We present a general framework for the characterization of the packet error probability achievable in cell-free Massive multiple-input multiple output (MIMO) architectures deployed to support ultra-reliable low-latency (URLLC) traffic. The framework is general and encompasses both centralized and distributed cell-free architectures, arbitrary fading channels and channel estimation algorithms at both network and user-equipment (UE) sides, as well as arbitrary combining and precoding schemes. The framework is used to perform numerical experiments on specific scenarios, which illustrate the superiority of cell-free architectures compared to cellular architectures in supporting URLLC traffic in uplink and downlink. Also, these numerical experiments provide the following insights into the design of cell-free architectures for URLLC: i) minimum mean square error (MMSE) spatial processing must be used to achieve the URLLC targets; ii) for a given total number of antennas per coverage area, centralized cell-free solutions involving single-antenna access points (APs) offer the best performance in the uplink, thereby highlighting the importance of reducing the average distance between APs and UEs in the URLLC regime; iii) this observation applies also to the downlink, provided that the APs transmit precoded pilots to allow the UEs to estimate accurately the precoded channel.

翻译：本文提出一个通用框架，用于表征支持超可靠低延迟（URLLC）业务的无蜂窝大规模多输入多输出（MIMO）架构中数据包错误概率的可达性特征。该框架具有通用性，涵盖集中式与分布式无蜂窝架构、任意衰落信道及网络侧与用户设备（UE）侧的信道估计算法，以及任意合并与预编码方案。基于该框架对特定场景进行数值实验，结果表明在上下行链路中支持URLLC业务时，无蜂窝架构相较于蜂窝架构具有显著优势。此外，这些数值实验为面向URLLC的无蜂窝架构设计提供了以下洞见：i）必须采用最小均方误差（MMSE）空间处理以实现URLLC目标；ii）在给定覆盖区域总天线数条件下，涉及单天线接入点（AP）的集中式无蜂窝方案在上行链路中性能最优，从而凸显了在URLLC场景中缩短AP与UE间平均距离的重要性；iii）若AP发送预编码导频使UE能准确估计预编码信道，该结论同样适用于下行链路。