  Illegitimate intelligent reflective surfaces (IRSs) can pose significant
physical layer security risks on multi-user multiple-input single-output
(MU-MISO) systems. Recently, a DISCO approach has been proposed an illegitimate
IRS with random and time-varying reflection coefficients, referred to as a
"disco" IRS (DIRS). Such DIRS can attack MU-MISO systems without relying on
either jamming power or channel state information (CSI), and classical
anti-jamming techniques are ineffective for the DIRS-based fully-passive
jammers (DIRS-based FPJs). In this paper, we propose an IRS-enhanced
anti-jamming precoder against DIRS-based FPJs that requires only statistical
rather than instantaneous CSI of the DIRS-jammed channels. Specifically, a
legitimate IRS is introduced to reduce the strength of the DIRS-based jamming
relative to the transmit signals at a legitimate user (LU). In addition, the
active beamforming at the legitimate access point (AP) is designed to maximize
the signal-to-jamming-plus-noise ratios (SJNRs). Numerical results are
presented to evaluate the effectiveness of the proposed IRS-enhanced
anti-jamming precoder against DIRS-based FPJs.
