Experiments on IMT-2000 using unmanned solar powered aircraft at an altitude of 20 km

Several countries are keenly interested in the research and development of novel wireless communication systems using high altitude platform stations (HAPS) because of their great potential benefits. In June and July 2002, we carried out the world's first experiment on International Mobile Telecommunication 2000 (IMT-2000) by radio relay via transponders onboard an unmanned solar powered aircraft at an altitude of 20 km. This experiment demonstrated for the first time the feasibility of applying the IMT-2000 system using a HAPS and a compact cellular phone designed for terrestrial cellular systems. Two experiments were carried out over Kauai Island in Hawaii, USA. In the first experiment, we were unable to establish an IMT-2000 link connection via the aircraft at 20 km because the onboard transponder for the reverse link received unexpected signals from a terrestrial cellular system in commercial use 200 km away on Oahu Island. However, we managed to establish the link by connecting a high gain external antenna to the cellular phone. After the first flight experiment, we developed an onboard receiving antenna with a low sidelobe level to suppress interference signals. In the second experiment, we successfully achieved IMT-2000 video communication with a cellular phone. This paper presents the experimental results and analysis of these IMT-2000 communication experiments.