Explicitly set CAD symbols, improve slot time calculation and adjust CW size accordingly (#5772)

src/mesh/LR11x0Interface.cpp

@@ -256,10 +256,17 @@ template <typename T> void LR11x0Interface<T>::startReceive()
 template <typename T> bool LR11x0Interface<T>::isChannelActive()
 {
     // check if we can detect a LoRa preamble on the current channel
+    ChannelScanConfig_t cfg = {.cad = {.symNum = NUM_SYM_CAD,
+                                       .detPeak = RADIOLIB_LR11X0_CAD_PARAM_DEFAULT,
+                                       .detMin = RADIOLIB_LR11X0_CAD_PARAM_DEFAULT,
+                                       .exitMode = RADIOLIB_LR11X0_CAD_PARAM_DEFAULT,
+                                       .timeout = 0,
+                                       .irqFlags = RADIOLIB_IRQ_CAD_DEFAULT_FLAGS,
+                                       .irqMask = RADIOLIB_IRQ_CAD_DEFAULT_MASK}};
     int16_t result;
 
     setStandby();
-    result = lora.scanChannel();
+    result = lora.scanChannel(cfg);
     if (result == RADIOLIB_LORA_DETECTED)
         return true;
 

src/mesh/RadioInterface.cpp

@@ -261,7 +261,7 @@ uint8_t RadioInterface::getCWsize(float snr)
     const uint32_t SNR_MIN = -20;
 
     // The maximum value for a LoRa SNR
-    const uint32_t SNR_MAX = 15;
+    const uint32_t SNR_MAX = 10;
 
     return map(snr, SNR_MIN, SNR_MAX, CWmin, CWmax);
 }
@@ -566,7 +566,7 @@ void RadioInterface::applyModemConfig()
     saveChannelNum(channel_num);
     saveFreq(freq + loraConfig.frequency_offset);
 
-    slotTimeMsec = computeSlotTimeMsec(bw, sf);
+    slotTimeMsec = computeSlotTimeMsec();
     preambleTimeMsec = getPacketTime((uint32_t)0);
     maxPacketTimeMsec = getPacketTime(meshtastic_Constants_DATA_PAYLOAD_LEN + sizeof(PacketHeader));
 
@@ -581,6 +581,25 @@ void RadioInterface::applyModemConfig()
     LOG_INFO("Slot time: %u msec", slotTimeMsec);
 }
 
+/** Slottime is the time to detect a transmission has started, consisting of:
+  - CAD duration;
+  - roundtrip air propagation time (assuming max. 30km between nodes);
+  - Tx/Rx turnaround time (maximum of SX126x and SX127x);
+  - MAC processing time (measured on T-beam) */
+uint32_t RadioInterface::computeSlotTimeMsec()
+{
+    float sumPropagationTurnaroundMACTime = 0.2 + 0.4 + 7; // in milliseconds
+    float symbolTime = pow(2, sf) / bw;                    // in milliseconds
+
+    if (myRegion->wideLora) {
+        // CAD duration derived from AN1200.22 of SX1280
+        return (NUM_SYM_CAD_24GHZ + (2 * sf + 3) / 32) * symbolTime + sumPropagationTurnaroundMACTime;
+    } else {
+        // CAD duration for SX127x is max. 2.25 symbols, for SX126x it is number of symbols + 0.5 symbol
+        return max(2.25, NUM_SYM_CAD + 0.5) * symbolTime + sumPropagationTurnaroundMACTime;
+    }
+}
+
 /**
  * Some regulatory regions limit xmit power.
  * This function should be called by subclasses after setting their desired power.  It might lower it

