  The trade-off between reliability, latency, and energy-efficiency is a
central problem in communication systems. Advanced hybrid automated repeat
request (HARQ) techniques can reduce the number of retransmissions required for
reliable communication, but they have a significant computational cost. On the
other hand, strict energy constraints apply mainly to devices, while the access
point receiving their packets is usually connected to the electrical grid.
Therefore, moving the computational complexity required for HARQ schemes from
the transmitter to the receiver may provide a way to overcome this trade-off.
To achieve this, we propose the Reinforcement-based Adaptive Feedback (RAF)
scheme, in which the receiver adaptively learns how much additional redundancy
it requires to decode a packet and sends rich feedback (i.e., more than a
single bit), requesting the coded retransmission of specific symbols.
Simulation results show that the RAF scheme achieves a better trade-off between
energy-efficiency, reliability, and latency, compared to existing HARQ
solutions and a fixed threshold-based policy. Our RAF scheme can easily adapt
to different modulation schemes, and since it relies on the posterior
probabilities of the codeword symbols at the decoder, it can generalize to
different channel statistics.
