*2.2. Highly Directive Channel*

The channel model of Bob *hB* can be obtained as:

$$\mathbf{h}\_{\mathbf{B}} = l\_{\mathbf{B}} \mathbf{s}\_{\mathbf{B}'} \tag{5}$$

where *lB* is the large-scale factor denoting the fixed pass loss and *sB* is the small-scale random vector containing *N* elements. The *lB* influenced by the free space pass loss (FSPL) and highly directive antennas is given by:

$$l\_B = \frac{\lambda \sqrt{G\_t G\_r}}{4\pi d\_1} \,\,\,\,\,\,\tag{6}$$

where *Gt* and *Gr*, respectively, represent the antenna gains of Alice and Bob, and *λ* stands for the wavelength, and *d*<sup>1</sup> is the distance between Alice and Bob.

Unlike the conventional channel on the microwave band where the small-scale fading follows normal distribution, *sB* on the THz band is usually represented by Nakagami-m distribution with the *i*-th element *sBi* ∼ *Nakagami*(*m*, 1), which has recently been proven by experiments [40,41]. Finally, according to Equation (3), the signal-to-noise ratio (SNR) of Bob is given by:

$$\text{SNR}\_B = S\_B \frac{L\_B P \eta}{\sigma\_n^2},\tag{7}$$

where *SB* ∼ *Gamma*(*mN*, *m*) and *LB* = *l* 2 *<sup>B</sup>* are given by Equation (6).
