• Journal of Communications and Networks
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• v.11 no.2
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• pp.175-186
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• 2009

A new medium access control (MAC) scheme embedding physical channels into multiband carrier sense multiple access/collision avoidance (CSMA/CA) networks is proposed to provide strict quality of service (QoS) guarantee to high priority users. In the proposed scheme, two priority classes of users, primary and secondary users, are supported. For primary users physical channels are provided to ensure strict QoS, whereas secondary users are provided with best-effort service using CSMA/CA modified for multiband operation. The performance of the proposed MAC scheme is investigated using a new multiband CSMA/CA Markov chain model capturing the primary user activity and the operation of secondary users in multiple bands. The throughput of secondary users is obtained as a function of the primary user activity and other CSMA/CA parameters. It is shown that the new MAC scheme yields larger throughput than the conventional single-band CSMA/CA when both schemes use the same bandwidth.

• Journal of KIISE
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• v.42 no.9
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• pp.1162-1174
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• 2015

These days, the networks have exhibited HBDP (High Bandwidth Delay Product) characteristics. The legacy TCP slowly increases the size of the congestion window and drastically decreases the size of a congestion window. The legacy TCP has been found to be unsuitable for HBDP networks. TCP mechanisms for solving the problems of the legacy TCP can be categorized into the loss-based TCP and the delay-based TCP. Most of the TCP mechanisms use the standard slow start phase, which leads to a heavy packet loss event caused by the overshoot. Also, in the case of congestion avoidance, the loss-based TCP has shown problems of wastage in terms of the bandwidth and RTT (Round Trip Time) fairness. The delay-based TCP has shown a slow increase in speed and low occupancy of the bandwidth. In this paper, we propose a new scheme for improving the over shoot, increasing the speed of the bandwidth and overcoming the bandwidth occupancy and RTT fairness issues. By monitoring the buffer condition in the bottleneck link, the proposed scheme does congestion control and solves problems of slow start and congestion avoidance. By evaluating performance, we prove that our proposed scheme offers better performance in HBDP networks compared to the previous TCP mechanisms.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.3A
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• pp.178-183
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• 2005

In Optical Burst Switching(OBS) networks, burst loss due to contention significantly affects the performance of networks. Even if many contention resolution schemes have been proposed, most of them have focused mainly on passively resolving contentions instead of avoiding them. In this paper, we propose an active contention avoidance scheme which controls a burst sending rate according to traffic load and burst loss in a network. Simulation results show that the proposed scheme can more effectively reduce the burst loss rate compared to existing OBS model with and without burst sending rate control, and at the same time guarantees throughput.

• The Journal of Korea Robotics Society
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• v.17 no.3
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• pp.359-364
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• 2022

This study presents a nonlinear model predictive control (NMPC)-based obstacle avoidance and whole-body motion planning method for the mobile manipulators. For the whole-body motion control, the mobile manipulator with an omnidirectional mobile base was modeled as a nine degrees-of-freedom (DoFs) serial open chain with the PPR (base) plus 6R (arm) joints, and a swept sphere volume (SSV) was applied to define a convex hull for collision avoidance. The proposed receding horizon control scheme can generate a trajectory to track the end-effector pose while avoiding the self-collision and obstacle in the task space. The proposed method could be calculated using an interior-point (IP) method solver with 100[ms] sampling time and ten samples of horizon size, and the validation of the method was conducted in the environment of Pybullet simulation.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.2B
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• pp.120-130
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• 2011

Recently the multicast based contents transmission is rapidly increasing due to the various multimedia services and the importance of switching technology to handle it is increasing as a consequence. Though the CICQ architecture has advantages that reduction of HoL blocking probability and simple scheduling using cross point buffer, it has disadvantage that the processing rate of multicast traffic can be significantly degraded corresponds to the traffic load increment. Several schemes have been proposed to solve this problem however they still can't provide enough processing ratio for multicast traffic. Therefore this paper proposes the BA cell assignment scheme and the VT scheme, and the processing rate of multicast traffic can be guaranteed by reducing the HoL blocking probability of multicast traffic and reservation of cross point buffer. Also simulation results verify that using the proposed scheme, the QoS of multicast service can be improved.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.1B
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• pp.154-161
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• 2010

In wireless sensor networks, due to the many-to-one convergence of upstream traffic, congestion more probably appears. The existing congestion control protocols avoid congestion by controlling incoming traffic, but the duty-cycle operation of MAC(Medium Access Control) layer has not considered. In this paper, we propose DCA(Duty-cycle Based Congestion Avoidance), an energy efficient congestion control scheme using duty-cycle adjustment for wireless sensor networks. The DCA scheme uses both a resource control approach by increasing the packet reception rate of the receiving node and a traffic control approach by decreasing the packet transmission rate of the sending node for the congestion avoidance. Our results show that the DCA operates energy efficiently and achieves reliability by its congestion control scheme in duty-cycled wireless sensor networks.

