Optimise packet handling for SX1280 fast data rates

Boards.h

@@ -445,7 +445,7 @@
         const int pin_dio = 33;
         const int pin_tcxo_enable = -1;
       #elif MODEM == SX1280
-        #define CONFIG_QUEUE_SIZE 40960
+        #define CONFIG_QUEUE_SIZE 6144
         #define DIO2_AS_RF_SWITCH false
         #define HAS_BUSY true
         #define HAS_TCXO true

Config.h

@@ -71,6 +71,9 @@
 	#define PHY_CRC_LORA_BITS          16
 	#define LORA_PREAMBLE_SYMBOLS_MIN  18
 	#define LORA_PREAMBLE_TARGET_MS    24
+	#define LORA_PREAMBLE_FAST_DELTA   18
+	#define LORA_FAST_THRESHOLD_BPS    30E3
+	#define LORA_LIMIT_THRESHOLD_BPS   60E3
 	long lora_preamble_symbols      =  LORA_PREAMBLE_SYMBOLS_MIN;
 	long lora_preamble_time_ms      =  0;
 	long lora_header_time_ms        =  0;
@@ -78,12 +81,14 @@
 	float lora_symbol_rate          =  0.0;
 	float lora_us_per_byte          =  0.0;
 	bool lora_low_datarate          =  false;
+	bool lora_limit_rate            =  false;
 
 	// CSMA Parameters
 	#define CSMA_SIFS_MS               0
 	#define CSMA_POST_TX_YIELD_SLOTS   3
 	#define CSMA_SLOT_MAX_MS           100
 	#define CSMA_SLOT_MIN_MS           24
+	#define CSMA_SLOT_MIN_FAST_DELTA   18
 	#define CSMA_SLOT_SYMBOLS          12
 	#define CSMA_CW_BANDS              4
 	#define CSMA_CW_MIN                0

RNode_Firmware.ino

@@ -553,6 +553,45 @@ void flush_queue(void) {
   #endif
 }
 
+void pop_queue() {
+  if (!queue_flushing) {
+    queue_flushing = true;
+    led_tx_on(); uint16_t processed = 0;
+
+    #if MCU_VARIANT == MCU_ESP32 || MCU_VARIANT == MCU_NRF52
+    if (!fifo16_isempty(&packet_starts)) {
+    #else
+    if (!fifo16_isempty_locked(&packet_starts)) {
+    #endif
+
+      uint16_t start = fifo16_pop(&packet_starts);
+      uint16_t length = fifo16_pop(&packet_lengths);
+      if (length >= MIN_L && length <= MTU) {
+        for (uint16_t i = 0; i < length; i++) {
+          uint16_t pos = (start+i)%CONFIG_QUEUE_SIZE;
+          tbuf[i] = packet_queue[pos];
+        }
+
+        transmit(length); processed++;
+      }
+      queue_height -= processed;
+      queued_bytes -= length;
+    }
+
+    lora_receive(); led_tx_off();
+  }
+
+  #if MCU_VARIANT == MCU_ESP32 || MCU_VARIANT == MCU_NRF52
+    update_airtime();
+  #endif
+
+  queue_flushing = false;
+
+  #if HAS_DISPLAY
+    display_tx = true;
+  #endif
+}
+
 void add_airtime(uint16_t written) {
   #if MCU_VARIANT == MCU_ESP32 || MCU_VARIANT == MCU_NRF52
     float lora_symbols = 0;
@@ -1376,9 +1415,8 @@ void tx_queue_handler() {
             cw_wait_passed += millis()-cw_wait_start; cw_wait_start   = millis();
             if (cw_wait_passed < cw_wait_target) { return; }                      // Contention window wait time has not yet passed, continue waiting
             else {                                                                // Wait time has passed, flush the queue
-              flush_queue();
+              if (!lora_limit_rate) { flush_queue(); } else { pop_queue(); }
               cw_wait_passed = 0; csma_cw = -1; difs_wait_start = -1; }
-
           }
         }
       }

Utilities.h

@@ -1101,12 +1101,20 @@ void updateBitrate() {
 			lora_bitrate = (uint32_t)(lora_sf * ( (4.0/(float)lora_cr) / ((float)(pow(2, lora_sf))/((float)lora_bw/1000.0)) ) * 1000.0);
 			lora_us_per_byte = 1000000.0/((float)lora_bitrate/8.0);
 			
+			bool fast_rate   = lora_bitrate > LORA_FAST_THRESHOLD_BPS;
+			lora_limit_rate  = lora_bitrate > LORA_LIMIT_THRESHOLD_BPS;
+
+			int csma_slot_min_ms = CSMA_SLOT_MIN_MS;
+			float lora_preamble_target_ms = LORA_PREAMBLE_TARGET_MS;
+			if (fast_rate) { csma_slot_min_ms        -= CSMA_SLOT_MIN_FAST_DELTA;
+											 lora_preamble_target_ms -= LORA_PREAMBLE_FAST_DELTA; }
+			
 			csma_slot_ms = lora_symbol_time_ms*CSMA_SLOT_SYMBOLS;
 			if (csma_slot_ms > CSMA_SLOT_MAX_MS) { csma_slot_ms = CSMA_SLOT_MAX_MS; }
-			if (csma_slot_ms < CSMA_SLOT_MIN_MS) { csma_slot_ms = CSMA_SLOT_MIN_MS; }
+			if (csma_slot_ms < CSMA_SLOT_MIN_MS) { csma_slot_ms = csma_slot_min_ms; }
 			difs_ms = CSMA_SIFS_MS + 2*csma_slot_ms;
 			
-			float target_preamble_symbols = LORA_PREAMBLE_TARGET_MS/lora_symbol_time_ms;
+			float target_preamble_symbols = lora_preamble_target_ms/lora_symbol_time_ms;
 			if (target_preamble_symbols < LORA_PREAMBLE_SYMBOLS_MIN) { target_preamble_symbols = LORA_PREAMBLE_SYMBOLS_MIN; }
 			else { target_preamble_symbols = (ceil)(target_preamble_symbols); }