  In this work, we develop low complexity, optimal power allocation algorithms
that would allow ultra reliable operation at any outage probability target with
minimum power consumption in the finite blocklength regime by utilizing
Karush-Kuhn-Tucker (KKT) conditions. In our setup, we assume that the
transmitter does not know the channel state information (CSI). First, we show
that achieving a very low packet outage probability by using an open loop setup
requires extremely high power consumption. Thus, we resort to retransmission
schemes as a solution, namely Automatic Repeat Request (ARQ), Chase Combining
Hybrid ARQ (CC-HARQ) and Incremental Redundancy (IR) HARQ. Countrary to
classical approaches, where it is optimal to allocate equal power with each
transmission, we show that for operation in the ultra reliable regime (URR),
the optimal strategy suggests transmission with incremental power in each
round. Numerically, we evaluate the power gains of the proposed protocol. We
show that the best power saving is given by IR-HARQ protocol. Further, we show
that when compared to the one shot transmission, these protocols enable large
average and maximum power gains. Finally, we show that the larger the number of
transmissions is, the larger power gains will be attained.
