This paper considers a cell-free integrated sensing and communication (ISAC) MIMO system, where distributed MIMO access points (APs) jointly serve the communication users and sense the target. For this setup, we derive a sensing SNR for multi-static sensing where both joint communication and sensing signals transmitted by different APs are utilized. With this sensing objective, we develop two baseline approaches that separately design the sensing and communication beamforming vectors, namely communication-prioritized sensing beamforming and sensing-prioritized communication beamforming. Then, we consider the joint sensing and communication (JSC) beamforming design and derive the optimal structure of these beamforming vectors based on a max-min fairness formulation. In addition, considering any pre-determined JSC beam design, we devise a power allocation approach. The results show that the developed JSC beamforming is capable of achieving nearly the same communication signal-to-interference-plus-noise ratio (SINR) of the communication-prioritized sensing beamforming solution with almost the same sensing SNR of the sensing-prioritized communication beamforming approach. The proposed JSC beamforming optimization also provides a noticeable gain over the power allocation with regularized zero-forcing beamforming, yielding a promising strategy for cell-free ISAC MIMO systems.

翻译：本文研究了一种无蜂窝集成感知与通信（ISAC）MIMO系统，其中分布式MIMO接入点（AP）协同服务于通信用户并对目标进行感知。针对该架构，我们推导了多静态感知的感知信噪比（SNR），该感知利用了不同AP发射的联合通信与感知信号。基于此感知目标，我们开发了两种分别设计感知与通信波束赋形向量的基线方法，即通信优先感知波束赋形和感知优先通信波束赋形。随后，我们考虑了联合感知与通信（JSC）波束赋形设计，并基于最大最小公平性准则推导了这些波束赋形向量的最优结构。此外，针对任意预设的JSC波束设计，我们提出了一种功率分配方法。结果表明，所开发的JSC波束赋形能够实现与通信优先感知波束赋形方案几乎相同的通信信干噪比（SINR），同时保持与感知优先通信波束赋形方法几乎相同的感知SNR。与采用正则化迫零波束赋形的功率分配相比，所提出的JSC波束赋形优化还提供了显著增益，从而为无蜂窝ISAC MIMO系统提供了一种有前景的策略。