FT1

FT1 is an experimental, general-purpose weak-signal digital mode for amateur radio, designed by KD9TAW. It chases something the established modes don’t: pulling more information out of every second of air time so contacts complete in a 4-second transmit/receive cycle — roughly 4× quicker turnaround than the 15-second frame behind FT8 and JS8. FT1 is the underlying mode, not a single application: it powers the chat-first Tempo app and a WSJT-X integration alike. Making a keyboard QSO move at the speed of a conversation is one of the things it unlocks — but the bigger story is a modern weak-signal protocol the ham bands haven’t had before.

Cellular tech on the ham bands

FT1’s two signature techniques come straight from modern 4G/LTE cellular systems:

• Turbo equalization — an iterative receiver where a maximum-likelihood trellis detector (BCJR) and the LDPC decoder trade soft information back and forth, each pass refining the other. It’s the same iterative-decoding idea behind turbo codes in 3G/4G.
• IR-HARQ (Incremental-Redundancy Hybrid ARQ) — retransmissions send new parity bits the receiver has never seen, soft-combined into a longer, lower-rate code. No existing amateur text mode does this; in cellular it’s standard.

On top of that, FT1 uses coherent 4-CPM modulation (continuous-phase, h=1/2, BT=0.3) — tracking carrier phase to wring the most information out of every second of air time.

The honest trade

FT1 doesn’t beat FT8 on raw single-shot sensitivity — it gives up roughly 3 dB to buy its fast cycle (a simulated AWGN threshold near −15 dB). What it offers is speed, plus an IR-HARQ path that claws sensitivity back over several short transmissions instead of one long one.

Every figure here is simulation-validated only — not yet confirmed on the air. On-air decode-rate-vs-SNR validation is the project’s #1 remaining gate.

The reference implementation

This repository is the Python reference and research codebase for the mode. It’s the foundation that the WSJT-X (FT1) fork and the chat-first Tempo app build on.

Want the full story? Read the FT1 protocol deep-dive → for the waveform, signal chain, message layer, and how IR-HARQ accumulates redundancy.