  This letter analyzes and compares the performance of full-duplex relaying
(FDR) and half-duplex relaying (HDR) for ultra-reliable short-packet
communications. Specifically, we derive both approximate and asymptotic
closed-form expressions of the block error rate (BLER) for FDR and HDR using
short packets with finite blocklength codes. We define and attain a closed-form
expression of a critical BLER, which can be used to efficiently determine the
optimal duplex mode for ultra-reliable low latency communication scenarios. Our
results unveil that FDR is more appealing to the system with relatively lower
transmit power constraint, less stringent BLER requirement and stronger loop
interference suppression.
