Impact of antenna pattern directivity on near-field focusing of planar array in large intelligent surface applications

Near-field focusing (NFF) is a fundamental enabler for a broad spectrum of applications utilizing large intelligent surfaces (LIS) within sub-10 GHz frequency bands. This paper investigates the impact of element pattern on NFF properties, when the LIS array is a large planar array. The NFF properties of maximum received power, focal width, and focal depth are analyzed for three typical antenna patterns of varying directivities at both the transmitter and receiver, along the array broadside. The results reveal that the peak received power occurs for the omni-directional pattern when the focal point approaches the array. In contrast, as the directivity increases, the peak power occurs when the focal point is farther from the array. Moreover, the peak received power positions of any given pair of transmitting-receiving antenna patterns is linearly proportional to array dimension. In addition, the more directive antenna has slightly broader focal width at distances closer to the array plane. The focal width approximates a Bessel function when the distance is close to the array plane, whereas the focal depth is narrower, and the sidelobe level lower. Similar behaviors are observed when the receiving antenna is laterally offset from the array broadside.