PACTOR-4 – the 4th-generation HF data protocol
PACTOR-4 (P4) uses state-of-the-art methods of adaptive channel equalization, channel coding and source compression. The result is a highly adaptive ARQ mode that adjusts almost perfectly to the varying conditions of the shortwave channel.
1. Operating principle
PACTOR-4 is an ARQ protocol (Automatic Repeat reQuest): data packets are transmitted synchronously and the receiving station acknowledges each block. The narrowly synchronized ARQ grid – similar to PACTOR-3, but with more speed levels and an improved switching algorithm – enables an extremely fast adaption to changing channel conditions.
On the lowest speed level, PACTOR-4 uses a newly developed 2-tone chirp modem. This chirp mode is robust against narrow-band and part-channel interference as well as against selective fading. Together with a significantly improved Memory-ARQ method it allows a link to continue down to an SNR of −20 dB @ 4 kHz.
2. Adaptive equalizer & notch filters
The channel equalizer operates iteratively and compensates for distortion caused by multi-path propagation up to a delay spread of 8.8 ms. Signal echoes are resolved and constructively superimposed. Even high Doppler spreads (e.g. ITU "poor channel") can be equalized. In contrast to OFDM methods, selective fading does not produce an irreducible noise floor – which is why a high-rate channel coding with relatively little overhead can be used.
Narrow-band interference is effectively removed in front of the adaptive equalizer by a newly developed minimum-phase-shift "real" N-times auto-notch filter (6 auto-notches in total).
3. Robustness & SNR range
PACTOR-4 reaches a higher data throughput than PACTOR-3 over the entire working range from −20 to +25 dB @ 4 kHz. The difference is more noticeable on typical shortwave channels with multipath propagation than on AWGN channels and lies, on average, about a factor 2 compared to PACTOR-3.
This makes PACTOR-4 equally suitable for channels with high signal-to-noise ratios and for those with very low SNR – the ideal protocol for safety-relevant data transmission.
4. Bandwidth & compatibility
PACTOR-4 was explicitly designed to be used on industry-standard SSB transceivers with a 2.4 kHz IF bandwidth – any modern amateur or marine transceiver will do. Thanks to the adaptive equalizer the shape of the IF filter curve is non-critical; even "heavily bent" filter curves are equalized with only a slightly higher SNR demand.
PACTOR-4 is fully backward compatible with PACTOR-1, PACTOR-2 and PACTOR-3. A connection to a station running an older protocol is established automatically in the appropriate mode.
5. Speed levels & data rates
| Gross data rate | up to 5,500 bps (uncompressed) |
|---|---|
| Net data rate (with PMC compression) | up to 10,500 bps |
| Speed levels | 10 adaptive speed levels + 1 chirp mode |
| Occupied bandwidth | 2,400 Hz |
| Auto-notch filters | 6 (automatic) |
| Delay spread compensation | up to 8.8 ms |
| Frequency correction range | ±280 Hz on connect (P4dragon) |
| Interference immunity | down to −20 dB @ 3 kHz SNR |
| Backward compatibility | PACTOR-1 / -2 / -3 |
6. Applications
The combination of high data rate, low bandwidth and extreme robustness makes PACTOR-4 the preferred solution wherever conventional infrastructure (internet, mobile, satellite) is not available or not trusted:
- Commercial shipping & yachting – email, weather, position reporting via Sailmail / Winlink
- Critical infrastructure – fail-safe backup communication
- Government & military – situational reporting, optional 2G ALE and AES-256
- Disaster relief & NGOs – Red Cross, civil defense, rescue services
- Research & expeditions – data transmission from remote regions
- Amateur radio – the fastest way to handle Winlink email traffic
Datasheets & protocol docs Available modems
The Ouroboros logo of the P4dragon series symbolizes both globe-circling HF communication and the many iterative methods used – without which PACTOR-4 could not approach the Shannon limit so closely.