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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

OFDM Implementation in GNU Radio

Matt Ettus, Thomas W. Rondeau, and Robert McGwier

Wireless@VT Symposium, 2007

signal detection
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.
ml estimation
ML Estimation

Delay to CP

Peak to 0

Gate symbol

Correlate to delayed CP

Hold angle for frequency correction

ml performance
ML Performance

Correlation

Peaks

Correlations occur every OFDM symbol

ml performance1
ML Performance

Correlation

Peaks

Peaks should occur every FFT length + CP length

ml performance2
ML Performance

Histogram of Peak Timing

Missing a peak inside a packet will cause a rotation in versus the other symbols

pn correlation
PN Correlation

Create sample in frequency with 0’s in every other bin:

IFFT

Repeated Known Symbol in time: ABCDABCD

synchronizer flow graph
Synchronizer Flow Graph

Delay first half

Detects edge

Signal Power

Regenerate for each symbol

Hold angle for frequency correction

Correlate to preamble

Gate symbol

pn sync performance
PN Sync Performance

Correlation

Match Filtered

Regen. Peaks

Correlations occur every packet

pn sync performance1
PN Sync Performance

Correlation

Match Filtered

Regen. Peaks

Regenerating peaks by a known amount

pn sync comparison
PN Sync Comparison

Noise and multipath affects against a perfect signal

pn sync performance2
PN Sync Performance

Histogram of Peak Timing

Differences do not affect this method as much because the rotation is constant in the packet

frame detection
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
fine frequency compensation
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

coarse frequency compensation corrected in frame detector
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

fine frequency error from pn sync
Fine Frequency Error from PN Sync

Frequency error calculated once per packet; does not fix for clock offset drift throughout packet.

profiling support
Profiling Support
  • Use OProfile
  • Collects amount of processing done by each component of a process
  • Used to tune performance
  • Started at 100 kHz bandwidth
  • 1.5 hours of profiling increased to 600 kHz
profiling support qpsk receiver
Profiling Support – QPSK Receiver

% symbol name

29.2 .loop2

11.5 .loop2

6.05 gr_fast_atan2f

5.14 gr_fft_filter_ccc::work

4.31 gr_single_threaded_scheduler::main_loop

4.10 gr_ofdm_correlator

3.80 .cleanup

3.15 gr_multiply_cc::work

2.44 gr_fir_ccf_simd::filter

1.81 gr_fir_fff_simd::filter

profiling support qpsk receiver1
Profiling Support – QPSK Receiver

1.7259 gr_divide_ff::work

1.5111 gr_frequency_modulator_fc::work

1.4963 gr_block_detail::input

1.4815 gr_peak_detector_fb::work

1.3111 gr_ofdm_correlator::coarse_freq_comp

1.2667 .loop1

1.0593 gr_complex_to_mag_squared::work

1.0222 .loop1

0.9556 gr_fft_vcc::work

0.8963 gr_ofdm_sampler::general_work

0.8593 gr_add_const_cc::work

0.8593 gr_ofdm_correlator::general_work

gnu radio interface
GNU Radio Interface
  • Command-line options allow selection of
    • FFT bins (any value, odd or even)
    • Occupied bins (must be < fft bins)
    • Cyclic prefix length (< fft bins)
  • Flow graphs are built at runtime
  • Should tie directly in with current method to use an PHY/MAC interface card

./benchmark –R A –f 2411M --fft-length=512 \

--occupied-tones=300 --cp-length=128

future work and goals
Future Work and Goals
  • Hit 1 MHz in receiver
  • More modulations
  • Implement a variation of the PN sync
  • Implement adaptive equalizer over the entire packet
  • SINR calculation
  • Forward Error Correction
  • Performance tuning
slide59
Thank You

Questions?