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Incremental Pilots in OFDM

Incremental Pilots in OFDM. Parichaya Shrivastava. What will we discuss?. Brief overview of OFDM Why use OFDM? How does it work? Pilots Why do we need them? Incremental Pilots Concept What am I trying to do?. Why use OFDM?. Higher spectral efficiency Lower multi-path distortion.

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Incremental Pilots in OFDM

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  1. Incremental Pilots in OFDM ParichayaShrivastava

  2. What will we discuss? • Brief overview of OFDM • Why use OFDM? • How does it work? • Pilots • Why do we need them? • Incremental Pilots • Concept • What am I trying to do?

  3. Why use OFDM? • Higher spectral efficiency • Lower multi-path distortion

  4. Higher spectral efficiency

  5. Example of an OFDM modem

  6. Multipath problem

  7. ‘A’ Multipath solution

  8. ‘The’ Multipath solution

  9. Example of an OFDM modem

  10. OFDM challanges • Synchronization requirements are much more strict : • Requires more complex algorithms for time / frequency synch • Peak-to-average power ratio • Large signal peaks require higher power amplifiers • Amplifier cost grows nonlinearly with required power

  11. Pilot based OFDM system model

  12. Main equation: Channel estimation equation

  13. Frequency domain channel estimation

  14. Pilot allocation 1

  15. Pilot allocation 2

  16. Why Incremental Pilots? • Some channels (e.g. where there is more frequency selective fading), increased number of pilots will help. • More pilots • Interpolation error small -> Better channel estimation -> Better BER

  17. P/S S/P 16-QAM Transmitter with channel encoder Data sub-carriers IFFT CP 010 010001 Channel encoder Pilot PN Sequence BPSK DAC Pilot subcarriers

  18. Incremental pilots trade-off Loss in redundancy acceptable Better CE Better BER Less redundancy: Worse BER

  19. Loss in redundancy. Crude figure

  20. Work to do • See the performance under different channel conditions • Different coding schemes (Convolutional, Turbo, LDPC) • Complexity of implementing higher coding rates for different coding schemes

  21. References: • Eric Phillip Lawrey, “Adaptive Techniques for multiuser OFDM” Phd Thesis. Online http://www.skydsp.com/resources/OFDM_thesis_lawrey.PDF • Marina Mondi, “Orthogonal Frequency Division Multiplexing/Modulation : OFDM”. Online. • Keithley Corporation, “An Introduction to Orthogonal Frequency Division Multiplex Technology”. Online http://www.ieee.li/pdf/viewgraphs/introduction_orthogonal_frequency_division_multiplex.pdf • Muhammad Saad Akram, “Pilot-based Channel Estimation in OFDM Systems” . Online http://www2.imm.dtu.dk/pubdb/views/edoc_download.php/5405/pdf/imm5405.pdf • Sung-Yoon Jung and Sunghwan Kim, “Channel Estimation and LDPC Code Puncturing Schemes Based on Incremental Pilots for OFDM”. ETRI Journal, Volume 32, Number 4, August 2010

  22. Thank you!

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