ofdm-transfer

The active repository is at https://codeberg.org/glv/ofdm-transfer

ofdm-transfer is a command-line program to send or receive data by software defined radio using the OFDM modulation.

Usage

ofdm-transfer [options] [filename]

Options: -a Use audio samples instead of IQ samples. -b (default: 38400 b/s) Bit rate of the OFDM transmission. -c (default: 0.0, can be negative) Correction for the radio clock. -d Dump a copy of the samples sent to or received from the radio. -e <fec[,fec]> (default: h128,none) Inner and outer forward error correction codes to use. -f (default: 434000000 Hz) Frequency of the OFDM transmission. -g (default: 0) Gain of the radio transceiver, or audio gain in dB. -h This help. -i (default: "") Transfer id (at most 4 bytes). When receiving, the frames with a different id will be ignored. -m (default: qpsk) Modulation to use for the subcarriers. -n <subcarriers[,cyclic prefix[,taper]]> (default: 64,16,4) Number of subcarriers, cyclic prefix length and taper length of the OFDM transmission. -o (default: 0 Hz, can be negative) Set the central frequency of the transceiver 'offset' Hz lower than the signal frequency to send or receive. -r (default: "") Radio to use. -s (default: 2000000 S/s) Sample rate to use. -T (default: 0 s) Number of seconds after which reception will be stopped if no frame has been received. A timeout of 0 means no timeout. -t Use transmit mode. -v Print debug messages. -w (default: 0.0 s) Wait a little before switching the radio off. This can be useful if the hardware needs some time to send the last samples it has buffered.

By default the program is in 'receive' mode. Use the '-t' option to use the 'transmit' mode.

In 'receive' mode, the samples are received from the radio, and the decoded data is written either to 'filename' if it is specified, or to standard output. In 'transmit' mode, the data to send is read either from 'filename' if it is specified, or from standard input, and the samples are sent to the radio.

Instead of a real radio transceiver, the 'io' radio type uses standard input in 'receive' mode, and standard output in 'transmit' mode. The 'file=path-to-file' radio type reads/writes the samples from/to 'path-to-file'. The IQ samples must be in 'complex float' format (32 bits for the real part, 32 bits for the imaginary part). The audio samples must be in 'signed integer' format (16 bits).

The gain parameter can be specified either as an integer to set a global gain, or as a series of keys and values to set specific gains (for example 'LNA=32,VGA=20'). When using the audio mode (with the '-a' option), the gain value in dB is applied to the audio samples.

Compilation

ofdm-transfer requires the following dependencies:

• libliquid (https://github.com/jgaeddert/liquid-dsp)
• libSoapySDR (https://github.com/pothosware/SoapySDR)

It can be compiled with the usual:

./autogen.sh
./configure
make

Supported radios

ofdm-transfer uses the SoapySDR API, therefore if your radio is supported by SoapySDR and you have the right driver installed, it should work.

When used with the '-h' option to print the help message, ofdm-transfer will also list the radios that it has detected.

Supported subcarrier modulations

• bpsk
• qpsk
• psk8
• apsk16
• apsk32
• apsk64
• apsk128
• apsk256

Supported FEC codes

The forward error correction codes that can be specified with the '-e' option are the codes that are supported by libliquid.

These should always be available:

• none
• rep3: repeat(3)
• rep5: repeat(5)
• h74: Hamming(7,4)
• h84: Hamming(8,4)
• h128: Hamming(12,8)
• g2412: Golay(24,12)
• secded2216: SEC-DED(22,16)
• secded3932: SEC-DED(39,32)
• secded7264: SEC-DED(72,64)

If your libliquid library has been compiled with support for the libfec library, the following codes should also be available:

• v27: convolutional r1/2 K=7
• v29: convolutional r1/2 K=9
• v39: convolutional r1/3 K=9
• v615: convolutional r1/6 K=15
• v27p23: convolutional r2/3 K=7 (punctured)
• v27p34: convolutional r3/4 K=7 (punctured)
• v27p45: convolutional r4/5 K=7 (punctured)
• v27p56: convolutional r5/6 K=7 (punctured)
• v27p67: convolutional r6/7 K=7 (punctured)
• v27p78: convolutional r7/8 K=7 (punctured)
• v29p23: convolutional r2/3 K=9 (punctured)
• v29p34: convolutional r3/4 K=9 (punctured)
• v29p45: convolutional r4/5 K=9 (punctured)
• v29p56: convolutional r5/6 K=9 (punctured)
• v29p67: convolutional r6/7 K=9 (punctured)
• v29p78: convolutional r7/8 K=9 (punctured)
• rs8: Reed-Solomon, 223/255

Bandwidth

The occupied bandwidth (99% power) of the generated signal is approximately (bit_rate * 1.6 * k) Hertz.

Where k is a coefficient depending on the subcarrier modulation:

• bpsk: 1
• qpsk: 1/2
• psk8: 1/3
• apsk16: 1/4
• apsk32: 1/5
• apsk64: 1/6
• apsk128: 1/7
• apsk256: 1/8

Examples

Send a file at 9600 b/s on 434 MHz using a HackRF:

ofdm-transfer -t \
              -r driver=hackrf \
              -s 4000000 \
              -f 434000000 \
              -o 100000 \
              -b 9600 \
              -g 30 \
              -w 1 \
              input_file

Receive a file at 9600 b/s on 434 MHz using a RTL-SDR:

ofdm-transfer -r driver=rtlsdr \
              -s 2000000 \
              -f 434000000 \
              -o 100000 \
              -b 9600 \
              -g 20 \
              -T 30 \
              output_file

Generate audio samples for a 1200 b/s transmission centered at 1500 Hz and play them:

ofdm-transfer -t \
              -r file=/tmp/samples.s16 \
              -a \
              -s 48000 \
              -f 1500 \
              -b 1200 \
              input_file
aplay -f S16_LE -r 48000 -c 1 /tmp/samples.s16

Send a file at 48 kb/s using an audio cable:

cat file.dat | ofdm-transfer -t -a -r io -s 48000 -f 12000 -m apsk16 -b 48000 | aplay -q -f S16_LE -r 48000 -c 1

Receive a file at 48 kb/s using an audio cable:

arecord -q -f S16_LE -r 48000 -c 1 | ofdm-transfer -a -r io -s 48000 -f 12000 -m apsk16 -b 48000 -T 10 > file.dat

Library

You can add OFDM transfer support to your programs easily by using the 'libofdm-transfer' library. The API is described in the 'ofdm-transfer.h' file.

The 'echo-server' example program shows how to use the API to make a server receiving messages from clients and sending them back in reverse order.

The 'full-duplex' example program shows how to use the API to make a full-duplex link using two devices.

The 'full-duplex-ppp.sh' script shows how to make a PPP connection between two machines using the 'full-duplex' example program.

The 'half-duplex.sh' script shows how to make a half-duplex link between two (not SDR) transceivers using audio, like a sound modem TNC.