• Proceedings of the KSRS Conference
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• 2003.11a
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• pp.811-813
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• 2003

The Satellite Technology Research Centre (SaTReC), Korea Advanced Institute of Science and Technology (KAIST) has developed and is to launch STSAT-1 (Science and Technology Satellite - 1) on 27$^{th}$ September 2003. The data collection system (DCS) is the one of its payloads. The DCS is a data relay system used for transmission from ground-based sensors through satellite to receiving station. This is one of the important methods collecting global data from the remote locations. In this paper, the DCS on the STSAT-1 will be introduced and the development of the mobile terminal (MT) will be reported.

• Journal of Communications and Networks
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• v.13 no.1
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• pp.32-42
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• 2011

Frequency reuse among relay stations (RSs) in a down-link access zone is widely adopted for throughput enhancement in IEEE 802.16j relay networks. Since the areas covered by the RSs or the base station (BS) may overlap, some mobile stations (MSs) at the border between two neighboring transmitting stations (RS or BS) using an identical frequency band may suffer severe interference or outage. This co-channel interference within the cell degrades the quality of service (QoS) fairness among the MSs as well as the system throughput. Exclusive use of a frequency band division (orthogonal resource allocation) among RSs can solve this problem but would cause degradation of the system throughput. We observe a trade-off between system throughput and QoS fairness in the previously reported schemes based on frequency reuse. In this paper, we propose a new frequency reuse scheme that achieves high system throughput with a high fairness level in QoS, positioning our scheme far above the trade-off curve formed by previous schemes. We claim that our scheme is beneficial for applications in which a high QoS level is required even for the MSs at the border. Exploiting the features of a directional antenna in the BS, we create a new zone in the frame structure. In the new zone, the RSs can serve the subordinate MSs at the border and prone to interference. In a 3-RS topology, where the RSs are located at points $120^{\circ}$ apart from one another, the throughput and Jain fairness index are 10.64 Mbps and 0.62, respectively. On the other hand, the throughput for the previously reported overlapped and orthogonal allocation schemes is 8.22 Mbps (fairness: 0.48) and 3.99 Mbps (fairness: 0.80), respectively. For a 6-RS topology, our scheme achieves a throughput of 18.38 Mbps with a fairness of 0.68; however, previous schemes with frequency reuse factors of 1, 2, 3, and 6 achieve a throughput of 15.24 Mbps (fairness: 0.53), 12.42 Mbps (fairness: 0.71),8.84 Mbps (fairness: 0.88), and 4.57 Mbps (fairness: 0.88), respectively.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.16 no.8
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• pp.2666-2687
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• 2022

As a new alternative communication scheme in 5G, unmanned aerial vehicle (UAV) is used as a relay in the remote base station (BS) for assistant communication. In order to ameliorate the quality of the backhaul link, a UAV backhaul transmission scheme based on improved Nash equilibrium (NE) strategy is proposed. First, the capacity of air-to-ground (A2G) channel by the link preprocess is maximized. Then, the maximum utility function of each UAV is used as the basis of obtaining NE point according to the backhaul channel and the backhaul congestion. Finally, the improved NE strategy is applied in multiple iterations until maximum utility functions of all the UAVs are reached, and the UAVs which are rejected by air-to-air (A2A) link during the process would participate in the source recovery process to construct a multi-hop backhaul network. Simulation results show that average effective backhaul rate, minimum effective backhaul rate increases by 10%, 28.5% respectively in ideal A2G channel, and 11.8%, 42.3% respectively in fading channel, comparing to pure NE strategy. And the average number of iterations is decreased by 5%.

• Journal of KIISE:Information Networking
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• v.33 no.5
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• pp.369-379
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• 2006

In recent years, there have been many researches about Ad-hoc Networks which is available to communicate freely between mobile devices by using multi-hop without any support of relay base or access point. TCP that used the most widely transport protocol in the Internet repeats packet loss and retransmission because it increases congestion window size by using reactive congestion control until packet loss occurs. As a result of this, energy of mobile device is wasted unnecessarily. In this paper, we propose TCP-New Veno in order to improve the energy efficiency of mobile device. According to the state of network, the scheme adjusts appropriate size of congestion window. Therefore, the energy efficiency of mobile device and utilization of bandwidth are improved by the scheme. From the simulation by using ns-2, we could see more improved energy efficiency with TCP-New Veno than those with TCP in Ad-hoc Networks.

