  The non-orthogonal multiple access (NOMA) and millimeter-wave (mmWave)
transmission enable the unmanned aerial vehicle (UAV) assisted wireless
networks to provide broadband connectivity over densely packed urban areas. The
presence of malicious receivers, however, compromise the security of the
UAV-to-ground communications link, thereby degrading secrecy rates. In this
work, we consider a NOMA-based transmission strategy in a mmWave UAV-assisted
wireless network, and investigate the respective secrecy-rate performance
rigorously. In particular, we propose a protected-zone approach to enhance the
secrecy-rate performance by preventing the most vulnerable subregion (outside
the user region) from the presence of malicious receivers. The respective
secrecy rates are then derived analytically as a function of the protected
zone, which verifies great secrecy rate improvements through optimizing shape
of the protected zone in use. Furthermore, we show that the optimal protected
zone shape for mmWave links appears as a compromise between protecting the
angle versus distance dimension, which would otherwise form to protect solely
the distance dimension for sub-6GHz links. We also numerically evaluate the
impact of transmission power, protected-zone size, and UAV altitude on the
secrecy-rate performance improvements as practical considerations.
