CDSS is a Fortran 2018 implementation of a digital modem based on chirp-assisted direct-sequence spread spectrum (DSSS), short-block Polar coding, soft-decision reception, and iterative receiver refinement.

The project provides a command-line modem, reusable Fortran modules, waveform file I/O, and a regression test suite intended to support maintenance, peer review, and reproducible scientific experiments.

• Encode binary payloads into complex baseband IQ waveforms.
• Decode synchronized IQ or audio captures back into payload bits.
• Synchronize captures by matched preamble correlation.
• Read and write cf32, mono WAV audio, and stereo WAV I/Q files.
• Run unit, integration, channel, and CLI regression tests with diagnostic logs.

The transmitter builds a protected frame, applies channel coding, spreads the coded symbols, and synthesizes a chirp-assisted waveform. The receiver performs preamble acquisition, soft demodulation, reliability weighting, Polar decoding, and CRC validation.

Payload bits
  -> CRC-16 protected header
  -> Polar channel coding
  -> BPSK symbol mapping
  -> Gold-code DSSS spreading
  -> chirp carrier synthesis
  -> complex baseband waveform

Received waveform
  -> preamble synchronization
  -> chip-level soft demodulation
  -> uncertainty/reliability weighting
  -> Polar SCL decoding
  -> CRC validation
  -> recovered payload bits

The frame layout is:

31-bit preamble | Polar-coded 48-bit header | Polar-coded payload

The header stores the payload length and a CRC-16/CCITT-FALSE checksum of the payload bits.

cdss/
  app/
    cdss_cli.f90              command-line interface entry point
  src/
    api/                      public modem API
    channel/                  AWGN, CFO, drift, tones, and clock error models
    codec/                    Polar encoder and SCL decoder
    core/                     constants, kinds, and shared derived types
    receiver/                 synchronization, demodulation, and refinement
    util/                     bit utilities, CRC, RNG, FFTW, and file I/O
    waveform/                 PN sequences, chirp carrier, and synthesis
  test/                       automated test programs
  fpm.toml                    Fortran Package Manager project definition
  Makefile                    documented build, test, install, and run targets
  LICENSE                     project license

• gfortran with Fortran 2018 support
• fpm (Fortran Package Manager)
• FFTW3 development files (fftw3.f03 and libfftw3)
• make
• POSIX shell utilities used by the Makefile and CLI tests

On Debian or Ubuntu systems:

sudo apt install gfortran fpm libfftw3-dev make

The Makefile exports the compiler and linker flags required by the FFTW Fortran interface.

make build       # optimized release build
make debug       # debug profile build
make install     # install cdss to PREFIX/bin, default ~/.local/bin
make clean       # remove FPM build artifacts

The installation prefix can be changed with:

make install PREFIX=/opt/cdss

Print command help:

cdss help

Print modem geometry and default parameters as JSON:

cdss info

Encode a bit string into a waveform file:

cdss mod --bit-string 101100111000 --output frame.cf32 --output-format cf32

Supported output formats:

• cf32: interleaved 32-bit floating-point I/Q samples
• wav-audio: mono 16-bit PCM audio projection
• wav-iq: stereo 16-bit PCM I/Q, with I on the left channel and Q on the right

Detect the preamble in a capture and write a frame-aligned waveform:

cdss sync --input capture.cf32 --output synced.cf32 --bits 64

Decode a frame-aligned waveform:

cdss demod --input synced.cf32 --input-format cf32 --max-payload-bits 128

Default modem constants are defined in src/core/cdss_types.f90 and src/core/cdss_constants.f90.

Run the complete regression suite with:

make test

The tests print diagnostic values for maintainers and reviewers, including CRC values, decoded lengths, synchronization scores, LLR ranges, channel model measurements, waveform round-trip error, CLI JSON output, and expected failure exit statuses.

The suite covers:

• bit conversion, bit-string parsing, CRC-16, and frame header handling
• Polar encoding and SCL decoding
• PN and Gold-code sequence generation
• waveform synthesis, carrier generation, envelopes, and deterministic output
• AWGN, CFO, drift, clock error, guard padding, and tone interference
• cf32, wav-audio, and wav-iq I/O round trips
• synchronization, soft demodulation, uncertainty weighting, and refinement
• zero-length, boundary-length, and multi-block payloads
• high-SNR integration and deterministic channel regressions
• CLI success paths and expected CLI error paths

Typical local workflow:

make debug
make test
make run ARGS='info'

CDSS is distributed under the GNU General Public License v3.0 or later. See LICENSE for the full license text.