  Exploiting short packets for communications is one of the key technologies
for realizing emerging application scenarios such as massive machine type
communications (mMTC) and ultra-reliable low-latency communications (uRLLC). In
this paper, we investigate short-packet communications to provide both
reliability and security guarantees simultaneously with an eavesdropper. In
particular, an outage probability considering both reliability and secrecy is
defined according to the characteristics of short-packet transmission, while
the effective throughput in the sense of outage is established as the
performance metric. Specifically, a general analytical framework is proposed to
approximate the outage probability and effective throughput. Furthermore,
closed-form expressions for these quantities are derived for the high
signal-to-noise ratio (SNR) regime. Both effective throughput obtained via a
general analytical framework and a high-SNR approximation are maximized under
an outage-probability constraint by searching for the optimal blocklength.
Numerical results verify the feasibility and accuracy of the proposed
analytical framework, and illustrate the influence of the main system
parameters on the blocklength and system performance under the
outage-probability constraint.
