| With the rapid development of wireless communication technologies and data services,the demand for information transmission rate and transmission capacity is getting higher.Transmission rate of conventional wireless communication systems is limited due to problems such as multipath fading of wireless channels and scarcity of available spectrum resources.MIMO technology can establish multiple parallel transmission channels between multiple antennas of the transceiver system,and uses these parallel channels to greatly increase the data transmission rate without adding additional operating bands and transmission power,which has become one of the core technologies for 4G and 5G.As a crucial device,the performance of antennas plays an important role in the quality of the entire communication system.The terminal devices are developing in the direction of multi-function and miniaturization,and how to design a high-isolation and high-performance MIMO antenna in a compact space is a challenge.Therefore,this thesis studies the high-isolation and high-performance MIMO antennas for terminal devices.The related theory of MIMO antennas,and miniaturization technologies,decoupling technologies and three antenna forms widely used in terminal device are introduced in the thesis.Based on practical requirement,three high-isolation MIMO antennas are designed.1.A dipole-like MIMO antenna with horizontal omnidirectional radiation characteristics and high isolation is designed.The antenna unit derives from dipole antenna,and consists of an IFA branch and a monopole branch,which has a balanced radiation mode.Then,the low-band and high-band antenna units are designed by using the proposed structure,and a dual-band four-port MIMO antenna is formed.The designed MIMO antenna can cover 2.4-2.5GHz and 5.1-5.9GHz bands with reflection coefficient less than-10 dB,and the isolation in the operating band is higher than 19dB without any additional decoupling structures.2.A PIFA antenna with reduced ground currents effects is proposed,and a MIMO antenna system with horizontal omnidirectional radiation and high isolation is designed based on the proposed antenna.Firstly,based on the structure of traditional PIFA antenna,the ground current of the antenna is limited by introducing a slab for capacitive loading.The low-band and high-band antenna elements are respectively designed by using the proposed structure,and then a dual-band four-port MIMO antenna is formed.To further restrain the ground current,three rows of shorting pins are loaded surrounding the feeding port,so that the current on the ground just distributes surrounding the antenna,increasing the horizontal gain.The designed MIMO antenna can cover 2.4-2.5GHz and 5.1-5.9GHz bands with reflection coefficient less than-10 dB,and the isolation in the operating band is more than 20dB.3.An eight-port MIMO antenna for 5G mobile phones is designed.The antenna unit operates in the 3.5 GHz band and has a compact size of only 7×5.2 mm~2 by employing a coupling feed method and a bended shorting branch.In order to improve the impedance matching and isolation,a T-shaped slot is etched on the system ground between the adjacent antennas.The designed eight-port MIMO antenna's reflection coefficient is less than-10 dB within the 3.4-3.6 GHz band,and the isolation between any two ports is higher than 16 dB.The calculated envelope correlation coefficient(ECC)between any two antennas is less than 0.02,which indicates that the proposed antenna has a competitive MIMO performance. |
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