src/mesh/RadioInterface.h

@@ -83,24 +83,22 @@ class RadioInterface
     float bw = 125;
     uint8_t sf = 9;
     uint8_t cr = 5;
-    /** Slottime is the minimum time to wait, consisting of:
+
-      - CAD duration (maximum of SX126x and SX127x);
+    const uint8_t NUM_SYM_CAD = 2;       // Number of symbols used for CAD, 2 is the default since RadioLib 6.3.0 as per AN1200.48
-      - roundtrip air propagation time (assuming max. 30km between nodes);
+    const uint8_t NUM_SYM_CAD_24GHZ = 4; // Number of symbols used for CAD in 2.4 GHz, 4 is recommended in AN1200.22 of SX1280
-      - Tx/Rx turnaround time (maximum of SX126x and SX127x);
+    uint32_t slotTimeMsec = computeSlotTimeMsec();
-      - MAC processing time (measured on T-beam) */
-    uint32_t slotTimeMsec = computeSlotTimeMsec(bw, sf);
     uint16_t preambleLength = 16;      // 8 is default, but we use longer to increase the amount of sleep time when receiving
     uint32_t preambleTimeMsec = 165;   // calculated on startup, this is the default for LongFast
     uint32_t maxPacketTimeMsec = 3246; // calculated on startup, this is the default for LongFast
     const uint32_t PROCESSING_TIME_MSEC =
         4500;                // time to construct, process and construct a packet again (empirically determined)
-    const uint8_t CWmin = 2; // minimum CWsize
+    const uint8_t CWmin = 3; // minimum CWsize
-    const uint8_t CWmax = 7; // maximum CWsize
+    const uint8_t CWmax = 8; // maximum CWsize
 
     meshtastic_MeshPacket *sendingPacket = NULL; // The packet we are currently sending
     uint32_t lastTxStart = 0L;
 
-    uint32_t computeSlotTimeMsec(float bw, float sf) { return 8.5 * pow(2, sf) / bw + 0.2 + 0.4 + 7; }
+    uint32_t computeSlotTimeMsec();
 
     /**
      * A temporary buffer used for sending/receiving packets, sized to hold the biggest buffer we might need

src/mesh/SX126xInterface.cpp

@@ -299,10 +299,17 @@ template <typename T> void SX126xInterface<T>::startReceive()
 template <typename T> bool SX126xInterface<T>::isChannelActive()
 {
     // check if we can detect a LoRa preamble on the current channel
+    ChannelScanConfig_t cfg = {.cad = {.symNum = NUM_SYM_CAD,
+                                       .detPeak = RADIOLIB_SX126X_CAD_PARAM_DEFAULT,
+                                       .detMin = RADIOLIB_SX126X_CAD_PARAM_DEFAULT,
+                                       .exitMode = RADIOLIB_SX126X_CAD_PARAM_DEFAULT,
+                                       .timeout = 0,
+                                       .irqFlags = RADIOLIB_IRQ_CAD_DEFAULT_FLAGS,
+                                       .irqMask = RADIOLIB_IRQ_CAD_DEFAULT_MASK}};
     int16_t result;
 
     setStandby();
-    result = lora.scanChannel();
+    result = lora.scanChannel(cfg);
     if (result == RADIOLIB_LORA_DETECTED)
         return true;
     if (result != RADIOLIB_CHANNEL_FREE)

src/mesh/SX128xInterface.cpp

@@ -275,10 +275,17 @@ template <typename T> void SX128xInterface<T>::startReceive()
 template <typename T> bool SX128xInterface<T>::isChannelActive()
 {
     // check if we can detect a LoRa preamble on the current channel
+    ChannelScanConfig_t cfg = {.cad = {.symNum = NUM_SYM_CAD_24GHZ,
+                                       .detPeak = 0,
+                                       .detMin = 0,
+                                       .exitMode = 0,
+                                       .timeout = 0,
+                                       .irqFlags = RADIOLIB_IRQ_CAD_DEFAULT_FLAGS,
+                                       .irqMask = RADIOLIB_IRQ_CAD_DEFAULT_MASK}};
     int16_t result;
 
     setStandby();
-    result = lora.scanChannel();
+    result = lora.scanChannel(cfg);
     if (result == RADIOLIB_LORA_DETECTED)
         return true;
     if (result != RADIOLIB_CHANNEL_FREE)