Wideband compact antennas for MIMO wireless communications MIMO無線通信のための広帯域コンパクトアンテナに関する研究
Wideband compact antennas for MIMO wireless communications
Le, Dinh Thanh
レ, ディン タン
Recent years, the demand of high data rates in wireless communication systems has rapidly increased. Among the promising candidates responding to the demand, multi-input multi-output (MIMO) systems have been received much research attention. Compared with the conventional singleinput single-output (SISO), a MIMO system can improve channel capacity without upgrading the transmitter’s power supply and widening the bandwidth. In MIMO techniques, antenna issues such as operating bandwidth, element radiation patterns, array configuration, element polarization, mutual coupling, and array size may have strong effects on channel capacity. Therefore, these issues should be taken into account in new antenna designs for MIMO systems. Recently, many types of compact MIMO antennas have been introduced. However, there are still several drawbacks such as high mutual coupling, narrow bandwidth, and large size. This dissertation will make a contribution on new designs of MIMO antennas with main focus on wide operating bandwidth and compact size issues. Furthermore, experiments on MIMO schemes, which utilize proposed antennas, are also conducted with detailed discussions and results. Firstly, two compact wideband MIMO antennas with tri-polarization are proposed. One has three ports formed in an “H” shape, and the other has six ports formed in a cube. Antenna elements are formed in compact sizes, yet mutual couplings between them are kept under -18 dB. These MIMO antennas can offer a relative bandwidth of over 16%. In addition, several measurements of MIMO systems utilizing the proposed antennas in two typical environments, line-of-sight (LOS) and non-line-of-sight (NLOS) propagations, have been carried. The results of these measurements show wideband MIMO characteristics of the antennas. Secondly, we propose a simple broadband antenna that is suitable for broadband wireless applications. The antenna can offer a bandwidth of over 50%. In addition, two MIMO antennas, of which elements are similar to the broadband antenna, are proposed. One consists of two ports, and the other consists of four ports. Measurement results show that mutual coupling between ports in these MIMO antennas are kept under -10 dB at low frequency region, and -20 dB at high frequency region of the bandwidth of the MIMO antennas. Furthermore, we utilized the broadband MIMO antennas in some MIMO experiments to examine channel capacity in a wide frequency range. Measurement results indicate the change of channel capacity over a wide frequency range. Finally, basing on the 50%-bandwidth antenna above, we introduce a novel ultra-wideband (UWB) antenna. This antenna offers a relative bandwidth of 95.5%, covering a frequency range of 3.5-9.9 GHz, making it suitable for UWB communications. Furthermore, since its design is simple and compact, we may utilize the antenna to form UWB-MIMO antennas.