SX1280 calculateTimeOnAir implementation question

[GusBon]

Hello,

By reading the implementation of the caclulateTimeOnAir() function of the SX1280 module, there is something that I do not understand.

RadioLib/src/modules/SX128x/SX128x.cpp

Lines 1298 to 1355 in a90197d

	RadioLibTime_t SX128x::calculateTimeOnAir(ModemType_t modem, DataRate_t dr, PacketConfig_t pc, size_t len) {
	switch(modem) {
	case (ModemType_t::RADIOLIB_MODEM_LORA): {
	// calculate number of symbols
	float N_symbol = 0;
	uint8_t sf = dr.lora.spreadingFactor;
	float cr = (float)dr.lora.codingRate;
	// get SF coefficients
	float coeff1 = 0;
	int16_t coeff2 = 0;
	int16_t coeff3 = 0;
	if(sf < 7) {
	// SF5, SF6
	coeff1 = 6.25;
	coeff2 = 4*sf;
	coeff3 = 4*sf;
	} else if(sf < 11) {
	// SF7. SF8, SF9, SF10
	coeff1 = 4.25;
	coeff2 = 4*sf + 8;
	coeff3 = 4*sf;
	} else {
	// SF11, SF12
	coeff1 = 4.25;
	coeff2 = 4*sf + 8;
	coeff3 = 4*(sf - 2);
	}
	// get CRC length
	int16_t N_bitCRC = 16;
	if(!pc.lora.crcEnabled) {
	N_bitCRC = 0;
	}
	// get header length
	int16_t N_symbolHeader = 20;
	if(pc.lora.implicitHeader) {
	N_symbolHeader = 0;
	}
	// calculate number of LoRa preamble symbols
	uint32_t N_symbolPreamble = pc.lora.preambleLength;
	// calculate the number of symbols
	N_symbol = (float)N_symbolPreamble + coeff1 + 8.0f + ceilf((float)RADIOLIB_MAX((int16_t)(8 * len + N_bitCRC - coeff2 + N_symbolHeader), (int16_t)0) / (float)coeff3) * cr;
	// get time-on-air in us
	return(((uint32_t(1) << sf) / dr.lora.bandwidth) * N_symbol * 1000.0f);
	}
	case (ModemType_t::RADIOLIB_MODEM_FSK):
	return((((float)(pc.fsk.crcLength * 8) + pc.fsk.syncWordLength + pc.fsk.preambleLength + (uint32_t)len * 8) / (dr.fsk.bitRate / 1000.0f)));
	default:
	return(RADIOLIB_ERR_WRONG_MODEM);
	}
	}

Here for every SF the last term "+cr" is excluded from the ceil function. This cr term is the equivalent to the "(CR+4)" that we have in the section 7.4.4.1 of the datasheet.

But in this datasheet, for SF < SF7, the "(CR+4)" is included in the ceil function.

Do you think it is a typo in the datasheet? Or is the packet engine working a bit differently for SF5 and SF6?

Thanks a lot for your great lib!
Gus

[jgromes]

I'm not fully certain the datasheet is correct in this case - for sure the formatting of it seems weird as the number of parentheses does not seem to match for the SF5/SF6 case:

Nevertheless, this calculation is covered in our unit tests, which compare its output with the numbers provided by Semtech calculator applications, and one of the tested cases is for SF5:

RadioLib/extras/test/unit/tests/TestCalculateTimeOnAir.cpp

Line 28 in a90197d

{ "SX128x", RADIOLIB_MODEM_LORA, {.lora={5,400,5}}, {.lora={8,false,true,false}}, {1,50,200}, {2580,10179,34180} }, // 2.54, 10.02, 33.65

So I am reasonably sure the calculation as implemented inthe library is correct.