Hybrid ARQ: Does Fading Diversity Help?

In the literature on hybrid automatic repeat request (HARQ), the quasi-static channel model and the independent identically distributed (i.i.d.) fading channel model are typically used to model slow varying channel and fast varying channel, respectively. Even though the i.i.d. fading channel results in lower outage probability thanks to the diversity from independent fading, somewhat surprisingly, we analytically show that fading diversity hurts throughput of Chase combining with a single retransmission, for all possible initial attempted rates. We show similar trends across a wide range of initial attempted rates for HARQ with incremental redundancy. The results provide a better understanding of the impact of channel correlation and fading diversity on the throughput of HARQ and offer guidance for designing HARQ with multiple parallel processes.