  Terahertz (THz) communications are envisioned to be a promising technology
for 6G thanks to its broad bandwidth. However, the large path loss, antenna
misalignment, and atmospheric influence of THz communications severely
deteriorate its reliability. To address this, hybrid automatic repeat request
(HARQ) is recognized as an effective technique to ensure reliable THz
communications. This paper delves into the performance analysis of HARQ with
incremental redundancy (HARQ-IR)-aided THz communications in the
presence/absence of blockage. More specifically, the analytical expression of
the outage probability of HARQ-IR-aided THz communications is derived, with
which the asymptotic outage analysis is enabled to gain meaningful insights,
including diversity order, power allocation gain, modulation and coding gain,
etc. Then the long term average throughput (LTAT) is expressed in terms of the
outage probability based on renewal theory. Moreover, to combat the blockage
effects, a multi-hop HARQ-IR-aided THz communication scheme is proposed and its
performance is examined. To demonstrate the superiority of the proposed scheme,
the other two HARQ-aided schemes, i.e., Type-I HARQ and HARQ with chase
combining (HARQ-CC), are used for benchmarking in the simulations. In addition,
a deep neural network (DNN) based outage evaluation framework with low
computational complexity is devised to reap the benefits of using both
asymptotic and simulation results in low and high outage regimes, respectively.
This novel outage evaluation framework is finally employed for the optimal rate
selection, which outperforms the asymptotic based optimization.
