Add weighted tx delay for flooding router

src/mesh/RadioInterface.cpp

@@ -180,7 +180,38 @@ uint32_t RadioInterface::getTxDelayMsec()
      */
     // const uint32_t MAX_TX_WAIT_MSEC = 2000; // stress test would still fail occasionally with 1000
 
-    return random(MIN_TX_WAIT_MSEC, shortPacketMsec);
+    return random((MIN_TX_WAIT_MSEC), (MIN_TX_WAIT_MSEC + shortPacketMsec));
+}
+
+
+/** The delay to use when we want to send something but the ether is busy */
+uint32_t RadioInterface::getTxDelayMsecWeighted(float snr)
+{
+    /** At the low end we want to pick a delay large enough that anyone who just completed sending (some other node)
+     * has had enough time to switch their radio back into receive mode.
+     */
+    const uint32_t MIN_TX_WAIT_MSEC = 100;
+
+    // The minimum value for a LoRa SNR
+    const uint32_t SNR_MIN = -20;
+
+    // The maximum value for a LoRa SNR
+    const uint32_t SNR_MAX = 15;
+
+    //  high SNR = Long Delay
+    //  low SNR = Short Delay
+    uint32_t delay = 0;
+
+    if (radioConfig.preferences.role == Role_Router) {
+        delay = map(snr, SNR_MIN, SNR_MAX, MIN_TX_WAIT_MSEC, (MIN_TX_WAIT_MSEC + (shortPacketMsec / 2)));
+        DEBUG_MSG("rx_snr found in packet. As a router, setting tx delay:%d\n", delay);
+    } else {
+        delay = map(snr, SNR_MIN, SNR_MAX, MIN_TX_WAIT_MSEC + (shortPacketMsec / 2), (MIN_TX_WAIT_MSEC + shortPacketMsec * 2));
+        DEBUG_MSG("rx_snr found in packet. Setting tx delay:%d\n", delay);
+    }
+
+
+    return delay;
 }
 
 void printPacket(const char *prefix, const MeshPacket *p)

src/mesh/RadioInterface.h

@@ -129,6 +129,10 @@ class RadioInterface
     /** The delay to use when we want to send something but the ether is busy */
     uint32_t getTxDelayMsec();
 
+    /** The delay to use when we want to send something but the ether is busy. Use a weighted scale based on SNR */
+    uint32_t getTxDelayMsecWeighted(float snr);
+
+
     /**
      * Calculate airtime per
      * https://www.rs-online.com/designspark/rel-assets/ds-assets/uploads/knowledge-items/application-notes-for-the-internet-of-things/LoRa%20Design%20Guide.pdf

src/mesh/RadioLibInterface.cpp

@@ -113,7 +113,19 @@ ErrorCode RadioLibInterface::send(MeshPacket *p)
 
     // We want all sending/receiving to be done by our daemon thread, We use a delay here because this packet might have been sent
     // in response to a packet we just received.  So we want to make sure the other side has had a chance to reconfigure its radio
-    startTransmitTimer(true);
+
+    /* We assume if rx_snr = 0 and rx_rssi = 0, the packet was not generated locally.
+    *   This assumption is valid because of the offset generated by the radio to account for the noise
+    *   floor.
+    */  
+    if (p->rx_snr == 0 && p->rx_rssi == 0) {
+        startTransmitTimer(true);
+    } else {
+        // If there is a SNR, start a timer scaled based on that SNR.
+        DEBUG_MSG("rx_snr found. hop_limit:%d rx_snr:%f\n", p->hop_limit, p->rx_snr);
+        startTransmitTimerSNR(p->rx_snr);
+    }
+    
 
     return res;
 #else
@@ -208,6 +220,16 @@ void RadioLibInterface::startTransmitTimer(bool withDelay)
     }
 }
 
+void RadioLibInterface::startTransmitTimerSNR(float snr)
+{
+    // If we have work to do and the timer wasn't already scheduled, schedule it now
+    if (!txQueue.empty()) {
+        uint32_t delay = getTxDelayMsecWeighted(snr);
+        // DEBUG_MSG("xmit timer %d\n", delay);
+        notifyLater(delay, TRANSMIT_DELAY_COMPLETED, false); // This will implicitly enable
+    }
+}
+
 void RadioLibInterface::handleTransmitInterrupt()
 {
     // DEBUG_MSG("handling lora TX interrupt\n");

src/mesh/RadioLibInterface.h

@@ -148,6 +148,13 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
      * */
     void startTransmitTimer(bool withDelay = true);
 
+    /** if we have something waiting to send, start a short scaled timer based on SNR so we can come check for collision before actually doing
+     * the transmit
+     *
+     * If the timer was already running, we just wait for that one to occur.
+     * */
+    void startTransmitTimerSNR(float snr);
+
     void handleTransmitInterrupt();
     void handleReceiveInterrupt();