  This work considers downlink incremental redundancy Hybrid Automatic Repeat
Request (IR-HARQ) over unreliable feedback channels. Since the impact of
positive feedback (i.e., ACK) error is smaller than that of negative feedback
(i.e., NACK) error, an asymmetric feedback detection scheme is proposed to
protect NACK and further reduce the outage probability. We formulate the HARQ
process as a Markov Decision Process (MDP) model to adapt to the transmission
rate of each transmission attempt without enriched feedback and additional
feedback cost. We aim to optimize the performance of HARQ process under certain
outage probability requirements by finding optimal asymmetric detection
thresholds. Numerical results obtained on the downlink Rayleigh fading channel
and 5G new radio (NR) PUCCH feedback channel show that by applying asymmetric
feedback detection and adaptive rate allocation, higher throughput can be
achieved under outage probability limitations.
