* CustomLR1110::getTimeOnAir(), copied from sx1262

Author: Scott Powell

src/helpers/CustomLR1110.h

@@ -9,6 +9,35 @@ class CustomLR1110 : public LR1110 {
   public:
     CustomLR1110(Module *mod) : LR1110(mod) { }
 
+    RadioLibTime_t getTimeOnAir(size_t len) override {
+      uint32_t symbolLength_us = ((uint32_t)(1000 * 10) << this->spreadingFactor) / (this->bandwidthKhz * 10) ;
+      uint8_t sfCoeff1_x4 = 17; // (4.25 * 4)
+      uint8_t sfCoeff2 = 8;
+      if(this->spreadingFactor == 5 || this->spreadingFactor == 6) {
+        sfCoeff1_x4 = 25; // 6.25 * 4
+        sfCoeff2 = 0;
+      }
+      uint8_t sfDivisor = 4*this->spreadingFactor;
+      if(symbolLength_us >= 16000) {
+        sfDivisor = 4*(this->spreadingFactor - 2);
+      }
+      const int8_t bitsPerCrc = 16;
+      const int8_t N_symbol_header = this->headerType == RADIOLIB_SX126X_LORA_HEADER_EXPLICIT ? 20 : 0;
+
+      // numerator of equation in section 6.1.4 of SX1268 datasheet v1.1 (might not actually be bitcount, but it has len * 8)
+      int16_t bitCount = (int16_t) 8 * len + this->crcTypeLoRa * bitsPerCrc - 4 * this->spreadingFactor  + sfCoeff2 + N_symbol_header;
+      if(bitCount < 0) {
+        bitCount = 0;
+      }
+      // add (sfDivisor) - 1 to the numerator to give integer CEIL(...)
+      uint16_t nPreCodedSymbols = (bitCount + (sfDivisor - 1)) / (sfDivisor);
+
+      // preamble can be 65k, therefore nSymbol_x4 needs to be 32 bit
+      uint32_t nSymbol_x4 = (this->preambleLengthLoRa + 8) * 4 + sfCoeff1_x4 + nPreCodedSymbols * (this->codingRate + 4) * 4;
+
+      return((symbolLength_us * nSymbol_x4) / 4);
+    }
+
     bool isReceiving() {
       uint16_t irq = getIrqStatus();
       bool detected = ((irq & LR1110_IRQ_HEADER_VALID) || (irq & LR1110_IRQ_HAS_PREAMBLE));