  In this paper, the blocklength-limited performance of a relaying system is
studied, where channels are assumed to experience quasi-static Rayleigh fading
while at the same time only the average channel state information (CSI) is
available at the source. Both the physical-layer performance
(blocklength-limited throughput) and the link-layer performance (effective
capacity) of the relaying system are investigated. We propose a simple system
operation by introducing a factor based on which we weight the average CSI and
let the source determine the coding rate accordingly. In particular, we prove
that both the blocklength-limited throughput and the effective capacity are
quasi-concave in the weight factor. Through numerical investigations, we show
the appropriateness of our theoretical model. In addition, we observe that
relaying is more efficient than direct transmission. Moreover, this performance
advantage of relaying under the average CSI scenario is more significant than
under the perfect CSI scenario. Finally, the speed of convergence (between the
blocklength-limited performance and the performance in the Shannon capacity
regime) in relaying system is faster in comparison to the direct transmission
under both the average CSI scenario and the perfect CSI scenario.
