This paper investigates the fundamental communication--sensing tradeoffs of uplink dual-functional integrated sensing and communication (ISAC) multiple access under finite blocklength (FBL) constraints. Unlike conventional asymptotic analyses, we explicitly account for the limitations under FBL constraints imposed by short packets and low-latency transmission. By examining the unbiased channel state sensing estimator, we establish a geometric decomposition of the sensing error, indicating that it is jointly determined by the signal-to-noise ratio and the correlation structure of the information codebook. This insight reveals how cross-correlation among active users in the codebook geometry fundamentally constrains dual-functional ISAC performance. Consequently, we derive achievability and converse bounds that characterize the tradeoff between communication code rate and sensing accuracy in the FBL regime, with the converse further bounded by Shannon capacity. Moreover, by treating channel state sensing as a high-level sensing objective, a universal Cramér--Rao bound is derived to link channel estimation accuracy to practical sensing parameters. Examples of parameter sensing are also provided based on 3GPP standard. Numerical results validate the theoretical analysis and demonstrate the impact of blocklength, antenna dimensions, and sensing requirements.

翻译：本文研究了有限块长约束下上行链路双功能集成感知与通信多址接入的基本通信-感知权衡。与传统渐近分析不同，我们明确考虑了短包和低延迟传输所施加的有限块长约束下的局限性。通过考察无偏信道状态感知估计器，我们建立了感知误差的几何分解，表明其由信噪比和信息码本的相关结构共同决定。这一见解揭示了码本几何中活跃用户间的互相关如何从根本上约束双功能ISAC性能。因此，我们推导了可达性与逆界，刻画了有限块长体制下通信码率与感知精度之间的权衡关系，其中逆界进一步受香农容量约束。此外，通过将信道状态感知视为高层感知目标，推导了通用的克拉美-罗下界，将信道估计精度与实际感知参数联系起来。本文还基于3GPP标准提供了参数感知的实例。数值结果验证了理论分析，并展示了块长、天线维度和感知要求的影响。