Emerging intelligent reflecting surfaces (IRSs) significantly improve system
performance, while also pose a huge risk for physical layer security. A disco
IRS (DIRS), i.e., an illegitimate IRS with random time-varying reflection
properties, can be employed by an attacker to actively age the channels of
legitimate users (LUs). Such active channel aging (ACA) generated by the
DIRS-based fully-passive jammer (FPJ) can be applied to jam multi-user
multiple-input single-output (MU-MISO) systems without relying on either
jamming power or LU channel state information (CSI). To address the significant
threats posed by the DIRS-based FPJ, an anti-jamming strategy is proposed that
requires only the statistical characteristics of DIRS-jammed channels instead
of their CSI. Statistical characteristics of DIRS-jammed channels are first
derived, and then the anti-jamming precoder is given based on the derived
statistical characteristics. Numerical results are also presented to evaluate
the effectiveness of the proposed anti-jamming precoder against the DIRS-based
FPJ