  Phase noise (PN) can become a major bottleneck for integrated sensing and
communications (ISAC) systems towards 6G wireless networks. In this paper, we
consider an OFDM ISAC system with oscillator imperfections and investigate the
impact of PN on monostatic sensing performance by performing a misspecified
Cram\'er-Rao bound (MCRB) analysis. Simulations are carried out under a wide
variety of operating conditions with regard to SNR, oscillator type
(free-running oscillators (FROs) and phase-locked loops (PLLs)), 3-dB bandwidth
of the oscillator spectrum, PLL loop bandwidth and target range. The results
provide valuable insights on when PN leads to a significant degradation in
range and/or velocity accuracy, establishing important guidelines for hardware
and algorithm design in 6G ISAC systems.
