• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.6
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• pp.825-835
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• 2019

When the tunnel is designed, the ground is classified into several grades and the corresponding support system is applied according to the support pattern table. However, a simple pattern application based on rock grade does not take into account the longitudinal stress transitions occurring at rock grade boundaries. In this study, three-dimensional numerical analysis was performed to estimate the stress change in the longitudinal rock grade change of NATM tunnel, and the influence zone of load transfer was investigated using the influence line and trend line. As a result, the downward change of rock grade in the direction of tunnel excavation occurs in the range of 0.35~0.7D from low-strength rock to high-strength rock around the grade change boundary. It is necessary to apply a downward pattern of about 1.0D to the safety direction in consideration of the influence range of 0.35D to 0.7D.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.2
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• pp.195-199
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• 2010

(255,223) RS(Reed-Solomon) code which is the CCSDS(Consultative Committee for Space Data Systems) standard was used in the STSAT-3 to correct errors during the downlink of payload data. The RS encoder developed by VHDL was implemented in MMU(Mass Memory Unit). Moreover, the RS decoder developed by C-language was implemented in the DRS(Data Receiving System) of ground station. In this paper, we reported the design and analysis results of RS(255,223) for STSAT-3. The BER(Bit Error Rate) performance from MMU to DRS was confirmed through the downlink test at 16 Mbps. Also, the error correction performance and capability of RS(255,223) was tested by the manual attenuation of the RF(Radio Frequency) signal in the X-band transmitter resulting in putting some errors in the communication line.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2000.10a
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• pp.196-199
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• 2000

In this paper, we have designed a low-noise amplifier for the down-converter to apply the K-band Mu-kung-hwa satellite downconvertion. We have designed on three-stage to satisfy the property of low-noise amplifier for the down-converter required at least 30dB gain. The simulaition results for the designed three-stage Low-noise amplifier are measured that 33dB, gain and 0.93dB, noise-figure From 19.200 to 20.200, and The experiment results of the fabric are measured that 25dB, gain and 1.5dB, noise-figure. Since Input reflection coefficient and otput resection coefficient are -25dB and -28dB, respectively, and VSWR is lower than 1.5, this amplifier can be used as a low-noise amplifier for the down-converter to apply the K-band Mu-kung-hwa satellite downconvertion.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.20 no.12
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• pp.2226-2234
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• 2016

Orthogonal Frequency Division Multiplexing (OFDM) technique uses multiple sub-carriers for the data transmission. Therefore, Inter Carrier Interference(ICI) is generated because of nonlinear high power amplifier and carrier frequency offset. Wireless OFDM transmission over Doppler fading channels also causes ICI. In OFDMA(Orthogonal Frequency Division Multiplexing Access), multiple sub-carriers are allocated to each user. Therefore, inter carrier interference causes interference to other users. I evaluate the BER performance of OFDMA systems in frequency selective fading channel, considering Multi-User Interference (MUI) owing to the carrier frequency offset, the nonlinear high power amplifier, and the Doppler fading. In the uplink OFDMA, multi-user interference introduces larger BER degradation than in the downlink. I explain the reason and obtain the required characteristics of the nonlinear amplifier and the value of frequency offset for good BER performance. And I also analyze the BER degradation upon Doppler fading channel.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.30 no.9A
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• pp.749-754
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• 2005

It is well-known that mobile communication systems are usually limited in their performance and capacity by three major impairments, which are multipath fading, delay spread and co-channel interference (CCI). OFDMA (OFDM-FDMA) system can cope with the multipath fading and delay spread easily due to the beneficial properties of OmM technology. Though OFDMA scheme avoids intra-cell interference using the orthogonality among subcarriers, the scheme contains difficulty of reducing co-channel interference. Therefore, in this paper, adaptive antenna techniques are deployed into OFDMA/TDD system to minimize the co-channel interference induced by adjacent cells and to enhance the uplink performance. For the improvement of downlink performance, we apply TxAA (Transmit Adaptive Array), a kind of transmit diversity technique, utilizing preamble symbols for training antenna may into OFDMA/TDD transmitter side. Simulation results show that the uplink and downlink performance under multipath Rayleigh fading channel improved 9dB and 7dB each compared with the case of single antenna system at target BER $10^{-3}$ .

