The interplay between large antenna apertures and high frequencies in future generations of wireless networks will give rise to near-field communications. In this paper, we focus on the hybrid analog and digital beamforming architecture of dynamic metasurface antennas, which constitutes a recent prominent enabler of extremely massive antenna architectures, and devise a near-field beam tracking framework that initiates near-field beam sweeping only when the base station estimates that its provided beamforming gain drops below a threshold from its theoretically optimum value. Novel analytical expressions for the correlation function between any two beam focusing vectors, the beamforming gain with respect to user coordinate mismatch, the direction of the user movement yielding the fastest beamforming gain deterioration, and the minimum user displacement for a certain performance loss are presented. We also design a non-uniform coordinate grid for effectively sampling the user area of interest at each position estimation slot. Our extensive simulation results validate our theoretical analysis and showcase the superiority of the proposed near-field beam tracking over benchmarks.

翻译：未来无线网络中，大天线孔径与高频段的相互作用将催生近场通信。本文聚焦于动态超表面天线的混合模拟与数字波束赋形架构——该架构是近期实现超大规模天线阵列的关键使能技术，并提出一种近场波束追踪框架：仅当基站估计其提供的波束赋形增益低于理论最优值阈值时，才启动近场波束扫描。本文推导了以下创新性解析表达式：任意两个波束聚焦向量间的相关函数、用户坐标失配时的波束赋形增益、导致波束赋形增益最快恶化的用户移动方向，以及特定性能损失所需的最小用户位移量。此外，我们设计了一种非均匀坐标网格，用于在每个位置估计时隙有效采样目标用户区域。大量仿真结果验证了理论分析的正确性，并证明了所提近场波束追踪方案相较于基准方法的优越性。