FT8 signals below the noise level?

Krátka odpoveď znie: áno, FT8 can actually decode signals that are physically weaker than the overall noise in the receiving channel, ale má to dôležitý technický háčik v tom, ako ten šum definujeme.

Here is a breakdown of how it works:

Reference bandwidth (2500 Hz)

When you see in the program WSJT-X data for example -21 dB, tento údaj nie je absolútny. Je vztiahnutý k štandardnej šírke pásma SSB filtra, čo je 2500 Hz.

• If the signal level is 0 dB, znamená to, že výkon signálu je rovnaký ako výkon šumu v celom 2500 Hz pásme.

• If the signal is -21 dB, znamená to, že je hlboko „utopený“ v šume, ak by sme sa naň pozerali ako na celok.

Narrowband gain

However, FT8 does not use the entire 2500 Hz. One FT8 signal occupies only about 50 Hz. Fyzika nepustí: ak rozdelíte šum z 2500 Hz pásma na malé 50 Hz úseky, v každom takomto malom úseku je podstatne menej šumu. Tým, že sa FT8 „sústredí“ na veľmi úzku šírku pásma, získava obrovskú výhodu oproti SSB.

You can listen to how signals sound with different bandwidths at https://olgierd.github.io/ft8-vs-cw/ (TNX for the tip Matej OK1TEH)

Why is FT8 better than CW or SSB?

Pre zrozumiteľnosť ľudským uchom potrebujeme určitý odstup signálu od šumu (SNR). Digital modes ako FT8 používajú techniky, ktoré ucho jednoducho nedokáže:

• Forward Error Correction (FEC): Data is transmitted with redundant information. Even if part of the tones is completely lost in noise, mathematical algorithms can calculate and fill in the missing pieces of the message.

• Časová integrácia: An FT8 session lasts 15 seconds. The software 'observes' the frequency for a full 15 seconds and looks for known patterns (tones) in it. It's like staring at a very dark photo for a long time until your eyes start to discern outlines.

Bandwidth is just the beginning of the story. What makes FT8 almost 'magical' in pulling signals from the digital grave is kombinácia matematiky, presného načasovania a veľmi špecifickej modulácie.

Here are the key pillars that make FT8 dominate:

Time synchronization and 'Costas Arrays'

Time is critical. FT8 operates in 15-second cycles.

• Prečo je to dôležité? The decoder knows exactly when the signal starts and when it ends. It doesn't have to waste computational power looking for the start of the message in noise.

• Costas Arrays: At the beginning, middle, and end of each session, special sequences of tones (synchronization markers) are transmitted. The decoder looks for them like beacons. When it finds them, it knows exactly how to 'align' the rest of the data, even when the signal is extremely weak.

8-GFSK modulation (8 tones)

FT8 uses not just 'a few tones', but exactly 8 frekvencií (8-ary Frequency Shift Keying).

• Each tone represents 3 bits of data.

• G (Gaussian): The tones switch smoothly (through a Gaussian filter), which minimizes side emissions and saves bandwidth.

• Konštantná amplitúda: Unlike SSB, where power fluctuates according to voice, FT8 transmits always at 'full throttle'. This allows the final stage of the transmitter to operate in an efficient mode without distortion.

LDPC: Mathematical superhero (Error Correction)

This is probably the biggest reason for success. FT8 uses LDPC (Low-Density Parity-Check) kódy.

• The message itself (callsign, locator) has only 77 bitov.

• However, after adding LDPC coding, a total of 174 bitov.

• Čo to robí? These extra bits are pure mathematics. If due to noise or interference up to 30 - 40% of tones are lost or damaged, the LDPC algorithm can recover the rest of the message vypočítať. Je to ako skladačka, kde vám chýba tretina dielikov, ale vy presne viete, čo na nich musí byť, aby obraz dával zmysel.

Structured protocol

Unlike CW (telegraphy), where you can send anything, FT8 is highly structured. The decoder knows in advance that it expects:

1. Callsign 1

2. Callsign 2

3. Locator or report

By having a 'dictionary' of the protocol limited, the probability of successful decoding at low signal-to-noise ratio (SNR) dramatically increases.

Summary: are the signals below the noise level?

From the perspective of measuring signal strength, FT8 mixes pears with apples. The strength is reported at a bandwidth of 2500 Hz, but the decoder works with approx. 50 Hz bandwidth.

A similar effect would be observed when receiving a CW signal - on a wide filter, the signal will be in noise, for a narrower filter it will be better, and for an even narrower filter best.

So don't let anyone pull the wool over your eyes - FT8 signals are not below the noise level.