arq: MFSK weak-signal backend + delivery-driven stop-and-wait rethink

Makefile

@@ -125,8 +125,11 @@ MERCURY_LINK_INPUTS = \
 	main.o common/cfg_utils.o common/iniparser/iniparser.o common/iniparser/dictionary.o \
 	datalink_arq/arq.o datalink_arq/arq_tnc.o datalink_arq/arith.o datalink_arq/arq_channels.o \
 	datalink_arq/arq_fsm.o datalink_arq/arq_protocol.o datalink_arq/arq_timing.o datalink_arq/arq_modem.o \
-	datalink_broadcast/broadcast.o datalink_broadcast/kiss.o modem/modem.o modem/framer.o modem/channel_busy.o modem/freedv/libfreedvdata.a \
-	audioio/audioio.a common/os_interop.o common/ring_buffer_posix.o common/shm_posix.o common/crc6.o common/hermes_log.o common/virtual_clock.o \
+	datalink_broadcast/broadcast.o datalink_broadcast/kiss.o modem/modem.o modem/modem_freedv.o \
+	modem/modem_mfsk.o modem/mfsk.o modem/mfsk_ofdm.o modem/mfsk_sync.o modem/mfsk_ldpc.o modem/mfsk_ldpc_1_16.o \
+	modem/mfsk_ldpc_2_16.o modem/mfsk_ldpc_3_16.o modem/mfsk_ldpc_5_16.o modem/mfsk_ldpc_8_16.o modem/framer.o \
+	modem/channel_busy.o modem/freedv/libfreedvdata.a audioio/audioio.a common/os_interop.o \
+	common/ring_buffer_posix.o common/shm_posix.o common/crc6.o common/hermes_log.o common/virtual_clock.o \
 	common/chan.o common/queue.o common/mercury_engine.o common/mercury_cli.o data_interfaces/tcp_interfaces.o data_interfaces/net.o \
 	gui_interface/ui_communication.o \
 	gui_interface/websocket/mongoose.o gui_interface/websocket/mercury_websocket.o \
@@ -196,8 +199,10 @@ MERCURY_CORE_OBJS = \
 	datalink_arq/arq.o datalink_arq/arq_tnc.o datalink_arq/arith.o datalink_arq/arq_channels.o \
 	datalink_arq/arq_fsm.o datalink_arq/arq_protocol.o datalink_arq/arq_timing.o datalink_arq/arq_modem.o \
 	datalink_broadcast/broadcast.o datalink_broadcast/kiss.o \
-	modem/modem.o modem/framer.o modem/channel_busy.o \
-	common/os_interop.o common/ring_buffer_posix.o common/shm_posix.o common/crc6.o common/hermes_log.o common/virtual_clock.o \
+	modem/modem.o modem/modem_freedv.o modem/modem_mfsk.o modem/mfsk.o modem/mfsk_ofdm.o modem/mfsk_sync.o \
+	modem/mfsk_ldpc.o modem/mfsk_ldpc_1_16.o modem/mfsk_ldpc_2_16.o modem/mfsk_ldpc_3_16.o \
+	modem/mfsk_ldpc_5_16.o modem/mfsk_ldpc_8_16.o modem/framer.o modem/channel_busy.o common/os_interop.o \
+	common/ring_buffer_posix.o common/shm_posix.o common/crc6.o common/hermes_log.o common/virtual_clock.o \
 	common/chan.o common/queue.o common/mercury_engine.o common/mercury_cli.o \
 	data_interfaces/tcp_interfaces.o data_interfaces/net.o \
 	gui_interface/ui_communication.o \

datalink_arq/arq.c

@@ -218,6 +218,22 @@ static void cb_send_tx_frame(int packet_type, int mode,
     arq_modem_enqueue(&action);
 }
 
+/* Enqueue a Welch-Costas MFSK pattern ACK for the modem TX worker.  Carries no
+ * coded frame — the modem synthesises the ack/break tone burst directly (see
+ * send_pattern_ack in modem.c).  `mode` is the current payload mode, used only
+ * so the worker can key at the right passband geometry. */
+static void cb_send_pattern_ack(int mode, int pattern_kind)
+{
+    arq_action_t action = {
+        .type         = ARQ_ACTION_TX_PATTERN,
+        .mode         = mode,
+        .frame_size   = 0,
+        .frame_count  = 1,
+        .pattern_kind = pattern_kind,
+    };
+    arq_modem_enqueue(&action);
+}
+
 static void cb_notify_connected(const char *remote_call, const char *local_call)
 {
     pthread_mutex_lock(&g_conn_lock);
@@ -837,22 +853,11 @@ void arq_handle_incoming_frame(uint8_t *data, size_t frame_size, float rx_snr)
     {
         switch (hdr.subtype)
         {
+        /* ACK: the coded DATAC16 ACK is used only for the post-ACCEPT connect
+         * confirmation now; in-session ACKs are the MFSK pattern (synthesised
+         * in modem.c), not a control frame here. */
         case ARQ_SUBTYPE_ACK:          ev.id = ARQ_EV_RX_ACK;           break;
         case ARQ_SUBTYPE_DISCONNECT:   ev.id = ARQ_EV_RX_DISCONNECT;    break;
-        case ARQ_SUBTYPE_TURN_REQ:     ev.id = ARQ_EV_RX_TURN_REQ;      break;
-        case ARQ_SUBTYPE_TURN_ACK:     ev.id = ARQ_EV_RX_TURN_ACK;      break;
-        case ARQ_SUBTYPE_KEEPALIVE:    ev.id = ARQ_EV_RX_KEEPALIVE;     break;
-        case ARQ_SUBTYPE_KEEPALIVE_ACK: ev.id = ARQ_EV_RX_KEEPALIVE_ACK; break;
-        case ARQ_SUBTYPE_MODE_REQ:
-            ev.id   = ARQ_EV_RX_MODE_REQ;
-            ev.mode = (frame_size > ARQ_FRAME_HDR_SIZE)
-                      ? (int)data[ARQ_FRAME_HDR_SIZE] : 0;
-            break;
-        case ARQ_SUBTYPE_MODE_ACK:
-            ev.id   = ARQ_EV_RX_MODE_ACK;
-            ev.mode = (frame_size > ARQ_FRAME_HDR_SIZE)
-                      ? (int)data[ARQ_FRAME_HDR_SIZE] : 0;
-            break;
         default:
             return;
         }
@@ -865,6 +870,16 @@ void arq_handle_incoming_frame(uint8_t *data, size_t frame_size, float rx_snr)
     evq_push(&ev);
 }
 
