• 한국통신학회논문지
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• 제31권12A호
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• pp.1182-1188
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• 2006

적응 변조 OFDM(Orthogonal Frequency Division Multiplexing) 전송 기법은 각 부반송파의 채널 상태에 따라 변조방식을 적절히 변화시켜 무선 채널의 다중 경로 페이딩에 의해 의한 영향을 최소화하여 시스템의 성능을 증가시키는 방식이다. 시스템이 적응적으로 전송하기위해서는 단말기에서 각 부반송파(subcarrier)별 채널 상태 정보 (Channel State Information : CSI)를 되먹임 채널을 통해 실시간으로 기지국으로 전송해 주어야한다. 하지만, 단말기에서 데이터를 처리할 때 걸리는 시간과, 단말기에서 기지국으로 CSI를 되먹임(feedback) 할 때 걸리는 시간으로 인한 되먹임 지연(feedback delay) d가 발생하게 된다. 이 되먹임 지연은 CSI 정보의 불일치를 발생시켜 적응 OFDM 시스템의 성능저하를 일으킨다. 본 논문에서는 CSI의 되먹임 지연 $d(\geq2)$를 적절히 보상하는 주파수 축 멀티 스탭 채널 예측기를 제안하고 이를 적응 전송 OFDM 시스템에 적용하고 모의실험을 통하여 기존의 OFDM 시스템, 기존의 채널 예측방식과의 성능을 MSE(mean square error), 비트오율(bit error rate : BER) 및 채널용량을 바탕으로 비교한다.

• 한국통신학회논문지
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• 제32권7A호
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• pp.762-769
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• 2007

적응 전송 MIMO(multiple input multiple output)-OFDM(orthogonal frequency division multiplexing) 시스템은 CSI(channel state information)의 되먹임을 이용하여 각 부반송파의 채널 상황에 따라 변조 방식을 다르게 전송하는 시스템이다. CSI 되먹임 채널인 상향링크 채널에는 송수신기 처리지연, 전송 지연, 프레임 지연 등 다중 지연 요소가 존재한다. 이러한 다중 지연요소로 인한 CSI 불일치는 채널 상황에 따라 적절한 변조 방식을 결정하는데 오류를 발생시키게 되어 시스템 성능을 떨어트린다. 본 논문에서는 적응 시스템에 내장되는 다중 지연 채널 예측 방식인 CTSBP(comb type samples based prediction)와 BTSBP(block type samples based prediction)에 대해 전송 지연 및 채널 신호 대 잡음비에 따른 MSE (mean square error), 데이터 율 등 성능을 비교한다. 이를 통하여 악조건 다중경로 채널환경에 강건한 적응 전송 SISO(single input single output)-OFDM/MIMO-OFDM을 설계한다. 또한 CSI 되먹임 오버헤드를 줄이기 위한 방안으로 선형보간 방법을 제안하고 선형 보간 간격에 따른 MSE를 도출한다.

• 대한전자공학회:학술대회논문집
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• 대한전자공학회 2006년도 하계종합학술대회
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• pp.213-214
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• 2006

To reduce the feedback overhead of predicted CSI (channel status information) of adaptive OFDM (orthogonal frequency division multiplexing), we use partial data of CSI and employ linear interpolation. Simulation results show estimated CSI and its MSE.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제7권12호
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• pp.3200-3219
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• 2013

This paper proposes an adaptive combined scalable video coding (CSVC) system for video transmission over MIMO-OFDM (Multiple-Input Multiple-Output-Orthogonal Frequency Division Multiplexing) broadband wireless communication systems. The scalable combination method of CSVC adaptively combines the medium grain scalable (MGS), the coarse grain scalable (CGS) and the scalable spatial modes with the limited feedback partially from channel state information (CSI) of MIMO-OFDM systems. The objective is to improve the average of peak signal-to-noise ratio (PSNR) and bit error rate (BER) of the received video stream by exploiting partial CSI of video sources and channel condition. Experimental results show that the delivered quality using the proposed adaptive CSVC over MIMO-OFDM system performs better than those proposed previously in the literature.

• Journal of Communications and Networks
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• 제18권3호
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• pp.387-396
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• 2016

Multiuser communication has been an important research area of underwater acoustic communications and networking. This paper studies the use of adaptive orthogonal frequency-division multiple access (OFDMA) in a downlink scenario, where a central node sends data to multiple distributed nodes simultaneously. In practical implementations, the instantaneous channel state information (CSI) cannot be perfectly known by the central node in time-varying underwater acoustic (UWA) channels, due to the long propagation delays resulting from the low sound speed. In this paper, we explore the CSI feedback for resource allocation. An adaptive power-bit loading algorithm is presented, which assigns subcarriers to different users and allocates power and bits to each subcarrier, aiming to minimize the bit error rate (BER) under power and throughput constraints. Simulation results show considerable performance gains due to adaptive subcarrier allocation and further improvement through power and bit loading, as compared to the non-adaptive interleave subcarrier allocation scheme. In a lake experiment, channel feedback reduction is implemented through subcarrier clustering and uniform quantization. Although the performance gains are not as large as expected, experiment results confirm that adaptive subcarrier allocation schemes based on delayed channel feedback or long term statistics outperform the interleave subcarrier allocation scheme.

