I. Introduction
Beamforming using phased array antennas is being applied to Sub-6 frequencies and millimeter wave 5th generation mobile communication systems (5G), and as frequencies become higher, the need for Beyond 5G (6G) is increasing [12]. To implement a high-frequency phased array, a method of stacking multiple analog front-end boards containing antenna elements is currently being considered [3]–[5]. In such a system, it is necessary to wire multi-bit phase control bus between boards. For example, if a 4 x 4 phased array is implemented using 8-bit phase shifters, a 128-bit control bus is required between the DSP and the array antenna. This becomes one of the implementation issues.