  Reconfigurable intelligent surface (RIS) has recently attracted a spurt of
interest due to its innate advantages over Massive MIMO on power consumption.
In this paper, we study the outage performance of multi-RIS system with the
help of hybrid automatic repeat request (HARQ) to improve the RIS system
reliability, where the destination received channels are modeled by Rician
fading and the phase shift setting only depends on the line-of-sight (LoS)
component. Both the exact and asymptotic outage probabilities under Type-I HARQ
and HARQ with chase combining (HARQ-CC) schemes are derived. Particulary, the
tractable asymptotic results empower us to derive meaningful insights for
HARQ-aided multi-RIS system. On the one hand, we find that both the Type-I and
the HARQ-CC schemes can achieve full diversity that is equal to the maximal
number of HARQ rounds. On the other hand, the closed-form expression of the
optimal phase shift setting with respect to outage probability minimization is
obtained. The optimal solution indicates that the reflecting link direction
should be consistent with direct link LoS component. Finally, the analytical
results are validated by Monte-Carlo simulations.