• Proceedings of the Korea Information Processing Society Conference
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• 2007.11a
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• pp.999-1002
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• 2007

널리 보급된 컴퓨터와 인터넷 그리고 모바일기기의 발달은 우리사회에 큰 변화를 주었다. 이는 이동성을 제공하면서 휴대 인터넷을 사용할 수 있는 서비스를 절실히 필요로 하게 되었고 그러한 요구에 부응하여 WiBro가 탄생하였다. 본 논문에서는 WiBro의 Downlink(DL)와 Uplink(UL)의 속도를 다양한 환경에서 현재 시판중인 WiBro 모뎀으로 성능을 평가해 보았고, 그 결과 WiBro에서 추구하는 DL:UL의 비율과 비슷하게 나옴을 확인하였다. 하지만 서비스 지역의 건물 내에서 WiBro의 서비스를 원활하게 받지 못하는 문제점을 발견하였고, 그 해결책으로 Fixed RS, Nomadic RS, 그리고 Mobile RS 3가지 방식을 제안하였다. 특히 건물의 경우는 Fixed RS(Relay Station)에서 Broadcast message 방식과 Wake-up 방식을 사용하도록 제안하였다.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5A
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• pp.381-390
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• 2009

In this paper, we propose a seamless intra MR-BS handover scheme based on IEEE802.16j transparent. The proposed scheme estimates the outage probability of mobile stations at a base station, finds the optimum relay user, and provides low handover latency for seamless data transmission. The simulation results show that the proposed scheme outperforms the conventional handover schemes in terms of the handover latency by 65% of conventional scheme. Moreover, the proposed scheme exhibits lower packet error rate compared with the conventional handover scheme when a mobile station moves to outside of the cell coverage and reduce both outage probability and the number of handover about 50% from setting forgetting factor and redundant threshold.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.6B
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• pp.403-410
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• 2008

In this paper, we investigate the performance of MMR system in Non-transparent mode. The IEEE 802.16j MMR system has two node of operation, Single-frame (in band) and Multi-frame (out band) mode. In the analysis, we assume that channel interference between MR-BS and RS, or between RSs anywhere in the given area is ignored. The performance is presented in terms of the delay and the frame efficiency by varying number of RS and BS coverage to RS coverage ratio and the maximum coverage area of a BS by varying traffic density. Analytical results show that the Single-frame is more efficient than Multi-frame in frame efficiency and coverage extension.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37 no.2B
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• pp.97-109
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• 2012

In this paper, we propose a modulation scheme for a network-coded bi-directional relaying (NBR) system over an asymmetric channel, which means that the qualities of the relay channel (the link between the BS and RS) and access channel (the link between the RS and MS) are not identical. The proposed scheme employs a dual constellation in such a way that the RS broadcasts the network-coded symbols modulated by two different constellations to the MS and BS over two consecutive transmission intervals. We derive an upper bound on the average bit error rate (BER) of the proposed scheme, and compare it with the hybrid constellation-based modulation scheme proposed for the asymmetric bi-directional link. Furthermore, we investigate the channel utilization of the existing bi-directional relaying schemes as well as the NBR system with the proposed dual constellation diversity-based modulation (DCD). From our simulation results, we show that the DCD gives better average BER performance about 3.5~4dB when $E_b/N_0$ is equal to $10^{-2}$, while maintaining the same spectral efficiency as the existing NBR schemes over the asymmetric bi-directional relaying channel.

• The Journal of the Korea institute of electronic communication sciences
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• v.10 no.10
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• pp.1087-1092
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• 2015

It was diversified demand for a wireless network to the rapid growth of the Internet, the time and space that are not in the new level of Internet technology, limits the Ad-hoc networks are needed. Ad-hoc networks do not communicate with the central station, each of the mobile nodes included in the network communicate with each other by the relay role. In recent years, the Ad-hoc wireless networks in a variety of routing protocols and network security, research is actively underway for the authentication method, but the security of wireless Internet and Ad-hoc networks, certification is incomplete situation. This paper considers the authentication and key agreement technique applicability of the USIM card using the DSR routing protocol of the Java Card and Ad-hoc networks, we propose a secure authentication mechanism between the mobile node.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.12A
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• pp.941-948
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• 2009

In the 3GPP LTE systems, SC-FDMA is adopted for the uplink owing to its good PAPR efficiency. In the frequency domain, however, the performance of SC-FDMA is degraded due to frequency selective fading. As a compromise, a Hybrid OFDMA/SC-FDMA was proposed in the LTE-Advanced systems. Hybrid scheme uses SC-FDMA in the edge of cell area, while it uses OFDMA nearby the base-station. In this paper, uplink relays based on hybrid scheme is proposed to increase the performance of the 3GPP LTE systems. With different positions of mobile and relay, the proposed scheme can adaptively select SC-FDMA or OFDMA to improve the link performance. Our simulation results show that the proposed scheme achieves a better performance in terms of end-to-end BER and power consumption.