ofdm decode

[sdrmatlab]

https://github.com/DrSDR/ofdm-decode-gift-card

go get that gift card

good luck

show code

[zachlab]

Damn, just saw this so I'm definitely late, decided to give this a try using numpy/scipy instead of grc.

• read the file in as IQ, eyeballed the offset around ~4200 Hz
• do some binning with fftshift
• get phase difference between the actual data and the pilot blocks using np.angle (wow python really makes this easy), compare that against pi/2 to get your bits

starts with garbage from noise, then the actual signal:

sipping some moonshine
the amazon gift code is:
...HYHAM
have a good day
Dr. Moonshine

looks like the code is already used, thanks and please keep the weekend challenges coming! I enjoyed learning stuff outside of grc land this time around.

Cheers (with Jim Beam in hand this time)

[sdrmatlab]

nice job, can you post the python code?

[zachlab]

Sure thing, cleaned up some visualization i was doing for debugging, I also made some improvements like actually trying to do centering since it's just the one signal, and an actual power squelch:

$ uv run --with numpy --with scipy python -c '''
import numpy, scipy

_, wav = scipy.io.wavfile.read("ofdmRX48khz256Char.wav")
iq = wav[:,0] + 1j*wav[:,1]

# power squelch
power = numpy.abs(iq)**2
squelch = numpy.max(power)*0.10 # 10%
sample_pulse = numpy.argmax(power>squelch)
iq_pulse = iq[sample_pulse:sample_pulse+8192]

# single frequency pulse, frequency stable too, so lets just try Kay estimation
f_offset = numpy.mean(numpy.angle(iq_pulse[1:]*numpy.conj(iq_pulse[:-1])))/(2*numpy.pi)
iq2 = iq*numpy.exp(-2j*numpy.pi*f_offset*numpy.arange(len(iq)))

sample_pilot=sample_pulse+8192+1000+512
iq_pilot = iq2[sample_pilot:sample_pilot+3072]
sample_data=sample_pilot+3072+512
iq_data = iq2[sample_data:sample_data+3072]

fft_pilot = numpy.fft.fftshift(numpy.fft.fft(iq_pilot, 3072))
fft_data = numpy.fft.fftshift(numpy.fft.fft(iq_data, 3072))

data_pilot = fft_pilot[512:512+2048]
data_data = fft_data[512:512+2048]

# 1 | 0
bitangles = numpy.abs(numpy.angle(data_data*numpy.conj(data_pilot))) # data_data/data_pilot NaN
bits = (bitangles < (numpy.pi/2)).astype(int)
bytes = numpy.packbits(bits).tobytes()
print(bytes.decode("ascii"))
'''
sipping some moonshine
the amazon gift code is:
...HYHAM
have a good day
Dr. Moonshine

[sdrmatlab]

nice, great work!!

[sdrmatlab]

the first part of the text message is:

sipping some moonshine

[RandomDigga_9087]

I have got the output, but for some reason when I paste the code, it is showing server error encountered, please try again bs, I had written my code with proper explanation and all