Ultra-reliable low latency communications (uRLLC) is adopted in the fifth generation (5G) mobile networks to better support mission-critical applications that demand high level of reliability and low latency. With the aid of well-established multiple-input multiple-output (MIMO) information theory, uRLLC in the future 6G is expected to provide enhanced capability towards extreme connectivity. Since the latency constraint can be represented equivalently by blocklength, channel coding theory at finite block-length plays an important role in the theoretic analysis of uRLLC. On the basis of Polyanskiy's and Yang's asymptotic results, we first derive the exact close-form expressions for the expectation and variance of channel dispersion. Then, the bound of average maximal achievable rate is given for massive MIMO systems in ideal independent and identically distributed fading channels. This is the study to reveal the underlying connections among the fundamental parameters in MIMO transmissions in a concise and complete close-form formula. Most importantly, the inversely proportional law observed therein implies that the latency can be further reduced at expense of spatial degrees of freedom.

翻译：超可靠低延迟通信（uRLLC）已被第五代（5G）移动网络采用，以更好地支持对高可靠性和低延迟有严格要求的任务关键型应用。借助成熟的多输入多输出（MIMO）信息理论，未来6G中的uRLLC有望提供向极致连接增强的能力。由于延迟约束可等效地由块长表示，有限块长下的信道编码理论在uRLLC的理论分析中发挥着重要作用。基于Polyanskiy和Yang的渐近结果，我们首先推导了信道色散的期望和方差的精确闭式表达式。然后，在理想独立同分布衰落信道中，给出了大规模MIMO系统的平均最大可达速率界。本研究以简洁且完整的闭式公式揭示了MIMO传输中基本参数之间的内在联系。最重要的是，其中观察到的反比定律表明，延迟可以以空间自由度为代价进一步降低。