• 한국추진공학회지
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• 제8권4호
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• pp.91-101
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• 2004

분사기와 연소실만을 가지는 연소기의 수학적 선형모델을 가지고, 액체추진제 로켓엔진 연소기에 대한 저주파 동특성 분석을 수행하였다. 연소실의 압력변화가 추진제 유량변화로 되먹임되는 구조를 가짐에 따라 저주파 섭동을 나타냈으며. 연소실의 시정수가 증가할수록, 분사기의 차압이 증가할수록, 연소시간지연이 짧을수록 시스템은 안정하였다. 분사기 시정수 변화가 안정성에 미치는 영향은 크지 않았다. 연소시간지연이 없는 경우의 시스템은 항상 안정하였으며, 지연시간이 증가할수록 섭동 주파수 및 감쇠율은 줄어들며, 결국 시스템은 불안정하게 되었다.

• ETRI Journal
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• 제38권1호
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• pp.185-194
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• 2016

In this paper, a novel rate control scheme for low delay video coding of High Efficiency Video Coding (HEVC) is proposed. The proposed scheme is developed by considering a new temporal prediction structure of HEVC. In the proposed scheme, the relationship between bit rate and quantization step is exploited firstly to formulate an accurate quadratic rate-quantization (R-Q) model. Secondly, a method of determining the quantization parameters (QPs) for the first frames within a group of pictures is proposed. Thirdly, an accurate frame-level bit allocation method is proposed for HEVC. Finally, based on the proposed R-Q model and the target bit allocated for the frame, the QPs are predicted for coding tree units by using rate-distortion (R-D) optimization. We compare our scheme against that of three other state-of-the-art rate control schemes. Experimental results show that the proposed rate control scheme can increase the Bjøntegaard delta peak signal-to-noise ratio by 0.65 dB and 0.09 dB on average compared with the JCTVC-I0094 and JCTVC-M0036 schemes, respectively, both of which have been implemented in an HEVC test model encoder; furthermore, the proposed scheme achieves a similar R-D performance to Wang's scheme, as well as obtaining the smallest bit rate mismatch error of all the schemes.

• Journal of Information Processing Systems
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• 제10권2호
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• pp.176-192
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• 2014

The realization of Wireless Multimedia Sensor Networks (WMSNs) has been fostered by the availability of low cost and low power CMOS devices. However, the transmission of bulk video data requires adequate bandwidth, which cannot be promised by single path communication on an intrinsically low resourced sensor network. Moreover, the distortion or artifacts in the video data and the adherence to delay threshold adds to the challenge. In this paper, we propose a two stage Quality of Service (QoS) guaranteeing scheme called Prioritized Multipath WMSN (PMW) for transmitting H.264 encoded video. Multipath selection based on QoS metrics is done in the first stage, while the second stage further prioritizes the paths for sending H.264 encoded video frames on the best available path. PMW uses two composite metrics that are comprised of hop-count, path energy, BER, and end-to-end delay. A color-coded assisted network maintenance and failure recovery scheme has also been proposed using (a) smart greedy mode, (b) walking back mode, and (c) path switchover. Moreover, feedback controlled adaptive video encoding can smartly tune the encoding parameters based on the perceived video quality. Computer simulation using OPNET validates that the proposed scheme significantly outperforms the conventional approaches on human eye perception and delay.

• 한국전자파학회논문지
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• 제25권3호
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• pp.269-281
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• 2014

본 논문에서는 루프 군지연을 이용하여 저위상 잡음을 갖는 5 GHz 전압제어 유전체 공진기 발진기(VTDRO: Voltage Tuned Dielectric Resonator Oscillator)의 체계적인 설계를 제안하였다. 설계된 발진기는 폐루프 구조로 공진부, 위상천이부, 증폭부로 구성된다. 먼저 기준 전압제어 유전체 공진기 발진기를 제작하고, 루프 군지연을 측정 후, 전기적 주파수 조정 범위 및 위상 잡음을 측정하였다. 기준 전압제어 유전체 공진기 발진기의 측정 결과로부터, 루프 군지연만 조정될 경우 새로운 전압제어 유전체 공진기 발진기의 전기적 주파수 조정 범위 및 위상 잡음을 수식적으로 예측할 수 있다. 루프군지연이 높을수록 위상 잡음은 개선되고, 전기적 주파수 조정 범위는 좁아지게 된다. 이러한 루프 군지연은 주로 공진부에 의해 결정된다. 따라서 높은 군지연 특성을 갖는 공진부 설계방법의 체계적인 절차를 제안하였다. 측정 결과, 루프군 지연은 약 700 nsec를 보였다. 또한, 5 GHz의 발진 주파수에서 위상 잡음은 수식으로부터 도출한 값과 근사한 100 kHz offset 주파수에서 -154.5 dBc/Hz를 얻었다. 전기적 주파수 조정 범위는 0~10 V의 조정 전압에서 448 kHz를 보였으며, 출력 전력은 약 4.39 dBm을 보였다.

