mirror of
https://github.com/aicodix/modem.git
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108 lines
3.1 KiB
Markdown
Vendored
108 lines
3.1 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 48000 Hz sample rate, 16 bits and only 1 (real) channel:
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```
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./encode encoded.wav 48000 16 1 1500 ANONYMOUS 176 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 48000 -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 2400 Hz.
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These are the durations for each differential modulation scheme:
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| Modulation | Short | Normal |
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| ---------- | ----- | ------ |
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| DBPSK | 1.5s | 2.6s |
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| DQPSK | 1.0s | 2.6s |
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| D8PSK | 1.9s | 3.4s |
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And these are the durations for each coherent modulation scheme:
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| Modulation | Short | Normal |
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| ---------- | ----- | ------ |
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| QAM16 | 1.0s | 2.6s |
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| QAM64 | 1.9s | 3.4s |
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| QAM256 | 1.5s | 2.6s |
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| QAM1024 | 2.2s | 4.0s |
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| QAM4096 | 1.9s | 3.4s |
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Currently all digital modes use a 1/2-rate forward error correction code.
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Therefore we have the following byte payloads for each modulation:
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| Modulation | Short | Normal |
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| ---------- | ----- | ------ |
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| DBPSK | 128B | 256B |
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| DQPSK | 128B | 512B |
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| D8PSK | 512B | 1024B |
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| QAM16 | 256B | 1024B |
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| QAM64 | 1024B | 2048B |
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| QAM256 | 1024B | 2048B |
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| QAM1024 | 2048B | 4096B |
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| QAM4096 | 2048B | 4096B |
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Which give us the following bitrates for each modulation:
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| Modulation | Short | Normal |
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| ---------- | ------- | ------- |
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| DBPSK | 681b/s | 789b/s |
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| DQPSK | 1070b/s | 1577b/s |
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| D8PSK | 2141b/s | 2398b/s |
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| QAM16 | 2141b/s | 3155b/s |
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| QAM64 | 4282b/s | 4795b/s |
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| QAM256 | 5449b/s | 6310b/s |
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| QAM1024 | 7493b/s | 8268b/s |
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| QAM4096 | 8563b/s | 9591b/s |
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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 - 48000 16 2 1500 ANONYMOUS 176 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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