• Journal of Communications and Networks
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• 제16권1호
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• pp.18-25
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• 2014

In this paper, we propose a codebook-based interference alignment (IA) scheme in the constant multiple-input multiple-output (MIMO) interference channel especially for the uplink scenario. In our proposed scheme, we assume cooperation among base stations (BSs) through reliable backhaul links so that global channel knowledge is available for all BSs, which enables BS to compute he transmit precoder and inform its quantized index to the associated user via limited rate feedback link. We present an upper bound on the rate loss of the proposed scheme and derive the scaling law of the feedback load to maintain a constant rate loss relative to IA with perfect channel knowledge. Considering the impact of overhead due to training, cooperation, and feedback, we address the effective degrees of freedom (DOF) of the proposed scheme and derive the maximization of the effective DOF. From simulation results, we verify our analysis on the scaling law to preserve the multiplexing gain and confirm that the proposed scheme is more effective than the conventional IA scheme in terms of the effective DOF.

• 한국통신학회논문지
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• 제29권6A호
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• pp.640-650
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• 2004

본 논문에서는 Multiple Put-Multiple output Orthogonal Frequency Division Multiplexing(MIMO-OFDM) 시스템을 위한 채널 추정 기법으로 space-time block c첨ing(STBC) 기법과 space-frequency block coding(SFBC) 기법을 제안한다. 제안된 방식은 각 송신 안테나에서 서로 직교성을 갖는 훈련 심볼을 전송함으로써 수신 안테나에서는 각 송수신 안테나 사인의 채널 계수를 추정한다. 제안된 알고리즘은 IEEE 802.11a 시스템 기반의 MIMO-OFDM 시스템에 적용해 성능 개선 효과를 확인해 보았다. 그 결과 제안된 방법이 기존의 방법보다 더 정확한 채별 계수 추정이 가능함을 확인할 수 있었다. 모의실험은 기존의 추정기법과 제안된 추정 기법이 적용된 시스템을 50ns 와 150ns 의 RMS 지연 확산을 갖는 다중 경로 페이딩 환경에서 수행되었으며 각 변복조 방식에 따른 비트 오류 성능 개선 정도를 확인해 보았다.

• Journal of Communications and Networks
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• 제7권3호
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• pp.307-312
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• 2005

In this work, we study the performance of a serial concatenated scheme comprising a convolutional code (CC) and an orthogonal space-time block code (STBC) separated by an inter-leaver. Specifically, we derive performance bounds for this concatenated scheme, clearly quantify the impact of using a CC in conjunction with a STBC, and compare that to using a STBC code only. Furthermore, we examine the impact of performing antenna selection at the receiver on the diversity order and coding gain of the system. In performing antenna selection, we adopt a selection criterion that is based on maximizing the instantaneous signal-to­noise ratio (SNR) at the receiver. That is, we select a subset of the available receive antennas that maximizes the received SNR. Two channel models are considered in this study: Fast fading and quasi-static fading. For both cases, our analyses show that substantial coding gains can be achieved, which is confirmed through Monte-Carlo simulations. We demonstrate that the spatial diversity is maintained for all cases, whereas the coding gain deteriorates by no more than $10\;log_{10}$ (M / L) dB, all relative to the full complexity multiple-input multiple-output (MIMO) system.

• 한국통신학회논문지
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• 제24권6A호
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• pp.782-790
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• 1999

본 논문은 slowly 레일리 페이딩에서 기존의 ATCMPSK(Adaptive Trellis-Coded Multiple Phase Shift Keying) 시스템을 트렐리스 코디드(Trellis-Coded) MQAM(Multiple Quadrature Amplitude Modulation)을 적용한 적응화 시스템 모델을 제안하였다. 이 제안한 적응화 시스템 모델은 채널 상태에 따라서 프래크메틱 부호화기를 사용하여 부호율과 변조 레벨을 가변하였다. 컴퓨터 모의 실험에서는 제안한 ATCMQAM(Adaptive Trellis-Coded MQAM)과 기존의 ATCMPSK(Adaptive Trellis-Coded MPSK) 모델에 대한 성능을 평가하였다. 결론적으로, MQAM을 적용한 시스템이 MPSK를 적용한 시스템에 비해 성능이 비슷할 때, 데이터 출력율면에서 보면 SNR이 증가함에 따라 0.5 ∼ 1비트이상 향상되었다. 또한 코딩 이득 (coding gain)도 2 ∼ 5dB정도 얻을 수 있었다.

• Journal of Communications and Networks
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• 제10권2호
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• pp.186-193
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• 2008

A two-hop multiple-input multiple-output (MIMO) relay network which comprises a multiple antenna source, an amplify-and-forward MIMO relay and many potential users are studied in this paper. Consider the achievable sum rate as the performance metric, a joint design method for the processing units of the BS and relay node is proposed. The optimal structures are given, which decompose the multiuser MIMO relay channel into several parallel single-input single-output relay channels. With these structures, the signal-to-noise ratio at the destination users is derived; and the power allocation is proved to be a convex problem. We also show that high sum rate can be achieved by pairing each link according to its magnitude. The sum rate of three broadcast strategies, time division multiple access (TDMA) to the strongest user, dirty paper coding (DPC), and beamforming (BF) are investigated. The sum rate bounds of these strategies and the sum capacity (achieved by DPC) gain over TDMA and BF are given. With these results, it can be easily obtained that how far away TDMA and BF are from being optimal in terms of the achievable sum rate.

