raw lora packets multiple of 5 bytes?

[robert-hh]

@kjm testing the length of the actual radio transmission with an oscilloscope, it shows indeed that the duration of the RF signal increases in steps. Using LoRaWan messages at DR=5 (SF7BW125), here are some results for increasing payload sizes. Only the values are shown at which the time changes. So the value for 3 - 6 bytes is the same.

Size   DR5 (ms)  Size DR0 (ms)
  1       46,4     1    1155
  3       51,8     3    1315
  7       56,8     8    1480
 10       61,6    13    1646
 14       66,8    18    1815
 17       72,0    23    1970
 21       77,4    28    2135
 24       82,3    33    2300

That may be related to the coding.
Edit: I repeated that for DR0. There, the Tx times increases at steps of 5 bytes.
Edit2: lora.stats() after a send seems to tell the airtime of the previous send. So repeating sending the same size twice tells you the time on air. That matches down to 1 ms what I see in the oscilloscope.

[jcaron]

@kjm in theory it should go up by 5 bits for each 4 of payload due to the coding rate (coding rate 1 is 5/4). See page 7 of the LoRa modem design guide. Not sure if that’s supposed to be rounded to entire bytes, I would have thought not.

IIRC the airtime returned by lora.stats is not measured but computed, so it’s not impossible it’s not correct, one would have to check the sources.