High-performance HF modem for amateur radio
Last updated: 2026-01-31
EXPERIMENTAL SOFTWARE - WORK IN PROGRESS
This project is under active development and is not ready for production use. Features may be incomplete, untested, or broken. APIs and protocols may change without notice. Performance numbers are from simulation only - real-world HF testing is ongoing. Use at your own risk for experimentation and development only.
ProjectUltra is a software modem that achieves reliable, high-speed data transfer over HF radio. It uses adaptive waveform selection to maintain communication across varying ionospheric conditions - from quiet bands to disturbed polar paths.
- Adaptive Waveforms: Automatically selects optimal waveform based on channel conditions
- Wide SNR Range: Operates from -11 dB to 30+ dB (DPSK tested at -11 dB)
- Strong FEC: LDPC codes with rates from R1/4 to R5/6
- ARQ Protocol: Reliable delivery with automatic retransmission
- GUI Application: Real-time waterfall, constellation display, and message log
- CLI Tools: Frame-level transmit/receive for testing and debugging
Verified Performance (loopback + acoustic testing):
| SNR | Mode | Throughput | Notes |
|---|---|---|---|
| 5+ dB | MC-DPSK (8 carriers) | 938 bps | 100% verified, ±50 Hz CFO |
| 10+ dB | OFDM-CHIRP R1/4 | 1.8 kbps | 100% verified, ±50 Hz CFO |
| 17+ dB | OFDM-COX R1/4 | 1.8 kbps | DATA phase verified |
| 20+ dB | OFDM DQPSK R1/2 | 3.6 kbps | 100% verified |
| 20+ dB | OFDM D8PSK R1/2 | 5.3 kbps | 100% AWGN, 88% fading |
| 25+ dB | OFDM DQPSK R2/3 | 5.4 kbps | 90% verified |
Theoretical (pending HF validation):
| SNR | Mode | Throughput | Notes |
|---|---|---|---|
| -5 to 5 dB | MC-DPSK | 938 bps | Hard floor at -5 dB |
| 25+ dB | OFDM 16QAM R3/4 | 5.8 kbps | Coherent mode, stable channels |
| 30+ dB | OFDM 32QAM R3/4 | 7.2 kbps | Maximum throughput |
When to use coherent modes (16QAM/32QAM): Stable propagation paths like ground wave or direct cable connection. These paths have stable phase, allowing coherent demodulation with pilot-assisted channel estimation.
ProjectUltra uses adaptive waveform selection based on channel conditions:
SNR Range Waveform Why
─────────────────────────────────────────────────────────────
5-10 dB MC-DPSK (8 carriers) Robust sync, differential encoding
10-17 dB OFDM-CHIRP 1024-FFT Dual chirp sync, 59 carriers
17+ dB OFDM-COX 1024-FFT Schmidl-Cox sync, max throughput
25+ dB (NVIS) OFDM 16QAM Coherent mode, 7.2 kbps
MC-DPSK (Multi-Carrier DPSK): 8 carriers with differential encoding. Works reliably at 5+ dB SNR with ±50 Hz CFO tolerance. Uses dual chirp synchronization for robust timing recovery.
D8PSK (8-Phase DPSK): +50% throughput over DQPSK (3 bits/symbol vs 2). Auto-selected on AWGN channels at SNR >= 20 dB. For fading channels, a two-pass decoder uses the embedded DQPSK grid to estimate and correct phase drift before D8PSK decoding.
Two OFDM sync modes:
- OFDM-CHIRP: Dual chirp preamble for robust sync, works at 10+ dB
- OFDM-COX: Schmidl-Cox sync for faster acquisition, requires 17+ dB
NVIS/Local optimization: For stable paths like NVIS (Near Vertical Incidence Skywave), 16QAM with pilots provides better performance than differential modes. The pilots track slow phase drift and frequency-selective fading.