• 대한전자공학회논문지TE
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• 제42권1호
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• pp.41-46
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• 2005

채널의 잡음 영향을 극복하기 위하여 새로운 멀티플 디스크립션 기법을 제안한다. 본 기법은 임베디드 부호화기법을 채택하고 있으며, 부대역 계수가 한번에 비트 플레인 단위로 부호화 되는데 이때 비적응적 산술부호화 방식을 사용한다. 각 영역의 중요도에 따라 관심영역을 부호화할 때, 다른 부분에 비해서 패스 횟수를 늘려서 부호화를 행함으로써 관심영역이 다른 부분에 비해서 고화질로 부호화되도록 한다. 또 채널 에러를 극복하기 위해서 부호화 과정에서 소스 데이터에 제어된 중복성을 허용하는 멀티플 디스크립션 기법을 채택한다. 본 방식은 고압축률을 지향하고, 특정한 영역이 다른 부분에 비해서 중요한 시스템의 경우에 보다 중요한 특성을 나타낸다.

• Journal of Communications and Networks
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• 제16권6호
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• pp.639-644
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• 2014

In low-density parity-check (LDPC) coded multiple-input multiple-output (MIMO) communication systems, probabilistic information are exchanged between an LDPC decoder and a MIMO detector. TheMIMO detector has to calculate probabilistic values for each bit which can be very complex. In [1], the authors presented a class of linear block codes named low-density MIMO codes (LDMC) which can reduce the complexity of MIMO detector. However, this code only supports the outer-iterations between the MIMO detector and decoder, but does not support the inner-iterations inside the LDPC decoder. In this paper, a new approach to construct LDMC codes is introduced. The new LDMC codes can be encoded efficiently at the transmitter side and support both of the inner-iterations and outer-iterations at the receiver side. Furthermore they can achieve the design rates and perform very well over MIMO channels.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권4호
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• pp.1535-1554
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• 2016

This paper proposes a novel channel assignment scheme called multi-cluster based dynamic channel assignment (MC-DCA) to improve system performance for the downlink of dense femtocell networks (DFNs) based on orthogonal frequency division multiple access (OFDMA) and frequency division duplexing (FDD). In order to dynamically assign channels for femtocell access points (FAPs), the MC-DCA scheme uses a heuristic method that consists of two steps: one is a multiple cluster assignment step to group FAPs using graph coloring algorithm with some extensions, while the other is a dynamic subchannel assignment step to allocate subchannels for maximizing the system capacity. Through simulations, we first find optimum parameters of the multiple FAP clustering to maximize the system capacity and then evaluate system performance in terms of the mean FAP capacity, unsatisfied femtocell user equipment (FUE) probability, and mean FAP power consumption for data transmission based on a given FUE traffic load. As a result, the MC-DCA scheme outperforms other schemes in two different DFN environments for commercial and office buildings.

• 한국산업정보학회논문지
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• 제20권6호
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• pp.1-10
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• 2015

본 논문에서는 음영 지역을 위한 다중 중계기 기반의 주파수 효율적인 OFDMA(orthogonal frequency division multiple access) 전송 시스템을 제안한다. 본 논문에서는 다중 중계기를 통해 확보된 다중 경로에 주파수 공간 블록 부호를 적용하여 음영 지역의 시스템 성능을 향상시킨다. 또한, 주기적 프리픽스 부족으로 인해 발생하는 채널 주기성 손실을 복원하는 과정이 포함된 주기적 프리픽스를 사용하지 않는 OFDMA 시스템을 제안한다. 본 논문에서 제안된 주기적 프리픽스를 사용하지 않는 OFDMA 시스템의 성능이 충분한 주기적 프리픽스를 사용하는 기존 OFDMA 시스템의 성능과 거의 동일하여, 기존 시스템 대비 주파수 효율이 향상되는 것을 실험을 통해서 확인할 수 있다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제13권2호
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• pp.635-656
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• 2019

In this paper, we investigate the degrees of freedom (DoF) of a multi-cell multi-user multiple-input multiple-output (MIMO) interference broadcast channel (IBC) with non-cooperation distributed base stations (BS), where each BS serves users of its corresponding cell. When all BSs simultaneously transmit their own signals over the same frequency band in the MIMO IBC, the edge users in each cell will suffer the inter-cell interference (ICI) and inter-user interference (IUI) signals. In order to eliminate the ICI and IUI signals, a distributed space time interference alignment (DSTIA) approach is proposed where each BS has only limited access to distributed moderately-delay channel state information at the transmitter (CSIT). It is shown that the DSTIA scheme can obtain the appreciate DoF gains. In addition, the DoF upper bound is asymptotically achievable as the number of antenna at each BS increases. It is shown that the DSTIA method can get DoF gains over other interference alignment schemes with delayed CSIT in literature. Moreover, the DSTIA method can attain higher DoFs than the IA schemes with global CSIT for certain antenna configurations.