Proposal of Antenna Selection Method for Terminal Antenna with Orthogonal Polarizations and Patterns for Outdoor Multiuser MIMO System

Multi-Input Multi-Output (MIMO) technology, which employs several antennas, enhances wireless data transmission even with a limited bandwidth [1]. Considering a multiuser environment, the spatial multiplexing of several users, which is called multiuser MIMO (MU-MIMO), is more effective in enhancing the total data transmission ratio than single user MIMO [2]. Various types of MIMO antennas have been studied and, in particular, small MIMO antennas and their downsizing techniques have been investigated for application to portable terminals [3]. Combining antennas that employ orthogonal polarizations and patterns is an important means to configure MIMO antennas into a compact configuration since this method provides a low level of antenna coupling even with a narrow antenna spacing [4]. The authors proposed a six-port compact MIMO antenna, and this antenna can be actualized within the size of a PC card by means of combining the antennas with orthogonal polarizations and patterns [5]. It was shown that the MIMO performance of the antenna is almost the same as that for three orthogonal dipoles, in which the size reduction of the dipole elements is not considered. On the other hand, the number of transmitters or receivers in mobile terminals is restricted in terms of battery power and hardware complexity. The antenna selection in MIMO systems is effective in extending the channel capacity without increasing the number of transmitters or receivers [6]. The use of both the combination of the orthogonal antennas and antenna selection is the key toward achieving a compact MIMO terminal with high data transmission performance even when the power consumption and size of the terminal are limited. The polarization selection method for Space Division Multiple Access (SDMA) was investigated and verified under outdoor environment conditions [7], however, the size restrictions and coupling effect of the small antennas must be taken into account. We need to extend this consideration to multi-antenna cooperative systems with practical small antenna configurations in order to consider the MU-MIMO performance. In this report, we propose an antenna selection method suitable for MU-MIMO terminals with orthogonal polarization and patterns. Two of three orthogonal antennas are selected based on only the signal-to-noise ratio (SNR) information. From the results of the measured channel properties in an urban microcell environment, the multiaccess channel (MAC) capacity is estimated and shown under the proposed antenna selection conditions, and the effectiveness of the proposed method is discussed.