• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.1
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• pp.72-80
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• 2015

In distributed generation systems, a grid-connected inverter should operate with synchronization to grid voltage. Considering that synchronization requires the phase angle of grid voltage, a phase locked loop (PLL) scheme is often used. The synchronous reference frame phase locked loop (SRF-PLL) is generally known to provide reasonable performance under ideal grid voltage. However, this scheme indicates performance degradation under the harmonic distorted or unbalanced grid voltage condition. To overcome this limitation, this paper proposes a phase and harmonic detection method of grid voltage using fast Fourier transform (FFT). To reduce the calculation time of FFT algorithm, minimum sampling data is taken from the voltage measurement to determine the phase angle and the magnitude of harmonic components. An experimental test setup for a grid-connected inverter system has been constructed. By comparative simulations and experiments under various abnormal grid voltage conditions, the proposed scheme has been proven to effectively track the phase angle of the grid voltage.

• The Journal of the Institute of Internet, Broadcasting and Communication
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• v.11 no.6
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• pp.31-39
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• 2011

For rapid formed the ad hoc wireless backbone network in disaster scene, It is necessary for real-time deployment scheme of wireless ad hoc relay devices by first responders without pre-planning. However, in order to realize this scheme, redundant deployment should be minimized, as well as optimal location of relay devices should be selected to expand communication coverage. Therefore, in this paper, we propose a new deployment scheme of relay devices to optimize communication coverage and then through simulations showed that improved performance of algorithm.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.12 no.3
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• pp.1330-1347
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• 2018

Mobile Ad hoc Network (MANET) is a collection of nodes or communication devices that wish to communicate without any fixed infrastructure and predetermined organization of available links. The effort has been made by proposing a scheme to overcome the critical security issue in MANET. The insufficiency of security considerations in the design of Ad hoc On-Demand Distance Vector protocol makes it vulnerable to the threats of collaborative black hole attacks, where hacker nodes attack the data packets and drop them instead of forwarding. To secure mobile ad hoc networks from collaborative black hole attacks, we implement our scheme and considered sensor's energy as a key feature with a better packet delivery ratio, less delay time and high throughput. The proposed scheme has offered an improved solution to diminish collaborative black hole attacks with high performance and benchmark results as compared to the existing schemes EDRIAODV and DRIAODV respectively. This paper has shown that throughput and packet delivery ratio increase while the end to end delay decreases as compared to existing schemes. It also reduces the overall energy consumption and network traffic by maintaining accuracy and high detection rate which is more safe and reliable for future work.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.14 no.3
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• pp.1363-1376
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• 2020

An improved clustering scheme based on user group is proposed. Every two users are grouped among N-users in the allowed system according to their link gain from large to small. Each user group is numbered sequentially. Two user clusters are obtained according to the principle of maximizing link gain difference for the users in the first and last user groups. The remaining user groups are added to the two existing user clusters according to the parity of the group number. The clustering should be clustered again among the users in either user cluster if the throughput summation of a user cluster in NOMA is less than that of these users in orthogonal multiple access. The simulation results show that the proposed clustering scheme can increase the system throughput by about 8% compared with the hybrid clustering scheme when the number of users requiring service is 12.

• Journal of Advanced Navigation Technology
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• v.14 no.5
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• pp.691-696
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• 2010

The main idea is to protect the signer of a document against the document being digitally distributed without the cooperation of signer. This paper proposes a new scheme of undeniable signature, which is so effective and improved D. Chaum's scheme. And our scheme which is zero-knowledge proved by using one-way function and partition - selection method, shows that its communication(challenge-response) only needs much fewer times during the confirmation protocol and disavowal protocol respectively, being very useful for wireless network environment. In the meantime our scheme allows the verifier to verify that the signature is valid, while the signer doesn't know the original message and the signature, to preserve the privacy of the verifier.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2008.10a
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• pp.655-658
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• 2008

