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https://github.com/aicodix/modem.git
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74 lines
2.1 KiB
Markdown
Vendored
74 lines
2.1 KiB
Markdown
Vendored
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### OFDM MODEM
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Quick start:
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Create file ```uncoded.dat``` with ```2048``` bits of random data:
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```
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dd if=/dev/urandom of=uncoded.dat bs=1 count=256
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```
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Encode file ```uncoded.dat``` to ```encoded.wav``` [WAV](https://en.wikipedia.org/wiki/WAV) audio file with 8000 Hz sample rate, 16 bits and only 1 (real) channel:
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```
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./encode encoded.wav 8000 16 1 1500 5 uncoded.dat
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```
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Start recording to ```recorded.wav``` audio file and stop after 5 seconds:
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```
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arecord -c 1 -f S16_LE -r 8000 -d 5 recorded.wav
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```
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Start playing ```encoded.wav``` audio file:
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```
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aplay encoded.wav
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```
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Decode ```recorded.wav``` audio file to ```decoded.dat``` file:
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```
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./decode recorded.wav decoded.dat
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```
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Compare original ```uncoded.dat``` with ```decoded.dat```:
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```
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diff -s uncoded.dat decoded.dat
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```
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### Supported Modes
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All modes need a bandwidth of 2000 Hz and use a 1/2-rate forward error correction code
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These use a differential modulation scheme:
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* Mode 0: DBPSK, 3 seconds and 256 bytes
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* Mode 1: DQPSK, 3 seconds and 512 bytes
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* Mode 2: D8PSK, 4 seconds and 1024 bytes
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And these a coherent modulation scheme:
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* Mode 3: BPSK, 3 seconds and 256 bytes
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* Mode 4: QPSK, 3 seconds and 512 bytes
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* Mode 5: QAM16, 1 second and 256 bytes
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* Mode 6: QAM16, 3 seconds and 1024 bytes
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* Mode 7: QAM64, 4 seconds and 2048 bytes
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### Simulating
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Prerequisite: [disorders](https://github.com/aicodix/disorders)
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Encode ```uncoded.dat``` to [analytic](https://en.wikipedia.org/wiki/Analytic_signal) audio signal, add [multipath](https://en.wikipedia.org/wiki/Multipath_propagation), [CFO, SFO](https://en.wikipedia.org/wiki/Carrier_frequency_offset), [AWGN](https://en.wikipedia.org/wiki/Additive_white_Gaussian_noise), decode and compare:
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```
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./encode - 8000 16 2 1500 5 uncoded.dat | ../disorders/multipath - - ../disorders/multipath.txt 10 | ../disorders/cfo - - 234.567 | ../disorders/sfo - - 147 | ../disorders/awgn - - -20 | ./decode - - | diff -q -s uncoded.dat -
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```
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### Reading
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* Robust frequency and timing synchronization for OFDM
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by Timothy M. Schmidl and Donald C. Cox - 1997
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* On Timing Offset Estimation for OFDM Systems
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by H. Minn, M. Zeng, and V. K. Bhargava - 2000
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