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Channel Estimation in OFDM Systems

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:

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Channel Estimation in OFDM Systems

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  1. Channel Estimation in OFDM Systems Zhibin WuYan LiuXiangpeng Jing

  2. OUTLINE • OFDM System Introduction • Channel Estimation Techniques • Performance Evaluation • Conclusion

  3. OFDM Overview • Divides 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 distortion • Controlled 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

  4. Introduction to OFDM Systems

  5. Time-Frequency View

  6. Some Assumptions • Usage of cyclic Prefix • Impulse response of the channel shorter than Cyclic Prefix • Slow fading effects so that the channel is time-invariant over the symbol interval • Rectangular Windowing of the transmitted pulses • Perfect Synchronization of transmitter and receiver • Additive, white, Gaussian channel noise

  7. System Architecture

  8. System Architecture (cont’d) • Input to time domain • Guard Interval • Channel • Guard Removal • Output to frequency domain • Output • Channel Estimation Channel ICI AWGN Estimated Channel

  9. Pilot for Channel Estimation • Comb 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 Time Carriers Time Carriers

  10. Block-type Channel Estimation • LS: Least Square Estimation

  11. Comb-type Estimation Np pilot signals uniformly inserted in X(k) L=Number of Carriers/Np xp(m) is the mth pilot carrier value {Hp(k) k=0,1,…,Np} , channel at pilot sub-carriers Xp input at the kth pilot sub-carrier Yp output at the kth pilot sub-carrier LS Estimate LMS Estimate Yp(k) Xp(k) - e(k) LMS +

  12. Interpolation for Comb-type • Linear Interpolation • Second Order Interpolation

  13. Simulation Parameters

  14. System structure in MATLAB Simulation

  15. OFDM Transmitter OFDM Receiver

  16. Received and Recovered Signals Received signal phases are distorted by multi-path fading

  17. Comb-LS Estimation • Combating multipath rayleigh fading with RLS adaptive equalization • A detail simulation with MATLAB • 20 multipath, random phase, and weibull distribution of amplitutde Symbol Error Rate

  18. Comb-LS Estimation

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

  20. Conclusion • OFDM System Introduction • Block Type • Direct or Decision Feedback • Comb Type • LS or LMS estimation at pilot frequencies • Interpolation Techniques • Linear • Second Order • Time Domain • Modulation • BPSK,QPSK,16QAM,DQPSK • Some Results: • Comb Type performs better since it tracks fast fading channels. • RLS algorithm vs. LMS algorithm

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