6G Communications in the Terahertz Band: The Impact of Near-Field Dynamics Under User Micromobility

The Terahertz (THz) frequency band (0.1-3 THz), which is anticipated to play a crucial role in 6G and future cellular systems, requires the deployment of massive antenna arrays that create highly directional radiation patterns. This, in turn, results in a portion of the cell operating within the near field. In these systems, even small rotations or displacements of User Equipment (UE) in the user - s hands - referred to as micromobility - may cause significant fluctuations in the Signal Received Power (SRP) in the near-field due to propagation specifics and in the far-field due to beam misalignment. Given that both these factors require different mitigation strategies, understanding their relative impacts is essential. This study aims to (i) assess the qualitative effects of beam misalignment and near-field propagation in line-ofsight (LoS) conditions, and (ii) identify which factor is the dominant contributor to SRP degradation in both near and far fields. To achieve this, we propose a unified model that simultaneously captures beam misalignment, as well as near-and far-field propagation dynamics, as influenced by UE micromobility. We then characterize the time-dependent SRP variations. Our findings reveal that the impact of beam misalignment and near-field dynamics is typically of similar magnitude. Specifically, for THz antennas of 128×128 elements, even minor UE rotations (e.g., half a degree) can cause significant performance degradation (up to 50 dB) due to near-field propagation. This can lead to outage conditions within just 500-700 ms. The impact of micromobility on beam misalignment in the far field and propagation specifics in the near field exhibits comparable influence under rotational micromobility, but near-field impact is of higher magnitude when the UE undergoes Cartesian displacements, leading to a 20-30 dB degradation over just a few centimeters.