In monostatic, decision-aided, or known-waveform integrated sensing and communications (ISAC) formulations, the sensing receiver is often modeled as knowing the transmitted waveform. This assumption is not suitable for passive, bistatic, or distributed settings where the sensing receiver knows the signaling rule but not the transmitted symbols. We study such a symbol-unaware ISAC model, where sensing is measured by the unconditioned mutual information $I(S;V)$ rather than the symbol-aware quantity $I(S;V|X)$. For discrete-input memoryless channels, we characterize the capacity-sensing region through an auxiliary time-sharing variable, showing that the optimal upper boundary is the upper concave envelope of the single-mode frontier. Thus, explicit time sharing is unnecessary when the single-mode frontier is already concave, but strictly beneficial when its upper concave envelope strictly dominates the frontier. For Rayleigh-fading BPSK, we further show that the curvature of the single-mode boundary is determined by the stochastic ordering of the communication- and sensing-side effective SNR distributions. Communication-side dominance yields a concave single-mode frontier and no time-sharing gain, sensing-side dominance yields a convex single-mode frontier and a strict time-sharing gain, and equality yields a linear boundary. The result extends to SIMO-BPSK through the ordering of post-combining SNR distributions. These findings explain when symbol-unaware ISAC optimally moves from data-symbol transmission to pilot-like sensing modes.

翻译：在单站、决策辅助或已知波形集成感知与通信（ISAC）框架中，感知接收机通常被建模为已知发射波形。该假设不适用于无源、双站或分布式场景，其中感知接收机知道信号规则但不知道发射符号。我们研究这种符号未知的ISAC模型，其中感知性能由无条件互信息$I(S;V)$而非符号感知量$I(S;V|X)$衡量。针对离散输入无记忆信道，我们通过辅助时间共享变量刻画容量-感知区域，证明最优上界是单模前沿的上凹包络。因此，当单模前沿已为凹函数时，显式时间共享不必要；但当其上凹包络严格优于前沿时，时间共享严格有益。针对瑞利衰落BPSK，我们进一步证明单模边界的曲率由通信侧和感知侧有效信噪比分布的随机序决定：通信侧主导产生凹单模前沿且无时间共享增益，感知侧主导产生凸单模前沿并存在严格时间共享增益，相等时边界呈线性。该结论通过合并后信噪比分布的序关系推广至SIMO-BPSK。这些发现解释了符号未知ISAC何时从数据符号传输最优地过渡到类导频感知模式。