• Title/Summary/Keyword: Time-varying Channels

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Low Complexity MMSE with Successive Interference Cancellation for OFDM Systems over Time-selective Channels (시변 채널 환경에서 OFDM 시스템을 위한 복잡도가 감소된 MMSE-SIC 등화기법)

  • Park, Ji-Hyun;Hwang, Seung-Hoon;Whang, Keum-Chan
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.33 no.7A
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    • pp.743-750
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    • 2008
  • Orthogonal frequency division multiplexing (OFDM) is a attractive modulation scheme for high data rate transmission in frequency-selective channels. However, the time selectivity of wireless channel introduces intercarrier interference (ICI), and consequently degrades system performance. In this paper, we first propose a novel recursive algorithm for minimum mean squared error (MMSE) with successive interference cancellation (SIC). The proposed algorithm can significantly reduce the complexity of the MMSE-SIC scheme and achieve the same performance when optimal ordering is known. Also, the further reduced scheme of the proposed algorithm can be developed based on ICI properties, while preserving performance.

Low Complexity Bilateral Search Successive Interference Cancellation for OFDM in Fast Time-Varying Channels (고속 시변 채널 OFDM을 위한 저복잡도 양방향 탐색 순차적 간섭 제거)

  • Lim, Dongmin
    • Journal of the Institute of Electronics and Information Engineers
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    • v.50 no.1
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    • pp.9-14
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    • 2013
  • In this paper, we propose a low complexity bilateral search SIC for OFDM in fast time-varying channels. Due to the possibility of error propagation in SIC, symbol detection ordering within the block of symbols has a significant effect on the overall performance. In this paper, the first symbol to be detected is determined based on CSEP values, and then the next symbol to be detected is selected according to the updated CSEP while bilaterally searching from the boundary of the detected symbol group. Through computer simulations, we show that the proposed method has performance improvements with almost the same computation complexity over the conventional methods in the high SNR region. It has a performance approaching the MFB, known as the performance upper bound, within 2dB at the BER of $10^{-5}$.

Linear Suppression of Intercarrier Interference in Time-Varying OFDM Systems: From the Viewpoint of Multiuser Detection

  • Li, Husheng
    • Journal of Communications and Networks
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    • v.12 no.6
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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.

Performance Evaluation of Bandwidth Efficient Adaptive QAM Schemes in Flat and Frquency Selective Fading Channels (균일 및 주파수 선택적 페이딩에서 대역폭 효율의 적응 QAM 성능분석)

  • 정연호
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.25 no.10A
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    • pp.1473-1479
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    • 2000
  • This paper presents the performance evaluation of an adaptive QAM scheme under flat and frequency selective fading channels for indoor wireless communication systems. The QAM modulation is combined with differential encoding and the demodulation process is carried out noncoherently. The adaptation is performed by varying the modulation level of QAM, depending upon received signal strength. The adaptation mechanism allows a 2- or 3-bit increase or decrease at a time, if the channel condition is considered to be significantly good or bad. Simulation results show that the average number of bits per symbol (ABPS) for each symbol block transmitted over a flat fading channel is higher than 5.0 and the BER performance is better than 10^-4 for a SNR value higher than 30 dB. For frequency selective fading channels, an oversampling technique in the receiver was employed. The BER performance obtained for frequency selective fading channels is better than 10^-4 with a SNR value of 40 dB and ABPS is found to be approximately 5.5. Therefore, this scheme is very useful in that it provides both very high bandwidth efficiency and acceptable performance with moderate SNR values over flat and frequency selective fading channels. In addition, this scheme provides reduced receiver complexity by way of noncoherent detection.

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Underwater Acoustic Communication Channel Modeling Regarding Magnitude Fluctuation Based on Ocean Surface Scattering Theory and BELLHOP Ray Model and Its Application to Passive Time-reversal Communication (해수면에 의한 신호 응답 강도의 시변동성 특성이 적용된 벨홉 기반의 수중음향 통신 채널 모델링 및 수동 시역전 통신 응용)

  • Kim, Joonsuk;Koh, Il-Suek;Lee, Yongshik
    • The Journal of the Acoustical Society of Korea
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    • v.32 no.2
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    • pp.116-123
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    • 2013
  • This paper represents generation of time-varying underwater acoustic channels by performing scattering simulation with time-varying ocean surface and Kirchhoff approximation. In order to estimate the time-varying ocean surface, 1D Pierson-Moskowitz ocean power spectrum and Gaussian correlation function were used. The computed scattering coefficients are applied to the amplitudes of each impulse of BELLHOP simulation result. The scattering coefficients are then compared with measured doppler spectral density of signal components which were scattered from ocean surface and the correlation time used in the Gaussian correlation function was estimated by the comparison. Finally, bit-error-rate and channel correlation simulations were performed with the generated time-varying channel based on passive time-reversal communication scenario.

