Because of growing demand in recent years of mobile communications, it is aimed to achieve more than multi-Gbps transmission speed in 5th Generation （5G） mobile networks. It is indispensable to develop more high-speed wireless backhaul systems to connect flexibly between mobile base station and core network. Therefore, JRC proposes a full duplex radio system in same polarization, Directional Division Duplex （DDD）. To investigate the feasibility, we have developed and fabricated the radio equipment for 40 GHz-band fixed wireless system which employs DDD, and performed a field experiment of the DDD radio system. The DDD is a radio duplex system that transmits and receives at the same time by using the same radio frequency channel and same polarization, and DDD radio system can ideally achieve twice frequency utilization efficiency as that of the traditional duplex system of Time Division Duplex （TDD） or Frequency Division Duplex （FDD） .