Technical research for the 6th-generation mobile communication system (6G) expected to be implemented in the 2030s has been advanced to achieve extreme high data rate/capacity, extreme low latency, extreme high reliability, and extreme coverage extension for non-terrestrial areas. In this article, we introduce the following core wireless technologies developed by NTT Network Innovation Laboratories toward 6G, i.e., orbital angular momentum multiple-input multiple-output (OAM-MIMO) multiplexing transmission, underwater acoustic communication, and wireless-link-quality prediction.