Having a proper model of channel is crucial in communication system that allows to have a better resource allocation and enhance the user experience. In optical wireless communication and mmWave, it is important to incorporate the random orientation of device and user mobility to have a more accurate model of the channel. However, the device orientation and mobility should not be modeled as two independent random process. This has been explained in more detail in the following paper:

Orientation model of mobile device for indoor VLC and millimeter wave systems:

https://www.pure.ed.ac.uk/ws/portalfiles/portal/80547328/VTC2018_fall.pdf

https://www.youtube.com/watch?v=5OQYVaiqD1k