8 default symbols in LoRa payload, What do they represent?

Continuing the discussion from Spreadsheet for LoRa airtime calculation:
After searching everywhere on Semtech’s documents ,LoRa calculator, and the LoRaWAN specifications. I still don’t understand why a Payload has at least 8 symbols that are independent than payload size, SF, CR,…
the 8 symbols i’m talking about aren’t those of the preamble. As you can see here:

Time on Air = T preamble + T payload
with
Tpreamble= (NbPreamble (8) + (symbols added by radio)4,25) x Tsymbol
and
Tpayload = NbPayloadSymbol x Tsymbol
And according to LoRa calculator, the NbPayloadSymbols is calculated with this formula:

I am looking for the meaning of those 8 symbols (red circle). In the
formulas of the calculator the nPayload is independant from the 8+4.25
symb of the preamble.

Does anybody have an explanation for those 8?
Much appreciated,

i geuss 8 symbols is use for (MHDR 1byte MIC:4byte, FHDR:7byte,Fport:1byte), i don’t understand the -4SF in the formula.

I check the patent EP2763321, the 8 default symbols should be the fine synchronisation symbols, and the -4FS should be the smoke bomb, so only 4 fine synchronisation symbols.

Please just explain, How number 4 with the part 4(SF-2DE) represents Coding Rate?
Regards

Welcome,

This is a very very old thread, so you may not get a response.

It would help however if you told us what your interest is in the formula.

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Thanks a lot, am trying to get ToA for 433MHz LoRa SX1278 at different (SF, BW, and CR) values.

Just use the calculator that Semtech provides, they do them for SX127x, SX126x, SX128x.

Which area of the World are you in that uses 433Mhz for TTN ?

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Middle East

I suspect not, not for TTN which is what this forum is about - but it’s a question of general interest, just be aware that for LoRaWAN we only do TTN here.

Have a look at App Notes > LoRa Modulation Basics

Generically the formula is
Time = size / rate

RateCode = 4/(4+CR)

Wondering why such a specific low level question is being asked here?