An adaptive HF modem for amateur radio digital communication

⚠️ Work in Progress — This project is under active development. No official release yet. APIs and protocols may change. Use for experimentation and testing only.

ProjectUltra is a software modem designed for reliable data transfer over HF radio. It combines modern signal processing techniques—OFDM, OTFS, LDPC codes, and adaptive equalization—to maintain communication across challenging ionospheric conditions including multipath fading, Doppler spread, and interference.

• Dual Waveform Support: OFDM for stable channels, OTFS for harsh conditions
• Strong Error Correction: LDPC codes with rates from 1/4 to 5/6
• Selective Repeat ARQ: Improved efficiency over stop-and-wait
• Adaptive Equalization: LMS/RLS tracking for time-varying channels
• Implicit Channel Probing: Automatic link adaptation without overhead
• File Transfer: Compressed transfers with integrity verification

Experimental — Performance measured through simulation (Watterson HF channel model). Real-world on-air testing is in progress.

Notes:

• Poor and flutter channel conditions require OTFS mode (in development)
• Supports modulation from BPSK to 256-QAM with LDPC rates R1/4 to R5/6
• Real-world performance depends on actual ionospheric conditions

OTFS is a 2D modulation scheme designed for doubly-selective channels—those with both multipath (delay spread) and Doppler spread. Unlike OFDM which suffers from inter-carrier interference under fast fading, OTFS spreads each data symbol across the entire time-frequency grid, providing diversity against both delay and Doppler.

When to use OTFS:

• Polar/auroral paths with flutter fading
• Mobile or aeronautical HF
• Long paths with significant Doppler shift
• Disturbed ionospheric conditions

The protocol supports both Stop-and-Wait and Selective Repeat ARQ modes:

Selective Repeat uses a sliding window with per-frame acknowledgments, improving efficiency on paths with long round-trip times.

Two equalizer algorithms track time-varying channel responses:

• LMS (Least Mean Squares): Lower complexity, good for slow fading
• RLS (Recursive Least Squares): Faster convergence, better for flutter

Decision-directed mode uses decoded symbols as references between pilots, enabling continuous channel tracking.

Link adaptation happens automatically without explicit probing overhead:

1. When station B decodes any frame from station A, the demodulator measures:

   • SNR (from pilot symbols)
   • Delay spread (from channel impulse response)
   • Doppler spread (from channel time variation)

2. Station B includes these measurements in its response (ACK or CONNECT_ACK)

3. Station A adjusts modulation/coding for subsequent transmissions

This eliminates dedicated probe frames while maintaining accurate channel awareness.

• macOS, Linux, or Windows
• HF transceiver with audio I/O
• SSB filter width: 2.8 kHz or wider (signal bandwidth is ~2.8 kHz)
• Sound card interface (SignaLink, RigBlaster, etc.) or direct connection

# Install dependencies
# macOS: brew install sdl2 cmake
# Linux: sudo apt install libsdl2-dev cmake build-essential

git clone --recursive https://github.com/secup/ProjectUltra.git
cd ProjectUltra
mkdir build && cd build
cmake ..
cmake --build .

# If you already cloned without --recursive:
git submodule update --init --recursive

./ultra_gui

1. Open Settings and enter your callsign
2. Select audio devices (input from radio, output to radio)
3. Use Test Mode to verify operation without transmitting
4. Switch to Operate Mode to go on-air

Two virtual stations (TEST1, TEST2) with cross-wired audio for development:

• Loopback: Single modem, TX audio loops to RX
• Protocol Test: Full two-modem simulation with channel model

• General HF Data: Day-to-day messaging between amateur stations
• File Exchange: Documents, images, small files over HF
• Portable/Field Ops: Lightweight software for field deployments
• NVIS Networks: Regional coverage via 40m/80m near-vertical skywave
• Emergency Comms: Reliable messaging when infrastructure is down
• Challenging Paths: Polar, auroral, or mobile HF with OTFS mode

┌─────────────────────────────────────────────────────────────────┐
│                    GUI Application (ImGui)                       │
├─────────────────────────────────────────────────────────────────┤
│         Protocol Engine (ARQ, Connection, File Transfer)         │
├─────────────────────────────────────────────────────────────────┤
│         Modem Engine (TX/RX coordination, carrier sense)         │
├─────────────────────────────────────────────────────────────────┤
│   LDPC Encoder/Decoder  │  Interleaver  │  OFDM/OTFS Mod/Demod  │
├─────────────────────────────────────────────────────────────────┤
│                      Audio I/O (SDL2)                            │
└─────────────────────────────────────────────────────────────────┘

What Works:

• OFDM modulation/demodulation with robust frame detection
• LDPC encoding/decoding (all rates R1/4 to R5/6)
• Stop-and-Wait ARQ protocol with retransmission
• File transfer with compression
• GUI with waterfall and constellation displays
• Channel simulation (ITU-R F.1487 Watterson model)
• Two-modem test mode (cross-wired virtual stations)
• Loopback testing with channel noise simulation

In Progress:

• On-air testing with real HF equipment
• OTFS mode optimization for poor/flutter channels
• Adaptive mode selection (auto modulation/coding)
• Selective Repeat ARQ (currently using Stop-and-Wait)

Planned:

• Multi-carrier bonding
• Extended AFC beyond ±20 Hz
• Mobile/portable optimizations

Contributions welcome! Priority areas:

• On-air testing across different paths and conditions
• OTFS optimization for specific channel types
• User interface improvements
• Documentation and tutorials

Please open an issue to discuss before submitting PRs.

Released under the MIT License. See LICENSE for details.

Built with Dear ImGui, SDL2, and miniz.