• 한국통신학회논문지
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• 제31권11C호
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• pp.1100-1106
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• 2006

하향 링크 이동통신 시스템에서, 채널 추정 시점과 실제 전송 시점 사이의 시간 지연으로 인해 채널 상태 정보의 정확도가 떨어지게 되고 이는 성능 열화로 이어진다. 이를 극복하고 전송률을 극대화하기 위해 채널 예측이 필수적이다. 본 논문에서는 채널 예측을 통한 적응 전송 기법을 제안하고 성능을 분석한다. 모의 실험을 통해 제안한 기법이 궤환 지연으로 인한 성능 열화를 극복하는데 효율적임을 보였다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제11권9호
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• pp.4242-4263
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• 2017

Multi-User Multiple-Input Multiple-Output (MU-MIMO) technology has recently attracted significant attention from academia and industry because of it is increasingly important role in improving networks' capacity and data rate. Moreover, MU-MIMO systems for the Fifth Generation (5G) have already been researched. High Quality of Service (QoS) and efficient operations at the Medium Access Control (MAC) layer have become key requirements. In this paper, we propose a downlink MU-MIMO MAC protocol based on adaptive Channel State Information (CSI) feedback (called MMM-A) for Wireless Local Area Networks (WLANs). A modified CSMA/CA mechanism using new frame formats is adopted in the proposed protocol. Specifically, the CSI is exchanged between stations (STAs) in an adaptive way, and a packet selection strategy which can guarantee a fairer QoS for scenarios with differentiated traffic is also included in the MMM-A protocol. We then derive the expressions of the throughput and access delay, and analyze the performance of the protocol. It is easy to find that the MMM-A protocol outperforms the commonly used protocols in terms of the saturated throughput and access delay through simulation and analysis results.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제5권11호
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• pp.1959-1975
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• 2011

Adaptive Bit Loading (ABL) in Multiple-Input Multiple-Output Orthogonal Frequency-Division Multiplexing (MIMO-OFDM) is often used to achieve the desired Bit Error Rate (BER) performance in wireless systems. In this paper, we discuss some of the bit loading algorithms, compare them in terms of the BER performance, and present an effective and concise Adaptive Grouped Loading (AGL) algorithm. Furthermore, we propose a "weight factor" for loading algorithm to converge rapidly to the final solution for various data rate with variable Signal to Noise Ratio (SNR) gaps. In particular, we consider the bit loading in near optimal Singular Value Decomposition (SVD) based MIMO-OFDM system. While using SVD based system, the system requires perfect Channel State Information (CSI) of channel transfer function at the transmitter. This scenario of SVD based system is taken as an ideal case for the comparison of loading algorithms and to show the actual enhancement achievable by our AGL algorithm. Irrespective of the CSI requirement imposed by the mode of the system itself, ABL demands high level of feedback. Grouped Loading (GL) would reduce the feedback requirement depending upon the group size. However, this also leads to considerable degradation in BER performance. In our AGL algorithm, groups are formed with a number of consecutive sub-channels belonging to the same transmit antenna, with individual gains satisfying predefined criteria. Simulation results show that the proposed "weight factor" leads a loading algorithm to rapid convergence for various data rates with variable SNR gap values and AGL requires much lesser CSI compared to GL for the same BER performance.

• Journal of Communications and Networks
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• 제18권2호
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• pp.210-217
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• 2016

In this paper, we propose a low complexity multiple-input multiple-output (MIMO) detection algorithm with adaptive interference mitigation in downlink multiuser MIMO (DL MU-MIMO) systems with quantization error of the channel state information (CSI) feedback. In DL MU-MIMO systems using the imperfect precoding matrix caused by quantization error of the CSI feedback, the station receives the desired signal as well as the residual interference signal. Therefore, a complexMIMO detection algorithm with interference mitigation is required for mitigating the residual interference. To reduce the computational complexity, we propose a MIMO detection algorithm with adaptive interference mitigation. The proposed algorithm adaptively mitigates the residual interference by using the maximum likelihood detection (MLD) error criterion (MEC). We derive a theoretical MEC by using the MLD error condition and a practical MEC by approximating the theoretical MEC. In conclusion, the proposed algorithm adaptively performs interference mitigation when satisfying the practical MEC. Simulation results show that the proposed algorithm reduces the computational complexity and has the same performance, compared to the generalized sphere decoder, which always performs interference mitigation.

• 대한전자공학회논문지TC
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• 제46권1호
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• pp.32-39
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• 2009

송신 단에서 채널 상태 정보를 알고 있는 폐루프 시스템은 개방 루프 시스템에 비하여 더 효율적인 전송을 수행할 수 있다. 그러나 실제 시스템은 제한된 피드백 채널을 가지므로 송신단에서 완벽한 채널 정보를 알 수 없으며, 따라서 부분 채널 정보를 활용하는 시스템의 설계가 중요한 요소로 부각되고 있다. 특히, 모바일 환경에서는 사용자의 이동성으로 인하여 채널 상태가 유동적으로 변화하며 이는 성능의 열화를 초래한다. 본 논문에서는 비트 인터리버와 결합한 부호화된 직교 주파수 다중 분할(BIC-OFDM; Bit-Interleaved Coded Orthogonal Frequency Division Multiplexing) 시스템을 위하여 채널 변화에 적절히 대응하는 적응 변조 코딩 기법을 제안한다. 합리적인 피드백 정보량을 통해, 제안하는 기법은 도플러 확산에 의한 채널 변화를 보상하여 향상된 성능을 제공한다. 실험 결과를 통해 제안하는 기법이 정확한 비트 에러율의 추정을 통한 성능 이득을 가짐을 확인한다.