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

End Devices (Nodes)
1

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,

2

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

3

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.

4

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

5

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.

1 Like
6

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

7

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 ?

1 Like
8

Middle East

9

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.

10

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?