LoRa RSSI calculation in Liligo TTGO LoRa32 v2.1

saeedmagsi · April 26, 2022, 8:35am

According to the SX1276 data sheet the RSSI calculation formula should be -157+PacketRssi+PacketSnr*0.25(when using HF port and SNR<0) but in there code there is no SNR part added. secondly the mid band threshold in the code is 868e6 but it should be 525e6. Am i wrong or is there anything wrong in the code? please let me know because i need a very accurate rssi and snr values for my research. Thank you

maxgerhardt · April 26, 2022, 8:23pm

If there is a bug in the libarry you’re using, please report it in their repository.

saeedmagsi · April 26, 2022, 10:06pm

Thank you for your response. Yes i reported in their repository in github but still no response.

maxgerhardt · April 26, 2022, 10:17pm

Can you link to the opened issue? There’s no reference to which library you’re using in your post.

saeedmagsi · April 26, 2022, 11:21pm

maxgerhardt · April 27, 2022, 9:32am

They are just using the Arduino-LMIC library for LoRa(WAN), but a seemingly outdated version fo it.

Current versions of the LMIC library have a better RSSI calculation

github.com

mcci-catena/arduino-lmic/blob/master/src/lmic/radio.c#L1316-L1332


      
          LMIC.snr  = readReg(LORARegPktSnrValue); // SNR [dB] * 4
          u1_t const rRssi = readReg(LORARegPktRssiValue);
          s2_t rssi = rRssi;
          if (LMIC.freq > SX127X_FREQ_LF_MAX)
              rssi += SX127X_RSSI_ADJUST_HF;
          else
              rssi += SX127X_RSSI_ADJUST_LF;
          if (LMIC.snr < 0)
              rssi = rssi - (-LMIC.snr >> 2);
          else if (rssi > -100) {
              // correct nonlinearity -- this is the same as multiplying rRssi * 16/15 initially.
              rssi += (rRssi / 15);
          }
          
          
LMIC_X_DEBUG_PRINTF("RX snr=%u rssi=%d\n", LMIC.snr/4, rssi);
          // ugh compatibility requires a biased range. RSSI
          LMIC.rssi = (s1_t) (RSSI_OFF + (rssi < -196 ? -196 : rssi > 63 ? 63 : rssi)); // RSSI [dBm] (-196...+63)

github.com

mcci-catena/arduino-lmic/blob/8d378ea410887d3fb08ea2c9acce95cc4c047788/src/lmic/radio.c#L244-L252


      
          // Parameters for RSSI monitoring
          #define SX127X_FREQ_LF_MAX      525000000       // per datasheet 6.3
          
          
// per datasheet 5.5.3 and 5.5.5:
          #define SX1272_RSSI_ADJUST      -139            // add to rssi value to get dB (LF)
          
          
// per datasheet 5.5.3 and 5.5.5:
          #define SX1276_RSSI_ADJUST_LF   -164            // add to rssi value to get dB (LF)
          #define SX1276_RSSI_ADJUST_HF   -157            // add to rssi value to get dB (HF)

saeedmagsi · April 27, 2022, 3:55pm

Thank you very much. I will try to use those libraries.