- Linux, macOS, or Windows
- CMake 3.16+
- SDL2 (for GUI)
- FFTW3 (required for fast chirp detection)
- C++20 compiler (GCC 10+, Clang 12+, MSVC 2019+)
# Install dependencies
# Ubuntu/Debian: sudo apt install libsdl2-dev libfftw3-dev cmake build-essential
# macOS: brew install sdl2 fftw cmake
# Windows: vcpkg install sdl2 fftw3
git clone https://github.com/mfrigerio/ProjectUltra.git
cd ProjectUltra
mkdir build && cd build
cmake ..
make -j4GUI Application:
./ultra_gui # Normal mode
./ultra_gui -sim # Simulator mode (no radio needed)CLI Tools (individual frames, not full protocol):
# Transmit single frame
./ultra ptx "Hello World" -s MYCALL -d THEIRCALL | aplay -f FLOAT_LE -r 48000
# Decode frames from audio
arecord -f FLOAT_LE -r 48000 | ./ultra prx
# Loopback test
./ultra ptx "Test message" -s A -d B | ./ultra prxNote: The CLI generates/decodes individual frames. It does not support full protocol sessions (PING→CONNECT→DATA→DISCONNECT). For interactive operation, use the GUI application.
- PING/PONG - Fast presence probe (~1 sec each) to check if remote station is listening
- CONNECT - Full callsign exchange after successful probe (FCC Part 97.119 compliance)
- MODE_CHANGE - Negotiates optimal modulation/coding based on measured SNR
- DATA - Transfers payload with per-frame acknowledgment
- DISCONNECT - Graceful termination with callsign ID (regulatory compliance)
The PING/PONG probe allows quick "anyone home?" detection before committing to the full CONNECT sequence. If no response after 5 PINGs (15 seconds), connection fails fast.
| Parameter | MC-DPSK | OFDM |
|---|---|---|
| Sample Rate | 48 kHz | 48 kHz |
| Bandwidth | ~2.4 kHz | ~2.8 kHz |
| Center Frequency | 1500 Hz | 1500 Hz |
| Carriers | 8 | 59 |
| FFT Size | N/A | 1024 |
| Symbol Rate | ~94 baud | ~42 baud |
| Sync Method | Dual Chirp | Dual Chirp or Schmidl-Cox |
| LDPC Codeword | 648 bits | 648 bits |
| Rate | Info Bytes | Use Case |
|---|---|---|
| R1/4 | 20 | Low SNR, maximum reliability |
| R1/2 | 40 | Moderate SNR, good balance |
| R2/3 | 54 | Good SNR, higher throughput |
| R3/4 | 60 | Very good SNR |
| R5/6 | 67 | Excellent SNR, maximum throughput |
┌─────────────────────────────────────────────────────────┐
│ GUI Application (ImGui + SDL2) │
├─────────────────────────────────────────────────────────┤
│ Protocol Engine (Connection, ARQ, File Transfer) │
├─────────────────────────────────────────────────────────┤
│ Modem Engine (TX/RX coordination, SNR est) │
├──────────────────────────┬────────────────────────────┤
│ OFDM Mod/Dem │ DPSK Mod/Dem │
├──────────────────────────┴────────────────────────────┤
│ LDPC Encoder/Decoder │ Interleaver │ Sync/CFO │
├─────────────────────────────────────────────────────────┤
│ Audio I/O (SDL2) │
└─────────────────────────────────────────────────────────┘
cd build
# Primary test tool - continuous RX with channel simulation
./test_iwaveform --snr 10 -w ofdm_chirp --frames 5
# Test D8PSK modulation (auto-selected at SNR >= 20 on AWGN)
./test_iwaveform --snr 20 --channel awgn -w ofdm_chirp --mod d8psk --frames 5
# Test D8PSK on fading (requires R1/4 for robustness)
./test_iwaveform --snr 20 --channel good -w ofdm_chirp --mod d8psk --rate r1_4 --frames 5
# Full protocol test (PING/CONNECT/DATA/DISCONNECT)
./cli_simulator --snr 20 --test
# Regression matrix (all waveforms, multiple SNR levels)
../tests/regression_matrix.sh
# Run unit tests
ctestManual Modulation Selection:
The --mod flag allows testing specific modulations:
dqpsk- 4-phase differential (default, 2 bits/symbol)d8psk- 8-phase differential (+50% throughput, 3 bits/symbol)dbpsk- 2-phase differential (most robust, 1 bit/symbol)
The --rate flag selects LDPC code rate:
r1_4- Most robust (required for fading channels)r1_2- Balanced (default for AWGN)r2_3,r3_4,r5_6- Higher throughput, needs better SNR
GUI Expert Settings:
To force specific modulation in the GUI application:
- Open Settings (gear icon)
- Go to Expert tab
- Set Forced Modulation to desired mode (D8PSK, DQPSK, etc.)
