Implement listen-before-talk mechanism

- Function setRandomDelay() calls either startTransmitTimer() or startTransmitTimerSNR()
- After coming back from Rx/Tx-ing, call setRandomDelay()
- If channel is currently busy, call setRandomDelay()

src/mesh/RadioLibInterface.cpp

@@ -118,20 +118,10 @@ ErrorCode RadioLibInterface::send(MeshPacket *p)
         return res;
     }
 
-    // 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
-
-    /* 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);
-    }
+    // set random transmit delay to let others reconfigure their radio,
+    // to avoid collisions and implement timing-based flooding
+    // DEBUG_MSG("Set random delay before transmitting.\n");
+    setRandomDelay();  
 
     return res;
 #else
@@ -164,8 +154,8 @@ bool RadioLibInterface::cancelSending(NodeNum from, PacketId id)
 /** radio helper thread callback.
 
 We never immediately transmit after any operation (either rx or tx).  Instead we should start receiving and
-wait a random delay of 50 to 200 ms to make sure we are not stomping on someone else.  The 50ms delay at the beginning ensures all
-possible listeners have had time to finish processing the previous packet and now have their radio in RX state.  The up to 200ms
+wait a random delay of 100ms to 100ms+shortPacketMsec to make sure we are not stomping on someone else.  The 100ms delay at the beginning ensures all
+possible listeners have had time to finish processing the previous packet and now have their radio in RX state.  The up to 100ms+shortPacketMsec
 random delay gives a chance for all possible senders to have high odds of detecting that someone else started transmitting first
 and then they will wait until that packet finishes.
 
@@ -192,20 +182,26 @@ void RadioLibInterface::onNotify(uint32_t notification)
     case TRANSMIT_DELAY_COMPLETED:
         // DEBUG_MSG("delay done\n");
 
-        // If we are not currently in receive mode, then restart the timer and try again later (this can happen if the main thread
+        // If we are not currently in receive mode, then restart the random delay (this can happen if the main thread
         // has placed the unit into standby)  FIXME, how will this work if the chipset is in sleep mode?
         if (!txQueue.empty()) {
             if (!canSendImmediately()) {
-                startTransmitTimer(); // try again in a little while
+                // DEBUG_MSG("Currently Rx/Tx-ing: set random delay\n");
+                setRandomDelay(); // currently Rx/Tx-ing: reset random delay
             } else {
-                // Send any outgoing packets we have ready
-                MeshPacket *txp = txQueue.dequeue();
-                assert(txp);
-                startSend(txp);
+                if (isChannelActive()) { // check if there is currently a LoRa packet on the channel
+                    // DEBUG_MSG("Channel is active: set random delay\n");
+                    setRandomDelay(); // reset random delay
+                } else {
+                    // Send any outgoing packets we have ready
+                    MeshPacket *txp = txQueue.dequeue();
+                    assert(txp);
+                    startSend(txp);
 
-                // Packet has been sent, count it toward our TX airtime utilization.
-                uint32_t xmitMsec = getPacketTime(txp);
-                airTime->logAirtime(TX_LOG, xmitMsec);
+                    // Packet has been sent, count it toward our TX airtime utilization.
+                    uint32_t xmitMsec = getPacketTime(txp);
+                    airTime->logAirtime(TX_LOG, xmitMsec);
+                }
             }
         } else {
             // DEBUG_MSG("done with txqueue\n");
@@ -216,6 +212,26 @@ void RadioLibInterface::onNotify(uint32_t notification)
     }
 }
 
+void RadioLibInterface::setRandomDelay()
+{
+    MeshPacket *p = txQueue.getFront();
+    // 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.
+
+    /* We assume if rx_snr = 0 and rx_rssi = 0, the packet was 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);
+    }
+}
+
 void RadioLibInterface::startTransmitTimer(bool withDelay)
 {
     // If we have work to do and the timer wasn't already scheduled, schedule it now

src/mesh/RadioLibInterface.h

@@ -132,6 +132,9 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
      */
     virtual void startReceive() = 0;
 
+    /** can we detect a LoRa preamble on the current channel? */
+    virtual bool isChannelActive() = 0;
+
     /** are we actively receiving a packet (only called during receiving state)
      *  This method is only public to facilitate debugging.  Do not call.
      */
@@ -142,17 +145,13 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
 
   private:
     /** if we have something waiting to send, start a short random timer 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.
-     * */
+     * the transmit */
+    void setRandomDelay();
+
+    /** random timer with certain min. and max. settings */
     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.
-     * */
+    /** timer scaled to SNR of to be flooded packet */
     void startTransmitTimerSNR(float snr);
 
     void handleTransmitInterrupt();