  Wireless communication technique at terahertz (THz) frequencies is regarded
as the most potential candidate for future wireless networks due to its wider
frequency bandwidth and higher data capacity when compared to that employing
radio frequency (RF) and millimeter wave (mmWave). Besides, a THz link can
achieve higher security at physical layer when it propagates in clear weather
due to its higher directionality, which reduces the possibility of
eavesdropping attacks. However, under adverse weather conditions (such as water
fog, dust fog, rain and snow), the link degradation due to weather particles
and gaseous molecules will affect the link secrecy performance seriously. In
this work, we present theoretical investigations on physical layer security of
a point-to-point THz link in rain and snow with a potential eavesdropper
locating outside of the legitimate link path. Signal degradation due to
rain/snow, gaseous attenuation and beam divergence are included in a
theoretical model to estimate the link performance. Secrecy capacity of the
link with carriers at 140, 220 and 340 GHz is calculated and compared. Insecure
regions are also presented and the secrecy performance is analyzed. We find
that the THz link suffers least eavesdropping attacks in rain and the maximum
data transmission rate decreases for higher carrier frequencies in rain and
snow.
