fix(firmware): stop nodes falling back to 1 Mbps 802.11b, which can fill the channel

firmware/esp32-csi-node/main/c6_sync_espnow.c

@@ -24,10 +24,13 @@
 #include "esp_idf_version.h"
 #include "freertos/FreeRTOS.h"
 #include "freertos/timers.h"
+#include "nvs_config.h"
 #include <string.h>
 
 static const char *TAG = "c6_espnow";
 
+extern nvs_config_t g_nvs_config;
+
 #define BEACON_MAGIC      0x53454E50u   /* 'SENP' little-endian */
 #define BEACON_PROTO_VER  0x01
 #define BEACON_PERIOD_MS  100
@@ -43,6 +46,26 @@ typedef struct __attribute__((packed)) {
 
 static const uint8_t s_broadcast_mac[6] = {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF};
 
+/* Send beacons at 6 Mbps OFDM, not ESP-NOW's 1 Mbps 802.11b default. When
+ * the STA's 11b rates are disabled (the default, see nvs_config.h)
+ * a 1 Mbps beacon would not be sendable at all; and OFDM carries the LTF that
+ * CSI is computed from, at about a quarter of the airtime. With 11b allowed
+ * (NVS allow_11b=1) the old default is left untouched. */
+static void espnow_set_beacon_rate(void)
+{
+    if (g_nvs_config.wifi_allow_11b) return;
+    esp_now_rate_config_t rc = {
+        .phymode = WIFI_PHY_MODE_11G,
+        .rate    = WIFI_PHY_RATE_6M,
+        .ersu    = false,
+        .dcm     = false,
+    };
+    esp_err_t r = esp_now_set_peer_rate_config(s_broadcast_mac, &rc);
+    if (r != ESP_OK) {
+        ESP_LOGW(TAG, "beacon rate 6M OFDM not set: %s", esp_err_to_name(r));
+    }
+}
+
 static uint64_t s_local_id     = 0;   /* 6-byte MAC packed into u64 */
 static uint64_t s_leader_id    = 0;
 static int64_t  s_offset_us    = 0;
@@ -212,6 +235,7 @@ esp_err_t c6_sync_espnow_init(void)
     if (r != ESP_OK && r != ESP_ERR_ESPNOW_EXIST) {
         ESP_LOGW(TAG, "esp_now_add_peer(broadcast) failed: %s", esp_err_to_name(r));
     }
+    espnow_set_beacon_rate();
 
     /* Start as candidate leader — will step down on receiving lower-id beacon. */
     s_is_leader    = true;

firmware/esp32-csi-node/main/main.c

@@ -245,6 +245,17 @@ static void wifi_init_sta(void)
         s_reconnect_timer = NULL;
     }
 
+    /* Keep rate control out of 802.11b (1-11 Mbps DSSS/CCK). It must be set
+     * after esp_wifi_init() and before esp_wifi_start(). Non-fatal: if the
+     * driver refuses, the node runs as before rather than not at all. */
+    if (!g_nvs_config.wifi_allow_11b) {
+        esp_err_t r11b = esp_wifi_config_11b_rate(WIFI_IF_STA, true);
+        ESP_LOGI(TAG, "802.11b rates on STA: %s (%s)",
+                 r11b == ESP_OK ? "disabled" : "STILL ENABLED", esp_err_to_name(r11b));
+    } else {
+        ESP_LOGW(TAG, "802.11b rates on STA: allowed (NVS allow_11b=1)");
+    }
+
     ESP_ERROR_CHECK(esp_wifi_start());
 
     ESP_LOGI(TAG, "WiFi STA initialized, connecting to SSID: %s", g_nvs_config.wifi_ssid);

firmware/esp32-csi-node/main/nvs_config.c

@@ -48,6 +48,7 @@ void nvs_config_load(nvs_config_t *cfg)
     cfg->dwell_ms       = 50;
     cfg->tdm_slot_index = 0;
     cfg->tdm_node_count = 1;
+    cfg->wifi_allow_11b = 0;    /* 802.11b rates off by default */
 
     /* ADR-039: Edge intelligence defaults from Kconfig. */
 #ifdef CONFIG_EDGE_TIER
@@ -192,6 +193,17 @@ void nvs_config_load(nvs_config_t *cfg)
         }
     }
 
+    /* 802.11b rate-control override (see nvs_config.h). */
+    uint8_t allow_11b_val;
+    if (nvs_get_u8(handle, "allow_11b", &allow_11b_val) == ESP_OK) {
+        if (allow_11b_val <= 1) {
+            cfg->wifi_allow_11b = allow_11b_val;
+            ESP_LOGI(TAG, "NVS override: wifi_allow_11b=%u", (unsigned)allow_11b_val);
+        } else {
+            ESP_LOGW(TAG, "NVS allow_11b=%u not 0/1, ignored", (unsigned)allow_11b_val);
+        }
+    }
+
     /* ADR-039: Edge intelligence overrides. */
     uint8_t edge_tier_val;
     if (nvs_get_u8(handle, "edge_tier", &edge_tier_val) == ESP_OK) {

firmware/esp32-csi-node/main/nvs_config.h

@@ -36,6 +36,19 @@ typedef struct {
     uint8_t  tdm_slot_index;                  /**< This node's TDM slot index (0-based). */
     uint8_t  tdm_node_count;                  /**< Total nodes in the TDM schedule. */
 
+    /* 802.11b rates on the STA interface (NVS key "allow_11b").
+     *
+     * 0 (default) = disabled: rate control cannot fall below 6 Mbps OFDM.
+     * With 11b allowed, a node whose link degrades can settle at 1-2 Mbps
+     * and stay there; each CSI datagram then takes tens of times the
+     * airtime, the whole fleet's channel fills up and frame rates slide.
+     * The TX queue also backs up, which shows as deep free-heap dips and
+     * send_fail bursts that look like a leak. On a 9-node ESP32-C6 fleet,
+     * disabling 11b took the channel from 80-85% busy to ~44%.
+     * 1 = allow (the previous behaviour), kept as a no-reflash rollback;
+     * needs a reboot, since it is applied before esp_wifi_start(). */
+    uint8_t  wifi_allow_11b;
+
     /* ADR-039: Edge intelligence configuration */
     uint8_t  edge_tier;                       /**< Processing tier (0=raw, 1=basic, 2=full). */
     float    presence_thresh;                 /**< Presence threshold (0 = auto-calibrate). */