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OFDM Receiver Design

Outline. OverviewIntroduction to OFDM802.11a system specificationBlock DescriptionSynchronizationFFTViterbiSVDSystem Integration and SimulationConclusion. Introduction to OFDM. Basic ideaUsing a large number of parallel narrow-band sub-carriers instead of a single wide-band carrier to tra

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OFDM Receiver Design

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    1. OFDM Receiver Design Yun Chiu, Dejan Markovic, Haiyun Tang, Ning Zhang {chiuyun, dejan, tangh, ningzh}@eecs.berkeley.edu EE225C Final Project Report, 12 December 2000 Department of EECS, UC Berkeley

    2. Outline

    3. Introduction to OFDM Basic idea Using a large number of parallel narrow-band sub-carriers instead of a single wide-band carrier to transport information Advantages Very easy and efficient in dealing with multi-path Robust again narrow-band interference Disadvantages Sensitive to frequency offset and phase noise Peak-to-average problem reduces the power efficiency of RF amplifier at the transmitter Adopted for various standards DSL, 802.11a, DAB, DVB

    4. OFDM Signal Representation Data set to be encoded on sub-carriers IFFT at transmitter FFT at receiver

    5. Cyclic Prefix

    6. 802.11a System Specification Sampling (chip) rate: 20MHz Chip duration: 50ns Number of FFT points: 64 FFT symbol period: 3.2ms Cyclic prefix period: 16 chips or 0.8ms Typical maximum indoor delay spread < 400ns OFDM frame length: 80 chips or 4ms FFT symbol length / OFDM frame length = 4/5 Modulation scheme QPSK: 2bits/sample 16QAM: 4bits/sample 64QAM: 6bits/sample Coding: rate ½ convolutional code with constraint length 7

    7. OFDM System Block Diagram

    8. Synchronization Frame detection Frequency offset compensation Sampling error Usually less 100ppm and can be ignored 100ppm = off 1% of a sample every 100 samples

    9. Channel Estimation

    10. System Pilot Structure

    12. Outline

    13. IEEE 802.11a OFDM Txer

    14. Short & Long Preambles

    15. Correlation of Short Preamble

    16. Synchronization

    17. Impairments: Multi-Path Channel

    18. Impairments: Frequency Offset

    19. Fine Frequency Offset Est.

    20. Coarse-Fine Joint Estimation & Decision Alignment Error Correction

    21. Frequency Offset Compensation

    22. Performance Summary

    24. Pipelined FFT Architecture

    25. Summary of FFT Block

    27. System Simulation

    28. Problems to be Handled Constant over packet frame – use long preamble Slowly varying frequency selective fading channel Symbol boundary offset: Varying across OFDM symbols – use pilot channels Phase offset: Residue frequency offset

    29. Frequency Selective Fading Channel

    30. Channel Compensation One-tap equalizer for each sub-carrier 1) 2) Soft-input Viterbi Decoder Improve BER with AWGN Protect against frequency selective fading

    31. Simulation Results

    32. Viterbi Decoder

    33. Viterbi Decoder: Summary

    35. Concept of SVD

    36. SVD Based OFDM System

    37. SVD Block for OFDM System

    38. Model of Channel Noise

    39. BER: Simulation Results

    40. SVD Hardware Realization

    41. Performance Summary (0.25mm)

    43. Overall System Summary

    44. Overall Hardware Breakdown

    45. Porting to a New Technology

    46. Entire Design in 0.18mm

    47. Conclusion and Future Work

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