OFDM MODEM

Quick start:

Create file uncoded.dat with 2048 bits of random data:

dd if=/dev/urandom of=uncoded.dat bs=1 count=256

Encode file uncoded.dat to encoded.wav WAV audio file with 8000 Hz sample rate, 16 bits and only 1 (real) channel:

./encode encoded.wav 8000 16 1 1500 5 uncoded.dat

Start recording to recorded.wav audio file and stop after 5 seconds:

arecord -c 1 -f S16_LE -r 8000 -d 5 recorded.wav

Start playing encoded.wav audio file:

aplay encoded.wav

Decode recorded.wav audio file to decoded.dat file:

./decode recorded.wav decoded.dat

Compare original uncoded.dat with decoded.dat:

diff -s uncoded.dat decoded.dat

Supported Modes

All modes need a bandwidth of 2000 Hz and use a 1/2-rate forward error correction code

These use a differential modulation scheme:

• Mode 0: DBPSK, 3 seconds and 256 bytes
• Mode 1: DQPSK, 3 seconds and 512 bytes
• Mode 2: D8PSK, 4 seconds and 1024 bytes

And these a coherent modulation scheme:

• Mode 3: BPSK, 3 seconds and 256 bytes
• Mode 4: QPSK, 3 seconds and 512 bytes
• Mode 5: QAM16, 1 second and 256 bytes
• Mode 6: QAM16, 3 seconds and 1024 bytes
• Mode 7: QAM64, 4 seconds and 2048 bytes

Simulating

Prerequisite: disorders

Encode uncoded.dat to analytic audio signal, add multipath, CFO, SFO, AWGN, decode and compare:

./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 -

Reading

• Robust frequency and timing synchronization for OFDM
  by Timothy M. Schmidl and Donald C. Cox - 1997
• On Timing Offset Estimation for OFDM Systems
  by H. Minn, M. Zeng, and V. K. Bhargava - 2000