###### Abstract  
The performance of 5G/6G cellular systems operating in millimeter wave (mmWave, 30-100 GHz) and sub-terahertz (sub-THz, 100-300 GHz) bands is conventionally assessed by utilizing the static distributions of user locations. The rationale is that the use of the beam tracking procedure allows for keeping the beams of a base station (BS) and user equipment (UE) aligned at all times. However, by introducing 3 GPP Reduced Capability (RedCap) UEs utilizing the Radio Resource Management (RRM) Relaxation procedure, this may no longer be the case, as UEs are allowed to skip synchronization signal blocks (SSB) to improve energy efficiency. Thus, to characterize the performance of such UEs, methods explicitly accounting for UE mobility are needed. In this paper, we will utilize the tools of the stochastic geometry and random walk theory to derive signal-to-noise ratio (SNR), spectral efficiency, and rate as an explicit function of time by accounting for mmWave/sub-THz specifics, including realistic directional antenna radiation patterns and micro- and micro-mobilities causing dynamic antenna misalignment. Different from other studies in the field that consider time-averaged performance measures, these metrics are obtained as an explicit function of time. Our numerical results illustrate that the macro-mobility specifies the overall trend of the time-dependent spectral efficiency, while local dynamics at 1-3 s scales are mainly governed by micro-mobility. The difference between spectral efficiency corresponding to perfectly synchronized UE and BS antennas and time-dependent spectral efficiency in a completely desynchronized system is rather negligible for realistic cell coverages and stays within approximately 5-10% for a wide range of system parameters. These conclusions are not affected by the utilized antenna array at the BS side. However, accounting for realistic radiation patterns is critical for a time-dependent performance analysis of 5G/6G mmWave/sub-THz systems. 5G; new radio; time-dependent; blockage; micro-mobility; spectral efficiency; rate +
Footnote †: journal: _Future Internet_  
Article  
Micro-Mobilities  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], extending the effective coverage of BSs. In such systems, to maintain the connectivity at all times, and efficiently conceal the impact of both the micro- and macro-mobility of users [6; 7; 8], these systems will rely on beam tracking functionality synchronizing the beam directions between UE and BS at fine-grained timescales [9]. Another challenging issue in mmWave/sub-THz systems is the human body blockage phenomenon that cannot be concealed by utilizing the standard physical layer techniques [10; 11]. Despite the inherent mobility of users, the performance of 5G/6G mmWave/sub-THz systems with directional antennas and blockage has been conventionally assessed by utilizing the tools of stochastic geometry, explicitly assuming static locations of users and blockers [12; 13; 14]. The rationale is that these assumptions not only drastically simplify the analysis, but are well aligned with the way these systems operate the performance of beam alignment dynamically over time. Assuming the beam tracking capabilities, such models well represent the system's \"snapshot\" at any arbitrarily chosen instant of time. The recently introduced 3 GPP Reduced Capabilities (RedCap) type of UE aims at aggressive power savings [15], where one of the most efficient energy conservation mechanisms is radio resource management (RRM) relaxation. According to this functionality, UE is allowed to skip the beam alignment procedures performed by utilizing the synchronization signal blocks (SSB) for a certain amount of time [16]. Conventional mathematical modeling frameworks proposed in the literature (see [17] for an overview) cannot characterize the performance of such UEs, as they target time-averaged performance metrics.