  This paper studies physical-layer secure transmissions from a transmitter to
a legitimate receiver against an eavesdropper over slow fading channels, taking
into account the impact of finite blocklength secrecy coding. A comprehensive
analysis and optimization framework is established to investigate secrecy
throughput for both single- and multi-antenna transmitter scenarios. Both
adaptive and non-adaptive design schemes are devised, in which the secrecy
throughput is maximized by exploiting the instantaneous and statistical channel
state information of the legitimate receiver, respectively. Specifically,
optimal transmission policy, blocklength, and code rates are jointly designed
to maximize the secrecy throughput. Additionally, null-space artificial noise
is employed to improve the secrecy throughput for the multi-antenna setup with
the optimal power allocation derived.Various important insights are developed.
In particular, 1) increasing blocklength benefits both reliability and secrecy
under the proposed transmission policy; 2) secrecy throughput monotonically
increases with blocklength; 3) secrecy throughput initially increases but then
decreases as secrecy rate increases, and the optimal secrecy rate maximizing
the secrecy throughput should be carefully chosen in order to strike a good
balance between rate and decoding correctness. Numerical results are eventually
presented to verify theoretical findings.
