Contents 1. Introduction
Radio services have been widely prevalent and become crucial not only for our life and business but also for advancement of science and technology. Thanks to rapid and widespread commercialization of mobile and Wi-Fi devices, the mobile services were exponentially grown all over the world. The demand for radio access has been further expanding due to the high-speed wireless terminals [1]. The current deployment of mobile services shows a tendency towards higher bit rates with higher radio carrier frequencies in order to satisfy customer demands for higher speed wireless access. In sensor networks with the radio technology as another example of radio services, while the data rate generated from each sensor will not be so high, a huge number of sensors will be ubiquitously distributed. Since a radio terminal in a sensor should be as low power consumption as possible, the communication distance becomes short. To realize such radio services, a cell size must become smaller from the physical viewpoint, leading to drastic increases in the number of antennas for radio access in order to cover a wide area [2]. To aggregate many small cells, the collaboration of the other transmission systems will not only be unavoidable but also contribute strongly to the offload of growing mobile traffic.
