  In this work, a hybrid free-space optics (FSO)/ teraHertz (THz) based
backhaul network is considered to provide high-data rate reliable communication
to the terrestrial mobile users (MUs) operating at millimeter wave (mmWave)
bands. The FSO link is affected by the atmospheric turbulence and pointing
error impairments which are characterized with Gamma-Gamma and Rayleigh fading,
respectively. At the FSO receiver, both the intensity modulated direct
detection and heterodyne detection techniques are considered. The multi-antenna
THz link suffers from both high path-loss and misalignment error. The THz
channel is characterized with $\alpha-\mu$ distribution and misalignment error
is characterized with Rayleigh fading. To minimize the effect of back-and-forth
switching, soft switching method is introduced at the access point (AP) to
select the signal coming through the hybrid FSO/THz link, and a comparison with
hard switching method is presented. Selective decode-and-forward relaying is
considered at the AP which is forwarding the successfully decoded backhaul
signals to the terrestrial MUs. In this context, we derive closed-form
expressions of the individual link's outage probability, end-to-end (E2E)
outage probability, asymptotic outage probability, ergodic capacity, and
average bit-error-rate of various modulation schemes. Finally, we study the
effect of different parameters such as atmospheric turbulence,
pointing/misalignment errors, link distance, atmospheric attenuation/path-loss,
fading parameters of the THz and access links, and number of antennas on the
network performance. Our results indicate that, with a proper switching method,
the joint implementation of FSO/THz links, improves the rate/reliability of the
backhaul links with limited switching overhead.