+void arq_post_pattern_ack(bool is_break)
+{
+    arq_event_t ev = {0};
+    ev.id       = ARQ_EV_RX_ACK;
+    ev.rx_flags = is_break ? ARQ_FLAG_HAS_DATA : 0;
+    /* session_id left 0: patterns carry none, and the dispatch session-ID
+     * gate treats 0 as "unknown/accept". */
+    evq_push(&ev);
+}
+
 /* ======================================================================
  * Public arq.h API
  * ====================================================================== */
@@ -904,6 +919,7 @@ int arq_init(size_t frame_size, int mode)
 
     static const arq_fsm_callbacks_t cbs = {
         .send_tx_frame       = cb_send_tx_frame,
+        .send_pattern_ack    = cb_send_pattern_ack,
         .notify_connected    = cb_notify_connected,
         .notify_pending      = cb_notify_pending,
         .notify_cancelpending = cb_notify_cancelpending,
@@ -1068,8 +1084,11 @@ bool arq_get_runtime_snapshot(arq_runtime_snapshot_t *snapshot)
     pthread_mutex_lock(&g_sess_lock);
     snapshot->initialized      = true;
     snapshot->connected        = (g_sess.conn_state == ARQ_CONN_CONNECTED);
+    snapshot->expect_pattern_ack = (g_sess.conn_state == ARQ_CONN_ACCEPTING ||
+                                    g_sess.conn_state == ARQ_CONN_CONNECTED);
     snapshot->trx              = trx;
-    snapshot->tx_backlog_bytes = backlog + g_sess.tx_inflight_bytes;
+    snapshot->tx_backlog_bytes = backlog +
+        (g_sess.tx_frame_present ? g_sess.tx_frame_len : 0);
     snapshot->speed_level      = g_sess.speed_level;
     snapshot->payload_mode      = g_sess.payload_mode;
     snapshot->peer_tx_mode      = g_sess.peer_tx_mode;

datalink_arq/arq.h

@@ -69,9 +69,19 @@ typedef enum
     ARQ_ACTION_NONE = 0,
     ARQ_ACTION_TX_CONTROL = 1,
     ARQ_ACTION_TX_PAYLOAD = 2,
-    ARQ_ACTION_MODE_SWITCH = 3
+    ARQ_ACTION_MODE_SWITCH = 3,
+    ARQ_ACTION_TX_PATTERN = 4  /* emit a Welch-Costas MFSK pattern ACK — no
+                                * coded frame; pattern_kind selects ACK vs
+                                * ACK+TURN (break).  Airtime ~0.64 s.        */
 } arq_action_type_t;
 
+/** @brief Pattern-ACK symbol kind (ARQ_ACTION_TX_PATTERN). */
+typedef enum
+{
+    ARQ_PATTERN_ACK   = 0,  /* plain ACK: "got your frame, keep the floor"   */
+    ARQ_PATTERN_BREAK = 1   /* ACK+TURN: "got it AND I have data" (HAS_DATA) */
+} arq_pattern_kind_t;
+
 /** @brief Single modem action item popped by modem TX worker. */
 typedef struct
 {
@@ -81,13 +91,20 @@ typedef struct
     int frame_count;     /* frames in this PTT burst (>= 1); the modem reads
                           * frame_count * frame_size bytes and modulates them
                           * behind a single preamble                          */
+    int pattern_kind;    /* arq_pattern_kind_t, valid for ARQ_ACTION_TX_PATTERN */
 } arq_action_t;
 
 /** @brief Snapshot of current ARQ runtime state for telemetry/decision making. */
 typedef struct
 {
     bool initialized;
     bool connected;
+    /* True when a Welch-Costas pattern ACK could legitimately arrive: the
+     * answerer awaiting the caller's connect-confirm (ACCEPTING) or an active
+     * session (CONNECTED).  The RX pattern detector runs its per-chunk
+     * correlation only when this is set — running it during CALLING/LISTENING/
+     * idle is pure overhead that slows the connect-critical DATAC16 decode. */
+    bool expect_pattern_ack;
     int trx;
     int tx_backlog_bytes;
     int speed_level;
@@ -266,6 +283,17 @@ void arq_notify_cq_tx_complete(void);
  */
 void arq_handle_incoming_frame(uint8_t *data, size_t frame_size, float rx_snr);
 
+/**
+ * @brief Post a synthesized pattern-ACK event (from the RX pattern detector).
+ *
+ * A Welch-Costas pattern ACK carries no coded header, so the modem RX worker
+ * synthesizes the FSM event directly.  In stop-and-wait only one frame is
+ * outstanding, so an ACK unambiguously acks it.
+ *
+ * @param is_break true = ACK+TURN (HAS_DATA piggyback), false = plain ACK.
+ */
+void arq_post_pattern_ack(bool is_break);
+
 /**
  * @brief Feed decoder/link metrics into ARQ adaptation.
  * @param sync Decoder sync flag.