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Synchronization and Channel Estimation in OFDM Systems
유철우,홍대식,Yu, Cheol-U,Hong, Dae-Sik 한국통신학회 2007 정보와 통신 Vol.24 No.8
차세대 이동 통신 시스템은 다양한 응용 분야들을 지원하기 위하여 광대역을 사용한 고속 데이터 전송이 가능해야 할 것이다. 이와 같은 요구 사항을 효율적으로 대처할 수 있는 중요한 전송 방식중의 하나가 OFDM (Orthogonal frequency division multiplexing) 기법이며, 여러 국제 표준단체에서 차세대 전송 방식의 중요 후보로써 채택하여 심도깊은 연구가 진행되고 있는 실정이다. 본 고에서는 OFDM 전송 방식의 기본적인 특징을 살펴보고, OFDM 시스템 상에서 동기 획득 (Synchronization) 및 채널 추정 (Channel estimation)을 위하여 사용될 수 있는 기본적인 알고리즘들에 대하여 간략히 소개하고 그 특징을 살펴본다.
Improved Iterative Decoding of Parallel and Serially Concatenated Trellis Coded Modulation
유철우,서동선,You, Cheol-Woo,Seo, Dong-Sun Institute of Korean Electrical and Electronics Eng 2007 전기전자학회논문지 Vol.11 No.4
본 논문에서는, 병렬 혹은 직렬적으로 연접된 트렐리스 부호화 변조 기법 (Trellis coded modulation: TCM)을 위한 간단한 구조를 가진 향상된 반복적 복호 기법들이 제안되며, 동시에 제안된 기법들의 성능을 기존 기법들과 비교 제시한다. 제안된 복호 알고리즘은 기존 알고리즘의 단순 변형을 통해서 구현될 수 있음에도 불구하고, 모의실험 결과는 제안된 기법들이 부가 백색 가우스 잡음 채널 (Additive white Gaussian noise channel: AWGN channel) 및 레일리 (Rayleigh) 페이딩 채널 상에서 상당한 부호 이득을 제공함을 보여 준다. For parallel and serially concatenated trellis coded modulation (TCM), improved iterative decoding schemes with a simple mechanism are proposed and their performances are compared with those of conventional decoding schemes. Simulation results have shown that the proposed schemes have provided a considerable decoding gain in additive white Gaussian noise (AWGN) channels and Rayleigh fading channels, even if they can be implemented by a simple modification of conventional decoding algorithms.
신경회로망 콘볼루션 복호기의 최적 성능에 대한 확률적 근사화
유철우,강창언,홍대식 대한전자공학회 1996 전자공학회논문지-A Vol.33 No.4
It is well known that the viterbi algorithm proposed as a mthod of decoding convolutional codes is in fact maximum likelihood (ML) and therefore optimal. But, because hardware complexity grows exponentially with the constraint length, there will be severe constraints on the implementation of the viterbi decoders. In this paper, the three-layered backpropagation neural networks are proposed as an alternative in order to get sufficiently useful performance and deal successfully with the problems of the viterbi decoder. This paper shows that the neural convolutional decoder (NCD) can make a decision in the point of ML in decoding and describes simulation results. The cause of the difference between stochastic results and simulation results is discussed, and then thefuture prospect of the NCD is described on the basis of the characteristic of the transfer function.
유철우,정영호,조성현 한국통신학회 2011 Journal of communications and networks Vol.13 No.4
Beamforming techniques have been successfully utilized for traffic channels in order to solve the interference problem. However, their use for control channels has not been sufficiently in-vestigated. In this paper, a (semi-) centralized beamforming strat-egy that adaptively changes beam patterns and controls the total transmit power of cells is proposed for the performance enhance-ment of the common channel in hierarchical cell structure (HCS)networks. In addition, some examples of its practical implementa-tion with low complexity are presented for two-tier HCS networks consisting of macro and pico cells. The performance of the pro-posed scheme has been evaluated through multi-cell system-level simulations under optimistic and pessimistic interference scenar-ios. The cumulative distribution function of user geometry or chan-nel quality has been used as a performance metric since in the case of common control channel the number of outage users is more important than the sum rate. Simulation results confirm that the proposed scheme provides a significant gain compared to the ran-dom beamforming scheme as well as conventional systems that do not use the proposed algorithm. Finally, the proposed scheme can be applied simultaneously to several adjacent macro and pico cells even if it is designed primarily for the pico cell within macro cells.
비선형 왜곡을 가진 위성 채널상에서 신경회로망 콘볼루션 복호기(NCD)의 성능
유철우,강창언,홍대식 한국통신학회 1996 韓國通信學會論文誌 Vol.21 No.8
The neural convolutional decoder(NCD) was proposed as a method of decoding convolutional codes. In this paper, simulation results are presented for coherent BPSK in memoryless AWGN channels and coherent QPSK in the satellite channels. The NCD can learn the nonlinear distortion caused by the charactersitics of the satellite channel including the filtering effects and the nonlinear effects of the travling wave tube amplifier(TWTA). Thus, as compared with the AWGN channel, the performance difference in the satellite channel between the NCD for the systematic code and the Viterbi decoder for the nonsystematic code is reduced.
새로운 다단계 복소 활성 함수를 이용한 신경회로망에 의한 M-ary QAM 신호의 적응 등화
유철우,홍대식 대한전자공학회 1998 電子工學會論文誌, C Vol.c35 No.1
For decreasing intersymbol interference (ISI) due to band-limited channels in digitalcommunication, the uses of equalization techniques are necessary. Among the useful adaptive equalization techniques, because of their ease of implementation and nonlinear capabilites, the neural networks have been used as an alternative for effectively dealing with the channel distortion. In this paepr, a complex-valued multilayer percepron is proposed as a nonlinear adaptive equalizer. After the important properties that a suitable complex-valued activation function must possess are discussed, a new complex-valued activation function is developed for the proposed schemes to deal with M-ary QAM signals of any constellation sizes. It has been further proven that by the nonlinear transformation of the proposed function, the correlation coefficient between the real and imaginary parts of input data decreases when they are jointly Gaussian random variables. Lastly, the effectiveness of the proposed scheme is demonstrated by simulations. The proposed scheme provides, compared with the linear equalizer using the least mean squares (LMS) algorith, an interesting improvement concerning Bit Error Rate (BER) when channel distortions are nonlinear.
Joint Beamforming and Power Optimization for Cooperative Downlink MIMO Systems
유철우 한국통신학회 2023 韓國通信學會論文誌 Vol.48 No.4
Herein, we propose a joint optimization algorithm to minimize the overall transmit power in multicell multiuser multiple-input multiple-output (MIMO) systems for B5G. The proposed algorithm is an iterative optimization algorithm that jointly computes the transmit/receive beamforming vectors and the transmit power. The superiority of the proposed method is demonstrated through simulations.