Add SNR-driven dynamic coding rate selection for repeater retransmits

When a repeater forwards a direct-routed pkt, it looks up the next-hop
neighbor's SNR, computes the link margin against the SF-specific
demodulation floor, then determines coding rate (RT) based on SNR.
Weaker links get higher CR. We apply CR on a per-packet basis and
restore to default configuration after TX.

Margin thresholds:
<3 dB   - CR 4/8
3-6 dB  - CR 4/7
6-10 dB - CR 4/6
>=10 dB - CR 4/5

Flood repeats adhere to configured CR, as do forwards to unknown
neighbors.

Author: weebl2000

examples/simple_repeater/MyMesh.cpp

@@ -1,5 +1,6 @@
 #include "MyMesh.h"
 #include <algorithm>
+#include <climits>
 
 /* ------------------------------ Config -------------------------------- */
 
@@ -87,6 +88,42 @@ void MyMesh::putNeighbour(const mesh::Identity &id, uint32_t timestamp, float sn
 #endif
 }
 
+int8_t MyMesh::findNeighbourSNR(const uint8_t* hash, uint8_t hash_size) {
+#if MAX_NEIGHBOURS
+  for (int i = 0; i < MAX_NEIGHBOURS; i++) {
+    if (neighbours[i].heard_timestamp != 0 && neighbours[i].id.isHashMatch(hash, hash_size)) {
+      return neighbours[i].snr;
+    }
+  }
+#endif
+  return INT8_MAX;
+}
+
+// Approximate SNR demod floor per SF (same as RadioLibWrappers.cpp)
+static float cr_snr_thresholds[] = {
+  -7.5f,  // SF7
+  -10.0f, // SF8
+  -12.5f, // SF9
+  -15.0f, // SF10
+  -17.5f, // SF11
+  -20.0f  // SF12
+};
+
+uint8_t MyMesh::selectCodingRateForPeer(const uint8_t* hash, uint8_t hash_size) {
+  int8_t snr4 = findNeighbourSNR(hash, hash_size);
+  if (snr4 == INT8_MAX) return 0;  // unknown neighbor, use default
+
+  float snr = snr4 / 4.0f;
+  float threshold = (_prefs.sf >= 7 && _prefs.sf <= 12)
+    ? cr_snr_thresholds[_prefs.sf - 7] : -15.0f;
+  float margin = snr - threshold;
+
+  if (margin < 3.0f)  return 8;
+  if (margin < 6.0f)  return 7;
+  if (margin < 10.0f) return 6;
+  return 5;  // good margin, use lightest CR
+}
+
 uint8_t MyMesh::handleLoginReq(const mesh::Identity& sender, const uint8_t* secret, uint32_t sender_timestamp, const uint8_t* data, bool is_flood) {
   ClientInfo* client = NULL;
   if (data[0] == 0) {   // blank password, just check if sender is in ACL
@@ -955,6 +992,7 @@ void MyMesh::begin(FILESYSTEM *fs) {
 
   radio_driver.setParams(_prefs.freq, _prefs.bw, _prefs.sf, _prefs.cr);
   radio_driver.setTxPower(_prefs.tx_power_dbm);
+  setDefaultCR(_prefs.cr);
 
   radio_driver.setRxBoostedGainMode(_prefs.rx_boosted_gain);
   MESH_DEBUG_PRINTLN("RX Boosted Gain Mode: %s",

examples/simple_repeater/MyMesh.h

@@ -120,6 +120,7 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks {
 #endif
 
   void putNeighbour(const mesh::Identity& id, uint32_t timestamp, float snr);
+  int8_t findNeighbourSNR(const uint8_t* hash, uint8_t hash_size);
   uint8_t handleLoginReq(const mesh::Identity& sender, const uint8_t* secret, uint32_t sender_timestamp, const uint8_t* data, bool is_flood);
   uint8_t handleAnonRegionsReq(const mesh::Identity& sender, uint32_t sender_timestamp, const uint8_t* data);
   uint8_t handleAnonOwnerReq(const mesh::Identity& sender, uint32_t sender_timestamp, const uint8_t* data);
@@ -156,6 +157,7 @@ class MyMesh : public mesh::Mesh, public CommonCLICallbacks {
   uint8_t getExtraAckTransmitCount() const override {
     return _prefs.multi_acks;
   }
+  uint8_t selectCodingRateForPeer(const uint8_t* hash, uint8_t hash_size) override;
 
