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[SX1276] Reduced range with low data rate optimization

Semtech Transceivers
#1

Hi all,

I am using a LoRa transceiver based on the SX1276 with TCXO and external power amplifier in the 169 MHz ISM band with the following parameters:

  • Frequency = 169.4375 MHz
  • Bandwidth = 15.6 kHz
  • Spreading factor = 9
  • Coding rate = 4/5
  • TX power = 13 dBm (PA off) and 27 dBm (PA on)

So far, I used the transceiver without CRC and low data rate optimization being enabled and obtained very good results when doing range tests.

After I read about the fact that low data rate optimization is mandatory for symbol times greater than 16 ms, I enabled it and also switched on CRC at the same time. Against my expectations, the range decreased drastically with this change in the parameters. Before, correct messages were received with RSSI values down to -135 dBm. After changing the parameters, the lowest RSSI value I noticed was rather around -125 dBm.

Is there any technical explanation for this of which I am not aware? Is the low data rate optimization option even necessary when having a TCXO? Can the extra two bytes due to the CRC have such a big influence on the range?

Thanks in advance for your help!