Function duty_cycle_rx_mode 

pub fn duty_cycle_rx_mode(
    sf: SpreadingFactor,
    bw: Bandwidth,
    rx_preamble: u16,
    tx_preamble: u16,
) -> RxMode

Pick the RX mode for a strategy at the given modulation settings.

The duty-cycle windows treat rx_preamble (the receiver’s configured acquisition length) as the number of preamble symbols that must land inside a single RX window for reliable detection. With rx = rx_preamble + 1 symbols awake and sleep = tx_preamble - 2*rx_preamble - 1 symbols asleep, the worst preamble alignment still puts rx_preamble symbols in one window with a symbol to spare: a preamble that starts too late in one window meets the next one after sleep + rx symbols, leaving tx_preamble - sleep - rx - 1 >= rx_preamble symbols of it to hear. (The chip’s detector actually fires on fewer symbols than the full acquisition length, so the real margin is wider.)

Falls back to continuous RX when the TX preamble is too short to leave any sleep (tx_preamble < 2*rx_preamble + 2 — the LR1110’s 16-symbol acquisition against the 32-symbol MeshCore preamble lands here) or when a window overflows the chip’s 24-bit timers.