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Extremely large-scale array (XL-array) has emerged as a promising technologyto enable near-field communications for achieving enhanced spectrum efficiencyand spatial resolution, by drastically increasing the number of antennas.However, this also inevitably incurs higher hardware and energy cost, which maynot be affordable in future wireless systems. To address this issue, we proposein this paper to exploit two types of sparse arrays (SAs) for enablingnear-field communications. Specifically, we first consider the linear sparsearray (LSA) and characterize its near-field beam pattern. It is shown thatdespite the achieved beam-focusing gain, the LSA introduces several undesiredgrating-lobes, which have comparable beam power with the main-lobe and arefocused on specific regions. An efficient hybrid beamforming design is thenproposed 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, weshow that compared with the LSA with the same array sparsity, the ECA cangreatly suppress the beam power of near-field grating-lobes thanks to theoffset effect of the two subarrays, albeit with a larger number ofgrating-lobes. This thus motivates us to propose a customized two-phase hybridbeamforming design for the ECA. Finally, numerical results are presented todemonstrate the rate performance gain of the proposed two SAs over theconventional uniform linear array (ULA).

Sparse Array Enabled Near-Field Communications: Beam Pattern Analysis and Hybrid Beamforming Design