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.5A
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• pp.493-500
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• 2006

This paper presents the efficient structure and parameter optimization of downlink beamforming transmitter in OFDMA/TDD system. To design downlink beamforming transmitter for multiple transmit antennas, an efficient beamforming structure for multiple users and the choice of word-length of each block are critical in the aspect of its performance and hardware complexity. We propose an efficient beamforming scheme, which stores the weights of subcarriers into memory without user identification at the receiver of base station and calculates the weights for corresponding user in a subcarrier unit of IFFT input at high speed. Also, we obtain the word-length of main data path and other design parameters by fixed-point simulation analysis. The proposed architecture could reduce the memory size proportional to the maximum number of users per frame, and the processing time of an OFDM symbol at the receiver of base station without the need of additional processing time for calculating the weights at the transmitter.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.32 no.3A
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• pp.247-260
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• 2007

This paper solves the problem of finding a suitable sub-channel and power joint allocation method for multiple users in 802.16e OFDMA/TDD cellular systems. The joint allocation is thatfirstly the sub-channel is allocated to the users and then suitable power is allocated. We propose a FASA (Fairness insured Aggressive Sub-channel Allocation) algorithm which is a dynamic channel allocation algorithm considering all users' channel state information conditionally to maximize fairness and throughput. The improved CHC algorithm, which is dynamic power allocation algorithm, is also proposed in this paper The Improved CHC algorithm collects the extra of the downlink transmit power and then re-allocates it to other users. Simulation results show that the proposed improved CHC algorithm additionally increases the fairness and sector throughput.

• Korean Journal of Materials Research
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• v.5 no.1
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• pp.123-131
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• 1995

Natural convection in bridgman growth and it's effect on macrosegregation in unidirectionally solidified off-eutectic alloys were examined in this study. AlCu off-eutectic alloys(27.5wt% ~35. 6wt% ) were solidified upward or downward for producing a different natural convection and then Cu concentrations of off-eutectic composites were measured as a function of solidified fraction. Solutal and temperature distributions ahead of the solid/liquid interface were measured on quenched specimen. When hypo-utectic AlCu alloys are directionally solidified with downward growth, considerable macrosegregation occurs due to flow induced by thermal and solutal convection in melt. Soultal convection affects the macrosegregation of hyper-eutectic AlCu alloys more severely than thermal convection. Solute composition at solid/liquid interface of offkutectic composites was eutectic and also temperature was near eutectic point without large undercooling.

• Journal of the Institute of Electronics and Information Engineers
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• v.51 no.5
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• pp.29-34
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• 2014

We introduce a number of antenna optimization problem for the zero-forcing beamforming (ZFBF) scheme to enhance energy-efficiency (EE) of the multiple-input-multiple-output broadcast channel. For proposed optimization problem, we assume an instantaneous channel gain of the ZFBF scheme as an average channel gain, given by $N_a-K+1$, in order to reduce a computational complexity of finding the number of active antennas $N_a$. Then, we convert a fractional-form objective function into a subtractive-form, and find a solution of $N_a$ and the maximum EE by an iterative process. Simulation results show that the maximum EE value obtained by proposed algorithm is almost identical to the optimal EE value by the exhaustive search method.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2000.05a
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• pp.199-204
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• 2000

In this paper, we study single-user receiver, which applicable to DS-CDMA downlink. DS-CDMA downlink mainly use wallsh code, its orthogonality may be lost by the multiple access interference(MAI) caused by the multipath channel. In this paper, we use linear channel equalizer to eliminate the interference due to multipath channel and then to recover orthogonalit and use code-matched filter to detect transmitted data. Unlike existing research, which mainly assumed ideal channel information, we use pilot channel to estimate the channel coefficients. Especially we use guard symbols which are inserted periodically to estimate channel coefficients exactly without interference from user signal, and we accept an approximately ideal. The results show that improvement can be achieved using proposed detectors compared with that of that conventional RAKE receiver, especially when the user population is high we accepts excellent performance improvement.