Non-data Aided Timing Phase Recovery Scheme for Digital Equalization of Chromatic Dispersion and Polarization Mode Dispersion

  • Park, Jang-Woo;Chung, Won-Zoo;Park, Jong-Sun;Kim, Sung-Chul
    • Journal of the Optical Society of Korea
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    • v.13 no.3
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    • pp.367-372
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    • 2009
  • In this paper we propose an electronic domain timing phase selection scheme for the optical communication systems suffering from inter-symbol-interference (ISI) distortion due to chromatic dispersion (CD) or polarization mode dispersion (PMD). In the presence of CD/PMD a proper timing phase selection is important for discrete time domain equalizers, since different timing phases produce different nonlinear ISI channels of different severity. The proposed timing phase recovery scheme based on dispersion minimization (DM) practically approximates the optimal minimum mean squared error (MMSE) timing phase without training signals which reduces overall throughput substantially, especially in time-varying channels such as PMD. The simulation results show that the proposed DM timing agrees with MMSE timing phase, under proper normalization of the received signals, for various dispersion and OSNR.

Performance Analysis of Pilot Patterns for Channel Estimation in OFDM Systems (OFDM 시스템에서 채널 추정을 위한 파일럿 패턴의 성능 분석)

  • Choe, Kwang-Don;Hyun, Deok-Soo;Park, Sang-Kyu
    • The Journal of Korean Institute of Communications and Information Sciences
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    • v.30 no.8A
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    • pp.664-670
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    • 2005
  • OFDM is a very attractive technique for achieving high-bit-rate data transmission and high spectrum efficiency in fading environment. However, the reliable detection of an OFDM signal in time-varying multipath fading channels is a challenging problem. Accordingly, various channel estimation methods have been proposed for performance improvement. But, conventional pilot patterns for channel estimation in OFDM systems have not robust characteristics relating to various mobile speed. To solve this drawback in conventional patterns, we propose the pilot patterns modified from conventional patterns to have a good error performance in time-varying fading channel. Simulation results show that the performance of the proposed pilot patterns is better than conventional patterns in fast time-varying channel.

Incremental Passivity Based Control for DC-DC Boost Converters under Time-Varying Disturbances via a Generalized Proportional Integral Observer

  • He, Wei;Li, Shihua;Yang, Jun;Wang, Zuo
    • Journal of Power Electronics
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    • v.18 no.1
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    • pp.147-159
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    • 2018
  • In this paper, the voltage tracking control of a conventional DC-DC boost converter affected by unknown, time-varying circuit parameter perturbations is investigated. Based on the fundamental property of incremental passivity, a passivity based control law is designed. Then, to obtain a better disturbance rejection property, two generalized proportional integral (GPI) observers are employed to estimate the time-varying uncertainties in the output voltage and inductor current channels, and the estimated values are applied as feedforward compensation. Moreover, the global trajectory tracking performance of a system with disturbances is ensured under the composite controller. Finally, simulation and experiment studies are provided to demonstrate the feasibility and effectiveness of the proposed method. The results show that the proposed controller delivers a promising disturbance rejection capability as well as a good nominal tracking performance.

An time-varying acoustic channel estimation using least squares algorithm with an average gradient vector based a self-adjusted step size and variable forgetting factor (기울기 평균 벡터를 사용한 가변 스텝 최소 자승 알고리즘과 시변 망각 인자를 사용한 시변 음향 채널 추정)

  • Lim, Jun-Seok
    • The Journal of the Acoustical Society of Korea
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    • v.38 no.3
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    • pp.283-289
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    • 2019
  • RLS (Recursive-least-squares) algorithm is known to have good convergence and excellent error level after convergence. However, there is a disadvantage that numerical instability is included in the algorithm due to inverse matrix calculation. In this paper, we propose an algorithm with no matrix inversion to avoid the instability aforementioned. The proposed algorithm still keeps the same convergence performance. In the proposed algorithm, we adopt an averaged gradient-based step size as a self-adjusted step size. In addition, a variable forgetting factor is introduced to provide superior performance for time-varying channel estimation. Through simulations, we compare performance with conventional RLS and show its equivalency. It also shows the merit of the variable forgetting factor in time-varying channels.

STBC Detection Algorithm Using Double-Decision-Feedback Scheme in Time-Varying Rayleigh-Fading Channel (시변 레일리 페이딩 채널에서 이중 판정 궤환 방식을 이용한 STBC 검출 알고리즘)

  • Park, Sung-Joon;Heo, Seo-Weon;Lee, Ho-Kyoung
    • The Journal of Korean Institute of Electromagnetic Engineering and Science
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    • v.18 no.11
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    • pp.1237-1242
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    • 2007
  • In this paper, we study STBC(Space Time Block Code) detection scheme in time varying Rayleigh fading channel. When the channel is varying during the time duration of STBC, the channel matrix of orthogonal STBC is not orthogonal. To get the optimum reception performance in this channel, joint ML detection scheme may be used, however this scheme requires high computation complexity. Decision feedback scheme is proposed to reduce the computation complexity with less reception performance. In this paper, we propose a novel STBC detection algorithm using double decision feedback which is less complex than the joint ML scheme and outperforms the conventional decision feedback scheme.