I have a question concerning the calculation of the time-on-air for the LoRa modem of the SX126x. On page 40 in the SX126x datasheet (V1.2) it is stated in Figure 6-2 and in the text (“The header is transmitted with maximum error correction code (4/8).”) that the header uses a fixed coding rate. To my understanding, the payload is transmitted with a user defined coding rate (4/5, 4/6, 4/7, or 4/8). As far as I understand, the coding rate has an influence on the time-on-air of a packet. In the time-on-air formula (p. 41) I don’t see where these facts are taken into account. As far as I can see, both N_byte_payload and N_sysmbol_header are multiplied by the same (user-defined) coding rate factor (CR+4). To my understanding, the header (N_byte_payload) should be multiplied by a different (fixed) coding rate factor (4+4).
I don’t understand why the unit of the variable “N_symbol_header” is symbols. The rest of the numerator in the formula on p. 41 appears to be in bits (bytes*8 or variables directly provided as number of bits). What would be the unit of the numerator if the unit of N_symbol_header would be symbols? Bits, Bytes, or Bit * Bytes?
I would appreciate any help with resolving the confusion.