Jaesung Lim | Ajou University South Korea (original) (raw)

Papers by Jaesung Lim

The Journal of Korean Institute of Communications and Information Sciences

Applied Sciences, 2021

A frequency hopping orthogonal frequency division multiple access (FH-OFDMA) can provide low prob... more A frequency hopping orthogonal frequency division multiple access (FH-OFDMA) can provide low probability of detection (LPD) and anti-jamming capabilities to users against adversary detectors. To obtain an extreme LPD capability that cannot be provided by the basic symbol-by-symbol (SBS)-based FH pattern, we proposed two FH patterns, namely chaotic standard map (CSM) and cat map for FH-OFDMA systems. In our previous work, through analysis of complexity to regenerate the transmitted symbol sequence, at the point of adversary detectors, we found that the CSM had a lower probability of intercept than the cat map and SBS. It is possible when a detector already knows symbol and frame structures, and the detector has been synchronized to the FH-OFDMA system. Unlike the previous work, here, we analyze whether the CSM provides greater LPD capability than the cat map and SBS by detection probability using spectrum sensing technique. We analyze the detection probability of the CSM and provide ...

2019 International Conference on Information Networking (ICOIN), 2019

Ground tactical networks are often limited in their operations when the line of sight (LOS) is no... more Ground tactical networks are often limited in their operations when the line of sight (LOS) is not guaranteed. To overcome this limitation, airborne relay communications using unmanned aerial vehicles (UAVs) are a possible solution. In airborne relay communication, it is important to reduce relaying delay because the airborne relay communication is extended to two-hop communication unlike the conventional communication environment. In this paper, we proposed a dynamic time-division multiple access (DTDMA) scheme based on time mirroring to reduce relaying delay in airborne relay communication and allow multiple nodes to access the network effectively. The proposed scheme also performs efficient resource allocation and supports seamless voice communication in order to accommodate the military communication environment where nodes join and leave frequently. Proposed scheme applies the time mirroring scheme to DTDMA. Time-mirroring is a scheme in which a relay node that receives a packet from a source node relays a packet to a destination node without any additional processing. The proposed scheme designates two-timeslots as one mirroring slot to minimize the delay caused in the slot assignment process of the relay node. Slot assignment is accomplished by dividing the idle mirroring slot into a number of mini-slots and allowing the node to randomly access the mini-slot. The simulation shows that the proposed scheme can significantly reduce relaying delay and support seamless voice communication.

All-to-all broadcast refers to the process by which every node broadcasts its certain piece of in... more All-to-all broadcast refers to the process by which every node broadcasts its certain piece of information to all other nodes in the network. The all-to-all broadcasting time is very large due to the long propagation delay time in the long-delay environment like the airborne network and the underwater network. In this letter, we propose an efficient all-to-all broadcast method based on location information in long-delay single-hop wireless networks. We propose a matrix transformation and an algorithm to avoid conflictions in parallel transmissions and reduce all-to-all broadcasting time. The effectiveness of the proposed scheme is proved through simulations.

The Journal of Korean Institute of Communications and Information Sciences, 2021

IEEE Systems Journal, 2019

Recently, radar frequency bands have been considered as promising candidates for cognitive radio ... more Recently, radar frequency bands have been considered as promising candidates for cognitive radio (CR) owing to their low utilization. From a temporal access perspective, a substantial number of spectral opportunities could be exploited if CR users could achieve rotational synchronization with radar. However, to the best of our knowledge, no research has been focused on this synchronization issue. In this article, we propose a rotational synchronization scheme between radar and CR networks to opportunistically access the radar band. First, we propose a framed structure with sensing and transmission phases reflecting the rotational characteristics of radar. Second, we provide a cooperative synchronization algorithm. To determine the appropriate spectrum sensing timing in the proposed frame, we use a statistical approach via maximum a posteriori estimation considering the sensing abilities of CR users. We analyze the synchronization error of our proposed scheme via simulations.