 #if ENV_INCLUDE_GPS == 1
   void applyGpsPrefs() {

src/Dispatcher.cpp

@@ -105,6 +105,9 @@ void Dispatcher::loop() {
       }
 
       _radio->onSendFinished();
+      if (outbound->_tx_cr != 0 && outbound->_tx_cr != _default_cr) {
+        _radio->setCodingRate(_default_cr);
+      }
       logTx(outbound, 2 + outbound->getPathByteLen() + outbound->payload_len);
       if (outbound->isRouteFlood()) {
         n_sent_flood++;
@@ -117,6 +120,9 @@ void Dispatcher::loop() {
       MESH_DEBUG_PRINTLN("%s Dispatcher::loop(): WARNING: outbound packed send timed out!", getLogDateTime());
 
       _radio->onSendFinished();
+      if (outbound->_tx_cr != 0 && outbound->_tx_cr != _default_cr) {
+        _radio->setCodingRate(_default_cr);
+      }
       logTxFail(outbound, 2 + outbound->getPathByteLen() + outbound->payload_len);
 
       releasePacket(outbound);  // return to pool
@@ -323,14 +329,21 @@ void Dispatcher::checkSend() {
     } else {
       memcpy(&raw[len], outbound->payload, outbound->payload_len); len += outbound->payload_len;
 
+      if (outbound->_tx_cr != 0 && outbound->_tx_cr != _default_cr) {
+        _radio->setCodingRate(outbound->_tx_cr);
+      }
+
       uint32_t max_airtime = _radio->getEstAirtimeFor(len)*3/2;
       outbound_start = _ms->getMillis();
       bool success = _radio->startSendRaw(raw, len);
       if (!success) {
         MESH_DEBUG_PRINTLN("%s Dispatcher::loop(): ERROR: send start failed!", getLogDateTime());
+        if (outbound->_tx_cr != 0 && outbound->_tx_cr != _default_cr) {
+          _radio->setCodingRate(_default_cr);
+        }
 
         logTxFail(outbound, outbound->getRawLength());
-  
+
         releasePacket(outbound);  // return to pool
         outbound = NULL;
         return;
@@ -359,6 +372,7 @@ Packet* Dispatcher::obtainNewPacket() {
   } else {
     pkt->payload_len = pkt->path_len = 0;
     pkt->_snr = 0;
+    pkt->_tx_cr = 0;
   }
   return pkt;
 }

src/Dispatcher.h

@@ -74,6 +74,8 @@ class Radio {
   */
   virtual bool isReceiving() { return false; }
 
+  virtual void setCodingRate(uint8_t cr) { }
+
   virtual float getLastRSSI() const { return 0; }
   virtual float getLastSNR() const { return 0; }
 };
@@ -136,6 +138,8 @@ class Dispatcher {
   MillisecondClock* _ms;
   uint16_t _err_flags;
 
+  uint8_t _default_cr;
+
   Dispatcher(Radio& radio, MillisecondClock& ms, PacketManager& mgr)
     : _radio(&radio), _ms(&ms), _mgr(&mgr)
   {
@@ -150,6 +154,7 @@ class Dispatcher {
     tx_budget_ms = 0;
     last_budget_update = 0;
     duty_cycle_window_ms = 3600000;
+    _default_cr = 5;
   }
 
