• 한국정보통신학회논문지
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• 제6권3호
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• pp.434-439
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• 2002

본 연구는 파장 분할 다중화(WDM：Wavelength Division Multiplexing) 방식 전송 시스템 (Transmission System)에 사용되는 어븀 첨가 광섬유 증폭기(EDFA : Erbium-Doped Fiber Amplifier)의 이득 제어(Gain Control) 방법에 관한 것으로 어븀 첨가 광섬유에서 상호 이득 포화(Cross Gain Saturation) 현상, 이득 비동질 (Gain In-homogeneity) 특성, 그리고 어븀 이온의 밀도 반전(Population Inversion)의 변화 에 의해 출력되는 다 파장 광 신호들의 광 세기가 각기 다르게 출력되는 현상을 고출력을 내도록 구성된 어븀 첨가 광섬유 증폭기와 고속 제어기를 구성하여 위 현상들을 억제하며 이득을 제어하기 위한 레이저 다이오드(Laser Diode : LD)의 제어전압 조사하고, 얻어진 결과들을 토대로 이득 제어에 적합한 방법을 제시하고 제어기를 설계한다.

• 한국정보과학회:학술대회논문집
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• 한국정보과학회 1999년도 가을 학술발표논문집 Vol.26 No.2 (3)
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• pp.369-374
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• 1999

The problem of bandwidth allocation and routing in VP based ATM networks was studied. A priori reservation of resources for VP's reduces the statistical multiplexing gain, resulting in increased Call Blocking Probability (CBP). The focus of this study is on how to reduce CBP by the efficient bandwidth allocation and routing algorithms. Equivalent capacity concept was used to calculate the required bandwidth by the call. and the effect of traffic dispersion was explored to achieve more statistical gain. A cost-effective traffic dispersion routing algorithm, CED, was designed. The algorithm finds the optimal number of dispersion paths for a call, where the gain balances the dispersion cost. Simulation study showed that CED could significantly reduce the CBP.

• Journal of electromagnetic engineering and science
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• 제10권4호
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• pp.263-269
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• 2010

Multiple input multiple output (MIMO) has been considered one of the key enablers of broadband wireless communications. The indoor environment is known to be favorable to ensure both high rank property and high signal-to-interference/noise ratio (SINR) to fully exploit MIMO spatial multiplexing (SM) gain. In this paper, we describe several practical deployment cases where repeater links (or relay links), such as those present with an indoor distributed antenna system (DAS), can act as keyholes to degenerate the rank property of MIMO communications. In this case, we cannot exploit MIMO SM gain in indoor environment. We propose a novel MIMO communication scheme which uses simple converter in the devices in repeater links to resolve the rank degeneration issue and to ensure MIMO SM gain in highly MIMO-favorable indoor environment. MIMO SM is possible over the indoor DAS with single cable line through use of simple converters, which enables practical deployment in real fields.

• 한국통신학회논문지
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• 제34권3B호
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• pp.247-254
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• 2009

본 논문은 다중안테나를 구비한 전송단과 수신단의 활성화된 안테나 개수를 조절하여 통신 시스템의 소비전력을 최소화함으로써 동작시간을 최대화하는 전력절감 MAC 기법에 관한 것이다. 제안기법은 전력절감을 목적으로 다중안테나 시스템의 전송 방식인 멀티플렉싱 기법과 다이버시티 기법을 채널 상황에 따라 선택한다. 이러한 다중안테나 전력절감을 위한 MAC 기법은 무선단말에서 소모하는 전력을 고려한다. 이때 전송전력 뿐 아니라 활성화된 안테나 개수와 비례하는 회로 소모 에너지를 고려하기 때문에 성능분석 결과, 기존의 용량증대 접근 방법과는 다른 다중안테나 방안을 선택한다. 따라서 제안하는 다중안테나 전력절감 방식은 무선단말의 소비에너지 면에서 성능 향상을 보인다.

• 한국전자파학회논문지
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• 제15권5호
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• pp.444-450
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• 2004

이동통신 환경에서 송신 다이버시티를 이용하는 STBC-OFDW(Space Time Block Code-Orthogonal Frequency Division Multiplexing) 시스템의 성능은 두 심볼을 전송하는 시간 동안 채널 특성이 변하지 않을 경우, MRRC (Maximal Ratio Receiver Combining)의 다이버시티 이득을 얻는다. 그러나 채널 특성이 변하게 되면 수신단에서 수신 심볼간에 서로 간섭영향을 주기 때문에 MRRC의 다이버시티 이득이 발생하지 않는다. 따라서 이동통신환경에서 송신 다이버시티를 적용한 이동체가 고속으로 이동할 경우 이러한 간섭영향에 의한 성능 감소의 영향을 줄일 수 있는 방안이 필요하다. 본 논문에서는 이동통신 환경에서 개선된 송신 다이버시티를 이용하여 STBC-OFDM 시스템의 성능을 개선시킬 수 있는 수신기 구조를 제안하였다. 제안하는 구조는 수신단에 ZF(Zero Forcing) 알고리즘을 이용한 간섭제거기를 사용하여 전송대역폭의 변화 없이 다이버시티 이득을 얻을 수 있다. 시뮬레이션 결과 도플러 주파수가 클수록 성능 개선 정도가 커지는 특징을 갖는다.

