We consider a cellular massive MIMO system where swarms of wireless repeaters are deployed to improve coverage. These repeaters are full-duplex relays with small form factors that receive and instantaneously retransmit signals. They can be deployed in a plug-and-play manner at low cost, while being transparent to the network--conceptually they are active channel scatterers with amplification capabilities. Two fundamental questions need to be addressed in repeater deployments: (I) How can we prevent destructive effects of positive feedback caused by inter-repeater interaction (i.e., each repeater receives and amplifies signals from others)? (ii) How much performance improvement can be achieved given that repeaters also inject noise and may introduce more interference? To answer these questions, we first derive a generalized Nyquist stability criterion for the repeater swarm system, and provide an easy-to-check stability condition. Then, we study the uplink performance and develop an efficient iterative algorithm that jointly optimizes the repeater gains, user transmit powers, and receive combining weights to maximize the weighted sum rate while ensuring system stability. Numerical results corroborate our theoretical findings and show that the repeaters can significantly improve the system performance, both in sub-6 GHz and millimeter-wave bands. The results also warrant careful deployment to fully realize the benefits of repeaters, for example, by ensuring a high probability of line-of-sight links between repeaters and the base station.

翻译：我们研究一种蜂窝大规模MIMO系统，其中部署无线中继器群以改善覆盖。这些中继器为小型全双工中继设备，能够接收信号并即时转发。它们可以即插即用方式低成本部署，同时对网络透明——从概念上讲，它们是具备放大能力的主动信道散射体。在中继器部署中需要解决两个基本问题：(I) 如何防止由中继器间交互（即每个中继器接收并放大来自其他中继器的信号）引起的正反馈破坏效应？(II) 考虑到中继器也会引入噪声并可能产生更多干扰，系统能实现多大程度的性能提升？为回答这些问题，我们首先推导了中继器群系统的广义奈奎斯特稳定性判据，并提供了易于验证的稳定性条件。随后，我们研究上行链路性能，开发了一种高效的迭代算法，该算法联合优化中继器增益、用户发射功率和接收合并权重，在确保系统稳定的同时最大化加权和速率。数值结果验证了我们的理论发现，表明中继器能显著提升系统性能（在6GHz以下频段和毫米波频段均有效）。结果也提示需要谨慎部署以充分发挥中继器优势，例如确保中继器与基站间存在高概率的视距链路。