HARQ and AMC: Friends or Foes?

Wireless communication systems use adaptive modulation and coding (AMC) as well as hybrid ARQ (HARQ) to mitigate the effects of time-varying and error-prone channels. In this work, we analyze the benefits of combining HARQ and AMC adopting the throughput as a unique performance measure. The analysis of block-fading channels and error-prone decoders allows us to conclude that i) adding HARQ on top of the AMC is counterproductive in the high average signal- to-noise ratio regime for fast fading channels, and ii) HARQ is useful for slow fading channels but the throughput gain due to HARQ is moderate. In the numerical results, we make use of incremental redundancy HARQ (HARQ-IR) to confirm the theoretical analysis.