IEEE Systems Journal, 2018

In this letter, we propose relay-based positioning in time division multiple access (TDMA) networ... more In this letter, we propose relay-based positioning in time division multiple access (TDMA) networks, called RPT. Although the concept of relay-based positioning is an improvement over the performance of relative navigation (RelNav), there are two problems to be solved when applying the relay concept to a TDMA network. First, measurement errors caused by mobility may degrade user accuracy because positioning references transmit navigation messages at their assigned times in TDMA, unlike conventional navigation systems, where all references continuously transmit. Second, a selection algorithm among airborne nodes is needed to relay messages to users. An RPT frame is designed to assign the slots for the broadcast and relay of navigation messages consecutively to minimize the effect of mobility. The selection algorithm determines the optimum set of airborne relays by selecting evenly distributed nodes depending on their distances to the master station. The simulation results indicate that RPT significantly improves user accuracy over RelNav.

Korean Journal of Clinical Neurophysiology, 2013

Background: Diffusion-weighted image (DWI) might be useful to predict the prognosis of acute hypo... more Background: Diffusion-weighted image (DWI) might be useful to predict the prognosis of acute hypoxic encephalopathy. The aim of our study was to test whether the early change and extent of DWI abnormalities can be an indicator of the clinical outcome of hypoxic encephalopathy. Methods: Forty-four patients who were diagnosed as hypoxic encephalopathy due to the cardiorespiratory arrest were retrospectively identified. Clinical variables were determined, and the DWI abnormalities were counted by four areas: cortex, subcortical white matter, cerebellum and deep grey matter, and were divided into three groups by the extent of lesions. Prognosis was classified as 'poor' (Glasgow coma scale (GSC) at 30 days after arrest <9 or death) and 'good' (GSC at 30 days after arrest ≥9). Results: GCS at day 3 (p<0.001), presence of seizure (p=0.01), and presence of lesion (p<0.001) were significantly different in prognosis, but statistically there is no association with the extent of lesions and prognosis (p=0.26). Conclusions: Presence of early DWI changes could predict the clinical outcome of hypoxic encephalopathy after cardiorespiratory arrest.

Sensors, 2016

The airborne relay-based positioning system (ARPS), which employs the relaying of navigation sign... more The airborne relay-based positioning system (ARPS), which employs the relaying of navigation signals, was proposed as an alternative positioning system. However, the ARPS has limitations, such as relatively large vertical error and service restrictions, because firstly, the user position is estimated based on airborne relays that are located in one direction, and secondly, the positioning is processed using only relayed navigation signals. In this paper, we propose an enhanced positioning algorithm to improve the performance of the ARPS. The main idea of the enhanced algorithm is the adaptable use of either virtual or direct measurements of reference stations in the calculation process based on the structural features of the ARPS. Unlike the existing two-step algorithm for airborne relay and user positioning, the enhanced algorithm is divided into two cases based on whether the required number of navigation signals for user positioning is met. In the first case, where the number of signals is greater than four, the user first estimates the positions of the airborne relays and its own initial position. Then, the user position is re-estimated by integrating a virtual measurement of a reference station that is calculated using the initial estimated user position and known reference positions. To prevent performance degradation, the re-estimation is performed after determining its requirement through comparing the expected position errors. If the navigation signals are insufficient, such as when the user is outside of airborne relay coverage, the user position is estimated by additionally using direct signal measurements of the reference stations in place of absent relayed signals. The simulation results demonstrate that a higher accuracy level can be achieved because the user position is estimated based on the measurements of airborne relays and a ground station. Furthermore, the service coverage is expanded by using direct measurements of reference stations for user positioning.

The Journal of Korean Institute of Electromagnetic Engineering and Science, 2015

In this paper, we propose an algorithm to identify Wobble PRI and Sinusoidal PRI among Radar puls... more In this paper, we propose an algorithm to identify Wobble PRI and Sinusoidal PRI among Radar pulses. We applied not only the DTOA(Difference Time Of Arrival) concept of radar pulse signals incoming to antennas but also a rising and falling cub characteristic of those PRIs. After making a program by such algorithm, we input each 40 data to Wobble PRI's and Sinusoidal PRI's identification programs and in result, those programs fully processed the data the according to expectations. In the future, those programs can be applied to the ESM, ELINT system.

