  This paper develops a stochastic geometry framework to characterize the
statistics of the downlink interference and coverage probability of a typical
user in a coexisting terahertz (THz) and radio frequency (RF) network. We first
characterize the exact Laplace Transform (LT) of the aggregate interference and
coverage probability of a user in a THz-only network. Then, for a coexisting
RF/THz network, we derive the coverage probability of a typical user
considering biased received signal power association (BRSP). The framework can
be customized to capture the performance of a typical user in various network
configurations such as THz-only, opportunistic RF/THz, and hybrid RF/THz. In
addition, asymptotic approximations are presented for scenarios where the
intensity of THz BSs becomes large or molecular absorption coefficient in THz
approaches to zero. Numerical results demonstrate the accuracy of the derived
expressions and extract insights related to the significance of the BRSP
association compared to the conventional reference signal received power (RSRP)
association in the coexisting network.
