I. Introduction
Ultra-reliable low-latency communications (URLLC), as a key enabler of the fifth generation (5G) and beyond 5G systems, are expected to support some mission-critical applications with stringent requirements, such as remote surgery, intelligent transport, and factory automation [1], [2], [3]. For example, the reliability is required to be no smaller than 99.999% with the end-to-end latency limited to within 1 millisecond (ms) [1]. To achieve high reliability of URLLC, multi-input multi-output (MIMO) techniques can be a promising solution by fully exploiting the spatial diversity gain and the antenna array gain. On the other hand, short packet transmission can be adopted to meet the latency requirement of URLLC, which has also been extensively studied in literature [4], [5], and [6]. Taking advantages of both techniques, short packet MIMO transmission has emerged as an attractive topic in the context of URLLC.