  Integrated sensing and communication (ISAC) is increasingly recognized as a
pivotal technology for next-generation cellular networks, offering mutual
benefits in both sensing and communication capabilities. This advancement
necessitates a re-examination of the fundamental limits within networks where
these two functions coexist via shared spectrum and infrastructures. However,
traditional stochastic geometry-based performance analyses are confined to
either communication or sensing networks separately. This paper bridges this
gap by introducing a generalized stochastic geometry framework in ISAC
networks. Based on this framework, we define and calculate the coverage and
ergodic rate of sensing and communication performance under resource
constraints. Then, we shed light on the fundamental limits of ISAC networks by
presenting theoretical results for the coverage rate of the unified
performance, taking into account the coupling effects of dual functions in
coexistence networks. Further, we obtain the analytical formulations for
evaluating the ergodic sensing rate constrained by the maximum communication
rate, and the ergodic communication rate constrained by the maximum sensing
rate. Extensive numerical results validate the accuracy of all theoretical
derivations, and also indicate that denser networks significantly enhance ISAC
coverage. Specifically, increasing the base station density from $1$
$\text{km}^{-2}$ to $10$ $\text{km}^{-2}$ can boost the ISAC coverage rate from
$1.4\%$ to $39.8\%$. Further, results also reveal that with the increase of the
constrained sensing rate, the ergodic communication rate improves
significantly, but the reverse is not obvious.
