Merge pull request #1394 from GUVWAF/master

Implement listen-before-talk mechanism

src/mesh/MeshPacketQueue.cpp

@@ -72,6 +72,16 @@ MeshPacket *MeshPacketQueue::dequeue()
     return p;
 }
 
+MeshPacket *MeshPacketQueue::getFront()
+{
+    if (empty()) {
+        return NULL;
+    }
+
+    auto *p = queue.front();
+    return p;
+}
+
 /** Attempt to find and remove a packet from this queue.  Returns a pointer to the removed packet, or NULL if not found */
 MeshPacket *MeshPacketQueue::remove(NodeNum from, PacketId id)
 {

src/mesh/MeshPacketQueue.h

@@ -28,6 +28,8 @@ class MeshPacketQueue
 
     MeshPacket *dequeue();
 
+    MeshPacket *getFront();
+
     /** Attempt to find and remove a packet from this queue.  Returns the packet which was removed from the queue */
     MeshPacket *remove(NodeNum from, PacketId id);
 };

src/mesh/RF95Interface.cpp

@@ -176,6 +176,25 @@ void RF95Interface::startReceive()
     enableInterrupt(isrRxLevel0);
 }
 
+bool RF95Interface::isChannelActive()
+{
+    // check if we can detect a LoRa preamble on the current channel
+    int16_t result;
+    setTransmitEnable(false);
+    setStandby();  // needed for smooth transition
+    result = lora->scanChannel();
+    
+    if (result == PREAMBLE_DETECTED) {
+        // DEBUG_MSG("Channel is busy!\n");
+        return true;
+    }
+
+    assert(result != ERR_WRONG_MODEM);
+    
+    // DEBUG_MSG("Channel is free!\n");
+    return false;
+}
+
 /** Could we send right now (i.e. either not actively receving or transmitting)? */
 bool RF95Interface::isActivelyReceiving()
 {

src/mesh/RF95Interface.h

@@ -40,6 +40,9 @@ class RF95Interface : public RadioLibInterface
      */
     virtual void enableInterrupt(void (*callback)()) { lora->setDio0Action(callback); }
 
+    /** can we detect a LoRa preamble on the current channel? */
+    virtual bool isChannelActive() override;
+
     /** are we actively receiving a packet (only called during receiving state) */
     virtual bool isActivelyReceiving() override;
 

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");
+    setTransmitDelay();  
 
     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");
+                setTransmitDelay(); // 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");
+                    setTransmitDelay(); // 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::setTransmitDelay()
+{
+    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.
      */
@@ -141,18 +144,14 @@ class RadioLibInterface : public RadioInterface, protected concurrency::Notified
     virtual bool cancelSending(NodeNum from, PacketId id) override;
 
   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.
-     * */
+    /** if we have something waiting to send, start a short (random) timer so we can come check for collision before actually doing
+     * the transmit */
+    void setTransmitDelay();
+
+    /** 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();

src/mesh/SX126xInterface.cpp

@@ -226,6 +226,23 @@ void SX126xInterface<T>::startReceive()
 #endif
 }
 
+/** Could we send right now (i.e. either not actively receving or transmitting)? */
+template<typename T>
+bool SX126xInterface<T>::isChannelActive()
+{
+    // check if we can detect a LoRa preamble on the current channel
+    int16_t result;
+
+    setStandby(); 
+    result = lora.scanChannel();
+    if (result == PREAMBLE_DETECTED) 
+        return true;
+    
+    assert(result != ERR_WRONG_MODEM);
+    
+    return false;
+}
+
 /** Could we send right now (i.e. either not actively receving or transmitting)? */
 template<typename T>
 bool SX126xInterface<T>::isActivelyReceiving()

src/mesh/SX126xInterface.h

@@ -46,6 +46,9 @@ class SX126xInterface : public RadioLibInterface
      */
     virtual void enableInterrupt(void (*callback)()) { lora.setDio1Action(callback); }
 
+    /** can we detect a LoRa preamble on the current channel? */
+    virtual bool isChannelActive() override;
+
     /** are we actively receiving a packet (only called during receiving state) */
     virtual bool isActivelyReceiving() override;