• ETRI Journal
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• 제22권1호
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• pp.30-37
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• 2000

Existing tree construction mechanisms are classified into source-based trees and center-based trees. The source-based trees produce a source-rooted tree with a low delay. However, for the applications with multiple senders, the management overheads for routing tables and resource reservations are too high. The center-based trees are easy to implement and manage, but a priori configuration of candidate center nodes is required, and the optimization mature such as tree cost and delay is not considered. In this paper, we propose a new multicast tree building algorithm. The proposal algorithm basically builds a non-center based shared tree. In particular, any center node is not pre-configured. In the purposed algorithm, a multicast node among current tree nodes is suitably assigned to each incoming user: Such a node is selected in a fashion that tree cost and the maximum end-to-end delay on the tree are jointly minimized. The existing and proposed algorithms are compared by experiments. In the simulation results, it is shown that the proposed algorithm approximately provides the cost saving of 30% and the delay saving of 10%, compared to the existing approaches. In conclusion, we see that the cost and delay aspects for multicast trees can be improved at the cost of additional computations.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 1999년도 하계종합학술대회 논문집
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• pp.1063-1066
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• 1999

The delay of cardiac depolarization wave detection in the conventional pacemakers or AICD (automatic implantable cardioverter/ defibrillator, or ICD) has been overlooked. However, it is known that the delay may cause hemodynamic problems and may prevent the proper operation of a new automatic feature, automatic capture verification, that is to be appeared in the near-future devices. In order to reduce the effects of the delay, a delay prevention algorithm was developed and tested by applying three human electrograms. The algorithm set the sensing threshold just above the measured noise level to reduce the detection delay. It is found that the low threshold was able to reduce the delay by 20msec(average) In most cases. The implementation results showed reliability and efficacy of the algorithm, and the algorithm could be applicable to the existing hardware and software of the conventional pacemakers and AICD without any significant modifications.

• 한국군사과학기술학회지
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• 제20권3호
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• pp.413-420
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• 2017

We investigate the performance of a true time delay(TTD) phase shifter to reduce the beam squint caused by frequency changes of a phased array antenna in wideband communication systems. To design a high gain phased array antenna, we need a long TTD, which causes high RF loss, low resolution and large dimension of TTD phase shifters. To overcome the problems, we propose a schematic of dual TTD phase shifters, which consists of short time delay(STD) in radio frequency(RF) part and long time delay(LTD) in intermediate frequency(IF) part. Our analysis results show that the proposed scheme reduces the required bits and delay time in RF band of the TTD compared to the conventional single TTD scheme.

• Journal of Communications and Networks
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• 제10권1호
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• pp.28-37
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• 2008

In this paper, we investigate the problem of minimizing the average transmission power of users while guaranteeing the average delay constraints in time-varying uplink channels. We design a scheduler that selects a user for transmission and determines the transmission rate of the selected user based on the channel and backlog information of users. Since it requires prohibitively high computation complexity to determine an optimal scheduler for multi-user systems, we propose a low-complexity scheduling scheme that can achieve near-optimal performance. In this scheme, we reduce the complexity by decomposing the multiuser problem into multiple individual user problems. We arrange the probability of selecting each user such that it can be determined only by the information of the corresponding user and then optimize the transmission rate of each user independently. We solve the user problem by using a dynamic programming approach and analyze the upper and lower bounds of average transmission power and average delay, respectively. In addition, we investigate the effects of the user selection algorithm on the performance for different channel models. We show that a channel-adaptive user selection algorithm can improve the energy efficiency under uncorrelated channels but the gain is obtainable only for loose delay requirements in the case of correlated channels. Based on this, we propose a user selection algorithm that adapts itself to both the channel condition and the backlog level, which turns out to be energy-efficient over wide range of delay requirement regardless of the channel model.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제9권5호
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• pp.1682-1701
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• 2015

In this paper, we propose an intergraded unified imperfect CSI model and investigate the joined effects of feedback delay and channel estimation errors (CEE) for two-hop relaying systems with transmit beamforming and relay selection. We derived closed-form expressions for important performance measures including the exact analysis and lower bounds of outage probability as well as error performance. The ergodic capacity is also included with closed-form results. Furthermore, diversity and coding gains based on the asymptotic analysis at high SNRs are also presented, which are simple and concise and provide new analytical insights into the corresponding power allocation scheme. The analysis indicates that delay effect results in the coding gain loss and the diversity order loss, while CEE will merely cause the coding gain loss. Numerical results verify the theoretical analysis and illustrate the system is more sensitive to transmit beamforming delay compared with relay selection delay and also verify the superiority of optimum power allocation. We further investigate the outage loss due to the CEE and feedback delays, which indicates that the effect of the CEE is more influential at low-to-medium SNR, and then it will hand over the dominate role to the feedback delay.

• Journal of Power Electronics
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• 제19권1호
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• pp.179-189
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• 2019

Four-quadrant converters (4QCs) are widely used as AC-DC power conversion interfaces in many areas. A control delay commonly exists in the digital implementation process of 4QCs, especially for high power 4QCs with a low switching frequency. This usually results in alternating current distortion, increased current harmonic content and system instability. In this paper, the control delay is divided into a computation delay and a PWM delay. The impact of the control delay on the performance of a 4QC is briefly analyzed. To obtain a fundamental value of AC current that is as accurately as possible, a specific sampling method considering the PWM pattern is introduced. Then a current predictive control based on a modified z-transform is proposed, which is effective in reducing the control delay and easy in terms of digital implementation. In addition, it does not depend on object models and parameters. The feasibility and effectiveness of the proposed predictive current control method is verified by simulation and experimental results.