I. Introduction
5G mobile communication systems require high capacity backhaul and fronthaul. The cell radius becomes smaller, while the number of the base stations increases, which makes it impossible to connect all base stations with optical fiber. Therefore, the necessity of wireless backhaul and fronthaul systems is widely recognized. However, the existing wireless systems in the microwave and millimeter-wave range have limited capacity which cannot satisfy the demands in 5G. The orbital angular momentum (OAM) of the electromagnetic wave has been known since the 1990s [1], and the OAM mode-multiplexing in the field of radio communication has become common knowledge after the experiments by [2]. The OAM mode-multiplexing technology is viewed as a breakthrough technology for expanding the capacity of radio communications. Several methods and experimental results of OAM mode-multiplexing have been reported. Experiments using spiral phase plates or holographic plate have been reported in [3] [4]. A uniform circular array (UCA) has been studied in [5], and experiments using UCA are reported in [6] [7] [8] [9]. In these experiments, analog circuits are used to generate the OAM modes in transmitters and to separate the OAM modes in receivers. The transmission distance in these experiments is not more than 10 m.