  In this paper, we consider a dual-hop mixed THz-RF system model for
backhaul-fronthaul applications where the link between source and destination
is established only through the relay node in which decode-and-forward relaying
protocol is used. The THz link suffers from the joint impact of antenna
misalignment and stochastic characteristics of wireless channels, including the
effect of environmental conditions such as pressure, humidity, and temperature.
The envelope of THz link in the first hop follows a generalized $\alpha-\mu$
distribution, and for the RF end, the Nakagami-$m$ distribution is considered.
In this context, we obtain new closed-form expressions of the cumulative
density function and the moment-generating function of the end-to-end
signal-to-noise ratio. Further, we derive the average symbol error rate
expressions for coherent rectangular quadrature amplitude modulation (RQAM) and
coherent hexagonal QAM (HQAM), as well as the non-coherent modulation scheme.
The asymptotic behavior is also discussed to examine the system's diversity.
Furthermore, the impact of several parameters, such as fading coefficients of
individual links and antenna misalignment, as well as the distance between
nodes, are also highlighted in the system's performance. Moreover, Monte Carlo
simulations are used to validate the presented analytical framework. Finally,
the presented numerical insights aid in the extraction of practical design
principles.