- Optionally set Forced Code Rate (R1/4 recommended for fading)
When D8PSK is forced on a fading channel, the two-pass decoder automatically activates to improve reliability (uses embedded DQPSK grid for phase correction).
Test Tools:
| Tool | Purpose |
|---|---|
test_iwaveform |
Primary - continuous RX, all waveforms |
cli_simulator |
Full protocol testing |
regression_matrix.sh |
Automated multi-config testing |
- SSB transceiver with 2.8+ kHz filter bandwidth
- Audio interface (SignaLink, RigBlaster, or direct soundcard connection)
- PTT control (VOX, CAT, or hardware)
- TX: Adjust for clean signal without ALC compression
- RX: Set for comfortable listening level (avoid clipping)
ProjectUltra uses ~2.8 kHz bandwidth. Operate in wideband digital segments, not narrow-band FT8/PSK31 areas.
| Band | Frequency (USB) | Notes |
|---|---|---|
| 80m | 3.590 MHz | Above narrow digital, below voice |
| 40m | 7.102 MHz | Common for VARA/Winlink |
| 30m | 10.145 MHz | Check for WSPR at 10.140 |
| 20m | 14.108 MHz | Above FT8 crowd, wideband digital |
| 15m | 21.110 MHz | Above narrow digital segment |
| 10m | 28.120 MHz | Plenty of room |
Avoid:
- 14.070-14.095 MHz (FT8/PSK31)
- Any .074 MHz frequencies (FT8)
- 14.100 MHz (NCDXF beacons)
Best practice: Listen for 10-15 seconds before transmitting. Use minimum power necessary. Be ready to QSY if causing interference.
Note: Core waveforms have been verified in loopback and acoustic testing. On-air HF testing is in progress.
| Waveform | SNR Range | CFO Tolerance | Status |
|---|---|---|---|
| MC-DPSK (8 carriers) | 5+ dB | ±50 Hz | 100% verified |
| OFDM-CHIRP (59 carriers) | 10+ dB | ±50 Hz | 100% verified |
| OFDM-COX (Schmidl-Cox sync) | 17+ dB | TBD | DATA phase working |
Acoustic Channel Test: Successfully decoded CONNECT and DATA frames through speaker → air → microphone path at 70% volume (~22 dB SNR). This validates real-world audio chain compatibility.
Working:
- All LDPC code rates (R1/4 through R5/6)
- OFDM with DQPSK/D8PSK modulation (differential)
- OFDM with QPSK/16QAM/32QAM modulation (coherent, for NVIS/local)
- NVIS high-speed mode: 1024 FFT, 59 carriers, up to 7.2 kbps
- Multi-carrier DPSK (8 carriers, robust sync)
- Full protocol: PING/PONG/CONNECT/DATA/DISCONNECT
- ARQ protocol with retransmission
- GUI with waterfall and constellation
- Expert mode for forcing waveform/modulation settings
- CLI transmit/receive tools
- HF channel simulator (ITU-R F.1487)
- Continuous streaming RX decoder
In Progress:
- On-air HF testing
- Automatic waveform switching based on SNR
- File transfer optimization
Goal: Exceed industry-leading HF modem speeds (currently ~8.5 kbps in 2.8 kHz bandwidth)
ProjectUltra is a research platform for investigating advanced modulation and coding techniques for HF channels. Our target is 10+ kbps throughput while maintaining robustness.
