Channel Estimation in OFDM Systems

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Channel Estimation in OFDM Systems. Zhibin Wu Yan Liu Xiangpeng Jing. OUTLINE. OFDM System Introduction Channel Estimation Techniques Performance Evaluation Conclusion. OFDM Overview. Divides high-speed serial information signal into multiple lower-speed sub-signals: - PowerPoint PPT Presentation

Text of Channel Estimation in OFDM Systems

  • Channel Estimation in OFDM SystemsZhibin Wu Yan Liu Xiangpeng Jing

  • OUTLINEOFDM System Introduction

    Channel Estimation Techniques

    Performance Evaluation


  • OFDM OverviewDivides high-speed serial information signal into multiple lower-speed sub-signals:Transmits simultaneously at different frequencies in parallel.Modulation ( BPSK, PSK,QPSK,16QAM, ).Pilot sub-carriers used to prevent frequency and phase shift errors.Usage of cyclic prefix for lower multi-path distortionControlled overlapping of bands in one channel Max spectral efficiency (Nyquist rate) Easy implementation using inverse FFTs Easy time-freq. Synchronization Modulate by switching between time and frequency domain

  • Introduction to OFDM Systems

  • Time-Frequency View

  • Some AssumptionsUsage of cyclic Prefix Impulse response of the channel shorter than Cyclic PrefixSlow fading effects so that the channel is time-invariant over the symbol intervalRectangular Windowing of the transmitted pulsesPerfect Synchronization of transmitter and receiverAdditive, white, Gaussian channel noise

  • System Architecture

  • System Architecture (contd)Input to time domainGuard Interval ChannelGuard RemovalOutput to frequency domain Output Channel EstimationICIAWGNChannelEstimated Channel

  • Pilot for Channel EstimationTimeCarriersTimeCarriersComb Type:Part of the sub-carriers are always reserved as pilot for each symbol

    Block Type:All sub-carriers is used as pilot in a specific period

  • Block-type Channel Estimation

    LS: Least Square Estimation

  • Comb-type EstimationNp pilot signals uniformly inserted in X(k)L=Number of Carriers/Npxp(m) is the mth pilot carrier value {Hp(k) k=0,1,,Np} , channel at pilot sub-carriersXp input at the kth pilot sub-carrierYp output at the kth pilot sub-carrierLS EstimateLMS EstimateXp(k) LMS+e(k)-Yp(k)

  • Interpolation for Comb-typeLinear Interpolation

    Second Order Interpolation

  • Simulation Parameters

  • System structure in MATLAB Simulation

  • OFDM TransmitterOFDM Receiver

  • Received and Recovered SignalsReceived signal phases are distorted by multi-path fading

  • Comb-LS EstimationCombating multipath rayleigh fading with RLS adaptive equalizationA detail simulation with MATLAB 20 multipath, random phase, and weibull distribution of amplitutdeSymbol Error Rate

  • Comb-LS Estimation

  • Filter length .vs. Sample RateKeep the ratio of F/S, increase SKeep S, increase F.Observed Symbol error rate with F ( filter length ) and S ( samples per symbol)

  • ConclusionOFDM System IntroductionBlock Type Direct or Decision FeedbackComb TypeLS or LMS estimation at pilot frequenciesInterpolation TechniquesLinearSecond OrderTime DomainModulation BPSK,QPSK,16QAM,DQPSKSome Results:Comb Type performs better since it tracks fast fading channels.RLS algorithm vs. LMS algorithm