• Journal of Communications and Networks
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• 제11권2호
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• pp.196-206
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• 2009

Over the last five years, relaying or multihop techniques have been intensively researched as means for potentially improving link performance of wireless networks. However, the data rates of relays are often limited because they cannot transmit and receive on the same frequency simultaneously. This limitation has come to the attention of researchers, and recently a number of relay techniques have been proposed specifically to improve the data efficiency of relaying protocols. This paper surveys transmission protocols that employ first single relays, then multiple relays and finally multiple antenna relays. A common feature of these techniques is that novel signal processing techniques are required in the relay network to support increased data rates. This paper presents results and discussion that highlight the advantages of these approaches.

• 방송공학회논문지
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• 제24권7호
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• pp.1221-1227
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• 2019

This paper describes a low-density parity-check (LDPC) coded index modulated orthogonal frequency division multiplexing with quasi-orthogonal sequence (IM-OFDM-QOS) and provides performance evaluations of the proposed system. By using QOS as the spreading code, IM-OFDM-QOS scheme can improve the reception performance than IM-OFDM-SS scheme for a given data rate. On the other hand, LDPC code is widely used to the latest wireless communication systems as forward error correction (FEC) scheme and has Shannon-limit approaching performance. Therefore, by applying LDPC code to IM-OFDM-QOS system as FEC scheme, the reception performance can be further improved. Simulation results show that significant signal-to-noise ratio (SNR) gains can be obtained for LDPC coded IM-OFDM-QOS system compared to the LDPC coded IM-OFDM-SS system and the SNR gain increases with the higher code rate.

• Journal of Communications and Networks
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• 제14권4호
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• pp.410-417
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• 2012

One of the premier mechanisms used in extracting unobserved signals from observed mixtures in signal processing is employing a blind source separation (BSS) algorithm. Orthogonal frequency division multiplexing (OFDM) techniques are playing a prominent role in the sphere of multicarrier communication. A set of remedial solutions taken to mitigate deteriorative effects caused within the air interface of OFDM transmission with aid of BSS schemes is presented. Four energy functions are used in deriving the filter coefficients. Energy criterion functions to be optimized and the performance is justified. These functions together with iterative fixed point rule for receive signal are used in determining the filter coefficients. Time correlation properties of the channel are taken advantage for BSS. It is tried to remove colored noise and jamming components from themixture at the receiver. Themethod is tested in a slow fading channel with a receiver containing equal gain combining to treat the channel state information values. The importance is that, these are quite low computational complexity mechanisms.

• 한국통신학회논문지
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• 제34권5A호
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• pp.331-340
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• 2009

In this paper, we propose a new power loading algorithm for orthogonalized spatial multiplexing(OSM) systems over flat-fading multiple-input multiple-output (MIMO) channels. Compared to SVD-based transmission scheme, the OSM scheme exhibits a good system performance with lower complexity and feedback overhead. To further improve the performance in OSM systems with power loading, we introduce a geometric approach on the Euclidean distance between the constellation points in the effective channel. Using this approach, we show that the optimal power loading parameters in terms of the minimum distance can be obtained. Simulation results demonstrate that our algorithm provides a 5dB gain at a bit error rate (BER) of $10^{-4}$ over that of no power loading case with both QPSK and 16-QAM. Consequently, our power loading algorithm allows us to significantly improve the system performance with one additional feedback value.

• Journal of Communications and Networks
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• 제14권3호
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• pp.257-266
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• 2012

A decode-and-forward two-way relay system benefits from orthogonal frequency division multiplexing (OFDM) and relay transmission. In this paper, we consider a decode-and-forward two-way relay system over OFDMwith two strategies: A joint subcarrier matching algorithm and a power allocation algorithm operating with a total power constraint for all subcarriers. The two strategies are studied based on average capacity using numerical analysis by uniformly allocating power constraints for each subcarrier matching group. An optimal subcarrier matching algorithm is proposed to match subcarriers in order of channel power gain for both transmission sides. Power allocation is defined based on equally distributing the capacity of each hop in each matching group. Afterward, a modified water-filling algorithm is also considered to allocate the power among all matching groups in order to increase the overall capacity of the network. Finally, Monte Carlo simulations are completed to confirm the numerical results and show the advantages of the joint subcarrier matching, power allocation and water filling algorithms, respectively.