  Wireless communications over Terahertz (THz)-band frequencies are vital
enablers of ultra-high rate applications and services in sixth-generation (6G)
networks. However, THz communications suffer from poor coverage because of
inherent THz features such as high penetration losses, severe path loss, and
significant molecular absorption. To surmount these critical challenges and
fully exploit the THz band, we explore a coexisting radio frequency (RF) and
THz finite indoor network in which THz small cells are deployed to provide high
data rates, and RF macrocells are deployed to satisfy coverage requirements.
Using stochastic geometry tools, we assess the performance of coexisting RF and
THz networks in terms of coverage probability and average achievable rate. The
accuracy of the analytical results is validated with Monte-Carlo simulations.
Several insights are devised for accurate tuning and optimization of THz system
parameters, including the fraction of THz access points (APs) to deploy, and
the THz bias. The obtained results recognize a clear coverage/rate trade-off
where a high fraction of THz AP improves the rate significantly but may degrade
the coverage performance. Furthermore, the location of the user in the finite
area highly affects the fraction of THz APs that optimizes the performance.
