On the Impact of Rotational Mobility in Directional Cellular Systems

The high data rates required for next generation applications necessitate the use of millimeter wave and terahertz bands where bandwidth is abundant. Due to high path loss in these bands, antenna arrays (AAs) are needed to focus the signal in highly directional beams in the desired directions. However, highly directional communication links in these bands are vulnerable to misalignment and blockages due to mobility. Thus, both mobile blockers and user equipment (UE) rotations can significantly increase the handover (HO) frequency, while HO delays and HO failures jeopardize system latency and reliability. Furthermore, the scanning angle of each AA is limited by the orientation of the device, the mounting angle of the AA, the element spacing and the grating sidelobes formed during beamforming, which is referred as Field-of-View (FoV). In scenarios with UE rotational mobility, limited FoV may lead to loss of connection with the source next-generation NodeB (gNB). In this paper, we analyze current HO and radio link monitoring protocols in scenarios with UE rotational mobility and mobile blockers. We use a Markov Chain based analytical model as well as MATLAB based system level simulations to show how user rotation can significantly increase the outage duration under various deployment configurations. We propose enhancements to enable faster HO under user rotations and demonstrate significant performance improvements using simulations.