  In this work, we investigate the secrecy performance in an intelligent
reflecting surface (IRS)-assisted downlink system. In particular, we consider a
base station (BS)-side IRS and as such, the BS-IRS channel is assumed to be
known perfectly. Of more importance, we consider the case, in which only
outdated channel state information (CSI) of the IRS-user channel is available.
We study the impact of outdated CSI on the secrecy performance numerically and
analytically. Furthermore, we propose an element subset selection (ESS) method
in order to improve the secrecy performance. A key observation is that minimal
secrecy outage probability (SOP) can be achieved using a subset of the IRS, and
the optimal number of selected reflecting elements can be effectively found by
closed-form expressions.