• JSTS:Journal of Semiconductor Technology and Science
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• v.15 no.2
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• pp.208-222
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• 2015

For the performance-efficient integration of IPs on an SoC utilizing heterochronous multi-clock domains, we propose a synchronization scheme that causes low latency overhead when data are crossing clock boundaries. The proposed synchronization scheme is composed of a clock predictor and a synchronizer. The clock predictor of a sender clock domain produces a predicted clock that is used in a receiver clock domain to detect possible synchronization failures in advance. When the possible synchronization failures are detected, a synchronizer at the receiver delays data-capture times to avoid the possible synchronization failures. From the simulation of the proposed scheme through SPICE modeling using a Chartered $0.18{\mu}m$ CMOS process, we verified the functionalities and timing behavior of the clock predictor and the synchronizer. The simulation results show that the clock predictor produces a predicted clock before a synchronization failure, and the synchronizer samples data correctly using the predicted clock.

• Journal of Institute of Control, Robotics and Systems
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• v.14 no.2
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• pp.141-147
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• 2008

In this paper, we present one approach to achieve safe navigation in indoor dynamic environment. So far, there have been various useful collision avoidance algorithms and path planning schemes. However, those algorithms have a fundamental limitation that the robot can avoid only "visible" obstacles. In real environment, it is not possible to detect all the dynamic obstacles around the robot. There exist a lot of "occluded" regions due to the limitation of field of view. In order to avoid possible collisions, it is desirable to consider visibility information. Then, a robot can reduce the speed or modify a path. This paper proposes a safe navigation scheme to reduce the risk of collision due to unexpected dynamic obstacles. The robot's motion is controlled according to a hybrid control scheme. The possibility of collision is dually reflected to a path planning and a speed control. The proposed scheme clearly indicates the structural procedure on how to model and to exploit the risk of navigation. The proposed scheme is experimentally tested in a real office building. The presented result shows that the robot moves along the safe path to obtain sufficient field of view, while appropriate speed control is carried out.

• ETRI Journal
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• v.42 no.2
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• pp.175-185
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• 2020

Inter-cell interference (ICI) is a major problem in heterogeneous networks, such as two-tier femtocell (FC) networks, because it leads to poor cell-edge throughput and system capacity. Dynamic ICI coordination (ICIC) schemes, which do not require prior frequency planning, must be employed for interference avoidance in such networks. In contrast to existing dynamic ICIC schemes that focus on homogeneous network scenarios, we propose a novel semi-distributed dynamic ICIC scheme to mitigate interference in heterogeneous network scenarios. With the goal of maximizing the utility of individual users, two separate algorithms, namely the FC base station (FBS)-level algorithm and FC management system (FMS)-level algorithm, are employed to restrict resource usage by dominant interference-creating cells. The distributed functionality of the FBS-level algorithm and low computational complexity of the FMS-level algorithm are the main advantages of the proposed scheme. Simulation results demonstrate improvement in cell-edge performance with no impact on system capacity or user fairness, which confirms the effectiveness of the proposed scheme compared to static and semi-static ICIC schemes.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.53 no.8
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• pp.585-593
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• 2004

Fault-tolerant gait planning in legged locomotion is to design gaits with which legged robots can maintain static stability and motion continuity against a failure in a leg. For planning a robust and deadlock-free fault-tolerant gait, kinematic constraints caused by a failed leg should be closely examined with respect to remaining mobility of the leg. In this paper, based on the authors's previous results, deadlock avoidance scheme for fault-tolerant gait planning is proposed for a hexapod robot walking over even terrain. The considered fault is a locked joint failure, which prevents a joint of a leg from moving and makes it locked in a known position. It is shown that for guaranteeing the existence of the previously proposed fault-tolerant tripod gait of a hexapod robot, the configuration of the failed leg must be within a range of kinematic constraints. Then, for coping with failure situations where the existence condition is not satisfied, the previous fault-tolerant tripod gait is improved by including the adjustment of the foot trajectory. The foot trajectory adjustment procedure is analytically derived to show that it can help the fault-tolerant gait avoid deadlock resulting from the kinematic constraint and does not make any harmful effect on gait mobility. The post-failure walking problem of a hexapod robot with the normal tripod gait is addressed as a case study to show the effectiveness of the proposed scheme.