For eMBB communication using millimeter-wave band, degradation of communication quality due to shielding is a major issue. In this report, as a part of the research on the cooperative beamforming technique for multiple radio units (RUs), which minimizes the degradation of the communication quality due to the shielding, we propose a scheme to realize the synchronization between multiple RUs. We plan to verify our scheme by simulations based on Open-Air-Interface.