This paper investigates the design of orthogonal frequency-division multiplexing (OFDM) waveforms for bistatic integrated sensing and communication (ISAC) systems. In the considered framework, an ISAC transmitter jointly optimizes subcarrier assignment and power allocation for a single OFDM waveform that simultaneously supports communication and sensing functionalities. Meanwhile, an ISAC receiver decodes information on communication subcarriers and estimates per-path propagation delays via exploiting pilot symbols on sensing subcarriers. We propose a joint path coefficient and delay estimation (JPCDE) scheme, revealing that the achievable communication data rate (CDR) is determined by the number of communication subcarriers, whereas the delay sensing accuracy is governed by the index distribution of sensing subcarriers. Building on this insight, we formulate an OFDM waveform optimization problem to maximize the CDR subject to sensing-accuracy and power-budget constraints. To solve this problem, we employ a quadratic transform and Lagrangian dual decomposition, which iteratively updates the subcarrier assignment and power allocation variables in closed-form. Our results reveal that a subcarrier is allocated for sensing if and only if its Fisher information gain exceeds the corresponding communication rate loss, while the power allocation for communication subcarriers exhibits a bounded water-filling structure. Simulation results demonstrate that the proposed frameworks substantially outperform existing baselines in both delay estimation accuracy and CDR.

翻译：本文研究了面向双基地集成传感与通信系统的正交频分复用波形设计。在所提出的框架中，ISAC发射机通过联合优化子载波分配与功率配置，使单个OFDM波形同时支持通信与感知功能。与此同时，ISAC接收机在通信子载波上解码信息，并通过感知子载波上的导频符号估计每条路径的传播时延。我们提出了一种联合路径系数与时延估计方案，揭示了可达通信数据率由通信子载波数量决定，而时延感知精度则由感知子载波的索引分布所主导。基于这一发现，我们构建了在感知精度与功率预算约束下最大化通信数据率的OFDM波形优化问题。通过采用二次变换与拉格朗日对偶分解方法，该问题可通过闭式解迭代更新子载波分配与功率配置变量。研究结果表明，当且仅当某子载波的费希尔信息增益超过相应通信速率损失时，该子载波才会被分配用于感知；而通信子载波的功率配置则呈现有界注水结构。仿真结果表明，所提框架在时延估计精度与通信数据率方面均显著优于现有基线方法。