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OFDM Implementation in GNU Radio

OFDM Implementation in GNU Radio. Matt Ettus, Thomas W. Rondeau, and Robert McGwier Wireless@VT Symposium, 2007. OFDM: Basic Principles. Transmitter Flow Graph. Receive Flow Graph. Signal Acquisition. Signal Detection. Two methods implemented: Maximum Likelihood

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OFDM Implementation in GNU Radio

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  1. OFDM Implementation in GNU Radio Matt Ettus, Thomas W. Rondeau, and Robert McGwier Wireless@VT Symposium, 2007

  2. OFDM: Basic Principles

  3. Transmitter Flow Graph

  4. Receive Flow Graph

  5. Signal Acquisition

  6. Signal Detection • Two methods implemented: • Maximum Likelihood • Correlate with cyclic prefix • J. van de Beek, M. Sandell, P. Ola. Borjesson, “ML Estimation of Time and Frequency Offset in OFDM Systems,” IEEE Trans. On Signal Processing, vol. 45, no. 7, pp. 1800—1805, July 1997. • PN Sequence correlation • Transmit known symbol with repetition in the time domain • T. M. Schmidl and D. C. Cox, “Robust Frequency and Timing Synchronization for OFDM,” IEEE Trans. Communications, vol. 45, no. 12, pp. 1613—1621, Dec. 1997.

  7. ML Estimation Delay to CP Peak to 0 Gate symbol Correlate to delayed CP Hold angle for frequency correction

  8. ML Performance Correlation Peaks Correlations occur every OFDM symbol

  9. ML Performance Correlation Peaks Peaks should occur every FFT length + CP length

  10. ML Performance Histogram of Peak Timing Missing a peak inside a packet will cause a rotation in versus the other symbols

  11. PN Correlation Create sample in frequency with 0’s in every other bin: IFFT Repeated Known Symbol in time: ABCDABCD

  12. Synchronizer Flow Graph Delay first half Detects edge Signal Power Regenerate for each symbol Hold angle for frequency correction Correlate to preamble Gate symbol

  13. PN Sync Performance Correlation Match Filtered Regen. Peaks Correlations occur every packet

  14. PN Sync Performance Correlation Match Filtered Regen. Peaks Regenerating peaks by a known amount

  15. PN Sync Comparison Noise and multipath affects against a perfect signal

  16. PN Sync Performance Histogram of Peak Timing Differences do not affect this method as much because the rotation is constant in the packet

  17. Frame Detection • Uses known symbols to correlate and find beginning of frame • Once correlated, use the known symbols and receive symbols to generate a 1-tap equalizer • Use the equalizer to correct phase shifts and multipath

  18. Fine Frequency compensation PN Frequency ambiguity -0.5 -3.5 -2.5 -1.5 0.5 1.5 2.5 3.5 Frequency ML Frequency ambiguity

  19. Coarse Frequency compensationcorrected in frame detector Shifted full FFT bin -0.5 -3.5 -2.5 -1.5 0.5 1.5 2.5 3.5 Frequency

  20. Fine Frequency Error from PN Sync Frequency error calculated once per packet; does not fix for clock offset drift throughout packet.

  21. Implementation Results

  22. QPSK

  23. QPSK Repeated Preamble (1/13)FFT length=512, Occupied Tones=200, CP Length=128

  24. QPSK Known Preamble 1 (2/13)FFT length=512, Occupied Tones=200, CP Length=128

  25. QPSK Known Preamble 3 (3/13)FFT length=512, Occupied Tones=200, CP Length=128

  26. QPSK Data (4/13)FFT length=512, Occupied Tones=200, CP Length=128

  27. QPSK Data (5/13) FFT length=512, Occupied Tones=200, CP Length=128

  28. QPSK Data (6/13) FFT length=512, Occupied Tones=200, CP Length=128

  29. QPSK Data (7/13) FFT length=512, Occupied Tones=200, CP Length=128

  30. QPSK Data (8/13) FFT length=512, Occupied Tones=200, CP Length=128

  31. QPSK Data (9/13) FFT length=512, Occupied Tones=200, CP Length=128

  32. QPSK Data (10/13) FFT length=512, Occupied Tones=200, CP Length=128

  33. QPSK Data (11/13) FFT length=512, Occupied Tones=200, CP Length=128

  34. QPSK Data (12/13) FFT length=512, Occupied Tones=200, CP Length=128

  35. QPSK Repeated Preamble (13/13) FFT length=512, Occupied Tones=200, CP Length=128

  36. QAM16

  37. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  38. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  39. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  40. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  41. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  42. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  43. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  44. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  45. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  46. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  47. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  48. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  49. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

  50. QAM16 Repeated Preamble (1/1) FFT length=512, Occupied Tones=200, CP Length=128

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