  This paper investigates the problem of power allocation for hybrid automatic
repeat request (HARQ) schemes over time-correlated Nakagami-m fading channels
under outage constraint. The presence of time correlation complicates the power
allocation problem due to the involvement of multiple correlated fading
channels. Under a general time-correlated Nakagami-m fading channel with
exponential correlation, outage probabilities for three widely adopted HARQ
schemes, including Type I HARQ, HARQ with chase combining (HARQ-CC) and HARQ
with incremental redundancy (HARQ-IR), are first derived. With these results,
power allocation schemes are proposed to minimize the average total
transmission power with guaranteed outage performance. Simulation results
demonstrate the accuracy of our outage analysis and the effectiveness of our
proposed power allocation schemes. It is shown that our proposed power
allocation schemes can achieve significant power savings when compared with
fixed power allocation. Moreover, under practical low outage constraint, the
power efficiency is further improved when the time correlation is reduced
and/or the fading order is increased.
