Beam Misalignment in 3GPP mmWave NR

TL;DR

This work addresses beam misalignment in 3GPP NR mmWave systems with analog beamforming by developing an analytical framework that couples stochastic mobility, SSB-based beam sweeping, and NR timing. It derives closed-form expressions for misalignment probabilities, average misalignment durations, and the resulting beamforming gain, incorporating SSB overhead and TDD frame structures. By combining stochastic geometry with a detailed SSB scheduling model, the paper reveals trade-offs between beam counts, mobility, and SSB timing, providing design guidelines for robust and efficient beam management. The results show that mobility and the number of beams predominantly drive misalignment, while inter-site distance has a more modest effect, guiding practical NR parameter tuning for high-frequency networks.

Abstract

This paper presents an analytical framework for evaluating beam misalignment in 3GPP mmWave NR systems implementing analog beamforming. Our approach captures the interaction between user mobility, beam sweeping mechanisms, and deployment configurations, focusing on long-term average performance metrics. Specifically, we model the beam misalignment rates at both the base station (BS) and user equipment (UE) as Poisson processes and derive expressions for the expected misalignment duration, misalignment fraction, and overall beamforming gain. The framework accounts for practical constraints in NR such as Synchronization Signal Blocks (SSB) periodicity, TDD frame structures, and SSB overhead. Through numerical evaluation based on 3GPP mmWave parameters, we identify key trade-offs between beam counts, user mobility, and SSB timing, providing actionable design insights for robust and efficient beam management in future high-frequency networks.

Figures (5)

• Figure 1: The considered SSB timing model showing two representative missalignment events.
• Figure 2: Sweep duration ($T_{\text{\scriptsize sweep}}$) against the number of requested SSBs ($N_{\text{\scriptsize SSB}}^{\text{\scriptsize req}}$) for different TDD frame configurations and subcarrier spacings.
• Figure 3: Average beam misalignment duration $\Gamma$ versus the number of BS beams $N_\text{\scriptsize beam}^\text{\scriptsize BS}$ for different mmWave cell configurations.
• Figure 4: Total misalignment fraction against number of beams at the BS ($N_{\text{\scriptsize beam}}^{\text{\scriptsize BS}}$) for different UE speeds ($v$). Case D ($\Delta f=120$ kHz), $\tau_{\text{\scriptsize SS}}=20$ ms. (left) $d_{\text{\scriptsize ISD}}=100$m (right) $d_{\text{\scriptsize ISD}}=200$m.
• Figure 5: Average BF gain $\mathbb{E}[G]$ as a function of the number of BS beams ($N_\text{\scriptsize beam}^\text{\scriptsize BS}$) for different UE speeds $v$. Case D, 120 kHz subcarrier spacing (Pattern A), $\tau_\text{\scriptsize SS} = 20$ ms, $d_{\text{\scriptsize ISD}}$ m.