Category:SWIFT-3k: Difference between revisions

Revision as of 18:18, 23 September 2026

Deeper look into the burst

SWIFT-3k (Also known as OFDM, Guest's modem) is a 49-tone OFDM PSK Russian military multi-channel modem. It features scrambled voice/ data-communication at a maximum data rate of 7333 bps. Each subcarrier can transmit up to 44.1 Bd.

This waveform is transmitted by the SWIFT-3k and the newer SWIFT-3k-3 modems in use by the multiple operators but Guest Radio operator mostly. These modems were designed to have many different devices plugged into them to form the modem’s input – most often teleprinters unique to the different military services and civilian agencies that use them, and also, vocoders and speech encipherment systems, but all are transmitted in the same fashion.

A characteristic trait of this signal is the unmodulated leader tone at 1500 Hz (But some people use different ones).

Air interface

Parameter	Value	Why
Sample rate	48 000 Hz	Native rate of nearly every sound device.
FFT size / spacing	1024 → 46.875 Hz	Symbol long enough for HF Doppler, short enough to track drift.
Guard interval	64 / 128 / 256 samples	Fast / Standard / Robust — trades throughput for tolerance of path delay spread and timing error.
Symbol rate	44.1 / 41.7 / 37.5 baud	Not signalled — the receiver identifies it from the reference symbol's own guard interval before reading anything else.
Carriers	49 (375–2625 Hz)	Fits a 300–2700 Hz SSB filter with room at both edges.
Pilots	5 at −24, −12, 0, +12, +24	Carry a known zero phase step; drive the phase loop.
Data carriers	44	Everything else.
Modulation	DBPSK / DQPSK / D8PSK / D16PSK	Differential in time, so no channel equaliser is needed.
FEC	Rate ½, K=7 (171/133)	Soft-decision Viterbi, ~5 dB of coding gain.
Interleaver	Coprime stride, per block	Spreads a 2 kB block over up to 9 s of air time.
Occupied bandwidth	≈ 2.3 kHz	−26 dB points at roughly 350 and 2650 Hz.

Throughput

Mode	Gross	Net payload	Reliable from
DBPSK + FEC	1 833 bit/s	821 bit/s	+4 dB
DQPSK + FEC	3 667 bit/s	1 700 bit/s	+4 dB
D8PSK + FEC	5 500 bit/s	2 571 bit/s	+9 dB
D16PSK + FEC	7 333 bit/s	3 290 bit/s	+17 dB
DQPSK raw	3 667 bit/s	3 182 bit/s	+14 dB
D8PSK raw	5 500 bit/s	4 841 bit/s	+26 dB

Logs: none yet

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-[[File:CIS-12.png|300px|thumb|right|Idle/Stand-by -> Traffic burst]]
+[[File:Obraz.png|300px|thumb|right|Leader tone -> Traffic burst]]
+[[File:Obraz_(1).png|300px|thumb|right|Deeper look into the burst]]
 '''SWIFT-3k''' (Also known as OFDM, Guest's modem) is a 49-tone OFDM PSK Russian military multi-channel modem. It features scrambled voice/ data-communication at a maximum data rate of 7333 bps. Each subcarrier can transmit up to 44.1 Bd.