Extremely large-scale array (XL-array) has emerged as a promising technology to enable near-field communications for achieving enhanced spectrum efficiency and spatial resolution, by drastically increasing the number of antennas. However, this also inevitably incurs higher hardware and energy cost, which may not be affordable in future wireless systems. To address this issue, we propose in this paper to exploit two types of sparse arrays (SAs) for enabling near-field communications. Specifically, we first consider the linear sparse array (LSA) and characterize its near-field beam pattern. It is shown that despite the achieved beam-focusing gain, the LSA introduces several undesired grating-lobes, which have comparable beam power with the main-lobe and are focused on specific regions. An efficient hybrid beamforming design is then proposed for the LSA to deal with the potential strong inter-user interference (IUI). Next, we consider another form of SA, called extended coprime array (ECA), which is composed of two LSA subarrays with different (coprime) inter-antenna spacing. By characterizing the ECA near-field beam pattern, we show that compared with the LSA with the same array sparsity, the ECA can greatly suppress the beam power of near-field grating-lobes thanks to the offset effect of the two subarrays, albeit with a larger number of grating-lobes. This thus motivates us to propose a customized two-phase hybrid beamforming design for the ECA. Finally, numerical results are presented to demonstrate the rate performance gain of the proposed two SAs over the conventional uniform linear array (ULA).

翻译：超大规模阵列（XL-array）通过大幅增加天线数量，已成为实现近场通信以提升频谱效率和空间分辨率的潜在技术。然而，这不可避免地导致更高硬件与能量成本，可能无法适应未来无线系统需求。针对该问题，本文提出利用两种稀疏阵列（SAs）赋能近场通信。具体而言，首先考虑线性稀疏阵列（LSA）并刻画其近场波束模式。研究表明，尽管LSA能实现波束聚焦增益，但其会引入若干与主瓣功率相当且聚焦于特定区域的非期望栅瓣。为此，我们针对LSA提出一种高效混合波束赋形设计方案以应对潜在的强用户间干扰（IUI）。其次，考虑另一种稀疏阵列形式——扩展互质阵列（ECA），该阵列由两个具有不同（互质）天线间距的LSA子阵列组成。通过分析ECA近场波束模式，我们发现：与相同稀疏度的LSA相比，ECA虽然栅瓣数量更多，但得益于两个子阵列的偏移效应，其近场栅瓣波束功率可被大幅抑制。这启发我们为ECA设计定制化的两阶段混合波束赋形方案。最后，数值结果展示了所提两种稀疏阵列相较于传统均匀线性阵列（ULA）的速率性能增益。