The Journal of Korean Institute of Communications and Information Sciences, 2016

Link-16 is a representative TDL operated by US air force and NATO and supports structure of Multi... more Link-16 is a representative TDL operated by US air force and NATO and supports structure of Multi-net. Under Multi-net, military operation can be conducted effectively since terminal nodes in Link-16 hop over total frequency band simultaneously. As air traffic is rapidly increasing, new aeronautical system is introduced or existing system should be expanded to accommodate increasing air traffic and frequency band assigned for operating this system is scarce. It is scheduled to implement frequency remapping to solve frequency scarcity. With limited frequency band for operating Link-16, as frequency remapping is implemented, degradation of Multi-net performance can happen since multiple access interference in Link-16 is increasing so it is difficult to conduct multiple military operations. Thus, Group-based frequency hopping scheme is proposed to solve this problem. We verified the performance of the proposed scheme is improved.

MILCOM 2015 - 2015 IEEE Military Communications Conference, 2015

In this paper, we propose a new code selection approach for partitioned cyclic code shift keying ... more In this paper, we propose a new code selection approach for partitioned cyclic code shift keying (PCCSK) to improve the multinet capability of Joint Tactical Information Distribution Systems (JTIDS). The multinet capability of JTIDS enables concurrent operations in battle fields by allowing simultaneous data transmission and reception. PCCSK is a spread spectrum technique that supports various data rates and improves the data rate of JTIDS. The existing approach to PCCSK employs Reed Solomon (RS) code selection, and allows the dissemination of multiple access interference from the contaminated pulse to other pulses, and degrades the multinet capability. To prevent this error propagation, the proposed code selection approach restricts the RS symbol to be transmitted within one pulse. Simulation results show that the waveforms obtained using the proposed scheme outperform the previous waveforms including the JTIDS waveform with respect to both the multinet capability and the data rate.

2014 IEEE Military Communications Conference, 2014

In this paper, we propose an anti-jamming resource allocation scheme based on reserve channel wit... more In this paper, we propose an anti-jamming resource allocation scheme based on reserve channel with priority (RCP) fit algorithm for multi-frequency, time division multiple access (MF-TDMA) technology in satellite networks. The RCP fit allocates the traffic of the same terminal on the same frequency channel. In the RCP fit, partial bandwidth noise jamming (PBNJ) can corrupt the traffic of the specific terminals in jamming channels, and results in a fairness problem. Therefore, we extend the RCP fit to frequency hopping RCP (FH-RCP) fit, which prevents PBNJ using two steps. The first step is control channel frequency hopping to prevent continued jamming of the control channel, which forces the terminals to log out. The second step is data traffic frequency hopping, which enables various kinds of traffic to hop frequencies. FH-RCP fit thus applies the FH technique in a media access control layer, especially in MF-TDMA. Simulation results show that the proposed scheme improves anti-jamming capability and increases fairness compared to the RCP fit under PBNJ.

Signal Processing, 1996

Two adaptive algorithms based on the conjugate gradient method are presented for finite impulse r... more Two adaptive algorithms based on the conjugate gradient method are presented for finite impulse response (FIR) block adaptive filters. First, the block conjugate gradient (BCG) algorithm is derived from minimization of an estimate of the block mean-square error (BMSE). Using the fast convolution technique, the BCG algorithm is then extended to the frequencydomain BCG (FBCG) algorithm that yields significant computational savings over the BCG algorithm, especially for a large filter-tap order. Through computer simulations, it is shown that although the adaptation accuracy of the BCG or the FBCG is nearly equal to that of the optimum block adaptive (OBA) algorithm, its convergence property is superior to that of the OBA algorithm under any input conditions. Moreover, it is also shown that their convergence rate is as fast as the recursive least-squares (RLS) algorithms for a relatively small eigenvalue spread.

IEICE Transactions on Communications, 2008

In this letter, we emphasize the performance associated problem of the TCP protocol in the wired-... more In this letter, we emphasize the performance associated problem of the TCP protocol in the wired-wireless networks. It is shown that the increase of TCP congestion window is strongly influenced by the wireless link. To accelerate the increase of TCP congestion window regardless of wireless link conditions we adopt a fast snoop agent that sends indirect acknowledgement to the sender. Simulation results show that the proposed scheme achieves higher throughput with small data size.

