We consider a user-centric cell-free massive MIMO wireless network with $L$ remote radio units, each with $M$ antennas, serving $K_{\rm tot}$ user equipments (UEs). Most of the literature considers the regime $LM \gg K_{\rm tot}$, where the $K$ UEs are active on each time-frequency slot, and evaluates the system performance in terms of ergodic rates. In this paper, we take a quite different viewpoint. We observe that the regime of $LM \gg K_{\rm tot}$ corresponds to a lightly loaded system with low sum spectral efficiency (SE). In contrast, in most relevant scenarios, the number of UEs is much larger than the total number of antennas, but users are not all active at the same time. To achieve high sum SE and handle $K_{\rm tot} \gg ML$, users must be scheduled over the time-frequency resource. The number of active users $K_{\rm act} \leq K_{\rm tot}$ must be chosen such that: 1) the network operates close to its maximum SE; 2) the active user set must be chosen dynamically over time in order to enforce fairness in terms of per-user time-averaged throughput rates. The fairness scheduling problem is formulated as the maximization of a concave componentwise non-decreasing network utility function of the per-user rates. Intermittent user activity imposes slot-by-slot coding/decoding which prevents the achievability of ergodic rates. Hence, we model the per-slot service rates using information outage probability. To obtain a tractable problem, we make a decoupling assumption on the CDF of the instantaneous mutual information seen at each UE $k$ receiver. We approximately enforce this condition with a conflict graph that prevents the simultaneous scheduling of users with large pilot contamination and propose an adaptive scheme for instantaneous service rate scheduling. Overall, the proposed dynamic scheduling is robust to system model uncertainties and can be easily implemented in practice.

翻译：考虑一个用户中心无蜂窝大规模MIMO无线网络，包含$L$个远程射频单元，每个单元配备$M$根天线，服务于$K_{\rm tot}$个用户设备（UE）。现有文献主要研究$LM \gg K_{\rm tot}$的场景，其中$K$个UE在每个时频资源块上处于活动状态，并以遍历速率评估系统性能。本文采取截然不同的视角：观察到$LM \gg K_{\rm tot}$场景对应低总频谱效率（SE）的轻负载系统。而在实际场景中，UE数量远大于总天线数，但用户并非同时激活。为获得高总SE并处理$K_{\rm tot} \gg ML$的情况，需在时频资源上调度用户。活跃用户数$K_{\rm act} \leq K_{\rm tot}$需满足：1）网络接近最大SE运行；2）动态选择活跃用户集以实现每个用户时间平均吞吐量速率的公平性。公平调度问题被形式化为最大化用户速率凹分量非递减网络效用函数。间歇性用户活动要求逐时隙编解码，导致无法实现遍历速率，因此采用信息中断概率建模每时隙服务速率。为获得可解问题，对每个UE $k$接收端的瞬时互信息累积分布函数进行解耦假设。通过构建冲突图避免同时调度导频污染严重的用户来近似实现该条件，并提出瞬时服务速率调度的自适应方案。整体而言，所提动态调度对系统模型不确定性具有鲁棒性，且易于实际部署。