Module sync_pattern 

Frame-head preamble for uvpacket’s coherent QPSK modem.

Replaces the 4-FSK Costas-4 head pattern that the Phase 1 design used (and that Phase 2 found could not survive the modulation pivot to coherent QPSK — Costas arrays are FSK-tone-index sequences, not constellation-point sequences).

The new sync is a 31-bit maximum-length sequence (PRBS, generator polynomial x⁵ + x² + 1) mapped to BPSK at the channel-symbol rate. 31 chips × 1 sym/chip = 26 ms preamble at 1200 baud. Autocorrelation sidelobes are bounded by 1/31 ≈ −15 dB amplitude, giving a clean correlator peak that the receiver uses for symbol-timing acquisition, frequency-offset estimation, and initial carrier-phase lock.

After the preamble, the receiver maintains phase via decision-directed PLL with periodic PILOT_SYMBOL_INTERVAL known-QPSK pilot symbols (one per 32 transmitted symbols ≈ 3.1 % overhead). The pilot’s constellation point is +1 + 0j — the QPSK symbol mapped from bit pair [0, 0].

Trait-level placeholder

UVPACKET_SYNC_BLOCKS is kept as a [Costas-4 at symbol 0] placeholder so that Protocol::SYNC_MODE = SyncMode::Block(...) has something non-empty to point at and protocol_invariants tests pass. The uvpacket TX / RX paths are bespoke and do not consult this constant — they use UVPACKET_PREAMBLE_BPSK_BITS and PILOT_SYMBOL_INTERVAL directly.

Constants

PILOT_QPSK_POINT

QPSK pilot constellation point. Chosen as constellation index 0 (= bit pair [0, 0]) so it maps to +1 + 0j — receiver-side phase reference is straightforward (the pilot’s expected angle is the carrier reference angle).

PILOT_SYMBOL_INTERVAL

Pilot interval: every PILOT_SYMBOL_INTERVALth transmitted symbol after the preamble is a known pilot, the rest are data. 32 means 1 pilot per 31 data → ~3.1 % overhead, comfortable margin against 10 Hz Doppler at 1200 baud (coherence time ≈ 100 ms = 120 symbols, so a pilot every 32 symbols is well inside the coherence interval).

PREAMBLE_LEN

Length of the m-sequence preamble in BPSK chips (= QPSK transmitted symbols since the preamble is BPSK-mapped onto the QPSK constellation’s I axis).

UVPACKET_COSTAS

Decorative 4-FSK Costas pattern kept around so Protocol:: SYNC_MODE = SyncMode::Block(&UVPACKET_SYNC_BLOCKS) has something to point at and protocol_invariants checks pass. Not consulted by crate::uvpacket::tx::encode / crate::uvpacket::rx::decode_known_layout / crate::uvpacket::rx::decode.

UVPACKET_PREAMBLE_BPSK_BITS

31-bit maximum-length sequence from a Fibonacci LFSR with polynomial x⁵ + x² + 1 and initial state [0, 0, 0, 0, 1]. Bits run in TX order. Reproducible: any LFSR walker with the same polynomial / initial state regenerates this exact sequence.

UVPACKET_SYNC_BLOCKS

Protocol::SYNC_MODE placeholder. See module docs.