoretical values of the probability of successful transmission. Our

Correlation of interference affects spatio-temporal aspects of various wireless mobile systems, such as retransmission, multiple antennas and cooperative relaying. In this paper, we study the spatial and temporal correlation of interference in mobile ad-hoc networks under a correlated shadowing environment. By modeling the node locations as a Poisson point process with an i.i.d. mobility model and considering Gudmundson (1991)'s spatially correlated shadowing model, we theoretically analyze the

In this paper, we analyze the performance of vehicle-to-infrastructure (V2I) communications on a highway, in which the transmission distance between a road side unit (RSU) and a vehicle rapidly changes due to movement of vehicles. By theoretically analyzing the interference received at an RSU, we derive a closedform expression of a desired performance metric of V2I communications. Furthermore, by considering the trade-off between the interference at an RSU and the transmission range of a running

Cooperative vehicle safety (CVS) systems have been receiving significant attention as key enablers of important intelligent transportation system (ITS) applications, such as cooperative collision warning, emergency braking, and auto driving. In CVS systems, vehicles periodically broadcast short packets that include various types of vehicular information, e.g., position and speed. In this paper, we propose an optimization method for the broadcast rate in vehicle-to-vehicle (V2V) broadcast communi

This paper studies the light-tailed asymptotics of the stationary distribution of the GI/G/1-type Markov chain. We consider three cases:(ⅰ) the tail decay rate is determined by a certain parameter $\theta$ associated with the transition block matrices $\{\boldsymbol{A}(k);k=0,\pm1,\pm2,\dots\}$ in the non-boundary levels; (ⅱ) by the convergence radius of the generating function of the transition block matrices $\{\boldsymbol{B}(k);k=1,2,\dots\}$ in the boundary level; and (ⅲ) by the convergence

Vehicle-to-vehicle (V2V) communication is a key technology for future intelligent transportations systems (ITSs). To support existing dedicated-short-range- protocols (DSRC), cellular-assisted V2V communications have recently been pro- posed, in which cellular base stations (BSs), such as E-UTRAN and eNodeB, relay the transmission from a vehicle to another. In this paper, we theoretically analyze the performance of such cellular-relay V2V communications, in which a vehicle first transmits a pack