To realize holographic communications, a potential technology for spectrum efficiency improvement in the future sixth-generation (6G) network, antenna arrays inlaid with numerous antenna elements will be deployed. However, the increase in antenna aperture size makes some users lie in the Fresnel region, leading to the hybrid near-field and far-field communication mode, where the conventional far-field channel estimation methods no longer work well. To tackle the above challenge, this paper considers channel estimation in a hybrid-field multipath environment, where each user and each scatterer can be in either the far-field or the near-field region. First, a joint angular-polar domain channel transform is designed to capture the hybrid-field channel's near-field and far-field features. We then analyze the power diffusion effect in the hybrid-field channel, which indicates that the power corresponding to one near-field (far-field) path component of the multipath channel may spread to far-field (near-field) paths and causes estimation error. We design a novel power-diffusion-based orthogonal matching pursuit channel estimation algorithm (PD-OMP). It can eliminate the prior knowledge requirement of path numbers in the far field and near field, which is a must in other OMP-based channel estimation algorithms. Simulation results show that PD-OMP outperforms current hybrid-field channel estimation methods.

翻译：为实现全息通信——未来第六代(6G)网络提升频谱效率的潜在技术，将部署嵌有大量天线单元的天线阵列。然而，天线孔径尺寸的增加使得部分用户位于菲涅尔区，导致混合近场与远场通信模式，传统远场信道估计方法在此场景下不再适用。针对上述挑战，本文研究了混合场多径环境中的信道估计问题，其中每个用户和散射体可处于远场或近场区域。首先，设计了一种联合角度-极化域信道变换，以捕获混合场信道的近场与远场特征。随后分析了混合场信道中的功率扩散效应，该效应表明多径信道中某近场（远场）路径分量对应的功率可能扩散至远场（近场）路径并导致估计误差。我们设计了一种基于功率扩散的正交匹配追踪信道估计算法（PD-OMP），该算法无需预知远场与近场路径数量先验信息，而其他基于OMP的信道估计算法必须依赖此先验信息。仿真结果表明，PD-OMP在性能上优于现有混合场信道估计方法。