In this paper, we explain the combined scheme of multi-code modulation and trans-orthogonal modulation. The name of the explained scheme is the multi-code trans-orthogonal modulation. Additionally, we propose a multi-code trans-biorthogonal modulation scheme, which is a little more improved multi-code trans-orthogonal modulation. We show the performance of the proposed schemes and compare the performance of the proposed schemes with the orthogonal modulation by using the computer simulation. According to the computer simulations, we can know that the bandwidth efficiency of the proposed schemes is better than that of the conventional orthogonal modulation. The proposed schemes can be used in physical layer of digital wireless telecommunication system to provide high speed data rate.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.4A
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• pp.309-315
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• 2009

Wireless Ad-hoc networks are dynamic networks that consist of mobile nodes. Nodes in Ad-hoc networks are usually laptops, PDAs or mobile phones. These devices feature Bluetooth and/or IEEE 802.11 (WiFi) network interfaces and communicate in a decentralized manner. Due to characteristics of Ad-hoc networks, Mobility is a key feature of routing protocol design. In this paper, we present an enhanced routing maintenance scheme that cope with topology changes pre-actively. The key feature of the proposed scheme is to switch next-hop node to alternative neighbor node before link breakage for preventing route failure. From extensive experiments by using NS2, the performance of the proposed scheme has been improved by comparison to AODV protocol.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.6 no.5
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• pp.1354-1372
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• 2012

Route establishment in Mobile Ad Hoc Network (MANET) is the key mechanism to a successful connection between a pair of source and destination nodes. An efficient routing protocol constructs routing path with minimal time, less routing overhead and capable of utilizing all possible link connectivity. In general, most on-demand MANET routing protocols operates over symmetrical and bidirectional routing path, which is infeasible due to the inherent heterogeneous properties of wireless devices. Simulation results show that the presence of unidirectional links on a network severely affect the performance of a routing protocol. In this paper, a robust protocol independent scheme is proposed, which enable immediate rediscovery of alternative route for a path blocked by a unidirectional link. The proposed scheme is efficient; route rediscovery is locally computed, which results in significant minimization of multiple route packets flooding. Nodes may exploit route information of immediate neighbors using the local reply broadcast technique, which then redirect the control packets around the unidirectional links, therefore maintaining the end-to-end bidirectional connection. The proposed scheme along with Ad Hoc On-demand Distance Vector (AODV) and AODV-Blacklist routing protocol is investigated over three types of mobility models. Simulation results show that the proposed scheme is extremely reliable under poor network conditions and the route connectivity can be improved by as much as 75%.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.8 no.5
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• pp.1744-1765
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• 2014

A client stores data in the cloud and uses remote data checking (RDC) schemes to check the integrity of the data. The client can detect the corruption of the data using RDC schemes. Recently, robust RDC schemes have integrated forward error-correcting codes (FECs) to ensure the integrity of data while enabling dynamic update operations. Thus, minor data corruption can be recovered by FECs, whereas major data corruption can be detected by spot-checking techniques. However, this requires high communication overhead for dynamic update, because a small update may require the client to download an entire file. The Variable Length Constraint Group (VLCG) scheme overcomes this disadvantage by downloading the RS-encoded parity data for update instead of the entire file. Despite this, it needs to download all the parity data for any minor update. In this paper, we propose an improved RDC scheme in which the communication overhead can be reduced by downloading only a part of the parity data for update while simultaneously ensuring the integrity of the data. Efficiency and security analysis show that the proposed scheme enhances efficiency without any security degradation.

• The Journal of the Korea institute of electronic communication sciences
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• v.13 no.1
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• pp.69-76
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• 2018

This paper studies the Turbo Codes, the concatenation of Turbo Codes and Reed-Solomon (RS) codes on Orthogonal frequency-division multiplexing (OFDM) in digital communication channel, and then analysis the performances of the new concatenated Forward Error Correct (FEC) scheme applied to an OFDM system based on the digital terrestrial video broadcasting (DVB-T) scheme. We compared the results with the standard DVB-T convolutional codes performances on Additive White Gaussian Noise (AWGN) channels. The simulation results shown that by employing turbo codes with only a few numbers of iterations, the performances of the overall system can be improved significantly. These results will be obtained by applying an appropriate interleaver technique in the concatenated FEC procedure.