lunedì 3 luglio 2017

K1JT announces new "FT8" Mode

Dear WSJT-X Beta-Test Colleagues,

Steve (K9AN) and I have developed a potential new mode for WSJT-X. We’re calling the mode “FT8” (Franke-Taylor design, 8-FSK modulation).

FT8 is designed for situations like multi-hop Es where signals may be weak and fading, openings may be short, and you want fast completion of reliable, confirmable QSOs.

Important characteristics of FT8:

– T/R sequence length: 15 s
– Message length: 75 bits + 12-bit CRC
– FEC code: LDPC(174,87)
– Modulation: 8-FSK, keying rate = tone spacing = 5.86 Hz
– Waveform: Continuous phase, constant envelope
– Occupied bandwidth: 47 Hz
– Synchronization: three 7×7 Costas arrays (start, middle, end of Tx)
– Transmission duration: 79*2048/12000 = 13.48 s
– Decoding threshold: 20 dB (perhaps -24 dB with AP decoding, TBD)
– Operational behavior: similar to HF usage of JT9, JT65
– Multi-decoder: finds and decodes all FT8 signals in passband
– Auto-sequencing after manual start of QSO

Comparison with slow modes JT9, JT65, QRA64: FT8 is a few dB less sensitive but allows completion of QSOs four times faster. Bandwidth is greater than JT9, but about 1/4 of JT65A and less than 1/2 QRA64.

Comparison with fast modes JT9E-H: FT8 is significantly more sensitive, has much smaller bandwidth, uses the vertical waterfall, and offers multi-decoding over the full displayed passband.

Still to come, not yet implemented: We plan to implement signal subtraction, two-pass decoding, and use of “a priori” (already known) information as it accumulates during a QSO.

Three extra bits are available in the message payload, with uses yet to be defined. We have in mind special message formats that might be used in contests, and the like. Your considered suggestions for use of these bits are very welcome!

K1JT, K9AN, and G4WJS have conducted on-the-air tests of FT8 with excellent results. We’re now at a stage where tests under a wider range of conditions are desirable. If you can build WSJT-X from source code revision r7750 or later, and would like to help, please do so and report your results to us! Pre-built installation packages will be made available after further testing is completed.

Suggestions for FT8 setup and examples of use can be found in a screen shot posted here: http://physics.princeton.edu/pulsar/k1jt/ft8.png

We look forward to receiving your feedback.

— 73, Joe, K1JT

Hi all,
It will help to have some initial frequency coordination among those testing the new FT8 mode in WSJT-X v1.7.1.

To help get things started, we suggest using the following frequencies:

Band MHz
————
160 1.841
80 3.579
40 7.079
30 10.141
20 14.079
17 18.105
15 21.079
12 24.920
10 28.079
6 50.313
4 70.094
————-

Most of these suggestions follow a simple guideline: add 3 kHz to the commonly used JT65 dial frequency.

For 6 meters we suggest using 50.313, which should be consistent with band plans in all IARU Regions. (Six meter activity that’s now on 50.276, world-wide, might be moved to 50.310.)

We are very much open to suggestions for changes in the above table.

— 73, Joe, K1JT

- - -
Joe, K1JT

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