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Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems

Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems. Authors: Sinem Coleri, Mustafa Ergen. Motivation for OFDM OFDM System Architecture Channel Estimation Techniques Performance Analysis Conclusion. Outline. Disadvantages of FDMA Bad Spectrum Usage

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Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems

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  1. Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems Authors: Sinem Coleri, Mustafa Ergen

  2. Motivation for OFDM OFDM System Architecture Channel Estimation Techniques Performance Analysis Conclusion Outline

  3. Disadvantages of FDMA Bad Spectrum Usage Disadvantages of TDMA Multipath Delay spread problem Motivation for OFDM

  4. OFDM: Use of Frequency Spectrum Frequency spectrum of the subcarriers • Efficient use of spectrum • Overlap in frequency spectrum of subcarriers • Null point of all other subcarriers at the center frequency of any particular subcarrier

  5. Multi-path delay spread definition Time spread between the arrival of the first and last multipath signal, seen by the receiver. Received radio signal consisting of a direct signal, plus reflections from objects Multi-path delay spread effect Inter-Symbol Interference (ISI) when the delayed multipath signal overlaps with the symbols following it Multipath Delay Spread

  6. OFDM: Eliminating ISI • Cyclic Prefix • Prepend the last part of the signal to the beginning of the signal • Duration of the CP larger than multipath delay spread • Orthogonality of the carriers not affected.

  7. 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 subcarriers used to prevent frequency and phase shift errors.

  8. Benefits of OFDM • Higher data rates • Overlap of subcarriers • Lower bandwidth than spread spectrum. • High spectral efficiency • Lower multi-path distortion • Usage of cyclic prefix

  9. 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 Our OFDM System Assumptions

  10. System Architecture-1

  11. System Architecture-2 Input to Time Domain 1 2 Guard Interval 3 Channel 4 Guard Removal 5 Output to Frequency Domain 6 Output Channel ICI AWGN 7 Channel Estimation Estimated Channel

  12. Pilot Arrangement • Block Type • All sub-carriers reserved for pilots with a specific period • Comb Type • Some sub-carriers are reserved for pilots for each symbol

  13. Channel Estimation @Block-Type LS estimate MMSE estimate

  14. Block Type Decision Feedback Interpolation Use same channel estimation for the whole symbol duration He -kth sub-carrier Channel Response Estimated Xe(k) -> signal demapper -> signal mapper -> Xpure(k) Channel Estimation @ Block-Type

  15. Channel Estimation @ Comb-Type Pilot • Nppilot signals uniformly inserted in X(k) • L=Number of Carriers/Np • {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

  16. Interpolation @ Comb-Type • Linear Interpolation • Second Order Interpolation • Low pass Interpolation • Spline Cubic Interpolation • Time Domain Interpolation

  17. Interpolation @ Comb-Type Linear Interpolation Second Order Interpolation Low Pass Interpolation (interp in MATLAB) • Insert zeros into the original sequence • Low-pass filter while passing original data unchanged • Interpolation such that mean-square error between ideal and interpolated values min. Time Domain Interpolation Spline Cubic Interpolation (spline in MATLAB)

  18. OFDM Setup

  19. Channels ATTC (Advanced Television Technology Center) and the Grande Alliance DTV laboratory`s ensemble E model Simplified version of (Digital Video Broadcasting) DVB-T channel model Channel 2 Channel 1 Time Varying Channel (AR Model)

  20. Simulation-1

  21. Simulation-2

  22. Simulation-3

  23. Simulation-4

  24. Simulation-5

  25. Simulation-6

  26. Simulation-7

  27. Conclusion • OFDM System • Block Type • Direct or Decision Feedback • Comb Type • LS or LMS estimation at pilot frequencies • Interpolation Techniques • Linear • Second Order • Low Pass • Spline • Time Domain • Modulation • BPSK,QPSK,16QAM,DQPSK • Results: • Comb Type performs better since it tracks fast fading channels. • Low-pass interpolation performs better since mean square error between the interpolated points and their ideal values is minimized.

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