IEEE Transactions on Wireless Communications, 2012

In WLAN systems, the difference in propagation delay among stations (STAs) can exceed the predefi... more In WLAN systems, the difference in propagation delay among stations (STAs) can exceed the predefined cyclic prefix length, leading to multiple access interference (MAI). To solve this MAI problem, it is necessary to use a longer cyclic prefix length for orthogonality between different subchannels; however, this sacrifices system efficiency due to the cyclic prefix overhead. In this paper, we propose a group contention-based OFDMA to solve the MAI problem with an adequate cyclic prefix length and support a larger number of STAs. The STAs within the cyclic prefix duration that are synchronized to an access point (AP), are included in the same contending group and compete with other STAs of the group in transmitting packets. The proposed scheme allows for the adoption of an adequate cyclic prefix length without MAI, and is able to reduce the cyclic prefix overhead. In addition, since no packet errors are induced by MAI in any of the STAs, the STAs can transmit simultaneously without redundant short interframe spaces (SIFSs) and other control packets. Through analysis and simulation, we show that for a large number of STAs, the proposed scheme achieves higher throughput than 802.11 protocols and a conventional CSMA combined with OFDMA.

IEEE Transactions on Aerospace and Electronic Systems, 2012

The growth of data traffic as air traffic increases is leading to bottlenecks. We propose a new t... more The growth of data traffic as air traffic increases is leading to bottlenecks. We propose a new time division multiple access (TDMA) protocol termed location-based TDMA (LBTM) to accommodate increasing data traffic and air-to-air communications. LBTM provides all three modes of communication: unicast, multicast, and broadcast. ACK needs a long guard time for reliable multicast in the aeronautical environment, because it is a long distance communication system. LBTM uses location information to overcome this problem. We can reduce the guard time of ACK and prevent collisions. In our scheme, multicast aircraft calculate ACK arrival time to ensure that ACKs arrive at the sender at different times. Therefore, LBTM reduces the amount of guard time of ACK and the total average time to complete a reliable multicast MAC request. This results in a significant increase in network utilization and throughput.

IEEE Communications Letters, 2013

In a wireless network with large propagation delay S-ALOHA(Slotted ALOHA) requires large guard ti... more In a wireless network with large propagation delay S-ALOHA(Slotted ALOHA) requires large guard time which causes lower normalized throughput. To reduce the large guard time in the ISA-ALOHA, a time alignment mechanism was proposed under the assumption of propagation delay estimation. In this letter, we propose a framed structure which is able to estimate propagation delay by employing coordinator beaconing. The framed structure consists of a time period for beaconing and a group of multiple time slots for random access. The proposed Beacon-based S-ALOHA(BS-ALOHA) can make packets generated during the time of beaconing evenly distributed over the random access period. Furthermore, we propose an analytical model considering overhead due to coordinator beaconing time and show that BS-ALOHA provides higher normalized throughput than both ALOHA and S-ALOHA employing the large guard time.

Journal of the Korea Institute of Military Science and Technology

박동석 1) ․ 오동한 2) ․ 최은호 3) ․ 임재성 *,4) 1) 합동참모본부 사이버․지휘통신참모부 2) 육군3사관학교 컴퓨터공학과 3) 아주대학교 장위국방연구소 4) 아... more 박동석 1) ․ 오동한 2) ․ 최은호 3) ․ 임재성 *,4) 1) 합동참모본부 사이버․지휘통신참모부 2) 육군3사관학교 컴퓨터공학과 3) 아주대학교 장위국방연구소 4) 아주대학교 NCW학과

The Journal of Korea Information and Communications Society

In this paper, we propose a traffic load control scheme, called fast congestion control (FCC), fo... more In this paper, we propose a traffic load control scheme, called fast congestion control (FCC), for a satellite channel using enhanced random access schemes. The packet repetition used by enhanced random access schemes increases not only the maximum throughput but also the sensitivity to traffic load. FCC controls traffic load by using an access probability, and estimates backlogged traffic load. If the backlogged traffic load exceeds the traffic load corresponding to the maximum throughput, FCC recognizes congestion state, and processes the backlogged traffic first. The new traffic created during the congestion state accesses the channel after the end of congestion state. During the congestion state, FCC guarantees fast transmission of the backlogged traffic. Therefore, FCC is very suitable for the military traffic which has to be transmit urgently. We simulate FCC and other traffic load control schemes, and validate the superiority of FCC in latency.