Abstract
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.
Highlights
Modern communication systems play an essential role in everyone’s daily life
We show that the optimization problem is convex, and that the globally optimal power allocation strategy suggests a transmission with increasing power in each transmission round when ultra low outage probability values are required
WORK In this work, we showed that operation in the ultra reliable regime (URR) would not be feasible under the open loop transmission setup, since very large powers are required to achieve ultra-reliability
Summary
Modern communication systems play an essential role in everyone’s daily life. Throughout the history of the development of these systems, the goal has been to enable communication with higher data rates. We focus on the problem of optimal power allocation for repetitive and parallel IR-HARQ transmission schemes under block fading channel conditions For this purpose, we cast an optimization problem to minimize the average power expenditure needed to meet a certain target outage probability. We develop power allocation algorithms of low (linear) complexity, which would allow us to achieve any target outage probability for repetition and parallel (IR-HARQ) retransmission protocols that would enable operation in the URR and the finite block-length regime at any number of transmissions. The motivation is that URLLC and MTC devices usually communicate on short packets where the channel fading coefficient is almost constant through the packet duration [8] This assumption simplifies the analysis and serves the goal of obtaining benchmark power allocation for the problem addressed. Since the variance of the noise is set to unity, it corresponds to the signal-to-noise ratio (SNR)
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