A Triple-Layer HFM-LFM-CAZAC Preamble Framework for Underwater Acoustic Integrated Sensing and Communication

We propose a three-functional-layer decomposition framework for underwater acoustic (UWA) integrated sensing and communication (ISAC) preambles, instantiated as P5. Two spectrally separated chirp layers-hyperbolic frequency modulation (HFM) for wideband Doppler invariance and linear frequency modulation (LFM) for sub-meter ranging-are carried under a common constant-amplitude zero-autocorrelation (CAZAC) envelope that supplies cell identification and despreading against a root-blind attacker. Closed-form screening conditions constrain the layers to a near-orthogonal subspace, and direct cross-ambiguity measurement confirms the realized separation. In matched-filter Monte Carlo simulation, P5 meets the continuous-sensing target (range root mean square error σR≤1 m at 10 dB signal-to-noise ratio) and has the smallest normalized matched-filter peak loss across twelve modeled UWA environments among four tested waveforms. Against four classical structure-aware attackers it stays below the strict Pd≤0.1 low-probability-of-intercept target at 0 dB attacker-input SNR. A 10-seed, 11.2-million-parameter spectrogram ResNet-18 reaches Pd=0.5 against P5 at mean +21.24 dB total-energy SNR (95% CI [+21.06,+21.42] dB) and Pd=0.1 at +18.72 dB ([+18.45,+19.00] dB); these crossings are lower bounds on adversary capability, not a security guarantee. The integration also has explicit costs: composite peak-sidelobe level (-7.60 dB default, -11.29 dB optimized) remains inferior to equal-aperture single-waveform baselines, and sixteen-cell identification falls to ≤0.07 under a +2 dB near-far interferer. All-60-sounding WATERMARK replay further gives adverse P5^def-B5 losses of -0.816 dB on NOF1 (sounding-cluster 95% CI [-1.021,-0.621] dB) and -0.950 dB on NCS1 ([-0.977,-0.923] dB) after all waveforms are scaled into the same measured 8-kHz band. The evidence is therefore simulation dominant and supplemented by measured-channel replay of band-scaled variants; native-band transducer, pool, and sea-trial validation remain future work.