  Volumetric video offers a highly immersive viewing experience, but poses
challenges in ensuring quality of experience (QoE) due to its high bandwidth
requirements. In this paper, we explore the effect of viewing distance
introduced by six degrees of freedom (6DoF) spatial navigation on user's
perceived quality. By considering human visual resolution limitations, we
propose a visual acuity model that describes the relationship between the
virtual viewing distance and the tolerable boundary point cloud density. The
proposed model satisfies spatial visual requirements during 6DoF exploration.
Additionally, it dynamically adjusts quality levels to balance perceptual
quality and bandwidth consumption. Furthermore, we present a QoE model to
represent user's perceived quality at different viewing distances precisely.
Extensive experimental results demonstrate that, the proposed scheme can
effectively improve the overall average QoE by up to 26% over real networks and
user traces, compared to existing baselines.
