Compact 3D Constellation Based on Equilateral-Square Pyramid for OCDM Over Hollow-Core Fiber

In this letter, a compact three-dimensional (3D) constellation based on equilateral-square pyramid (ESP) is proposed for orthogonal chirp division multiplexing (OCDM) over hollow-core fiber (HCF). By exploiting the equal-length property of the base and side edges in the ESP unit, 16- and 32-point constellations with enhanced neighbor density, spatial compactness, and inherent scalability are constructed. The modular ESP structure allows flexible extension to higher-order constellations without compromising geometric regularity. Integrated into a 3D-OCDM system using two-dimensional inverse discrete Fresnel transform (2D-IDFnT) modulation, the proposed scheme is experimentally validated over a 2 km HCF link. Compared with conventional cube-shaped orthogonal frequency division multiplexing (Cube-OFDM), the ESP-OCDM system achieves sensitivity improvements of 0.69 dB and 1.28 dB for the 16- and 32-point formats, respectively, demonstrating superior performance and strong potential for scalable high-density 3D modulation.