• 한국통신학회논문지
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• 제30권6C호
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• pp.481-489
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• 2005

In this letter, we propose an efficient training sequence structure for adaptive linear multiuser detectors in space-time block coded multiuser systems, by exploiting a particular property of the minimum mean square error multiuser detectors used in these systems. The proposed structure wastes less overall system capacity than the straightforward training structure, without any corresponding loss of performance, as confirmed by the simulation results.

• Journal of Communications and Networks
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• 제12권6호
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• pp.605-615
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• 2010

Intercarrier interference (ICI) in orthogonal frequency division multiplexing (OFDM) systems, which causes substantial performance degradation in time-varying fading channels, is analyzed. An equivalent spreading code formulation is derived based on the analogy of OFDM and code division multiple access (CDMA) systems. Techniques as linear multiuser detection in CDMA systems are applied to suppress the ICI in OFDM systems. The performance of linear detection, measured using multiuser efficiency and asymptotic multiuser efficiency, is analyzed given the assumption of perfect channel state information (CSI), which serves as an upper bound for the performance of practical systems. For systems without CSI, time domain and frequency domain channel estimation based linear detectors are proposed. The performance gains and robustness of a linear minimum mean square error (LMMSE) filter over a traditional filter (TF) and matched filter (MF) in the high signal-to-noise ratio (SNR) regime are demonstrated with numerical simulation results.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제5권9호
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• pp.1573-1595
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• 2011

In this paper, the outage performance of multiuser multiple-input single-output (MISO) systems exploiting joint spatial and multiuser diversities is investigated for Rayleigh fading channels with outdated feedback. First, we derive closed-form exact outage probabilities for the joint diversity schemes that combine user scheduling with different spatial diversity techniques, including: 1) transmit maximum-ratio combining (TMRC); 2) transmit antenna selection (TAS); and 3) orthogonal space-time block coding (OSTBC). Then the asymptotic outage probabilities are analyzed to gain more insights into the effect of feedback delay. It is observed that with outdated feedback, the asymptotic diversity order of the multiuser OSTBC (M-OSTBC) scheme is equal to the number of transmit antennas at the base station, while that of the multiuser TMRC (M-TMRC) and the multiuser TAS (M-TAS) schemes reduce to one. Further by comparing the asymptotic outage probabilities, it is found that the M-TMRC scheme outperforms the M-TAS scheme, and the M-OSTBC scheme can perform best in the outage regime of practical interest when the feedback delay is large. Theoretical analysis is verified by simulation results.

• 한국통신학회논문지
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• 제33권10A호
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• pp.980-986
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• 2008

Spatial multiuser access is investigated for the reverse link of multiuser multiple-input multiple-output (MIMO) systems. In particular, we consider two alternative a aches to spatial multiuser access that adopt the same detection algorithm at the base station: one is a closed-loop approach based on singular value decomposition (SVD) of the channel matrix, whereas the other is an open-loop approach based in space-time block coding (STBC). We develop multiuser detection algorithms for these two spatial multiuser access schemes based on the minimum mean square error (MMSE) criterion. Then, we compare the bit error rate (BER) performance of the two schemes and a single-user MIMO scheme. Interestingly, it is found that the STBC approach can provide much better BER performance than the SVD approach as well as than a single-user MIMO scheme.

• 한국통신학회논문지
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• 제30권3C호
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• pp.153-157
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• 2005

이동 통신 시스템에서 도플러 효과에 의해 주파수 선택적 페이딩 현상이 발생한다. DS-CDMA 시스템은 주파수 선택적 채널 하에서 다중 사용자 간섭의 영향에 의해서 성능이 급격하게 저하 된다. 이를 극복하기 위해서 다중 반송파 변조 방법을 채택하여 주파수 선택적 채널 효과를 극복할 수 있다. 그리고 다중 사용자 접속으로 인한 간섭 문제는 다중 사용자 검출 기법을 적용하여 해결 할 수 있다. 본 논문에서는 블라인드 다중 사용자 검출 기법과 병렬 간섭 제거기를 결합한 형태의 블라인드 MOE-PIC 다중 사용자 검출 기법을 제안하였다. 모의실험 결과 제안한 방식이 기존의 방식보다 더 좋은 성능을 나타내는 것을 알 수 있었다.

