Repeat batch ACKs to cover RX latency

- Add ACK_REPEAT_COUNT/ACK_REPEAT_DELAY_MS and repeat ACK sends
- Update README with repeat-ACK behavior

README.md

@@ -239,6 +239,7 @@ Notes:
 - Total power is computed on receiver as `p1 + p2 + p3`.
 - Sender error counters are carried in the batch header and applied to all samples.
 - Receiver ACKs MeterBatch as soon as the batch is reassembled, before MQTT/web/UI work, to avoid missing the sender ACK window.
+- Receiver repeats ACKs (`ACK_REPEAT_COUNT`) spaced by `ACK_REPEAT_DELAY_MS` to cover sender RX latency.
 
 ## Device IDs
 - Derived from WiFi STA MAC.

include/config.h

@@ -76,6 +76,8 @@ constexpr uint32_t METER_SEND_INTERVAL_MS = 30000;
 constexpr uint32_t BATTERY_SAMPLE_INTERVAL_MS = 60000;
 constexpr float BATTERY_CAL = 1.083f;
 constexpr uint32_t BATCH_ACK_TIMEOUT_MS = 3000;
+constexpr uint8_t ACK_REPEAT_COUNT = 3;
+constexpr uint32_t ACK_REPEAT_DELAY_MS = 200;
 constexpr uint8_t BATCH_MAX_RETRIES = 2;
 constexpr uint8_t METER_BATCH_MAX_SAMPLES = 30;
 constexpr uint8_t BATCH_QUEUE_DEPTH = 10;

src/main.cpp

@@ -497,10 +497,17 @@ static void send_batch_ack(uint16_t batch_id, uint16_t sender_id) {
   write_u16_le(&ack.payload[0], batch_id);
   write_u16_le(&ack.payload[2], sender_id);
   write_u16_le(&ack.payload[4], g_short_id);
+  uint8_t repeats = ACK_REPEAT_COUNT == 0 ? 1 : ACK_REPEAT_COUNT;
+  for (uint8_t i = 0; i < repeats; ++i) {
     if (SERIAL_DEBUG_MODE) {
-    serial_debug_printf("ack: tx batch_id=%u to sender=%04X", batch_id, sender_id);
+      serial_debug_printf("ack: tx repeat %u/%u batch_id=%u", static_cast<unsigned>(i + 1),
+                          static_cast<unsigned>(repeats), batch_id);
     }
     lora_send(ack);
+    if (i + 1 < repeats && ACK_REPEAT_DELAY_MS > 0) {
+      delay(ACK_REPEAT_DELAY_MS);
+    }
+  }
   lora_receive_continuous();
 }
 
@@ -852,7 +859,7 @@ static void sender_loop() {
       if (ack_sender == g_short_id && ack_receiver == ack_pkt.device_id_short &&
           g_batch_ack_pending && ack_id == g_last_sent_batch_id) {
         g_last_acked_batch_id = ack_id;
-        serial_debug_printf("ack: ok batch_id=%u", ack_id);
+        serial_debug_printf("ack: rx ok batch_id=%u", ack_id);
         finish_inflight_batch();
       } else if (SERIAL_DEBUG_MODE) {
         serial_debug_printf("ack: reject batch_id=%u sender=%u receiver=%u exp_batch=%u exp_sender=%u",