# Adaptive Physical Communication System (APCS)

> APCS is an open-source (MIT) Flutter app for offline, phone-to-phone file transfer using light and sound. Light streams animated, fountain-coded QR codes from the sender's screen to the receiver's camera (estimated 1.3–2.5 KB/s). Sound is a 16-ary multi-tone FSK data-over-sound modem with Reed-Solomon error correction and LT fountain coding, including an inaudible 18.3–19.9 kHz band (nominal 3.4–36 B/s). No Internet, Wi-Fi, Bluetooth, NFC or mobile data is used in the data path. A third channel, vibration, is experimental and unreliable on real phones. The app runs on Android, iOS and Chrome (web). The adaptive channel-selection engine runs in the Simulation Lab and developer tools; in the normal Send/Receive screens the user picks the channel.

Performance figures are nominal (calculated) or estimated (from headless camera and room simulators), not measured across many phones. None of the channels is encrypted.

## Start here

- [README (complete technical manual)](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/README.md): what every part does, with exact numbers and derivations
- [Documentation index](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/README.md): the documentation set by topic and by reader need
- [User Guide](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/getting-started/USER_GUIDE.md): step-by-step use of the app
- [FAQ](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/getting-started/FAQ.md): common questions and short answers

## Channels and algorithms

- [Light Channel](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/channels/LIGHT_CHANNEL.md): animated fountain QR, frame format, camera pipeline
- [Sound Channel](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/channels/SOUND_CHANNEL.md): multi-tone FSK profiles, sync, Reed-Solomon, Silent band
- [Vibration Channel](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/channels/VIBRATION_CHANNEL.md): pulse-width keying design (experimental)
- [Fountain Code](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/algorithms/FOUNTAIN_CODE.md): LT code, degree distribution, exact decoding
- [Adaptive Engine](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/algorithms/ADAPTIVE_ENGINE.md): channel scoring and hysteresis

## Architecture and development

- [Architecture](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/architecture/ARCHITECTURE.md): layers, send and receive sequences
- [Data Formats](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/architecture/DATA_FORMATS.md): byte-level envelope, frame and packet layouts
- [API Reference](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/development/API_REFERENCE.md): public classes and functions
- [Testing](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/development/TESTING.md): test suite and device test checklist

## Optional

- [Performance](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/operations/PERFORMANCE.md): how each speed figure is derived
- [Known Issues](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/project/KNOWN_ISSUES.md): current limitations and bugs
- [Changelog](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/docs/project/CHANGELOG.md): release history
- [Citation](https://raw.githubusercontent.com/harsharaj-s/adaptive-physical-communication-system/main/CITATION.cff): how to cite APCS