Single-Layer Microstripline Third-Order Butler Matrix and Its Application to Orbital Angular Momentum Multiplexing

This paper proposes a third-order Butler matrix to extend orbital angular momentum (OAM) multiplexing to orders that are multiples of three for short-range wireless communications in fog/edge computing applications. The proposed network employs a single-layer microstripline structure, facilitating simplified fabrication and miniaturization. To validate its performance, we fabricated a prototype and conducted network evaluations and OAM multiplexing transmission experiments at a center frequency of 2.5 GHz. At this frequency, the prototype exhibited key characteristics required for OAM mode generation: low insertion loss (< 0.44 dB), well-balanced amplitude (error < 0.41 dB) and precise phase characteristics among the ports (error < 1.7 deg). Furthermore, transmission experiments were conducted using four array configurations under various conditions to evaluate system performance. In particular, the circularly polarized array demonstrated notable robustness compared to a co-polarized array, maintaining high mode orthogonality and a high achievable rate even under axial rotation between the transmitting and receiving arrays. This work contributes not only to the simplification, miniaturization, and low-power consumption of networks for edge devices but also to enhancing the scalability of seamless communication for fog/edge computing by enabling flexible multiplexing tailored to adaptive use cases.