• 한국정보통신설비학회:학술대회논문집
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• 한국정보통신설비학회 2008년도 정보통신설비 학술대회
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• pp.12-16
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• 2008

In this paper, we study precoding techniques for co-channel interference suppression in multiuser MIMO systems. DPC is optimal techniques to achieve the system capacity of multiuser MIMO downlink channels. DPC is not proper in practical wireless systems because complexity is very high. So block diagonal precoding for multiuser MIMO downlink channel is studied. The block diagonal precoding is used to suppress co-channel interference between multiuser. Block diagonal precoding method, whose complexity is reduced by modified null space operation, change multiuser MIMO channel to multiple single-user MIMO channel. We also use V-BLAST decoder in receiver. V-BLAST decoder can achieve increased system capacity in proportion to the number of users. We show improved system performance by using computer simulation.

• 한국통신학회논문지
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• 제35권3A호
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• pp.260-268
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• 2010

Channel inversion is one of the simplest techniques for multiuser downlink systems with single-antenna users. In this paper, we extend the regularized channel inversion technique developed for the single-antenna user case to multiuser multiple-input multiple-output (MIMO) channels with multiple-antenna users. We first employ the multiuser preprocessing to project the multiuser signals near the null space of the unintended users based on the MMSE criterion, and then the single-user preprocessing is applied to the decomposed MIMO interference channels. In order to reduce the complexity, we focus on non-iterative solutions for the multiuser transmit beamforming and use a linear receiver based on an MMSE criterion. Simulation results show that the proposed scheme outperforms existing joint iterative algorithms in most multiuser configurations.

• 한국위성정보통신학회논문지
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• 제6권1호
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• pp.19-27
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• 2011

본 논문에서는 블록 대각화 프리코딩 기법을 사용하는 다중 사용자 MIMO 하향링크 시스템에서 전체 채널 용량을 최대화하기 위한 데이터 스트림 할당 기법을 제안한다. 기존의 블록 대각화 프리코딩 기법은 모든 사용자에게 동일한 수의 데이터 스트림을 할당하고 water-filling에 의한 전력제어 만으로 전체 채널 용량을 최대화함으로써 데이터 스트림의 수의 차이에 의한 채널 용량의 이득을 스스로 제한했다. 제안하는 블록 대각화 프리코딩의 데이터 스트림 할당 기법은 시스템의 전체 채널 용량을 최대화하기 위하여 평균 채널이득, 순시 채널 이득의 크기를 비교하여 채널 이득이 좋은 사용자에게 많은 수의 데이터 스트림을 할당하고 water-filling을 통한 전력 제어를 하는 방법으로 채널 이득에 대한 채널 용량을 최대화 시킨다. 본 논문에서 제안된 데이터 스트림 할당 기법을 이용하여 시스템의 성능을 향상시킬 수 있음을 모의실험을 통해 증명했다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제10권7호
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• pp.3010-3025
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• 2016

Lattice reduction (LR) has been used widely in conventional multiple-input multiple-output (MIMO) systems to enhance the performance. However, LR is hard to be applied to the relay systems which are important but more complicated in the wireless communication theory. This paper introduces a new viewpoint for utilizing LR in multiuser MIMO relay systems. The vector precoding (VP) is designed along with zero force (ZF) criterion and minimum mean square error (MMSE) criterion and enhanced by LR algorithm. This implementable precoder design combines nonlinear processing at the base station (BS) and linear processing at the relay. This precoder is capable of avoiding multiuser interference (MUI) at the mobile stations (MSs) and achieving excellent performance. Moreover, it is shown that the amount of feedback information is much less than that of the singular value decomposition (SVD) design. Simulation results show that the proposed scheme using the complex version of the Lenstra--Lenstra--Lovász (LLL) algorithm significantly improves system performance.

• ETRI Journal
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• 제40권2호
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• pp.218-226
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• 2018

Multiuser detection (MUD) and channel estimation techniques in space-division multiple-access aided orthogonal frequency-division multiplexing systems recently has received intensive interest in receiver design technologies. The maximum likelihood (ML) MUD that provides optimal performance has the cost of a dramatically increased computational complexity. The minimum mean-squared error (MMSE) MUD exhibits poor performance, although it achieves lower computational complexity. With almost the same complexity, an MMSE with successive interference cancellation (SIC) scheme achieves a better bit error rate performance than a linear MMSE multiuser detector. In this paper, hybrid ML-MMSE with SIC adaptive multiuser detection based on the joint channel estimation method is suggested for signal detection. The simulation results show that the proposed method achieves good performance close to the optimal ML performance at low SNR values and a low computational complexity at high SNR values.