   virtual DispatcherAction onRecvPacket(Packet* pkt) = 0;
@@ -184,6 +189,8 @@ class Dispatcher {
   uint32_t getNumSentDirect() const { return n_sent_direct; }
   uint32_t getNumRecvFlood() const { return n_recv_flood; }
   uint32_t getNumRecvDirect() const { return n_recv_direct; }
+  void setDefaultCR(uint8_t cr) { _default_cr = cr; }
+
   void resetStats() {
     n_sent_flood = n_sent_direct = n_recv_flood = n_recv_direct = 0;
     _err_flags = 0;

src/Mesh.cpp

@@ -96,9 +96,10 @@ DispatcherAction Mesh::onRecvPacket(Packet* pkt) {
 
       if (!_tables->hasSeen(pkt)) {
         removeSelfFromPath(pkt);
+        pkt->_tx_cr = selectCodingRateForPeer(pkt->path, pkt->getPathHashSize());
 
         uint32_t d = getDirectRetransmitDelay(pkt);
-        return ACTION_RETRANSMIT_DELAYED(0, d);  // Routed traffic is HIGHEST priority 
+        return ACTION_RETRANSMIT_DELAYED(0, d);  // Routed traffic is HIGHEST priority
       }
     }
     return ACTION_RELEASE;   // this node is NOT the next hop (OR this packet has already been forwarded), so discard.

src/Mesh.h

@@ -70,6 +70,14 @@ class Mesh : public Dispatcher {
    */
   virtual uint8_t getExtraAckTransmitCount() const;
 
+  /**
+   * \brief  Select a coding rate for the next-hop peer based on link quality.
+   * \param  hash       the hash of the next-hop peer (from path[0..hash_size-1])
+   * \param  hash_size  bytes per hash entry
+   * \returns  0 = use node default CR, 5-8 = override CR for this packet
+   */
+  virtual uint8_t selectCodingRateForPeer(const uint8_t* hash, uint8_t hash_size) { return 0; }
+
   /**
    * \brief  Perform search of local DB of peers/contacts.
    * \returns  Number of peers with matching hash

src/Packet.cpp

@@ -8,6 +8,8 @@ Packet::Packet() {
   header = 0;
   path_len = 0;
   payload_len = 0;
+  _snr = 0;
+  _tx_cr = 0;
 }
 
 bool Packet::isValidPathLen(uint8_t path_len) {

src/Packet.h

@@ -49,6 +49,7 @@ class Packet {
   uint8_t path[MAX_PATH_SIZE];
   uint8_t payload[MAX_PACKET_PAYLOAD];
   int8_t _snr;
+  uint8_t _tx_cr;   // 0 = use node default, 5-8 = override CR for this TX (not serialized to wire)
 
   /**
    * \brief calculate the hash of payload + type

src/helpers/radiolib/CustomLLCC68Wrapper.h

@@ -25,6 +25,8 @@ class CustomLLCC68Wrapper : public RadioLibWrapper {
   float getLastRSSI() const override { return ((CustomLLCC68 *)_radio)->getRSSI(); }
   float getLastSNR() const override { return ((CustomLLCC68 *)_radio)->getSNR(); }
 
+  void setCodingRate(uint8_t cr) override { ((CustomLLCC68 *)_radio)->setCodingRate(cr); }
+
   float packetScore(float snr, int packet_len) override {
     int sf = ((CustomLLCC68 *)_radio)->spreadingFactor;
     return packetScoreInt(snr, sf, packet_len);

src/helpers/radiolib/CustomLR1110Wrapper.h

@@ -31,6 +31,8 @@ class CustomLR1110Wrapper : public RadioLibWrapper {
     _radio->setPreambleLength(preambleLengthForSF(getSpreadingFactor())); // overcomes weird issues with small and big pkts
   }
 
+  void setCodingRate(uint8_t cr) override { ((CustomLR1110 *)_radio)->setCodingRate(cr); }
+
   float getLastRSSI() const override { return ((CustomLR1110 *)_radio)->getRSSI(); }
   float getLastSNR() const override { return ((CustomLR1110 *)_radio)->getSNR(); }