Explicitly set CAD symbols, improve slot time calculation and adjust CW size accordingly (#5749)

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

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
@@ -637,4 +656,4 @@ size_t RadioInterface::beginSending(meshtastic_MeshPacket *p)
 
     sendingPacket = p;
     return p->encrypted.size + sizeof(PacketHeader);
-}
+}