Applying integrated sensing and communication (ISAC) to a cell-free massive multiple-input multiple-output (CF mMIMO) architecture has attracted increasing attention. This approach equips CF mMIMO networks with sensing capabilities and resolves the problem of unreliable service at cell edges in conventional cellular networks. However, existing studies on CF-ISAC systems have focused on the application of traditional integrated signals. To address this limitation, this study explores the employment of the orthogonal time frequency space (OTFS) signal as a representative of innovative signals in the CF-ISAC system, and the system's overall performance is optimized and evaluated. A universal downlink spectral efficiency (SE) expression is derived regarding multi-antenna access points (APs) and optional sensing beams. To streamline the analysis and optimization of the CF-ISAC system with the OTFS signal, we introduce a lower bound on the achievable SE that is applicable to OTFS-signal-based systems. Based on this, a power allocation algorithm is proposed to maximize the minimum communication signal-to-interference-plus-noise ratio (SINR) of users while guaranteeing a specified sensing SINR value and meeting the per-AP power constraints. The results demonstrate the tightness of the proposed lower bound and the efficiency of the proposed algorithm. Finally, the superiority of using the OTFS signals is verified by a 13-fold expansion of the SE performance gap over the application of orthogonal frequency division multiplexing signals. These findings could guide the future deployment of the CF-ISAC systems, particularly in the field of millimeter waves with a large bandwidth.

翻译：将集成感知与通信（ISAC）应用于无蜂窝大规模多输入多输出（CF mMIMO）架构已引起越来越多的关注。该方法为CF mMIMO网络赋予了感知能力，并解决了传统蜂窝网络中小区边缘服务不可靠的问题。然而，现有关于CF-ISAC系统的研究主要集中在传统集成信号的应用上。为克服这一局限，本研究探索将正交时频空间（OTFS）信号作为创新信号的代表应用于CF-ISAC系统，并对系统整体性能进行优化与评估。针对多天线接入点（APs）和可选感知波束，推导了一个通用的下行链路频谱效率（SE）表达式。为简化基于OTFS信号的CF-ISAC系统分析与优化，我们引入了一个适用于OTFS信号系统的可达SE下界。在此基础上，提出了一种功率分配算法，在保证指定感知信号干扰噪声比（SINR）值并满足每AP功率约束的前提下，最大化用户的最小通信SINR。结果验证了所提下界的紧致性以及所提算法的有效性。最后，通过SE性能差距相较于正交频分复用信号应用扩大13倍，验证了采用OTFS信号的优越性。这些发现可为未来CF-ISAC系统的部署提供指导，特别是在大带宽毫米波领域。