| Technique | Expected Gain | Status | References |
|---|---|---|---|
| OTFS | 20% SE over OFDM, full diversity | Implemented, needs HF testing | arxiv.org/pdf/2302.14224 |
| AFDM | >1 dB over OTFS, lower pilots | Not yet implemented | arxiv.org/html/2507.21704v3 |
| Turbo DPSK | 4.65 dB on Rayleigh fading | Planned | Turbo DPSK paper |
| Faster-than-Nyquist | 25% more bits (Mazo limit) | Planned | IEEE ComSoc overview |
| SC-LDPC | 0.07 dB from capacity | Planned | SC-LDPC tutorial |
| OFDM-IM | 2x SE via index modulation | Planned | arxiv.org/html/2501.15437v1 |
OTFS/AFDM: Traditional OFDM suffers from inter-carrier interference (ICI) when Doppler spread is high. OTFS and AFDM work in the delay-Doppler domain, achieving full diversity over doubly-selective channels. Literature shows AFDM outperforms OTFS by >1 dB with simpler channel estimation.
Turbo DPSK: Our single-carrier DPSK already works to -11 dB SNR. Adding iterative (turbo) processing between the DPSK demodulator and LDPC decoder could push this to -15 dB or improve throughput at higher SNR.
Faster-than-Nyquist: By accepting controlled intersymbol interference and using iterative detection, FTN can transmit 25% more symbols in the same bandwidth (Mazo limit: τ=0.8).
Spatially-Coupled LDPC: Our current LDPC codes are from IEEE 802.11n. SC-LDPC codes achieve threshold saturation, approaching capacity within 0.07 dB on fading channels.
| Mode | ProjectUltra | Industry Leader | Gap |
|---|---|---|---|
| Max throughput (verified) | 5.4 kbps | 8.5 kbps | -36% |
| Max throughput (theoretical) | 7.2 kbps | 8.5 kbps | -15% |
| Low-SNR floor (verified) | 5 dB | ~0 dB | Needs work |
| CFO tolerance | ±50 Hz | ±100 Hz | TBD |
We're looking for collaborators interested in:
- Implementing AFDM modulation
- Adding turbo processing to DPSK
- Testing OTFS on real HF recordings
- SC-LDPC code design and optimization
- FTN with iterative ISI cancellation
If you have expertise in these areas, please open an issue or reach out!
Contributions welcome! Areas of interest:
- On-air testing and performance reports
- Documentation and tutorials
- Bug fixes and optimizations
Please open an issue before submitting large PRs.
We need real-world HF recordings to validate the modem beyond simulation.
If you're a licensed amateur radio operator, you can help by recording your own transmissions via WebSDR. This is legitimate amateur experimentation under FCC Part 97 (advancement of the radio art).
How to record your own signal:
- Pick a frequency from the recommended list above (e.g., 14.108 MHz USB)
- Open a WebSDR receiver (websdr.org or kiwisdr.com) and tune to that frequency
- Start recording on the WebSDR (or use Audacity to capture system audio)
- Transmit using ProjectUltra (see options below)
- Stop recording and save the file
What to transmit:
| Frame Type | Duration | Best For |
|---|---|---|
| PING probe | ~1 second | Quick propagation test |
| Full CONNECT | ~8 seconds | Complete handshake test |
-
PING probe: Short DPSK burst with "ULTR" marker. Fast way to check if your signal is reaching the WebSDR. In GUI: click Connect, recording starts immediately with the PING. Cancel after 2-3 seconds if you just want the probe.
-
Full CONNECT: Complete connection attempt with callsign exchange (3 codewords). More comprehensive test of sync, LDPC decode, and protocol parsing.
What to submit:
- Recording file (48kHz mono WAV or F32 preferred)
- Your callsign and transmit location
- Band/frequency and time (UTC)
- WebSDR location used (e.g., "Twente WebSDR, Netherlands")
- Approximate path distance
- Band conditions if known (S-meter readings)
- Frame type transmitted (PING or CONNECT)
Why this helps: Real ionospheric propagation has characteristics that simulation can't capture. Even "failed" recordings where the signal didn't decode are valuable - they help us improve sync detection and weak-signal performance.
How to submit: Open a GitHub issue with the "Recording" label.
MIT License. See LICENSE for details.
- Dear ImGui - GUI framework
- SDL2 - Audio and windowing
- FFTW3 - Fast Fourier Transform (required for chirp detection)
- miniz - Compression
