Microwave linear analog computer (MiLAC) has emerged as a promising architecture for implementing linear multiple-input multiple-output (MIMO) processing in the analog domain, with radio frequency (RF) signals. Existing studies on MiLAC-aided communications rely on idealized channel models and neglect antenna mutual coupling. However, since MiLAC performs processing at RF, mutual coupling becomes critical and alters the implemented operation, not only the channel characteristics. In this paper, we develop a physics-compliant model for MiLAC-aided MIMO systems accounting for mutual coupling with multiport network theory. We derive end-to-end system models for scenarios with MiLACs at the transmitter, the receiver, or both, showing how mutual coupling impacts the linear transformation implemented by the MiLACs. Furthermore, we formulate and solve a mutual coupling aware MiLAC optimization problem, deriving a closed-form globally optimal solution that maximizes the received signal power. We establish the fundamental performance limits of MiLAC with mutual coupling, and derive three analytical results. First, mutual coupling is beneficial in MiLAC-aided systems, on average. Second, with mutual coupling, MiLAC performs as digital architectures equipped with a matching network, while having fewer RF chains. Third, with mutual coupling, MiLAC always outperforms digital architectures with no matching network. Numerical simulations confirm our theoretical findings.

翻译：微波线性模拟计算机（MiLAC）已成为一种在模拟域利用射频信号实现线性多输入多输出处理的有前景架构。现有关于MiLAC辅助通信的研究依赖于理想化信道模型，并忽略了天线互耦效应。然而，由于MiLAC在射频端进行处理，互耦效应变得至关重要，它不仅改变信道特性，还会改变所实现的运算操作。本文基于多端口网络理论，为考虑互耦效应的MiLAC辅助多输入多输出系统建立了物理合规模型。我们推导了在发射端、接收端或两端均配置MiLAC场景下的端到端系统模型，揭示了互耦如何影响MiLAC实现的线性变换。进一步地，我们构建并求解了考虑互耦的MiLAC优化问题，推导出以最大化接收信号功率为目标的闭式全局最优解。我们确立了存在互耦时MiLAC的基本性能极限，并得出三项解析结论：首先，平均而言，互耦对MiLAC辅助系统具有积极作用；其次，在互耦存在时，MiLAC能以更少的射频链实现与配备匹配网络的数字架构相当的性能；第三，在互耦存在时，MiLAC始终优于无匹配网络的数字架构。数值仿真验证了我们的理论发现。