  We introduce a phase hopping scheme for reconfigurable intelligent surfaces
(RISs) in which the phases of the individual RIS elements are randomly varied
with each transmitted symbol. This effectively converts slow fading into fast
fading. We show how this can be leveraged to significantly improve the outage
performance especially for small outage probabilities without channel state
information (CSI) at the transmitter and RIS. Furthermore, the same result can
be accomplished even if only two possible phase values are available. Since we
do not require perfect CSI at the transmitter or RIS, the proposed scheme has
no additional communication overhead for adjusting the phases. This enables
robust ultra-reliable communications with a reduced effort for channel
estimation.
