Radio receive calibration and recovery

Noise calibration collects 64 idle RSSI samples at least 50 ms apart. A quiet block takes about 3.2 seconds. Both continuous RX and temporary RXPS calibration windows allow up to 30 seconds; a partial block times out without publishing. Periodic requests coalesce while a block is active. RXPS resumes after a complete block, including a held block, or at timeout. Receive and transmit activity are never interrupted just to finish calibration.

The block estimate is the 12.5th percentile: the average of the eighth and ninth sorted samples, retaining fractional dBm. A published update uses 25% of the previous floor and 75% of the new estimate, with a -120 dBm lower bound. Initial calibration and explicit gain/tuning changes seed a new baseline. The old published value remains available while collecting. An AGC reset drops partial samples while retaining a valid baseline for channel checks.

An estimate more than 15 dB above the established floor is held twice. The third consecutive complete high block is accepted, allowing recovery after a permanent ambient rise. A normal or quieter complete block clears the hold. Partial blocks do not advance it. There is no admission threshold based on the old floor, so an old low baseline cannot reject every sample from a higher floor.

Hardware captures with sustained different-SF/BW traffic showed the median and lower quartile following the interferer. The 12.5th percentile retained the quiet background in those captures. It needs at least nine background samples out of 64 to avoid higher-valued traffic entirely. Nearly continuous interference or enough low outliers can still mislead it. The rise hold delays large changes but cannot identify their source.

RSSI sampling waits 20 ms after every receive re-arm, including CAD and TX completion, to avoid unsettled frontend readings. The ten-second trial window often failed to complete during heavy traffic with that settling guard; the 30-second cap allows more idle opportunities. Partial blocks still time out without publishing. Longer collection spends more time in continuous RX when RXPS is enabled. A quiet minimum is about 3.2 seconds; real completion time also depends on packet activity and main-loop scheduling.

On radios with a mode probe, each eligible RSSI sample first checks that the chip has not silently left RX. Confirmed standby/non-RX discards the partial block without changing the published floor. Rejected attempts are also spaced 50 ms apart. BUSY and packet ownership block sampling; families without a mode probe retain their existing RSSI path.

CAD has a bounded wait, stops RX duty cycling before scanning, and re-arms the configured receive mode on success, busy detection, or error. A completed or currently receiving packet owns the radio and blocks CAD. A packet interrupt arriving during re-arm is retained. Failed re-arm and a newly arrived packet both defer transmission.

SX126x wrappers additionally run a recovery watchdog at most once per 10 seconds while continuous RX is expected. RXPS sleep, busy SPI, pending packets, and invalid status reads do not count as non-RX evidence. Two confirmed non-RX observations request soft recovery; a third uses the existing hard recovery path where supported. An observed healthy RX state clears the sequence. Recovery preserves tuning, TX power, receive gain, and the operator's power-saving intent. The existing radio liveness and RXPS watchdogs remain active for other failures.

The radio liveness watchdog measures time without a successfully received packet. Only a positive recvRaw() result restarts its observation window; TX completion, CAD interrupts, and failed packet reads do not. Reception counts even if application parsing rejects the packet or the packet pool is full. The check runs after pending RX is drained and TX completes, before the next queued transmission, so sustained outbound traffic cannot postpone recovery.

The default silence thresholds are 30 minutes for a soft RX/AGC re-arm and 12 hours for a radio-only hard recovery where supported. MQTT observer radio.watchdog settings can override the soft interval. A successful hard recovery starts a fresh observation window to limit repeated resets on a quiet mesh; a failed hard recovery remains escalated and retries after 30 seconds. Targets with RADIO_LIVENESS_SOFT_ONLY repeat soft recovery at the soft interval because their radio cannot be reset independently. These recoveries do not reboot the MCU. The repeater's rx.watchdog is a separate whole-board reboot watchdog, enabled by default with a 24-hour sliding deadline since the last received packet passed MeshCore parsing. Its timer is updated before receive delays and routing filters; raw LoRa reads rejected by the parser do not count. MeshCore parsing checks the packet structure, not sender authentication or decryption of traffic addressed to other nodes. Temporary radio windows of at least 12 hours use a 12-hour deadline; shorter windows disable that reboot watchdog until the window ends. The temporary timing overrides are held in RAM and do not replace saved normal-mode preferences.

The status read uses the two-byte GetStatus transaction. The pinned RadioLib getStatus() helper requests zero copied data bytes, so its return value is not usable for this mode check. The dedicated read restores the normal SPI command layout afterward and does not wake a duty-cycling radio to inspect its mode.

Validation lives in test_noise_floor_estimator, test_rx_power_saving, test_radio_liveness, test_radio_receive_contract.py, and test_sx126x_receive_mode.py.