$h(t)= \frac{1}{M}\sum_{k=0}^{M-1}\delta(t-t_k)\cdot \exp(-\alpha \cdot \ell / \cos(\gamma_k))$

$H(f) = \frac{1}{M}\sum_{k=0}^{M-1}\exp(j 2 \pi f t_k)\cdot \exp(-\alpha \cdot \ell / \cos(\gamma_k))$

with:

$t_k = \frac{\ell}{c_0 \cdot n_K \cdot \cos(\gamma_k) }$

$\gamma_k = (\frac{2k}{M-1} - 1)\cdot\gamma_{crit},~~ k \in\{0, \cdots, M-1\}$

$M$ number of modes

$\alpha$ attenuation in dB/m