Function generate_ofdm_preamble 

pub fn generate_ofdm_preamble(
    preamble: &OfdmPreamble,
    cfg: &OfdmConfig,
) -> Vec<Complex32> 

Generates a repeated-segment preamble: a deterministic, reproducible pseudo-random unit-average-energy base sequence of repeat_len samples, tiled num_repeats times, followed by the training symbol (time-domain, CP included) if preamble.training_symbol is present.

The repeat base sequence and the training symbol’s frequency-domain pattern are both generated from fixed seeds (not derived from cfg), so the same OfdmPreamble always produces the same preamble on both the TX and RX side without requiring shared external state.

cfg.gain is applied to the result, exactly as OfdmMod applies it to every data symbol. This is not cosmetic: the preamble and the payload must share one amplitude scale or the frame is undecodable.

• The Schmidl & Cox timing metric normalizes against received energy, so a preamble that is quiet relative to the payload collapses the score. At gain = 121 with an unscaled preamble the best score falls from 1.00 to 0.095 — below the streaming receiver’s 0.5 acceptance threshold, so no candidate is ever accepted and nothing decodes.
• EqualizerMethod::TrainingSymbolHold estimates the channel from the training symbol. If the training symbol is unscaled while the payload is not, the estimate omits the gain, the equalizer never divides it out, and the demapper’s LLRs are miscalibrated by that factor.

cfg’s other fields remain unused here. In particular rf_hz is still not applied — a caller using a nonzero rf_hz gets a baseband preamble ahead of an upconverted body. Applying it correctly requires phase continuity with the symbols that follow, which the per-block modulator construction does not currently provide; modulate at rf_hz = 0.0 and upconvert the whole burst with one continuous Rotator instead.