  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.
